@article {1522191, title = {Evaluating Carlquist{\textquoteright}s Law from a physiological perspective}, journal = {IAWA Journal}, year = {2023}, abstract = {{\textquotedblleft}Carlquist{\textquoteright}s Law{\textquotedblright} is a striking pattern of association between anatomical features in the wood of vessel-bearing plants. It derives from Sherwin Carlquist{\textquoteright}s observation that xylem vessels tend to be solitary when embedded in a matrix of imperforate tracheary elements that appear to be conductive, whereas xylem vessels tend to be grouped when surrounded by seemingly non-conductive cells. Vessel-vessel contacts (vessel grouping) allow water to travel between conduits, but also provide pathways for air to propagate from embolized (air-filled) vessels into functional vessels. If the background matrix is conductive, it is conceivable that water could bypass embolized vessels, providing an alternative transport route in species with conductive backgrounds and solitary vessels. Much remains to be tested in this hypothesis, including the topology of the vessel networks in species with solitary versus grouped vessels and how conductive the different imperforate tracheary element types are. Exploring Carlquist{\textquoteright}s Law promises to provide key insight into the causes of embolism in plant conduits, the modes of embolism passage between conduits, and how vessels and the cells in which they are imbedded may interact to govern the pathways of water flow through plants.}, url = {https://brill.com/view/journals/iawa/aop/article-10.1163-22941932-bja10134/article-10.1163-22941932-bja10134.xml?rskey=jNvkER\&result=1}, author = {Kate M. Johnson and Sophie R. Everbach and N. Michele Holbrook and Mark E. Olson} } @article {1522221, title = {Sieve tube structural variation in Austrobaileya scandens and its significance for lianescence}, journal = {Plant, Cell \& Environment }, volume = {45}, year = {2022}, pages = {2460-2475}, abstract = {Lianas combine large leaf areas with slender stems, features that require an efficient vascular system. The only extant member of the Austrobaileyaceae is an endemic twining liana of the tropical Australian forests with well-known xylem hydraulics, but the vascular phloem continuum aboveground remains understudied. Microscopy analysis across leaf vein orders and stems of\ Austrobaileya\ scandens\ revealed a low foliar xylem:phloem ratio, with isodiametric vascular elements along the midrib, but tapered across vein orders. Sieve plate pore radii increased from 0.08 {\textmu}m in minor veins to 0.12 {\textmu}m in the petiole, but only to 0.20 {\textmu}m at the stem base, tens of metres away. In easily bent searcher branches, phloem conduits have pectin-rich walls and simple plates, whereas in twining stems, conduits were connected through highly angled and densely porated sieve plates. The hydraulic resistance of phloem conduits in the twisted and elongated stems of\ A. scandens\ is large compared with trees of similar stature; phloem hydraulic resistance decreases from leaves to stems, consistent with the efficient delivery of photoassimilates from sources under M{\"u}nch predictions. Sink strength of a continuously growing canopy might be stronger than in self-supporting understory plants, favoring resource allocation to aerial organs and the attainment of vertical stature.}, url = { https://doi.org/10.1111/pce.14361}, author = {Juan M. Losada and Zhe He and N. Michele Holbrook} } @article {1522201, title = {A tale to astonish: Ant-Man at the plasmodesmal gates}, journal = {Journal of Plant Physiology}, volume = {261}, year = {2021}, url = {https://doi.org/10.1016/j.jplph.2021.153431}, author = {Melissa H. Mai and N. Michele Holbrook} } @article {1393227, title = {Idioblasts and peltate hairs as distribution networks for water absorbed by xerophilous leaves.}, journal = {Plant Cell Environ}, year = {2020}, pages = {1{\textendash} 15}, url = {https://doi.org/10.1111/pce.13985}, author = {J.M. Losada and D{\'\i}az, M. and N. M. Holbrook} } @article {1391499, title = {Combined influence of soil moisture and atmospheric evaporative demand is important for accurately predicting US maize yields }, journal = {Nature Food}, volume = {1}, year = {2020}, pages = {127-133}, author = {Rigden, A.J. and N.D. Mueller and N. M. Holbrook and N. Pillai and P. Huybers} } @article {1391497, title = {Ontogenetic scaling of phloem sieve tube hydraulic resistance with tree height in Quercus rubra}, journal = {American Journal of Botany }, volume = {107}, year = {2020}, pages = {852-863}, author = {Clerx, L.E. and F.E. Rockwell and J.A. Savage and N. M. Holbrook} } @article {1391494, title = {Advanced vascular function discovered in a widespread moss}, journal = {Nature Plants}, volume = {6}, year = {2020}, pages = {273-279}, author = {T. J. Brodribb and M. Carriqui and S. Delzon and S.A.M. McAdam and N. M. Holbrook} } @article {1391488, title = {Leaf carbon export and non-structural carbohydrates in relation to diurnal water dynamics in mature oak trees}, journal = {Plant Physiology}, volume = {183}, year = {2020}, pages = {1612{\textendash}1621}, author = {Gersony, J.T. and U. Hochberg and F.E. Rockwell and M. Park and P.P. Gauthier and N. M. Holbrook} } @article {1380812, title = {Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species}, journal = {Plant, Cell \& Environment}, volume = {43}, year = {2020}, pages = {3048{\textendash} 3067}, url = {https://doi.org/10.1111/pce.13891}, author = {Ziemi{\'n}ska, K and Rosa, E and Gleason, SM and N. M. Holbrook} } @article {1391503, title = {Scaling of phloem hydraulic resistance in stems and leaves of the understory angiosperm shrub Illicium parviflorum (Illiciaceae)}, journal = {American Journal of Botany }, volume = {106}, year = {2019}, pages = {244-259}, author = {Losada, J. and N. M. Holbrook} } @article {1391501, title = {Visualizing embolism propagation in gas-injected leaves}, journal = {Plant Physiology}, volume = {180}, year = {2019}, pages = {874-881}, author = {U. Hochberg and A. Ponomarenko and Y.-J. Zhang and F.E. Rockwell and N. M. Holbrook} } @article {1153309, title = {Impact of hemlock woolly adelgid (Adelges tsugae) infestation on xylem structure and function and leaf physiologyin eastern hemlock (Tsuga canadensis)}, journal = {Functional Plant Biology}, volume = {45}, year = {2018}, pages = {501{\textendash}508}, url = {https://doi.org/10.1071/FP17233}, author = {Brett A. Huggett and Jessica A. Savage and Guang-You Hao and Evan L. Preisser and N. Michele Holbrook} } @article {1153306, title = {Where does M{\"u}nch flow begin? Sucrose transport in the pre-phloem path}, journal = {Current Opinion in Plant Biology}, volume = {43}, year = {2018}, pages = {101-107}, url = {https://www.sciencedirect.com/science/article/pii/S1369526617301760}, author = {Fulton E Rockwell and Jessica T Gersony and N Michele Holbrook} } @article {1153305, title = {Physiology and metabolism: Phloem: a supracellular highway for the transport of sugars, signals, and pathogens}, journal = {Current Opinion in Plant Biology}, volume = {43}, year = {2018}, pages = {iii-vii}, url = {http://dx.doi.org/10.1016/j.pbi.2018.05.013}, author = {N. Michele Holbrook and Michael Knoblauch} } @article {1153304, title = {Coordinated responses of plant hydraulic architecture with the reduction of stomatal conductance under elevated CO2 concentration}, journal = {Tree Physiology}, volume = {38}, year = {2018}, pages = {1041{\textendash}1052}, url = {https://academic.oup.com/treephys/article/38/7/1041/4833883}, author = {Guang-You Hao and N. Michele Holbrook and Maciej A. Zwieniecki and Vincent P. Gutschick and Hormoz BassiriRad} } @article {1153303, title = {Iso/Anisohydry: A Plant{\textendash}Environment Interaction Rather Than a Simple Hydraulic Trait}, journal = {Trends in Plant Science}, volume = {23}, year = {2018}, pages = {112-120}, author = {UriHochberg and Fulton E. Rockwell and N. Michele Holbrook and Herv{\'e} Cochard} } @article {1153301, title = {Comparing different methods for determining forest evapotranspiration and its components at multiple temporal scales}, journal = {Science of The Total Environment}, volume = {633}, year = {2018}, pages = {12-29}, author = {QiangTie and Hongchang Hu and Fuqiang Tian and N. Michele Holbrook} } @article {1391510, title = {Stomatal Closure, Basal Leaf Embolism, and Shedding Protect the Hydraulic Integrity of Grape Stems}, journal = {Plant Physiology}, volume = {174}, year = {2017}, pages = {764-775}, author = {U. Hochberg and A. Ponomarenko and F.E. Rockwell and N. M. Holbrook} } @article {1391509, title = {Divergences in hydraulic architecture form an important basis for niche differentiations between diploid and polyploid Betula species in NE China}, journal = {Tree Physiology}, volume = {37}, year = {2017}, pages = {604-616}, author = {Zhang, W.-W. and J. Song and M. Wang and Y.-Y. Liu and Y.-J. Zhang and N. M. Holbrook and G.-Y. Hao} } @article {1391508, title = {Global relationships between cropland intensification and summer temperature extremes over the last 50 years}, journal = {Journal of Climate}, volume = {30}, year = {2017}, pages = {7505-7528}, author = {N.D. Mueller and A. Rhines and D.K. Ray and S. Siebert and N. M. Holbrook and P. Huybers} } @article {1391507, title = {Leaf hydraulic architecture and stomatal conductance: a functional perspective}, journal = {Plant Physiology}, volume = {174}, year = {2017}, pages = {1996-2007}, author = {F.E. Rockwell and N. M. Holbrook} } @article {1391506, title = {Maintenance of carbohydrate transport in tall trees}, journal = {Nature Plants}, volume = {3}, year = {2017}, pages = {965 - 972}, author = {J.A. Savage and S.D. Beecher and L. Clerx and Gersony, J.T. and J. Knoblauch and J.M. Losada and K. H. Jensen and M. Knoblauch and N. M. Holbrook} } @article {1391511, title = {Reversible Leaf Xylem Collapse: A Potential "Circuit Breaker" against Cavitation}, journal = {Plant Physiology}, volume = {172}, year = {2016}, pages = {2261-2274}, author = {Y.-J. Zhang and F.E. Rockwell and A.C. Graham and T. Alexander and N. M. Holbrook} } @article {850021, title = {Testing the M{\"u}nch hypothesis of long distance phloem transport in plants}, journal = {eLife}, volume = {5}, year = {2016}, month = {2 June 2016}, abstract = {

Long distance transport in plants occurs in sieve tubes of the phloem. The pressure flow hypothesis introduced by Ernst M{\"u}nch in 1930 describes a mechanism of osmotically generated pressure differentials that are supposed to drive the movement of sugars and other solutes in the phloem, but this hypothesis has long faced major challenges. The key issue is whether the conductance of sieve tubes, including sieve plate pores, is sufficient to allow pressure flow. We show that with increasing distance between source and sink, sieve tube conductivity and turgor increases dramatically inIpomoea nil. Our results provide strong support for the M{\"u}nch hypothesis, while providing new tools for the investigation of one of the least understood plant tissues.

}, author = {Michael Knoblauch and Jan Knoblauch and Daniel L Mullendore and Jessica A Savage and Benjamin A Babst and Sierra D Beecher and Adam C Dodgen and Kaare H Jensen and Noel Michele Holbrook} } @article {930016, title = {Easy Come, Easy Go: Capillary Forces Enable Rapid Refilling of Embolized Primary Xylem Vessels}, journal = {Plant Physiology American Society of Plant Biologists }, year = {2015}, month = {June 2015}, pages = {1636-1647}, abstract = {

Protoxylem plays an important role in the hydraulic function of vascular systems of both herbaceous and woody plants, but relatively little is known about the processes underlying the maintenance of protoxylem function in long-lived tissues. In this study, embolism repair was investigated in relation to xylem structure in two cushion plant species, Azorella macquariensis and Colobanthus muscoides, in which vascular water transport depends on protoxylem. Their protoxylem vessels consisted of a primary wall with helical thickenings that effectively formed a pit channel, with the primary wall being the pit channel membrane. Stem protoxylem was organized such that the pit channel membranes connected vessels with paratracheal parenchyma or other protoxylem vessels and were not exposed directly to air spaces. Embolism was experimentally induced in excised vascular tissue and detached shoots by exposing them briefly to air. When water was resupplied, embolized vessels refilled within tens of seconds (excised tissue) to a few minutes (detached shoots) with water sourced from either adjacent parenchyma or water-filled vessels. Refilling occurred in two phases: (1) water refilled xylem pit channels, simplifying bubble shape to a rod with two menisci; and (2) the bubble contracted as the resorption front advanced, dissolving air along the way. Physical properties of the protoxylem vessels (namely pit channel membrane porosity, hydrophilic walls, vessel dimensions, and helical thickenings) promoted rapid refilling of embolized conduits independent of root pressure. These results have implications for the maintenance of vascular function in both herbaceous and woody species, because protoxylem plays a major role in the hydraulic systems of leaves, elongating stems, and roots.

}, author = {Rolland, V. and D.M. Bergstrom and T. Lenne and G. Bryant and H.Chen and J. Wolfe and N M. Holbrook and D. E. Stanton and M.C. Ball} } @article {924906, title = {Impacts of elevated atmospheric CO2 on nutrient content of important food crops}, journal = {Scientific Data}, year = {2015}, month = {July 2015}, author = {Lee H. Dietterich and Antonella Zanobetti and Itai Kloog and Peter Huybers and Andrew D.B. Leakey and Arnold J. Bloom and Eli Carlisle and Nimesha Fernando and Glenn Fitzgerald and Toshihiro Hasegawa and N. Michele Holbrook and Randall L. Nelson and Robert Norton and Michael J. Ottman and Victor Raboy and Hidemitsu Sakai and Karla A. Sartor and Joel Schwartz and Saman Seneweera and Yasuhiro Usui and Satoshi Yoshinaga and Samuel S. Myers} } @article {924886, title = {The tomato plastidic fructokinase SIFRK3 plays a role in xylem development}, journal = {New Phytologist }, year = {2015}, pages = {1484-1495}, author = {Ofer Stein and Hila Damari-Weissler and Francesca Secchi and Shimon Rachamilevitch and Marcelo A. German and Yelena Yeselson and Rachel Amir and Arthur Schaffer and N. Michele Holbrook and Roni Aloni and Maciej A. Zwieniecki and David Granot} } @article {844726, title = {A site for sori: Ecophysiology of fertile{\textendash}sterile leaf dimorphy in ferns}, journal = {American journal of botany }, volume = {103}, year = {2015}, month = {2016}, pages = {845-855}, abstract = {

PREMISE OF THE STUDY:\ Reproduction often requires significant investment and can move resources away from growth and maintenance; maintaining a balance between reproduction and growth can involve trade-offs. Extreme functional specialization has separated reproduction and photosynthesis in most seed plants, yet ferns use the laminar surface of their fronds for both reproduction and photosynthesis. This dual function selects for a variety of frond morphologies that range from no specialization (monomorphy) to extreme dimorphy between fertile and sterile fronds (holodimorphy). Here we examined the ecological and physiological consequences of variation in frond dimorphy in ferns, evaluated reproductive trade-offs across a dimorphy gradient, and speculate on factors controlling the occurrence of holodimorphy.

METHODS:\ Ecophysiological measurements of photosynthetic rate, water potential, hydraulic conductivity, and gross morphological comparisons of frond area and angle were used to evaluate differences between fertile and sterile fronds. We examined three temperate and three tropical fern species that vary in degree of fertile{\textendash}sterile dimorphy.

KEY RESULTS:\ Holodimorphic species produced fewer fertile fronds, which had significantly higher respiratory rates than in sterile fronds on the same plant or in any frond produced on monomorphic species; hemidimorphic species were frequently intermediate. We found no differences in vulnerability to cavitation between fertile and sterile fronds. In dimorphic species, fertile fronds had higher (less negative) water potential and lower stipe hydraulic conductivity relative than in sterile fronds.

CONCLUSIONS:\ Fertile{\textendash}sterile dimorphy in ferns appears to come at considerable carbon cost in holodimorohic species. It is possible that the relative costs of this reproductive system are offset by increased spore dispersal, yet such trade-offs require further exploration.

}, author = {James E. Watkins and Amber C. Churchill and Noel Michele Holbrook} } @article {778551, title = {Cooling of US Midwest summer temperature extremes from cropland intensification}, journal = {Nature Climate Change }, year = {2015}, url = {http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2825.html}, author = {Nathaniel D. Mueller, and Ethan E. Butler and Karen A. McKinnon and Andrew Rhines and Marin Tingley and N. Michele Holbrook and Peter Huybers} } @article {552431, title = {Scaling of phloem structure and optimality of photoassimilate transport in conifer needles}, journal = {Proc Biol Sci}, volume = {282}, number = {1801}, year = {2015}, note = {

Ronellenfitsch, HenrikLiesche, JohannesJensen, Kaare HHolbrook, N MicheleSchulz, AlexanderKatifori, EleniengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2015/01/09 06:00Proc Biol Sci. 2015 Feb 22;282(1801):20141863. doi: 10.1098/rspb.2014.1863.

}, month = {Feb 22}, pages = {20141863}, abstract = {

The phloem vascular system facilitates transport of energy-rich sugar and signalling molecules in plants, thus permitting long-range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 (Picea omorika) to 35 cm (Pinus palustris). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits and takes into account the transport strategy and natural constraints. This minimal model predicts a power law phloem distribution consistent with transport energy minimization, suggesting that energetics are more important than translocation speed at the leaf level.

}, isbn = {1471-2954 (Electronic)0962-8452 (Linking)}, author = {Ronellenfitsch, H. and Liesche, J. and Jensen, K. H. and Holbrook, N. M. and Schulz, A. and Katifori, E.} } @article {930311, title = {Leaf hydraulics I: Scaling transport properties from single cells to tissue}, journal = {Journal of Theoretical Biology}, volume = {340}, year = {2014}, month = {October 2013}, pages = {251-266}, abstract = {

In leaf tissues, water may move through the symplast or apoplast as a liquid, or through the airspace as vapor, but the dominant path remains in dispute. This is due, in part, to a lack of models that describe these three pathways in terms of experimental variables. We show that, in plant water relations theory, the use of a hydraulic capacity in a manner analogous to a thermal capacity, though it ignores mechanical interactions between cells, is consistent with a special case of the more general continuum mechanical theory of linear poroelasticity. The resulting heat equation form affords a great deal of analytical simplicity at a minimal cost: we estimate an expected error of less than 12\%, compared to the full set of equations governing linear poroelastic behavior. We next consider the case for local equilibrium between protoplasts, their cell walls, and adjacent air spaces during isothermal hydration transients to determine how accurately simple volume averaging of material properties (a {\textquoteleft}composite{\textquoteright} model) describes the hydraulic properties of leaf tissue. Based on typical hydraulic parameters for individual cells, we find that a composite description for tissues composed of thin walled cells with air spaces of similar size to the cells, as in photosynthetic tissues, is a reasonable preliminary assumption. We also expect isothermal transport in such cells to be dominated by the aquaporin-mediated cell-to-cell path. In the non- isothermal case, information on the magnitude of the thermal gradients is required to assess the dominant phase of water transport, liquid or vapor.

}, author = {Fulton E. Rockwell and N. Michele Holbrook and Abraham D. Stroock} } @article {930306, title = {The Physicochemical Hydrodynamics of Vascular Plants}, journal = {Annual Review of Fluid Mechanics}, volume = {46}, year = {2014}, month = {2014}, pages = {615-642}, abstract = {

Plants live dangerously, but gracefully. To remain hydrated, they exploit liquid water in the thermodynamically metastable state of negative pressure, similar to a rope under tension. This tension allows them to pull water out of the soil and up to their leaves. When this liquid rope breaks, owing to cavitation, they catch the ends to keep it from unraveling and then bind it back together. In parallel, they operate a second vascular system for the circulation of metabolites though their tissues, this time with positive pressures and flow that passes from leaf to root. In this article, we review the current state of understanding of water management in plants with an emphasis on the rich coupling of transport phenomena, thermodynamics, and active biological processes. We discuss efforts to replicate plant function in synthetic systems and point to opportunities for physical scientists and engineers to benefit from and contribute to the study of plants.

}, author = {Stroock, A. D. and Vinay V. Pagay and M. A. Zwieniecki and N. M. Holbrook} } @article {930036, title = {Payback time for soil carbon and sugar-cane ethanol}, journal = {nature climate change}, year = {2014}, month = {June 2014}, pages = {605-609}, author = {Mello, F.C. and C.E.P. Cerri and C.A. Davies and N. M. Holbrook and K. Paustian and S.M.F. Maia and M.V. Galdos and M. Bernoux and C.C. Cerri} } @article {552421, title = {Cavitation and its discontents: opportunities for resolving current controversies}, journal = {Plant Physiol}, volume = {164}, number = {4}, year = {2014}, note = {

Rockwell, Fulton EWheeler, James KHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Review2014/02/07 06:00Plant Physiol. 2014 Apr;164(4):1649-60. doi: 10.1104/pp.113.233817. Epub 2014 Feb 5.

}, month = {Apr}, pages = {1649-60}, abstract = {

Cavitation has long been recognized as a key constraint on the structure and functional integrity of the xylem. Yet, recent results call into question how well we understand cavitation in plants. Here, we consider embolism formation in angiosperms at two scales. The first focuses on how air-seeding occurs at the level of pit membranes, raising the question of whether capillary failure is an appropriate physical model. The second addresses methodological uncertainties that affect our ability to infer the formation of embolism and its reversal in plant stems. Overall, our goal is to open up fresh perspectives on the structure-function relationships of xylem.

}, keywords = {*Plant Physiological Phenomena, Air, Circadian Rhythm/physiology, Membranes/physiology, Models, Biological, Xylem/anatomy \& histology/*physiology}, isbn = {1532-2548 (Electronic)0032-0889 (Linking)}, author = {Rockwell, F. E. and Wheeler, J. K. and Holbrook, N. M.} } @article {552406, title = {The competition between liquid and vapor transport in transpiring leaves}, journal = {Plant Physiol}, volume = {164}, number = {4}, year = {2014}, note = {

Rockwell, Fulton EwingHolbrook, N MicheleStroock, Abraham DuncanengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2014/02/28 06:00Plant Physiol. 2014 Apr;164(4):1741-58. doi: 10.1104/pp.114.236323. Epub 2014 Feb 26.

}, month = {Apr}, pages = {1741-58}, abstract = {

In leaves, the transpirational flux of water exits the veins as liquid and travels toward the stomata in both the vapor and liquid phases before exiting the leaf as vapor. Yet, whether most of the evaporation occurs from the vascular bundles (perivascular), from the photosynthetic mesophyll cells, or within the vicinity of the stomatal pore (peristomatal) remains in dispute. Here, a one-dimensional model of the competition between liquid and vapor transport is developed from the perspective of nonisothermal coupled heat and water molecule transport in a composite medium of airspace and cells. An analytical solution to the model is found in terms of the energy and transpirational fluxes from the leaf surfaces and the absorbed solar energy load, leading to mathematical expressions for the proportions of evaporation accounted for by the vascular, mesophyll, and epidermal regions. The distribution of evaporation in a given leaf is predicted to be variable, changing with the local environment, and to range from dominantly perivascular to dominantly peristomatal depending on internal leaf architecture, with mesophyll evaporation a subordinate component. Using mature red oak (Quercus rubra) trees, we show that the model can be solved for a specific instance of a transpiring leaf by combining gas-exchange data, anatomical measurements, and hydraulic experiments. We also investigate the effect of radiation load on the control of transpiration, the potential for condensation on the inside of an epidermis, and the impact of vapor transport on the hydraulic efficiency of leaf tissue outside the xylem.

}, keywords = {Air, Biological Transport, Gases/*metabolism, Helianthus/physiology, Light, Models, Biological, Photons, photosynthesis, Plant Leaves/*physiology, Plant Transpiration/*physiology, Porosity, Quercus/physiology, Temperature, Water/*metabolism}, isbn = {1532-2548 (Electronic)0032-0889 (Linking)}, author = {Rockwell, F. E. and Holbrook, N. M. and Stroock, A. D.} } @article {552381, title = {Increasing CO2 threatens human nutrition}, journal = {Nature}, volume = {510}, number = {7503}, year = {2014}, note = {

Myers, Samuel SZanobetti, AntonellaKloog, ItaiHuybers, PeterLeakey, Andrew D BBloom, Arnold JCarlisle, EliDietterich, Lee HFitzgerald, GlennHasegawa, ToshihiroHolbrook, N MicheleNelson, Randall LOttman, Michael JRaboy, VictorSakai, HidemitsuSartor, Karla ASchwartz, JoelSeneweera, SamanTausz, MichaelUsui, Yasuhiroeng8UL1TR000170-0/TR/NCATS NIH HHS/Meta-AnalysisResearch Support, N.I.H., ExtramuralResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2014/05/09 06:00Nature. 2014 Jun 5;510(7503):139-42. doi: 10.1038/nature13179. Epub 2014 May 7.

}, month = {Jun 5}, pages = {139-42}, abstract = {

Dietary deficiencies of zinc and iron are a substantial global public health problem. An estimated two billion people suffer these deficiencies, causing a loss of 63 million life-years annually. Most of these people depend on C3 grains and legumes as their primary dietary source of zinc and iron. Here we report that C3 grains and legumes have lower concentrations of zinc and iron when grown under field conditions at the elevated atmospheric CO2 concentration predicted for the middle of this century. C3 crops other than legumes also have lower concentrations of protein, whereas C4 crops seem to be less affected. Differences between cultivars of a single crop suggest that breeding for decreased sensitivity to atmospheric CO2 concentration could partly address these new challenges to global health.

}, keywords = {*Nutritional Status, Air/analysis, Atmosphere/chemistry, Australia, Breeding, Carbon Dioxide/analysis/*pharmacology, Cereals/chemistry/drug effects/metabolism, Crops, Agricultural/*chemistry/*drug effects/metabolism, Diet, Fabaceae/chemistry/drug effects/metabolism, Global Health/trends, Humans, Iron/analysis/deficiency/metabolism, Japan, Nutritive Value/*drug effects, Photosynthesis/drug effects, Phytic Acid/analysis/metabolism, Public Health/*trends, United States, Zinc/analysis/deficiency/metabolism}, isbn = {1476-4687 (Electronic)0028-0836 (Linking)}, author = {Myers, S. S. and Zanobetti, A. and Kloog, I. and P. Huybers and Leakey, A. D. and Bloom, A. J. and Carlisle, E. and Dietterich, L. H. and Fitzgerald, G. and Hasegawa, T. and Holbrook, N. M. and Nelson, R. L. and Ottman, M. J. and Raboy, V. and Sakai, H. and Sartor, K. A. and Schwartz, J. and Seneweera, S. and Tausz, M. and Usui, Y.} } @article {552411, title = {Leaf hydraulics II: vascularized tissues}, journal = {J Theor Biol}, volume = {340}, year = {2014}, note = {

Rockwell, Fulton EHolbrook, N MicheleStroock, Abraham DengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2013/09/10 06:00J Theor Biol. 2014 Jan 7;340:267-84. doi: 10.1016/j.jtbi.2013.08.027. Epub 2013 Sep 4.

}, month = {Jan 7}, pages = {267-84}, abstract = {

Current models of leaf hydration employ an Ohm{\textquoteright}s law analogy of the leaf as an ideal capacitor, neglecting the resistance to flow between cells, or treat the leaf as a plane sheet with a source of water at fixed potential filling the mid-plane, neglecting the discrete placement of veins as well as their resistance. We develop a model of leaf hydration that considers the average conductance of the vascular network to a representative areole (region bounded by the vascular network), and represent the volume of tissue within the areole as a poroelastic composite of cells and air spaces. Solutions to the 3D flow problem are found by numerical simulation, and these results are then compared to 1D models with exact solutions for a range of leaf geometries, based on a survey of temperate woody plants. We then show that the hydration times given by these solutions are well approximated by a sum of the ideal capacitor and plane sheet times, representing the time for transport through the vasculature and tissue respectively. We then develop scaling factors relating this approximate solution to the 3D model, and examine the dependence of these scaling factors on leaf geometry. Finally, we apply a similar strategy to reduce the dimensions of the steady state problem, in the context of peristomatal transpiration, and consider the relation of transpirational gradients to equilibrium leaf water potential measurements.

}, keywords = {Acer/physiology, Ailanthus/physiology, Elasticity, Membrane Potentials, Models, Biological, Models, Theoretical, Plant Leaves/*physiology, Plant Physiological Phenomena, Plant Stomata/physiology, Plant Transpiration, Quercus/physiology, Species Specificity, Tilia/physiology, Trees/physiology, Water/chemistry}, isbn = {1095-8541 (Electronic)0022-5193 (Linking)}, author = {Rockwell, F. E. and Holbrook, N. M. and Stroock, A. D.} } @article {552471, title = {The making of giant pumpkins: how selective breeding changed the phloem of Cucurbita maxima from source to sink}, journal = {Plant Cell Environ}, year = {2014}, note = {

Savage, Jessica AHaines, Dustin FHolbrook, N MicheleENG2014/12/30 06:00Plant Cell Environ. 2014 Dec 25. doi: 10.1111/pce.12502.

}, month = {Dec 25}, abstract = {

Despite the success of breeding programmes focused on increasing fruit size, relatively little is known about the anatomical and physiological changes required to increase reproductive allocation. To address this gap in knowledge, we compared fruit/ovary anatomy, vascular structure and phloem transport of two varieties of giant pumpkins, and their smaller fruited progenitor under controlled environmental conditions. We also modelled carbon transport into the fruit of competitively grown plants using data collected in the field. There was no evidence that changes in leaf area or photosynthetic capacity impacted fruit size. Instead, giant varieties differed in their ovary morphology and contained more phloem on a cross-sectional area basis in their petioles and pedicels than the ancestral variety. These results suggest that sink activity is important in determining fruit size and that giant pumpkins have an enhanced capacity to transport carbon. The strong connection observed between carbon fixation, phloem structure and fruit growth in field-grown plants indicates that breeding for large fruit has led to changes throughout the carbon transport system that could have important implications for how we think about phloem transport velocity and carbon allocation.

}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Savage, J. A. and Haines, D. F. and Holbrook, N. M.} } @article {552191, title = {Measurement of flow velocity and inference of liquid viscosity in a microfluidic channel by fluorescence photobleaching}, journal = {Langmuir}, volume = {30}, number = {16}, year = {2014}, note = {

Carroll, Nick JJensen, Kaare HParsa, ShimaHolbrook, N MicheleWeitz, David AengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2014/04/16 06:00Langmuir. 2014 Apr 29;30(16):4868-74. doi: 10.1021/la404891g. Epub 2014 Apr 14.

}, month = {Apr 29}, pages = {4868-74}, abstract = {

We present a simple, noninvasive method for simultaneous measurement of flow velocity and inference of liquid viscosity in a microfluidic channel. We track the dynamics of a sharp front of photobleached fluorescent dye using a confocal microscope and measure the intensity at a single point downstream of the initial front position. We fit an exact solution of the advection diffusion equation to the fluorescence intensity recovery curve to determine the average flow velocity and the diffusion coefficient of the tracer dye. The dye diffusivity is correlated to solute concentration to infer rheological properties of the liquid. This technique provides a simple method for simultaneous elucidation of flow velocity and liquid viscosity in microchannels.

}, isbn = {1520-5827 (Electronic)0743-7463 (Linking)}, author = {Carroll, N. J. and Jensen, K. H. and Parsa, S. and Holbrook, N. M. and Weitz, D. A.} } @article {552386, title = {Modelled hydraulic redistribution by sunflower (Helianthus annuus L.) matches observed data only after including night-time transpiration}, journal = {Plant Cell Environ}, volume = {37}, number = {4}, year = {2014}, note = {

Neumann, Rebecca BCardon, Zoe GTeshera-Levye, JenniferRockwell, Fulton EZwieniecki, Maciej AHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2013/10/15 06:00Plant Cell Environ. 2014 Apr;37(4):899-910. doi: 10.1111/pce.12206. Epub 2013 Oct 24.

}, month = {Apr}, pages = {899-910}, abstract = {

The movement of water from moist to dry soil layers through the root systems of plants, referred to as hydraulic redistribution (HR), occurs throughout the world and is thought to influence carbon and water budgets and ecosystem functioning. The realized hydrologic, biogeochemical and ecological consequences of HR depend on the amount of redistributed water, whereas the ability to assess these impacts requires models that correctly capture HR magnitude and timing. Using several soil types and two ecotypes of sunflower (Helianthus annuus L.) in split-pot experiments, we examined how well the widely used HR modelling formulation developed by Ryel et al. matched experimental determination of HR across a range of water potential driving gradients. H. annuus carries out extensive night-time transpiration, and although over the last decade it has become more widely recognized that night-time transpiration occurs in multiple species and many ecosystems, the original Ryel et al. formulation does not include the effect of night-time transpiration on HR. We developed and added a representation of night-time transpiration into the formulation, and only then was the model able to capture the dynamics and magnitude of HR we observed as soils dried and night-time stomatal behaviour changed, both influencing HR.

}, keywords = {*Darkness, *Models, Biological, biomass, Helianthus/growth \& development/*physiology, Plant Roots/physiology, Plant Transpiration/*physiology, Soil, Water/*metabolism}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Neumann, R. B. and Cardon, Z. G. and Teshera-Levye, J. and Rockwell, F. E. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552351, title = {Relationship between hexokinase and the aquaporin PIP1 in the regulation of photosynthesis and plant growth}, journal = {PLoS One}, volume = {9}, number = {2}, year = {2014}, note = {

Kelly, GilorSade, NirAttia, ZivSecchi, FrancescaZwieniecki, MaciejHolbrook, N MicheleLevi, AsherAlchanatis, VictorMoshelion, MenachemGranot, DavidengResearch Support, Non-U.S. Gov{\textquoteright}t2014/02/06 06:00PLoS One. 2014 Feb 3;9(2):e87888. doi: 10.1371/journal.pone.0087888. eCollection 2014.

}, pages = {e87888}, abstract = {

Increased expression of the aquaporin NtAQP1, which is known to function as a plasmalemma channel for CO(2) and water, increases the rate of both photosynthesis and transpiration. In contrast, increased expression of Arabidopsis hexokinase1 (AtHXK1), a dual-function enzyme that mediates sugar sensing, decreases the expression of photosynthetic genes and the rate of transpiration and inhibits growth. Here, we show that AtHXK1 also decreases root and stem hydraulic conductivity and leaf mesophyll CO(2) conductance (g(m)). Due to their opposite effects on plant development and physiology, we examined the relationship between NtAQP1 and AtHXK1 at the whole-plant level using transgenic tomato plants expressing both genes simultaneously. NtAQP1 significantly improved growth and increased the transpiration rates of AtHXK1-expressing plants. Reciprocal grafting experiments indicated that this complementation occurs when both genes are expressed simultaneously in the shoot. Yet, NtAQP1 had only a marginal effect on the hydraulic conductivity of the double-transgenic plants, suggesting that the complementary effect of NtAQP1 is unrelated to shoot water transport. Rather, NtAQP1 significantly increased leaf mesophyll CO(2) conductance and enhanced the rate of photosynthesis, suggesting that NtAQP1 facilitated the growth of the double-transgenic plants by enhancing mesophyll conductance of CO(2).

}, keywords = {*Plant Development, Aquaporins/*metabolism, Arabidopsis Proteins/*metabolism, Arabidopsis/*growth \& development/metabolism, Biological Transport, Hexokinase/*metabolism, Lycopersicon esculentum/growth \& development/metabolism, Photosynthesis/*physiology, Plant Leaves/growth \& development/metabolism, Plant Roots/growth \& development/metabolism, Plant Stems/growth \& development/metabolism, Plant Transpiration, Plants, Genetically Modified/growth \& development/metabolism, Tobacco/growth \& development/metabolism, Water}, isbn = {1932-6203 (Electronic)1932-6203 (Linking)}, author = {Kelly, G. and Sade, N. and Attia, Z. and Secchi, F. and Zwieniecki, M. and Holbrook, N. M. and Levi, A. and Alchanatis, V. and Moshelion, M. and Granot, D.} } @article {552566, title = {Reversible Deformation of Transfusion Tracheids in Taxus baccata Is Associated with a Reversible Decrease in Leaf Hydraulic Conductance}, journal = {Plant Physiol}, volume = {165}, number = {4}, year = {2014}, note = {

Zhang, Yong-JiangRockwell, Fulton EWheeler, James KHolbrook, N MicheleENG2014/06/21 06:00Plant Physiol. 2014 Jun 19;165(4):1557-1565.

}, month = {Jun 19}, pages = {1557-1565}, abstract = {

Declines in leaf hydraulic conductance (Kleaf) with increasing water stress have been attributed to cavitation of the leaf xylem. However, in the leaves of conifers, the reversible collapse of transfusion tracheids may provide an alternative explanation. Using Taxus baccata, a conifer species without resin, we developed a modified rehydration technique that allows the separation of declines in Kleaf into two components: one reversible and one irreversible upon relaxation of water potential to -1 MPa. We surveyed leaves at a range of water potentials for evidence of cavitation using cryo-scanning electron microscopy and quantified dehydration-induced structural changes in transfusion tracheids by cryo-fluorescence microscopy. Irreversible declines in Kleaf did not occur until leaf water potentials were more negative than -3 MPa. Declines in Kleaf between -2 and -3 MPa were reversible and accompanied by the collapse of transfusion tracheids, as evidenced by cryo-fluorescence microscopy. Based on cryo-scanning electron microscopy, cavitation of either transfusion or xylem tracheids did not contribute to declines in Kleaf in the reversible range. Moreover, the deformation of transfusion tracheids was quickly reversible, thus acting as a circuit breaker regulating the flux of water through the leaf vasculature. As transfusion tissue is present in all gymnosperms, the reversible collapse of transfusion tracheids may be a general mechanism in this group for the protection of leaf xylem from excessive loads generated in the living leaf tissue.

}, isbn = {1532-2548 (Electronic)0032-0889 (Linking)}, author = {Zhang, Y. J. and Rockwell, F. E. and Wheeler, J. K. and Holbrook, N. M.} } @article {552561, title = {Seasonal dynamics in photosynthesis of woody plants at the northern limit of Asian tropics: potential role of fog in maintaining tropical rainforests and agriculture in Southwest China}, journal = {Tree Physiol}, volume = {34}, number = {10}, year = {2014}, note = {

Zhang, Yong-JiangHolbrook, N MicheleCao, Kun-FangengResearch Support, Non-U.S. Gov{\textquoteright}tCanada2014/10/10 06:00Tree Physiol. 2014 Oct;34(10):1069-78. doi: 10.1093/treephys/tpu083. Epub 2014 Oct 8.

}, month = {Oct}, pages = {1069-78}, abstract = {

The lowland tropical rainforests in Xishuangbanna, Southwest (SW) China, mark the northern limit of Asian tropics. Fog has been hypothesized to play a role in maintaining rainforests and tropical crop production in this region, but the physiological mechanism has not been studied. The goals of this study were to characterize the seasonal dynamics in photosynthesis and to assess the potential for fog to mitigate chilling-induced photodamage for tropical trees and crops in Xishuangbanna. We measured seasonal dynamics in light-saturated net photosynthetic rate (Aa), stomatal conductance (gs), intercellular CO2 concentration, quantum yield of Photosystem II (Fv/Fm) and maximum P700 changes (Pm; indicates the amount of active PSI complex), as well as chilling resistance and fog (light/shading) effects on low temperature-induced decline in Fv/Fm and Pm for native tree and introduced lower latitude tree or woody shrub species grown in a tropical botanical garden. Despite significant decreases in Aa, gs, Pm and Fv/Fm, most species maintained considerably high Aa during the cool season (2.51-14.6 mumol m(-2) s(-1)). Shaded leaves exposed to seasonal low temperatures had higher Fv/Fm than sun-exposed leaves in the cool season. All species could tolerate 1.4 degrees C in the dark, whereas a combined treatment of low temperature and high light caused a distinctly faster decline in Pm and Fv/Fm compared with low temperature treatment alone. Because fog persistence avoids or shortens the duration of high light condition in the morning when the temperatures are still low, our results provide support for the hypothesis that fog reduces chilling damage to tropical plants in this region and thus plays a role in maintaining tropical rainforests and agriculture in SW China.

}, isbn = {1758-4469 (Electronic)0829-318X (Linking)}, author = {Zhang, Y. J. and Holbrook, N. M. and Cao, K. F.} } @article {552556, title = {The stability of xylem water under tension: a long, slow spin proves illuminating}, journal = {Plant Cell Environ}, volume = {37}, number = {12}, year = {2014}, note = {

Zhang, Yong-JiangHolbrook, N MicheleengComment2014/06/20 06:00Plant Cell Environ. 2014 Dec;37(12):2652-3. doi: 10.1111/pce.12387. Epub 2014 Jul 17.

}, month = {Dec}, pages = {2652-3}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Zhang, Y. J. and Holbrook, N. M.} } @article {552396, title = {Water storage dynamics in the main stem of subtropical tree species differing in wood density, growth rate and life history traits}, journal = {Tree Physiol}, year = {2014}, note = {

Oliva Carrasco, LaureanoBucci, Sandra JDi Francescantonio, DeboraLezcano, Oscar ACampanello, Paula IScholz, Fabian GRodriguez, SabrinaMadanes, NCristiano, Piedad MHao, Guang-YouHolbrook, N MicheleGoldstein, GuillermoENG2014/11/28 06:00Tree Physiol. 2014 Nov 25. pii: tpu087.

}, month = {Nov 25}, abstract = {

Wood biophysical properties and the dynamics of water storage discharge and refilling were studied in the trunk of canopy tree species with diverse life history and functional traits in subtropical forests of northeast Argentina. Multiple techniques assessing capacitance and storage capacity were used simultaneously to improve our understanding of the functional significance of internal water sources in trunks of large trees. Sapwood capacitances of 10 tree species were characterized using pressure-volume relationships of sapwood samples obtained from the trunk. Frequency domain reflectometry was used to continuously monitor the volumetric water content in the main stems. Simultaneous sap flow measurements on branches and at the base of the tree trunk, as well as diurnal variations in trunk contraction and expansion, were used as additional measures of stem water storage use and refilling dynamics. All evidence indicates that tree trunk internal water storage contributes from 6 to 28\% of the daily water budget of large trees depending on the species. The contribution of stored water in stems of trees to total daily transpiration was greater for deciduous species, which exhibited higher capacitance and lower sapwood density. A linear relationship across species was observed between wood density and growth rates with the higher wood density species (mostly evergreen) associated with lower growth rates and the lower wood density species (mostly deciduous) associated with higher growth rates. The large sapwood capacitance in deciduous species may help to avoid catastrophic embolism in xylem conduits. This may be a low-cost adaptation to avoid water deficits during peak water use at midday and under temporary drought periods and will contribute to higher growth rates in deciduous tree species compared with evergreen ones. Large capacitance appears to have a central role in the rapid growth patterns of deciduous species facilitating rapid canopy access as these species are less shade tolerant than evergreen species.

}, isbn = {1758-4469 (Electronic)0829-318X (Linking)}, author = {Oliva Carrasco, L. and Bucci, S. J. and Di Francescantonio, D. and Lezcano, O. A. and Campanello, P. I. and Scholz, F. G. and Rodriguez, S. and Madanes, N. and Cristiano, P. M. and Hao, G. Y. and Holbrook, N. M. and Goldstein, G.} } @article {930321, title = {The role of leaf hydraulic conductance dynamics on the timing of leaf senescence}, journal = {Functional Plant Biology}, volume = {41}, year = {2013}, month = {August 2013}, pages = {37-47}, abstract = {

We tested the hypothesis that an age-dependent reduction in leaf hydraulic conductance (Kleaf) influences the timing of leaf senescence via limitation of the stomatal aperture on xylem compound delivery to leaves of tomato (Solanum lycopersicum L.), the tropical trees Anacardium excelsum Kunth, Pittoniotis trichantha Griseb, and the temperate trees Acer saccharum Marsh. and Quercus rubra L. The onset of leaf senescence was preceded by a decline in Kleaf in tomato and the tropical trees, but not in the temperate trees. Age-dependent changes in Kleaf in tomato were driven by a reduction in leaf vein density without a proportional increase in the xylem hydraulic supply. A decline in stomatal conductance accompanied Kleaf reduction with age in tomato but not in tropical and temperate tree species. Experimental manipulations that reduce the flow of xylem-transported compounds into leaves with open stomata induced early leaf senescence in tomato and A. excelsum, but not in P. trichantha, A. saccharum and Q. rubra leaves. We propose that in tomato, a reduction in Kleaf limits the delivery of xylem-transported compounds into the leaves, thus making them vulnerable to senescence. In the tropical evergreen tree A. excelsum, xylem-transported compounds may play a role in signalling the timing of senescence but are not under leaf hydraulic regulation; leaf senescence in the deciduous trees A. trichanta, A. saccharum and Q. rubra is not influenced by leaf vascular transport.

}, author = {Juan Pablo Giraldo and James K. Wheeler and Brett A. Huggett and N. Michele Holbrook} } @article {552546, title = {Cutting xylem under tension or supersaturated with gas can generate PLC and the appearance of rapid recovery from embolism}, journal = {Plant Cell Environ}, volume = {36}, number = {11}, year = {2013}, note = {

Wheeler, James KHuggett, Brett ATofte, Alena NRockwell, Fulton EHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2013/05/25 06:00Plant Cell Environ. 2013 Nov;36(11):1938-49. doi: 10.1111/pce.12139. Epub 2013 Jun 20.

}, month = {Nov}, pages = {1938-49}, abstract = {

We investigated the common assumption that severing stems and petioles under water preserves the hydraulic continuity in the xylem conduits opened by the cut when the xylem is under tension. In red maple and white ash, higher percent loss of conductivity (PLC) in the afternoon occurred when the measurement segment was excised under water at native xylem tensions, but not when xylem tensions were relaxed prior to sample excision. Bench drying vulnerability curves in which measurement samples were excised at native versus relaxed tensions showed a dramatic effect of cutting under tension in red maple, a moderate effect in sugar maple, and no effect in paper birch. We also found that air injection of cut branches (red and sugar maple) at pressures of 0.1 and 1.0 MPa resulted in PLC greater than predicted from vulnerability curves for samples cut 2 min after depressurization, with PLC returning to expected levels for samples cut after 75 min. These results suggest that sampling methods can generate PLC patterns indicative of repair under tension by inducing a degree of embolism that is itself a function of xylem tensions or supersaturation of dissolved gases (air injection) at the moment of sample excision. Implications for assessing vulnerability to cavitation and levels of embolism under field conditions are discussed.

}, keywords = {Acer/physiology, Air, Betula/physiology, Circadian Rhythm/physiology, Gases/*metabolism, Pressure, Trees/physiology, Water, Xylem/*physiology}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Wheeler, J. K. and Huggett, B. A. and Tofte, A. N. and Rockwell, F. E. and Holbrook, N. M.} } @article {552291, title = {Investigating xylem embolism formation, refilling and water storage in tree trunks using frequency domain reflectometry}, journal = {J Exp Bot}, volume = {64}, number = {8}, year = {2013}, note = {

Hao, Guang-YouWheeler, James KHolbrook, N MicheleGoldstein, GuillermoengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2013/04/16 06:00J Exp Bot. 2013 May;64(8):2321-32. doi: 10.1093/jxb/ert090. Epub 2013 Apr 12.

}, month = {May}, pages = {2321-32}, abstract = {

Trunks of large trees play an important role in whole-plant water balance but technical difficulties have limited most hydraulic research to small stems, leaves, and roots. To investigate the dynamics of water-related processes in tree trunks, such as winter embolism refilling, xylem hydraulic vulnerability, and water storage, volumetric water content (VWC) in the main stem was monitored continuously using frequency domain moisture sensors in adult Betula papyrifera trees from early spring through the beginning of winter. An air injection technique was developed to estimate hydraulic vulnerability of the trunk xylem. Trunk VWC increased in early spring and again in autumn, concurrently with root pressure during both seasons. Diurnal fluctuations and a gradual decrease in trunk VWC through the growing season were observed, which, in combination with VWC increase after significant rainfall events and depletion during periods of high water demand, indicate the importance of stem water storage in both short- and long-term water balance. Comparisons between the trunk air injection results and conventional branch hydraulic vulnerability curves showed no evidence of {\textquoteright}vulnerability segmentation{\textquoteright} between the main stem and small branches in B. papyrifera. Measurements of VWC following air injection, together with evidence from air injection and xylem dye perfusion, indicate that embolized vessels can be refilled by active root pressure but not in the absence of root pressure. The precise, continuous, and non-destructive measurement of wood water content using frequency domain sensors provides an ideal way to probe many hydraulic processes in large tree trunks that are otherwise difficult to investigate.

}, keywords = {Betula/physiology, Electromagnetic Radiation, Plant Roots/physiology, Plant Stems/*physiology, Rain, Seasons, Trees/*physiology, Water/analysis/metabolism, Wood/chemistry, Xylem/*physiology}, isbn = {1460-2431 (Electronic)0022-0957 (Linking)}, author = {Hao, G. Y. and Wheeler, J. K. and Holbrook, N. M. and Goldstein, G.} } @article {552346, title = {Optimal concentration for sugar transport in plants}, journal = {J R Soc Interface}, volume = {10}, number = {83}, year = {2013}, note = {

Jensen, Kaare HSavage, Jessica AHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2013/03/22 06:00J R Soc Interface. 2013 Mar 20;10(83):20130055. doi: 10.1098/rsif.2013.0055. Print 2013 Jun 6.

}, month = {Jun 6}, pages = {20130055}, abstract = {

Vascular plants transport energy in the form of sugars from the leaves where they are produced to sites of active growth. The mass flow of sugars through the phloem vascular system is determined by the sap flow rate and the sugar concentration. If the concentration is low, little energy is transferred from source to sink. If it is too high, sap viscosity impedes flow. An interesting question is therefore at which concentration is the sugar flow optimal. Optimization of sugar flow and transport efficiency predicts optimal concentrations of 23.5 per cent (if the pressure differential driving the flow is independent of concentration) and 34.5 per cent (if the pressure is proportional to concentration). Data from more than 50 experiments (41 species) collected from the literature show an average concentration in the range from 18.2 per cent (all species) to 21.1 per cent (active loaders), suggesting that the phloem vasculature is optimized for efficient transport at constant pressure and that active phloem loading may have developed to increase transport efficiency.

}, keywords = {*Carbohydrate Metabolism, Biological Transport, Carbohydrates/chemistry, Least-Squares Analysis, Phloem/metabolism/physiology, Plants/*metabolism, Pressure, Viscosity}, isbn = {1742-5662 (Electronic)1742-5662 (Linking)}, author = {Jensen, K. H. and Savage, J. A. and Holbrook, N. M.} } @article {552331, title = {Optimal concentrations in transport systems}, journal = {J R Soc Interface}, volume = {10}, number = {83}, year = {2013}, note = {

Jensen, Kaare HKim, WonjungHolbrook, N MicheleBush, John W MengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2013/04/19 06:00J R Soc Interface. 2013 Apr 17;10(83):20130138. doi: 10.1098/rsif.2013.0138. Print 2013 Jun 6.

}, month = {Jun 6}, pages = {20130138}, abstract = {

Many biological and man-made systems rely on transport systems for the distribution of material, for example matter and energy. Material transfer in these systems is determined by the flow rate and the concentration of material. While the most concentrated solutions offer the greatest potential in terms of material transfer, impedance typically increases with concentration, thus making them the most difficult to transport. We develop a general framework for describing systems for which impedance increases with concentration, and consider material flow in four different natural systems: blood flow in vertebrates, sugar transport in vascular plants and two modes of nectar drinking in birds and insects. The model provides a simple method for determining the optimum concentration copt in these systems. The model further suggests that the impedance at the optimum concentration muopt may be expressed in terms of the impedance of the pure (c = 0) carrier medium mu0 as muopt 2(alpha)mu0, where the power alpha is prescribed by the specific flow constraints, for example constant pressure for blood flow (alpha = 1) or constant work rate for certain nectar-drinking insects (alpha = 6). Comparing the model predictions with experimental data from more than 100 animal and plant species, we find that the simple model rationalizes the observed concentrations and impedances. The model provides a universal framework for studying flows impeded by concentration, and yields insight into optimization in engineered systems, such as traffic flow.

}, keywords = {*Carbohydrate Metabolism, *Drinking Behavior, *Models, Biological, Animals, Biological Transport, Biomechanical Phenomena, Birds/*physiology, Blood Circulation/*physiology, Insects/*physiology, Plants/metabolism, Transportation}, isbn = {1742-5662 (Electronic)1742-5662 (Linking)}, author = {Jensen, K. H. and Kim, W. and Holbrook, N. M. and Bush, J. W.} } @article {552476, title = {Phloem transport velocity varies over time and among vascular bundles during early cucumber seedling development}, journal = {Plant Physiol}, volume = {163}, number = {3}, year = {2013}, note = {

Savage, Jessica AZwieniecki, Maciej AHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2013/09/28 06:00Plant Physiol. 2013 Nov;163(3):1409-18. doi: 10.1104/pp.113.225359. Epub 2013 Sep 26.

}, month = {Nov}, pages = {1409-18}, abstract = {

We use a novel dye-tracing technique to measure in vivo phloem transport velocity in cucumber (Cucumis sativus) plants during early seedling development. We focus on seedlings because of their importance in plant establishment and because they provide a simple source and sink model of phloem transport. The dye-tracing method uses a photodiode to track the movement of a bleach front of fluorescent dye traveling in the phloem from the cotyledons (source) to the roots (sink). During early seedling development, phloem transport velocity in this direction can change 2-fold depending on vascular connectivity and the number of actively growing sinks. Prior to leaf expansion, vascular bundles attached to the first developing leaf demonstrate a decline in basipetal phloem transport that can be alleviated by the leaf{\textquoteright}s removal. At this stage, seedlings appear carbon limited and phloem transport velocity is correlated with cotyledon area, a pattern that is apparent both during cotyledon expansion and after source area manipulation. When the first leaf transitions to a carbon source, seedling growth rate increases and basipetal phloem transport velocity becomes more stable. Because bundles appear to operate autonomously, transport velocity can differ among vascular bundles. Together, these results demonstrate the dynamic and heterogeneous nature of phloem transport and underline the need for a better understanding of how changes in phloem physiology impact growth and allocation at this critical stage of development.

}, keywords = {Biological Transport, Cotyledon/growth \& development/metabolism, Cucumis sativus/growth \& development/*metabolism, Kinetics, Phloem/*metabolism, Plant Leaves/growth \& development/metabolism, Plant Vascular Bundle/*metabolism, Seedling/growth \& development/*metabolism, Time Factors}, isbn = {1532-2548 (Electronic)0032-0889 (Linking)}, author = {Savage, J. A. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552286, title = {Polyploidy enhances the occupation of heterogeneous environments through hydraulic related trade-offs in Atriplex canescens (Chenopodiaceae)}, journal = {New Phytol}, volume = {197}, number = {3}, year = {2013}, note = {

Hao, Guang-YouLucero, Mary ESanderson, Stewart CZacharias, Elizabeth HHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2012/12/05 06:00New Phytol. 2013 Feb;197(3):970-8. doi: 10.1111/nph.12051. Epub 2012 Dec 3.

}, month = {Feb}, pages = {970-8}, abstract = {

Plant hydraulic characteristics were studied in diploid, tetraploid and hexaploid cytotypes of Atriplex canescens (Chenopodiaceae) to investigate the potential physiological basis underlying the intraspecific habitat differentiation among plants of different ploidy levels. Populations of A. canescens from different habitats of the Chihuahuan Desert (New Mexico, USA) were analyzed using flow cytometry to determine ploidy levels. Traits related to xylem water transport efficiency and safety against drought-induced hydraulic failure were measured in both stems and leaves. At the stem level, cytotypes of higher ploidy showed consistently lower leaf-specific hydraulic conductivity but greater resistance to drought-induced loss of hydraulic conductivity. At the leaf level, comparisons in hydraulics between cytotypes did not show a consistent pattern, but exhibited high plasticity to proximal environmental conditions related to soil water availability. The results suggest that a trade-off between stem hydraulic efficiency and safety across ploidy levels underlies niche differentiation among different cytotypes of A. canescens. Polyploidization may have been facilitated by environmental heterogeneity related to water availability, and variation in water-related physiology found in the present study suggests an important functional basis for the niche differentiation and coexistence of A. canescens cytotypes in desert environments.

}, keywords = {*Polyploidy, Adaptation, Biological/genetics, Atriplex/*genetics/metabolism/physiology, Environment, New Mexico, Plant Leaves/metabolism, Water/*metabolism}, isbn = {1469-8137 (Electronic)0028-646X (Linking)}, author = {Hao, G. Y. and Lucero, M. E. and Sanderson, S. C. and Zacharias, E. H. and Holbrook, N. M.} } @article {930711, title = {Hydraulic differences along the water transport system of South American Nothofagus species: do leaves protect the stem functionality?}, journal = {Tree Physiology}, volume = {32}, year = {2012}, month = {June 2012}, pages = {880-893}, author = {Sandra J. Bucci and Fabian G. Scholz and Paula I. Campanello and Lia Montti and Mylthon Jimenez-Castillo and Fulton A. Rockwell and Ludmila La Manna and Pedro Guerra and Pablo Lopez Bernal and Oscar Troncoso and Juan Enricci and Michele N. Holbrook and Guillermo Goldstein} } @article {930336, title = {Measurements of stem xylem hydraulic conductivity in the laboratory and field}, journal = {Methods in Ecology and Evolution }, year = {2012}, month = {2012}, pages = {685-694}, author = {Peter J. Melcher and N. Michele Holbrook and Michael J. Burns and Maciej A. Zwieniecki and Alexander R. Cobb and Timothy J. Brodribb and Brendan Choat and Lawren Sack} } @article {552486, title = {Freeze/Thaw-induced embolism: probability of critical bubble formation depends on speed of ice formation}, journal = {Front Plant Sci}, volume = {3}, year = {2012}, note = {

Sevanto, SannaHolbrook, N MicheleBall, Marilyn CengSwitzerland2012/06/12 06:00Front Plant Sci. 2012 Jun 6;3:107. doi: 10.3389/fpls.2012.00107. eCollection 2012.

}, pages = {107}, abstract = {

Bubble formation in the conduits of woody plants sets a challenge for uninterrupted water transportation from the soil up to the canopy. Freezing and thawing of stems has been shown to increase the number of air-filled (embolized) conduits, especially in trees with large conduit diameters. Despite numerous experimental studies, the mechanisms leading to bubble formation during freezing have not been addressed theoretically. We used classical nucleation theory and fluid mechanics to show which mechanisms are most likely to be responsible for bubble formation during freezing and what parameters determine the likelihood of the process. Our results confirm the common assumption that bubble formation during freezing is most likely due to gas segregation by ice. If xylem conduit walls are not permeable to the salts expelled by ice during the freezing process, osmotic pressures high enough for air seeding could be created. The build-up rate of segregated solutes in front of the ice-water interface depends equally on conduit diameter and freezing velocity. Therefore, bubble formation probability depends on these variables. The dependence of bubble formation probability on freezing velocity means that the experimental results obtained for cavitation threshold conduit diameters during freeze/thaw cycles depend on the experimental setup; namely sample size and cooling rate. The velocity dependence also suggests that to avoid bubble formation during freezing trees should have narrow conduits where freezing is likely to be fast (e.g., branches or outermost layer of the xylem). Avoidance of bubble formation during freezing could thus be one piece of the explanation why xylem conduit size of temperate and boreal zone trees varies quite systematically.

}, isbn = {1664-462X (Electronic)1664-462X (Linking)}, author = {Sevanto, S. and Holbrook, N. M. and Ball, M. C.} } @article {552356, title = {Ion induced changes in the structure of bordered pit membranes}, journal = {Front Plant Sci}, volume = {3}, year = {2012}, note = {

Lee, JinkeeHolbrook, N MicheleZwieniecki, Maciej AengSwitzerland2012/05/31 06:00Front Plant Sci. 2012 Mar 21;3:55. doi: 10.3389/fpls.2012.00055. eCollection 2012.

}, pages = {55}, abstract = {

Ion-mediated changes in xylem hydraulic resistance are hypothesized to result from hydrogel like properties of pectins located in the bordered pit membranes separating adjacent xylem vessels. Although the kinetics of the ion-mediated changes in hydraulic resistance are consistent with the swelling/deswelling behavior of pectins, there is no direct evidence of this activity. In this report we use atomic force microscopy (AFM) to investigate structural changes in bordered pit membranes associated with changes in the ionic concentration of the surrounding solution. When submerged in de-ionized water, AFM revealed bordered pit membranes as relatively smooth, soft, and lacking any sharp edges surface, in contrast to pictures from scanning electron microscope (SEM) or AFM performed on air-dry material. Exposure of the bordered pit membranes to 50 mM KCl solution resulted in significant changes in both surface physical properties and elevation features. Specifically, bordered pit membranes became harder and the fiber edges were clearly visible. In addition, the membrane contracted and appeared much rougher due to exposed microfibers. In neither solution was there any evidence of discrete pores through the membrane whose dimensions were altered in response to the ionic composition of the surrounding solution. Instead the variable hydraulic resistance appears to involve changes in the both the permeability and the thickness of the pit membrane.

}, isbn = {1664-462X (Electronic)1664-462X (Linking)}, author = {Lee, J. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552256, title = {On measuring the response of mesophyll conductance to carbon dioxide with the variable J method}, journal = {J Exp Bot}, volume = {63}, number = {1}, year = {2012}, note = {

Gilbert, Matthew EdmundPou, AliciaZwieniecki, Maciej AndrzejHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2011/09/15 06:00J Exp Bot. 2012 Jan;63(1):413-25. doi: 10.1093/jxb/err288. Epub 2011 Sep 13.

}, month = {Jan}, pages = {413-25}, abstract = {

The response of mesophyll conductance to CO(2) (g(m)) to environmental variation is a challenging parameter to measure with current methods. The {\textquoteright}variable J{\textquoteright} technique, used in the majority of studies of g(m), assumes a one-to-one relationship between photosystem II (PSII) fluorescence and photosynthesis under non-photorespiratory conditions. When calibrating this relationship for Populus trichocarpa, it was found that calibration relationships produced using variation in light and CO(2) were not equivalent, and in all cases the relationships were non-linear-something not accounted for in previous studies. Detailed analyses were performed of whether different calibration procedures affect the observed g(m) response to CO(2). Past linear and assumed calibration methods resulted in systematic biases in the fluorescence estimates of electron transport. A sensitivity analysis on modelled data (where g(m) was held constant) demonstrated that biases in the estimation of electron transport as small as 2\% ( approximately 0.5 mumol m(-2) s(-1)) resulted in apparent changes in the relationship of g(m) to CO(2) of similar shape and magnitude to those observed with past calibration techniques. This sensitivity to biases introduced during calibrations leads to results where g(m) artefactually decreases with CO(2), assuming that g(m) is constant; if g(m) responds to CO(2), then biases associated with past calibration methods would lead to overestimates of the slope of the relationship. Non-linear calibrations were evaluated; these removed the bias present in past calibrations, but the method remained sensitive to measurement errors. Thus measurement errors, calibration non-linearities leading to bias, and the sensitivity of variable J g(m) hinders its use under conditions of varying CO(2) or light.

}, keywords = {Calibration, Carbon Dioxide/*metabolism, Populus/*metabolism, Spectrometry, Fluorescence}, isbn = {1460-2431 (Electronic)0022-0957 (Linking)}, author = {Gilbert, M. E. and Pou, A. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552341, title = {Modeling the hydrodynamics of Phloem sieve plates}, journal = {Front Plant Sci}, volume = {3}, year = {2012}, note = {

Jensen, Kaare HartvigMullendore, Daniel LeroyHolbrook, Noel MicheleBohr, TomasKnoblauch, MichaelBruus, HenrikengSwitzerland2012/07/20 06:00Front Plant Sci. 2012 Jul 13;3:151. doi: 10.3389/fpls.2012.00151. eCollection 2012.

}, pages = {151}, abstract = {

Sieve plates have an enormous impact on the efficiency of the phloem vascular system of plants, responsible for the distribution of photosynthetic products. These thin plates, which separate neighboring phloem cells, are perforated by a large number of tiny sieve pores and are believed to play a crucial role in protecting the phloem sap from intruding animals by blocking flow when the phloem cell is damaged. The resistance to the flow of viscous sap in the phloem vascular system is strongly affected by the presence of the sieve plates, but the hydrodynamics of the flow through them remains poorly understood. We propose a theoretical model for quantifying the effect of sieve plates on the phloem in the plant, thus unifying and improving previous work in the field. Numerical simulations of the flow in real and idealized phloem channels verify our model, and anatomical data from 19 plant species are investigated. We find that the sieve plate resistance is correlated to the cell lumen resistance, and that the sieve plate and the lumen contribute almost equally to the total hydraulic resistance of the phloem translocation pathway.

}, isbn = {1664-462X (Electronic)1664-462X (Linking)}, author = {Jensen, K. H. and Mullendore, D. L. and Holbrook, N. M. and Bohr, T. and Knoblauch, M. and Bruus, H.} } @article {552281, title = {Ecology of hemiepiphytism in fig species is based on evolutionary correlation of hydraulics and carbon economy}, journal = {Ecology}, volume = {92}, number = {11}, year = {2011}, note = {

Hao, Guang-YouGoldstein, GuillermoSack, LawrenHolbrook, N MicheleLiu, Zhi-HuiWang, Ai-YingHarrison, Rhett DSu, Zhi-HuiCao, Kun-FangengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2011/12/15 06:00Ecology. 2011 Nov;92(11):2117-30.

}, month = {Nov}, pages = {2117-30}, abstract = {

Woody hemiepiphytic species (Hs) are important components of tropical rain forests, and they have been hypothesized to differ from non-hemiepiphytic tree species (NHs) in adaptations relating to water relations and carbon economy; but few studies have been conducted comparing ecophysiological traits between the two growth forms especially in an evolutionary context. Using common-garden plants of the genus Ficus, functional traits related to plant hydraulics and carbon economy were compared for seven NHs and seven Hs in their adult terrestrial "tree-like" growth phase. We used phylogenetically independent contrasts to test the hypothesis that differences in water availability selected for contrasting suites of traits in Hs and NHs, driving evolutionary correlations among functional traits including hydraulic conductivity and photosynthetic traits. Species of the two growth forms differed in functional traits; Hs had substantially lower xylem hydraulic conductivity and stomatal conductance, and higher instantaneous photosynthetic water use efficiency. Leaf morphological and structural traits also differed strikingly between the two growth forms. The Hs had significantly smaller leaves, higher leaf mass per area (LMA), and smaller xylem vessel lumen diameters. Across all the species, hydraulic conductivity was positively correlated with leaf gas exchange indicating high degrees of hydraulic-photosynthetic coordination. More importantly, these correlations were supported by correlations implemented on phylogenetic independent contrasts, suggesting that most trait correlations arose through repeated convergent evolution rather than as a result of chance events in the deep nodes of the lineage. Vatiation in xylem hydraulic conductivity was also centrally associated with a suite of other functional traits related to carbon economy and growth, such as LMA, water use efficiency, leaf nutrient concentration, and photosynthetic nutrient use efficiency, indicating important physiological constraints or trade-offs among functional traits. Shifts in this trait cluster apparently related to the adaptation to drought-prone canopy growth during the early life cycle of Hs and clearly affected ecophysiology of the later terrestrial stage of these species. Evolutionary flexibility in hydraulics and associated traits might be one basis for the hyper-diversification of Ficus species in tropical rain forests.

}, keywords = {*Biological Evolution, Animals, Carbon/*metabolism, Ecosystem, Ficus/*genetics/*physiology, photosynthesis, Principal Component Analysis, Water/*metabolism}, isbn = {0012-9658 (Print)0012-9658 (Linking)}, author = {Hao, G. Y. and Goldstein, G. and Sack, L. and Holbrook, N. M. and Liu, Z. H. and Wang, A. Y. and Harrison, R. D. and Su, Z. H. and Cao, K. F.} } @article {552491, title = {Effects of the hydraulic coupling between xylem and phloem on diurnal phloem diameter variation}, journal = {Plant Cell Environ}, volume = {34}, number = {4}, year = {2011}, note = {

Sevanto, SannaHoltta, TeemuHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}t2011/01/19 06:00Plant Cell Environ. 2011 Apr;34(4):690-703. doi: 10.1111/j.1365-3040.2011.02275.x. Epub 2011 Feb 18.

}, month = {Apr}, pages = {690-703}, abstract = {

Measurements of diurnal diameter variations of the xylem and phloem are a promising tool for studying plant hydraulics and xylem-phloem interactions in field conditions. However, both the theoretical framework and the experimental verification needed to interpret phloem diameter data are incomplete. In this study, we analytically evaluate the effects of changing the radial conductance between the xylem and the phloem on phloem diameter variations and test the theory using simple manipulation experiments. Our results show that phloem diameter variations are mainly caused by changes in the radial flow rate of water between the xylem and the phloem. Reducing the hydraulic conductance between these tissues decreases the amplitude of phloem diameter variation and increases the time lag between xylem and phloem diameter variation in a predictable manner. Variation in the amplitude and timing of diameter variations that cannot be explained by changes in the hydraulic conductance, could be related to changes in the osmotic concentration in the phloem.

}, keywords = {Acer/anatomy \& histology/*physiology, Betula/anatomy \& histology/*physiology, Biological Transport, Models, Biological, Phloem/*anatomy \& histology/physiology, Plant Stems/anatomy \& histology/physiology, Plant Transpiration/physiology, Quercus/anatomy \& histology/*physiology, Time Factors, Water/physiology, Xylem/anatomy \& histology/*physiology}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Sevanto, S. and Holtta, T. and Holbrook, N. M.} } @article {552416, title = {Hydraulic conductivity of red oak (Quercus rubra L.) leaf tissue does not respond to light}, journal = {Plant Cell Environ}, volume = {34}, number = {4}, year = {2011}, note = {

Rockwell, F EHolbrook, N MZwieniecki, M Aeng2011/02/12 06:00Plant Cell Environ. 2011 Apr;34(4):565-79. doi: 10.1111/j.1365-3040.2011.02263.x. Epub 2011 Feb 11.

}, month = {Apr}, pages = {565-79}, abstract = {

The permeability of leaf tissue to water has been reported to increase under illumination, a response reputed to involve aquaporins. We studied this {\textquoteright}light response{\textquoteright} in red oak (Quercus rubra L.), the species in which the phenomenon was first detected during measurements of leaf hydraulic conductance with the high-pressure flow meter (HPFM). In our HPFM measurements, we found that pre-conditioning leaves in darkness was not sufficient to bring them to their minimum conductance, which was attained only after an hour of submersion and pressurization. However, pre-conditioning leaves under anoxic conditions resulted in an immediate reduction in conductance. Leaves light- and dark-acclimated while on the tree showed no differences in the time course of HPFM measurement under illumination. We also studied the effect of light level and anoxia on rehydration kinetics, finding that anoxia slowed rehydration, but light had no effect either in the lab (rehydration under low light, high humidity) or on the tree (acclimation under high light, 10 min of dark prior to rehydration). We conclude that the declines in conductance observed in the HPFM must involve a resistance downstream of the extracellular air space, and that in red oak the hydraulic conductivity of leaf tissue is insensitive to light.

}, keywords = {*Light, Aquaporins/metabolism, Biological Transport, Cryoelectron Microscopy, Gene Expression Regulation, Plant/radiation effects, Permeability/radiation effects, Plant Leaves/anatomy \& histology/physiology/radiation effects, Plant Stomata/anatomy \& histology, Plant Transpiration/*physiology/radiation effects, Quercus/anatomy \& histology/*physiology/radiation effects, Time Factors, Water/*metabolism}, isbn = {1365-3040 (Electronic)0140-7791 (Linking)}, author = {Rockwell, F. E. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552261, title = {Independent variation in photosynthetic capacity and stomatal conductance leads to differences in intrinsic water use efficiency in 11 soybean genotypes before and during mild drought}, journal = {J Exp Bot}, volume = {62}, number = {8}, year = {2011}, note = {

Gilbert, Matthew EZwieniecki, Maciej AHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2011/02/23 06:00J Exp Bot. 2011 May;62(8):2875-87. doi: 10.1093/jxb/erq461. Epub 2011 Feb 21.

}, month = {May}, pages = {2875-87}, abstract = {

Intrinsic water use efficiency (WUE(intr)), the ratio of photosynthesis to stomatal conductance to water, is often used as an index for crop water use in breeding projects. However, WUE(intr) conflates variation in these two processes, and thus may be less useful as a selection trait than knowledge of both components. The goal of the present study was to determine whether the contribution of photosynthetic capacity and stomatal conductance to WUE(intr) varied independently between soybean genotypes and whether this pattern was interactive with mild drought. Photosynthetic capacity was defined as the variation in WUE(intr) that would occur if genotypes of interest had the same stomatal conductance as a reference genotype and only differed in photosynthesis; similarly, the contribution of stomatal conductance to WUE(intr) was calculated assuming a constant photosynthetic capacity across genotypes. Genotypic differences in stomatal conductance had the greatest effect on WUE(intr) (26\% variation when well watered), and was uncorrelated with the effect of photosynthetic capacity on WUE(intr). Thus, photosynthetic advantages of 8.3\% were maintained under drought. The maximal rate of Rubisco carboxylation, generally the limiting photosynthetic process for soybeans, was correlated with photosynthetic capacity. As this trait was not interactive with leaf temperature, and photosynthetic capacity differences were maintained under mild drought, the observed patterns of photosynthetic advantage for particular genotypes are likely to be consistent across a range of environmental conditions. This suggests that it is possible to employ a selection strategy of breeding water-saving soybeans with high photosynthetic capacities to compensate for otherwise reduced photosynthesis in genotypes with lower stomatal conductance.

}, keywords = {*Droughts, Genotype, Photosynthesis/*physiology, Plant Stomata/*physiology, Soybeans/*genetics/*physiology, Vapor Pressure, Water/*metabolism}, isbn = {1460-2431 (Electronic)0022-0957 (Linking)}, author = {Gilbert, M. E. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552336, title = {Optimality of the Munch mechanism for translocation of sugars in plants}, journal = {J R Soc Interface}, volume = {8}, number = {61}, year = {2011}, note = {

Jensen, K HLee, JBohr, TBruus, HHolbrook, N MZwieniecki, M AengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2011/01/20 06:00J R Soc Interface. 2011 Aug 7;8(61):1155-65. doi: 10.1098/rsif.2010.0578. Epub 2011 Jan 18.

}, month = {Aug 7}, pages = {1155-65}, abstract = {

Plants require effective vascular systems for the transport of water and dissolved molecules between distal regions. Their survival depends on the ability to transport sugars from the leaves where they are produced to sites of active growth; a flow driven, according to the Munch hypothesis, by osmotic gradients generated by differences in sugar concentration. The length scales over which sugars are produced (Lleaf) and over which they are transported (L(stem)), as well as the radius r of the cylindrical phloem cells through which the transport takes place, vary among species over several orders of magnitude; a major unsettled question is whether the Munch transport mechanism is effective over this wide range of sizes. Optimization of translocation speed predicts a scaling relation between radius r and the characteristic lengths as r approximately (Lleaf Lstem)1/3. Direct measurements using novel in vivo techniques and biomimicking microfluidic devices support this scaling relation and provide the first quantitative support for a unified mechanism of sugar translocation in plants spanning several orders of magnitude in size. The existence of a general scaling law for phloem dimensions provides a new framework for investigating the physical principles governing the morphological diversity of plants.

}, keywords = {*Carbohydrates, *Models, Biological, Biological Transport/physiology, Carbohydrate Metabolism/*physiology, Microfluidic Analytical Techniques/instrumentation/methods, Phloem/*metabolism, Plants/*metabolism}, isbn = {1742-5662 (Electronic)1742-5662 (Linking)}, author = {Jensen, K. H. and Lee, J. and Bohr, T. and Bruus, H. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552361, title = {Structural and hydraulic correlates of heterophylly in Ginkgo biloba}, journal = {New Phytol}, volume = {189}, number = {2}, year = {2011}, note = {

Leigh, AZwieniecki, M ARockwell, F EBoyce, C KNicotra, A BHolbrook, N MengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2010/10/01 06:00New Phytol. 2011 Jan;189(2):459-70. doi: 10.1111/j.1469-8137.2010.03476.x. Epub 2010 Sep 29.

}, month = {Jan}, pages = {459-70}, abstract = {

This study investigates the functional significance of heterophylly in Ginkgo biloba, where leaves borne on short shoots are ontogenetically distinct from those on long shoots. Short shoots are compact, with minimal internodal elongation; their leaves are supplied with water through mature branches. Long shoots extend the canopy and have significant internodal elongation; their expanding leaves receive water from a shoot that is itself maturing. Morphology, stomatal traits, hydraulic architecture, Huber values, water transport efficiency, in situ gas exchange and laboratory-based steady-state hydraulic conductance were examined for each leaf type. Both structure and physiology differed markedly between the two leaf types. Short-shoot leaves were thinner and had higher vein density, lower stomatal pore index, smaller bundle sheath extensions and lower hydraulic conductance than long-shoot leaves. Long shoots had lower xylem area:leaf area ratios than short shoots during leaf expansion, but this ratio was reversed at shoot maturity. Long-shoot leaves had higher rates of photosynthesis, stomatal conductance and transpiration than short-shoot leaves. We propose that structural differences between the two G. biloba leaf types reflect greater hydraulic limitation of long-shoot leaves during expansion. In turn, differences in physiological performance of short- and long-shoot leaves correspond to their distinct ontogeny and architecture.

}, keywords = {Gases/metabolism, Ginkgo biloba/*anatomy \& histology/cytology/*physiology, Organ Size, Permeability, Plant Leaves/*anatomy \& histology/cytology/*physiology, Plant Shoots/anatomy \& histology, Water/*physiology}, isbn = {1469-8137 (Electronic)0028-646X (Linking)}, author = {Leigh, A. and Zwieniecki, M. A. and Rockwell, F. E. and Boyce, C. K. and Nicotra, A. B. and Holbrook, N. M.} } @article {930751, title = {Phenology, Lignotubers, and Water Relations of Cochlospermum vitifolium, a Pioneer Tropical Dry Forest Tree in Costa Rica}, journal = {Biotropica}, volume = {42}, year = {2010}, pages = {104-111}, abstract = {

We examined structural and physiological traits relevant to the phenology of the tropical dry forest (TDF) pioneer tree Cochlospermum vitifolium. Despite marked seasonality in rainfall, meristem activity occurred throughout the year. Leaves were produced almost continuously during the rainy season, while leaf shedding started early during drought, before changes in soil water content were observed. Phenological activity under drought included flowering and fruiting of leafless trees; bud break and shoot extension took place before the end of the dry season. Low wood density of C. vitifolium stems (0.17 g/cm3) and lignotubers (0.14 g/cm3) provided water and starch storage needed to support phenological events such as branch extension, leaf flushing, and reproduction during the dry season, and probably also contributed to survival following mechanical damage and fire, typical of early TDF successional stages. Lignotuber water and starch contents showed substantial seasonal variation, declining from the beginning of the dry season to their lowest levels at the time of reproduction and dry-season flushing. Stems progressively replaced lignotubers as main storage organs as tree size increased. Evidence for a role of water stores in buffering daily water deficits was weak. Leaf water potentials remained above 1.2 MPa and stomatal conductance below 350 mmol/m2/s, suggesting that gas exchange during the rainy season was limited to prevent xylem cavitation. Leaf shedding occurred when early-morning and mid-day CL converged at the rainy{\textendash}dry season transition, without changes in lignotuber or soil water content, suggesting that leaves of C. vitifolium are closely tuned to atmospheric drought.

Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp
Key words: dry-season flushing; fire; pioneer species; reserve storage; stem-succulent; stomatal conductance; succession.

}, author = {Lottie Fallas-Cedeno and N. Michele Holbrook and Oscar J. Rocha and Nelly Vasquez and Marco V. Gutierrez-Soto} } @article {552541, title = {Hydraulic properties of fern sporophytes: Consequences for ecological and evolutionary diversification}, journal = {Am J Bot}, volume = {97}, number = {12}, year = {2010}, note = {

Watkins, James E JrHolbrook, N MicheleZwieniecki, Maciej Aeng2011/05/28 06:00Am J Bot. 2010 Dec;97(12):2007-19. doi: 10.3732/ajb.1000124. Epub 2010 Nov 24.

}, month = {Dec}, pages = {2007-19}, abstract = {

* PREMISE OF THE STUDY: Ferns are an important component of both tropical and temperate forests; yet, our understanding of the water relations of their sporophyte generation is limited. Indeed, to date there has been no large scale survey that attempts to clarify how ferns fit into current ideas of plant water relations. This study examines several tropical ferns with the goal of understanding how these characters vary between species from various habitats and across life forms * METHODS: We measured stipe hydraulic conductivity, water potential, and vulnerability to cavitation along with photosynthetic variables and leaf allometry of 21 species from 14 genera to identify physiological trait assemblages across taxa. * KEY RESULTS: Epiphytic ferns have significantly lower hydraulic conductivity and a vascular system more resistant to cavitation (i.e., higher P(50) values). They reached lower mid-day water potentials and produced leaves with reduced stipe lengths and reduced laminar area relative to terrestrial species. Xylem specific hydraulic conductivity (K(S)) was correlated with the mean hydraulic diameter of tracheids in terrestrial species, but not in epiphytes. There was no evidence of safety-efficiency trade-offs in any group. * CONCLUSIONS: When compared across life forms, our data shed light on physiological mechanisms that may have allowed for terrestrial ferns to move into the epiphytic habit. When compared across a diverse assemblage of terrestrial plants, we find that resistance to water flow in fern stipes is significantly higher than that recorded from the stems of seed plants.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Watkins, J. E., Jr. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {931186, title = {Geographic distributions and physiological characteristics of co-existing Flaveria species in South-central Mexico}, journal = {Flora}, year = {2009}, month = {January 2008}, pages = {89-98}, abstract = {

The genus Flaveria consists of 23 species with significant variation in photosynthetic physiologies. We tested whether photosynthetic pathway variation in seven co-existing Flaveria species corresponds to geographic distributions or physiological performance in C3, C4, and intermediate species growing under natural conditions in south-central Mexico. We found that Flaveria pringlei (C3) was the most widely distributed species with multiple growth habits. Numerous populations of Flaveria kochiana (C4), a recently described species with a previously unknown distribution, were located in the Mixtec region of Oaxaca. Flaveria cronquistii (C3) and Flaveria ramosissima (C3-C4) were only located in the Tehuaca ́ n Valley region while Flaveria trinervia (C4) was widely distributed. Only one population of Flaveria angustifolia (C3-C4) and Flaveria vaginata (C4-like) were located near Izu ́ car de Matamoros. Midday leaf water potential differed significantly between Flaveria species, but did not vary according to growth habit or photosynthetic pathway. The quantum yield of photosystem II did not vary between species, despite large differences in leaf nitrogen content, leaf shape, plant size and life histories. We did not find a direct relationship between increasing C4 cycle characteristics and physiological performance in the Flaveria populations examined. Furthermore, C3 species were not found at higher elevation than C4 species as expected. Our observations indicate that life history traits and disturbance regime may be the primary controllers of Flaveria distributions in south-central Mexico.

r 2008 Elsevier GmbH. All rights reserved.

}, author = {Erika A. Sudderth and Fancisco J. Espinosa-Garcia and N. Michele Holbrook} } @article {931181, title = {LeFRK2 is required for phloem and xylem differentiation and the transport of both sugar and water}, journal = {Planta}, year = {2009}, month = {July 2009}, pages = {795-805}, author = {Hila Damari-Weissler and Shimon Rachamilevitch and Roni Aloni and Marcelo A. German and Shabtai Cohen and Maciej A. Zwieniecki and N. Michele Holbrook and David Granot} } @article {552586, title = {Confronting Maxwell{\textquoteright}s demon: biophysics of xylem embolism repair}, journal = {Trends Plant Sci}, volume = {14}, number = {10}, year = {2009}, note = {

Zwieniecki, Maciej AHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.ReviewEngland2009/09/04 06:00Trends Plant Sci. 2009 Oct;14(10):530-4. doi: 10.1016/j.tplants.2009.07.002. Epub 2009 Aug 31.

}, month = {Oct}, pages = {530-4}, abstract = {

Embolism results in a dramatic loss of xylem hydraulic transport capacity that can lead to decreased plant productivity and even death. The ability to refill embolized conduits despite the presence of tension in the xylem seems to be widespread, but how this occurs is not known. To promote discussion and future research on this topic, we describe how we believe refilling under tension might take place. Our scenario includes: (i) an osmotic role for low-molecular weight sugars; (ii) an apoplastic sugar-sensing mechanism to activate refilling; (iii) the contribution of vapor transport in both the influx of water and removal of entrapped gases; and (iv) the need for a mechanism that can synchronize reconnection to the transpiration stream through multiple bordered pits.

}, keywords = {Biological Transport/*physiology, Models, Biological, Plant Transpiration/physiology, Water/metabolism, Xylem/*metabolism}, isbn = {1878-4372 (Electronic)1360-1385 (Linking)}, author = {Zwieniecki, M. A. and Holbrook, N. M.} } @article {552326, title = {Tensioning the helix: a mechanism for force generation in twining plants}, journal = {Proc Biol Sci}, volume = {276}, number = {1667}, year = {2009}, note = {

Isnard, SandrineCobb, Alexander RHolbrook, N MicheleZwieniecki, MaciejDumais, JacquesengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2009/04/24 09:00Proc Biol Sci. 2009 Jul 22;276(1667):2643-50. doi: 10.1098/rspb.2009.0380. Epub 2009 Apr 22.

}, month = {Jul 22}, pages = {2643-50}, abstract = {

Twining plants use their helical stems to clasp supports and to generate a squeezing force, providing stability against gravity. To elucidate the mechanism that allows force generation, we measured the squeezing forces exerted by the twiner Dioscorea bulbifera while following its growth using time-lapse photography. We show that the development of the squeezing force is accompanied by stiffening of the stem and the expansion of stipules at the leaf base. We use a simple thin rod model to show that despite their small size and sparse distribution, stipules impose a stem deformation sufficient to account for the measured squeezing force. We further demonstrate that tensioning of the stem helix, although counter-intuitive, is the most effective mechanism for generating large squeezing forces in twining plants. Our observations and model point to a general mechanism for the generation of the twining force: a modest radial stem expansion during primary growth, or the growth of lateral structures such as leaf bases, causes a delayed stem tensioning that creates the squeezing forces necessary for twining plants to ascend their supports. Our study thus provides the long-sought answer to the question of how twining plants ascend smooth supports without the use of adhesive or hook-like structures.

}, keywords = {Biomechanical Phenomena/*physiology, Dioscorea/*growth \& development, Plant Stems/*growth \& development, Pressure}, isbn = {0962-8452 (Print)0962-8452 (Linking)}, author = {Isnard, S. and Cobb, A. R. and Holbrook, N. M. and Zwieniecki, M. and Dumais, J.} } @article {552466, title = {Water relations of Chusquea ramosissima and Merostachys claussenii in Iguazu National Park, Argentina}, journal = {Plant Physiol}, volume = {149}, number = {4}, year = {2009}, note = {

Saha, SonaliHolbrook, Noel MMontti, LiaGoldstein, GuillermoCardinot, Gina KnustengResearch Support, Non-U.S. Gov{\textquoteright}t2009/02/13 09:00Plant Physiol. 2009 Apr;149(4):1992-9. doi: 10.1104/pp.108.129015. Epub 2009 Feb 11.

}, month = {Apr}, pages = {1992-9}, abstract = {

Bamboos are prominent components of many tropical ecosystems, yet little is known about the physiological mechanisms utilized by these gigantic forest grasses. Here, we present data on the water transport properties of Chusquea ramosissima and Merostachys claussenii, monocarpic bamboo grasses native to the subtropical Atlantic forests of Argentina. C. ramosissima and M. claussenii differed in their growth form and exhibited contrasting strategies of water transport. Maximum xylem hydraulic conductivity of C. ramosissima culms was 2-fold higher than that of M. claussenii. C. ramosissima cavitated at relatively high water potentials (50\% loss of conductivity at \>or=1 MPa), whereas M. claussenii was more drought tolerant (50\% loss at \<or=3 mpa).="" both="" species="" exhibited="" significant="" loss="" of="" hydraulic="" conductivity="" during="" the="" day,="" which="" was="" reversed="" overnight="" due="" to="" generation="" root="" pressure.="" photosynthetic="" capacities="" bamboo="" species,="" estimated="" based="" on="" electron="" transport="" rates,="" were="" moderate,="" reflecting="" large="" amount="" leaf="" area="" supported="" by="" culms="" and="" diurnal="" cavitation.="" conductance="" also="" relatively="" low="" for="" congruent="" with="" their="" modest="" capacities.="" within="" its="" native="" range,="" c.="" ramosissima="" is="" highly="" invasive="" ability="" colonize="" persist="" in="" forest="" gaps="" land="" cleared="" agriculture.="" we="" propose="" that="" a="" vulnerable="" vasculature,="" coupled="" pressure="" an="" allometry="" allows="" substantial="" be="" slender="" culms,="" are="" key="" traits="" contributing="" ecological="" success="" ramosissima.\<="" div=""\>

}, keywords = {*Conservation of Natural Resources, Argentina, Chlorophyll/metabolism, Circadian Rhythm/physiology, Climate, Droughts, Fluorescence, Plant Roots/physiology, Plant Stomata/physiology, Poaceae/*physiology, Pressure, Water/*physiology, Xylem/physiology}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Saha, S. and Holbrook, N. M. and Montti, L. and Goldstein, G. and Cardinot, G. K.} } @article {931191, title = {Transporting water to the top of trees}, journal = {Physics Today }, year = {2008}, month = {2008}, pages = {76-77}, author = {N. Michele Holbrook and Maciej A. Zwieniecki} } @article {552551, title = {Cell-to-cell pathway dominates xylem-epidermis hydraulic connection in Tradescantia fluminensis (Vell. Conc.) leaves}, journal = {Planta}, volume = {227}, number = {6}, year = {2008}, note = {

Ye, QingHolbrook, N MicheleZwieniecki, Maciej AengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Germany2008/02/15 09:00Planta. 2008 May;227(6):1311-9. doi: 10.1007/s00425-008-0703-7. Epub 2008 Feb 14.

}, month = {May}, pages = {1311-9}, abstract = {

A steady supply of water is indispensable for leaves to fulfil their photosynthetic function. Understanding water movement in leaves, especially factors that regulate the movement of water flux from xylem to epidermis, requires that the nature of the transport pathway be elucidated. To determine the hydraulic linkage between xylem and epidermis, epidermal cell turgor pressure (P (t)) in leaves of Tradescantia fluminensis was monitored using a cell pressure probe in response to a 0.2 MPa step change in xylem pressure applied at the leaf petiole. Halftime of P (t) changes (T(x)(1/2)) were 10-30 times greater than that of water exchange across an individual cell membrane (T(m)(1/2)) suggesting that cell-to-cell water transport constitutes a significant part of the leaf hydraulic path from xylem to epidermis. Furthermore, perfusion of H(2)O(2) resulted in increases of both T(m)(1/2) and T(x)(1/2) by a factor of 2.5, indicating that aquaporins may play a role in the xylem to epidermis hydraulic link. The halftime for water exchange (T(m)(1/2)) did not differ significantly between cells located at the leaf base (2.5 s), middle (2.6 s) and tip (2.5 s), indicating that epidermal cell hydraulic properties are similar along the length of the leaf. Following the pressure application to the xylem (0.2 MPa), P (t) changed by 0.12, 0.06 and 0.04 MPa for epidermal cells at the base, middle and the tip of the leaf, respectively. This suggests that pressure dissipation between xylem and epidermis is significant, and that the pressure drop along the vein may be due to its structural similarities to a porous pipe, an idea which was further supported by measurements of xylem hydraulic resistance using a perfusion technique.

}, keywords = {Cell Communication, Cell Physiological Phenomena, Cell Wall/physiology, Elasticity, Plant Epidermis/cytology/*physiology, Plant Leaves/cytology/*physiology, Pressure, Tradescantia/cytology/*physiology, Xylem/cytology/*physiology}, isbn = {0032-0935 (Print)0032-0935 (Linking)}, author = {Ye, Q. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552441, title = {Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption}, journal = {Proc Natl Acad Sci U S A}, volume = {105}, number = {5}, year = {2008}, note = {

Sack, LawrenDietrich, Elisabeth MStreeter, Christopher MSanchez-Gomez, DavidHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2008/01/30 09:00Proc Natl Acad Sci U S A. 2008 Feb 5;105(5):1567-72. doi: 10.1073/pnas.0709333105. Epub 2008 Jan 28.

}, month = {Feb 5}, pages = {1567-72}, abstract = {

Leaf venation is a showcase of plant diversity, ranging from the grid-like network in grasses, to a wide variety of dendritic systems in other angiosperms. A principal function of the venation is to deliver water; however, a hydraulic significance has never been demonstrated for contrasting major venation architectures, including the most basic dichotomy, "pinnate" and "palmate" systems. We hypothesized that vascular redundancy confers tolerance of vein breakage such as would occur during mechanical or insect damage. We subjected leaves of woody angiosperms of contrasting venation architecture to severing treatments in vivo, and, after wounds healed, made detailed measurements of physiological performance relative to control leaves. When the midrib was severed near the leaf base, the pinnately veined leaves declined strongly in leaf hydraulic conductance, stomatal conductance, and photosynthetic rate, whereas palmately veined leaves were minimally affected. Across all of the species examined, a higher density of primary veins predicted tolerance of midrib damage. This benefit for palmate venation is consistent with its repeated evolution and its biogeographic and habitat distribution. All leaves tested showed complete tolerance of damage to second- and higher-order veins, demonstrating that the parallel flow paths provided by the redundant, reticulate minor vein network protect the leaf from the impact of hydraulic disruption. These findings point to a hydraulic explanation for the diversification of low-order vein architecture and the commonness of reticulate, hierarchical leaf venation. These structures suggest roles for both economic constraints and risk tolerance in shaping leaf morphology during 130 million years of flowering plant evolution.

}, keywords = {*Biological Evolution, *Plant Transpiration, *Water, Angiosperms/anatomy \& histology/physiology, Plant Leaves/*anatomy \& histology/*physiology}, isbn = {1091-6490 (Electronic)0027-8424 (Linking)}, author = {Sack, L. and Dietrich, E. M. and Streeter, C. M. and Sanchez-Gomez, D. and Holbrook, N. M.} } @article {552506, title = {Low leaf hydraulic conductance associated with drought tolerance in soybean}, journal = {Physiol Plant}, volume = {132}, number = {4}, year = {2008}, note = {

Sinclair, Thomas RZwieniecki, Maciej AHolbrook, Noel MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Denmark2008/03/13 09:00Physiol Plant. 2008 Apr;132(4):446-51. doi: 10.1111/j.1399-3054.2007.01028.x.

}, month = {Apr}, pages = {446-51}, abstract = {

Lack of water is the most serious environmental constraint on agricultural production. More efficient use of water resources is a key solution for increased plant productivity in water-deficit environments. We examined the hydraulic characteristics of a {\textquoteright}slow wilting{\textquoteright} phenotype in soybean (Glycine max Merr.), PI 416937, which has been shown to have relatively constant transpiration rates above a threshold atmospheric vapor pressure deficit (VPD). The VPD response of PI 416937 was confirmed. Three experiments are reported to examine the hypothesis that the VPD response was a result of low hydraulic conductance in leaves as compared to two other soybean genotypes. Results are reported from experiments to measure transpiration response to VPD when xylem water potential was maintained at zero, leaf rehydration response and leaf carbon assimilation response to petiole cutting. Major interspecific differences in leaf hydraulic properties were observed. The observed low leaf hydraulic conductance in PI 416937 is consistent with an increased water use efficiency, and an increased water conservation by limiting transpiration rates under high evaporative conditions but allowing normal gas exchange rates under more moderate evaporative conditions.

}, keywords = {*Adaptation, Physiological, Plant Leaves/*physiology, Soybeans/*physiology, Water/*metabolism}, isbn = {1399-3054 (Electronic)0031-9317 (Linking)}, author = {Sinclair, T. R. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552271, title = {Nitrate control of root hydraulic properties in plants: translating local information to whole plant response}, journal = {Plant Physiol}, volume = {148}, number = {2}, year = {2008}, note = {

Gorska, AnnaYe, QingHolbrook, N MicheleZwieniecki, Maciej AengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2008/08/30 09:00Plant Physiol. 2008 Oct;148(2):1159-67. doi: 10.1104/pp.108.122499. Epub 2008 Aug 27.

}, month = {Oct}, pages = {1159-67}, abstract = {

The sessile lifestyle of plants constrains their ability to acquire mobile nutrients such as nitrate. Whereas proliferation of roots might help in the longer term, nitrate-rich patches can shift rapidly with mass flow of water in the soil. A mechanism that allows roots to follow and capture this source of mobile nitrogen would be highly desirable. Here, we report that variation in nitrate concentration around roots induces an immediate alteration of root hydraulic properties such that water is preferentially absorbed from the nitrate-rich patch. Further, we show that this coupling between nitrate availability and water acquisition results from changes in cell membrane hydraulic properties and is directly related to intracellular nitrate concentrations. Split-root experiments in which nitrate was applied to a portion of the root system showed that the response is both localized and reversible, resulting in rapid changes in water uptake to the portions of the roots exposed to the nitrate-rich patch. At the same time, water uptake by roots not supplied with nitrate was reduced. We believe that the increase in root hydraulic conductance in one part causes a decline of water uptake in the other part due to a collapse in the water potential gradient driving uptake. The translation of local information, in this case nitrate concentration, into a hydraulic signal that can be transmitted rapidly throughout the plant and thus coordinate responses at the whole plant level, represents an unexpected, higher level physiological interaction that precedes the level of gene expression.

}, keywords = {Adaptation, Physiological, Cell Membrane Permeability, Cell Membrane/metabolism, Cucumis sativus/drug effects/*metabolism, Lycopersicon esculentum/drug effects/*metabolism, Nitrates/*metabolism/pharmacology, Osmotic Pressure, Plant Roots/drug effects/*metabolism, Time Factors, Water/metabolism}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Gorska, A. and Ye, Q. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552276, title = {Nitrate induction of root hydraulic conductivity in maize is not correlated with aquaporin expression}, journal = {Planta}, volume = {228}, number = {6}, year = {2008}, note = {

Gorska, AnnaZwieniecka, AnnaHolbrook, N MicheleZwieniecki, Maciej AengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Germany2008/08/06 09:00Planta. 2008 Nov;228(6):989-98. doi: 10.1007/s00425-008-0798-x. Epub 2008 Aug 5.

}, month = {Nov}, pages = {989-98}, abstract = {

Some plant species can increase the mass flow of water from the soil to the root surface in response to the appearance of nitrate in the rhizosphere by increasing root hydraulic conductivity. Such behavior can be seen as a powerful strategy to facilitate the uptake of nitrate in the patchy and dynamically changing soil environment. Despite the significance of such behavior, little is known about the dynamics and mechanism of this phenomenon. Here we examine root hydraulic response of nitrate starved Zea mays (L.) plants after a sudden exposure to 5 mM NO(3)(-) solution. In all cases the treatment resulted in a significant increase in pressure-induced (pressure gradient approximately 0.2 MPa) flow across the root system by approximately 50\% within 4 h. Changes in osmotic gradient across the root were approximately 0.016 MPa (or 8.5\%) and thus the results could only be explained by a true change in root hydraulic conductance. Anoxia treatment significantly reduced the effect of nitrate on xylem root hydraulic conductivity indicating an important role for aquaporins in this process. Despite a 1 h delay in the hydraulic response to nitrate treatment, we did not detect any change in the expression of six ZmPIP1 and seven ZmPIP2 genes, strongly suggesting that NO(3)(-) ions regulate root hydraulics at the protein level. Treatments with sodium tungstate (nitrate reductase inhibitor) aimed at resolving the information pathway regulating root hydraulic properties resulted in unexpected findings. Although this treatment blocked nitrate reductase activity and eliminated the nitrate-induced hydraulic response, it also produced changes in gene expression and nitrate uptake levels, precluding us from suggesting that nitrate acts on root hydraulic properties via the products of nitrate reductase.

}, keywords = {Aquaporins/*metabolism, Biological Transport/drug effects/physiology, Hydroponics, Nitrates/*pharmacology, Plant Roots/*metabolism, Water/metabolism, Zea mays/drug effects/*metabolism}, isbn = {0032-0935 (Print)0032-0935 (Linking)}, author = {Gorska, A. and Zwieniecka, A. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552391, title = {Optimal vein density in artificial and real leaves}, journal = {Proc Natl Acad Sci U S A}, volume = {105}, number = {27}, year = {2008}, note = {

Noblin, XMahadevan, LCoomaraswamy, I AWeitz, D AHolbrook, N MZwieniecki, M AengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2008/07/05 09:00Proc Natl Acad Sci U S A. 2008 Jul 8;105(27):9140-4. doi: 10.1073/pnas.0709194105. Epub 2008 Jul 1.

}, month = {Jul 8}, pages = {9140-4}, abstract = {

The long evolution of vascular plants has resulted in a tremendous variety of natural networks responsible for the evaporatively driven transport of water. Nevertheless, little is known about the physical principles that constrain vascular architecture. Inspired by plant leaves, we used microfluidic devices consisting of simple parallel channel networks in a polymeric material layer, permeable to water, to study the mechanisms of and the limits to evaporation-driven flow. We show that the flow rate through our biomimetic leaves increases linearly with channel density (1/d) until the distance between channels (d) is comparable with the thickness of the polymer layer (delta), above which the flow rate saturates. A comparison with the plant vascular networks shows that the same optimization criterion can be used to describe the placement of veins in leaves. These scaling relations for evaporatively driven flow through simple networks reveal basic design principles for the engineering of evaporation-permeation-driven devices, and highlight the role of physical constraints on the biological design of leaves.

}, keywords = {Environment, Humidity, Microfluidics, Plant Leaves/*anatomy \& histology, Plant Transpiration}, isbn = {1091-6490 (Electronic)0027-8424 (Linking)}, author = {Noblin, X. and Mahadevan, L. and Coomaraswamy, I. A. and Weitz, D. A. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552226, title = {Diversity of hydraulic traits in nine Cordia species growing in tropical forests with contrasting precipitation}, journal = {New Phytol}, volume = {175}, number = {4}, year = {2007}, note = {

Choat, BrendanSack, LawrenHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2007/08/11 09:00New Phytol. 2007;175(4):686-98.

}, pages = {686-98}, abstract = {

Inter- and intraspecific variation in hydraulic traits was investigated in nine Cordia (Boraginaceae) species growing in three tropical rainforests differing in mean annual precipitation (MAP). Interspecific variation was examined for the different Cordia species found at each site, and intraspecific variation was studied in populations of the widespread species Cordia alliodora across the three sites. Strong intra- and interspecific variation were observed in vulnerability to drought-induced embolism. Species growing at drier sites were more resistant to embolism than those growing at moister sites; the same pattern was observed for populations of C. alliodora. By contrast, traits related to hydraulic capacity, including stem xylem vessel diameter, sapwood specific conductivity (K(s)) and leaf specific conductivity (K(L)), varied strongly but independently of MAP. For C. alliodora, xylem anatomy, K(s), K(L) and Huber value varied little across sites, with K(s) and K(L) being consistently high relative to other Cordia species. A constitutively high hydraulic capacity coupled with plastic or genotypic adjustment in vulnerability to embolism and leaf water relations would contribute to the ability of C. alliodora to establish and compete across a wide precipitation gradient.

}, keywords = {*Rain, *Tropical Climate, Cordia/*classification/*metabolism, Species Specificity, Time, Trees/*physiology}, isbn = {0028-646X (Print)0028-646X (Linking)}, author = {Choat, B. and Sack, L. and Holbrook, N. M.} } @article {552426, title = {Dry-season leaf flushing of Enterolobium cyclocarpum (ear-pod tree): above- and below-ground phenology and water relations}, journal = {Tree Physiol}, volume = {27}, number = {11}, year = {2007}, note = {

Rojas-Jimenez, KeilorHolbrook, Noel MGutierrez-Soto, Marco VengResearch Support, Non-U.S. Gov{\textquoteright}tCanada2007/08/03 09:00Tree Physiol. 2007 Nov;27(11):1561-8.

}, month = {Nov}, pages = {1561-8}, abstract = {

Above- and belowground phenology and water relations of Enterolobium cyclocarpum Jacq. trees in the dry forest of Santa Rosa National Park, Costa Rica were studied during two consecutive phenological cycles, from November 1998 to June 2000. Aboveground phenological activity, including leaf shedding, growth and maturation of dormant fruits, new leaf flushing and flowering, occurred during the dry season. Measurements of leaf water potential, stomatal conductance and sap flow indicated that stomata of newly flushed leaves remained essentially closed until the onset of the first rains, suggesting that the main factor accounting for the favorable water balance of dry-season flushed leaves was their capacity to restrict water loss. Evidence of a contribution from stem and root water stores to shoot expansion was mixed because only the first dry-season flushing episode monitored was accompanied by a marked decrease in stem and root water potentials. Fine root production did not precede leaf flushing, occurred only after the onset of the rainy season and stopped under drought conditions, suggesting that soil water content was the most important variable controlling fine root dynamics in this species.

}, keywords = {*Seasons, Fabaceae/*physiology, Microclimate, Plant Leaves/*physiology, Plant Roots/physiology, Plant Stems/physiology, Time Factors, Trees/*physiology, Tropical Climate, Water/*physiology}, isbn = {0829-318X (Print)0829-318X (Linking)}, author = {Rojas-Jimenez, K. and Holbrook, N. M. and Gutierrez-Soto, M. V.} } @article {552266, title = {Dynamic changes in root hydraulic properties in response to nitrate availability}, journal = {J Exp Bot}, volume = {58}, number = {10}, year = {2007}, note = {

Gloser, VitZwieniecki, Maciej AOrians, Colin MHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2007/06/15 09:00J Exp Bot. 2007;58(10):2409-15. Epub 2007 Jun 11.

}, pages = {2409-15}, abstract = {

Changes in root hydraulic resistance in response to alterations in nitrate supply were explored in detail as a potential mechanism that allows plants to respond rapidly to changes in their environment. Sunflower (Helianthus annuus cv. Holiday) plants grown hydroponically with limited nitrate availability (200 micromol l(-1)) served as our model system. Experimental plants were 6-9-weeks-old with total dry mass of 2-4 g. Root pressurization of intact plants and detached root systems was used to elucidate the temporal dynamics of root hydraulic properties in sunflower plants following changes in external nitrate availability. The response was rapid, with a 20\% decrease in hydraulic resistance occurring within the first hour after the addition of 5 mM nitrate and the magnitude of the effect was dependent on nitrate concentration. The change in root hydraulic resistance was largely reversible, although the temporal dynamics of the response to nitrate addition versus nitrate withdrawal was not symmetric (a gradual decrease in resistance versus its fast increase), raising the possibility that the underlying mechanisms may also differ. Evidence is presented that the observed changes in root hydraulic properties require the assimilation of nitrate by root cells. The hydraulic resistance of roots, previously stimulated by the addition of nitrate, increased more than in control plants in low nitrate under anoxia and that suggests a key role of aquaporin activity in this response. It is proposed that a rapid decrease in root hydraulic resistance in the presence of increased nitrate availability is an important trait that could enhance a plant{\textquoteright}s ability to compete for nitrate in the soil.

}, keywords = {Adaptation, Physiological, Aquaporins/physiology, Helianthus/drug effects/*metabolism/physiology, Nitrates/*metabolism/pharmacology, Plant Proteins/physiology, Plant Roots/metabolism/physiology, Water/*metabolism}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Gloser, V. and Zwieniecki, M. A. and Orians, C. M. and Holbrook, N. M.} } @article {552236, title = {Dynamics of freeze-thaw embolism in Smilax rotundifolia (Smilacaceae)}, journal = {Am J Bot}, volume = {94}, number = {4}, year = {2007}, note = {

Cobb, Alexander RChoat, BrendanHolbrook, N Micheleeng2007/04/01 00:00Am J Bot. 2007 Apr;94(4):640-9. doi: 10.3732/ajb.94.4.640.

}, month = {Apr}, pages = {640-9}, abstract = {

Freeze-thaw cycles pose a major physiological challenge for all temperate perennial plants, but monocotyledonous vines face a still greater risk because their few large vessels are especially susceptible to embolism and are not replaced by secondary growth. The genus Smilax is particularly remarkable because it is widespread in the tropics but includes species that survive the hard frosts of New England winters. Smilax rotundifolia was monitored for a year for evidence of stem xylem freeze-thaw cavitation and refilling. Embolism of metaxylem was complete by late November and was completely reversed by late April, when root pressures rose as high as 100 kPa. Protoxylem remained full of sap throughout the year in cryogenic scanning electron micrographs. Three methods were used to quantify embolism: percent loss conductivity (PLC), gravimetric air fraction (GAF: mass of water in stem xylem relative to capacity), and cryogenic scanning electron microscopy (cryo-SEM). The three methods corroborated one another well and gave quantitatively similar results. Osmolality of xylem sap extracted from exuding stems was 64 mol/kg (+/-7.0, N = 8), consistent with the root pressures observed. Strong root pressure can account for Smilax{\textquoteright}s survival in temperate regions with severe frosts, where few monocots with persistent aboveground organs are found.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Cobb, A. R. and Choat, B. and Holbrook, N. M.} } @article {552521, title = {Effects of carbon dioxide and oxygen on sapwood respiration in five temperate tree species}, journal = {J Exp Bot}, volume = {58}, number = {6}, year = {2007}, note = {

Spicer, RachelHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2007/02/07 09:00J Exp Bot. 2007;58(6):1313-20. Epub 2007 Feb 5.

}, pages = {1313-20}, abstract = {

The gaseous environment surrounding parenchyma in woody tissue is low in O2 and high in CO2, but it is not known to what extent this affects respiration or might play a role in cell death during heartwood formation. Sapwood respiration was measured in two conifers and three angiosperms following equilibration to levels of O2 and CO2 common within stems, using both inner and outer sapwood to test for an effect of age. Across all species and tissue ages, lowering the O2 level from 10\% to 5\% (v/v) resulted in about a 25\% decrease in respiration in the absence of CO2, but a non-significant decrease at 10\% CO2. The inhibitory effect of 10\% CO2 was smaller and only significant at 10\% O2, where it reduced respiration by about 14\%. Equilibration to a wider range of gas combinations in Pinus strobus L. showed the same effect: 10\% CO2 inhibited respiration by about 15\% at both 20\% and 10\% O2, but had no net effect at 5\% O2. In an extreme treatment, 1\% O2+20\% CO2 increased respiration by over 30\% relative to 1\% O2 alone, suggesting a shift in metabolic response to high CO2 as O2 decreases. Although an increase in respiration would be detrimental under limiting O2, this extreme gas combination is unlikely to exist within most stems. Instead, moderate reductions in respiration under realistic O2 and CO2 levels suggest that within-stem gas composition does not severely limit respiration and is unlikely to cause the death of xylem parenchyma during heartwood formation.

}, keywords = {Acer/drug effects/physiology, Carbon Dioxide/*pharmacology, Climate, Coniferophyta/drug effects/*physiology, Fraxinus/drug effects/physiology, Oxygen/*pharmacology, Plant Stems/drug effects/physiology, Quercus/drug effects/physiology, Respiration/drug effects, Trees/drug effects/growth \& development/*physiology}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Spicer, R. and Holbrook, N. M.} } @article {552576, title = {Hydraulic design of leaves: insights from rehydration kinetics}, journal = {Plant Cell Environ}, volume = {30}, number = {8}, year = {2007}, note = {

Zwieniecki, Maciej ABrodribb, Timothy JHolbrook, N MicheleengComparative StudyResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2007/07/10 09:00Plant Cell Environ. 2007 Aug;30(8):910-21.

}, month = {Aug}, pages = {910-21}, abstract = {

We examined the leaf hydraulic design in 10 species based on their rehydration kinetics. In all cases, a biphasic response described the temporal pattern of water uptake, with time constants of approximately 30 to 800 s and approximately 800 to 8000 s. The time constants of the fast phase were significantly shorter in the six angiosperms (30 to 110 s) compared with the two single-veined conifer species (\>400 s) examined, while the two multi-veined gymnosperm species, Gnetum gnemon and Ginkgo biloba, had time constants for the fast phase of approximately 150 s. Among angiosperm species, the fast phase constituted 50-90\% of the total water absorbed, whereas in gymnosperms 70-90\% of the water uptake could be assigned to the slow phase. In the four gymnosperms, the relative water uptake corresponding to the fast phase matched to a good degree the relative volume of the venation and bundle sheath extension; whereas in the angiosperm species, the relatively larger water influx during the fast phase was similar in relative volume to the combined venation, bundle sheath extension, epidermis and (in four species) the spongy mesophyll. This suggests a general trend from a design in which the epidermis is weakly connected to the veins (all four gymnosperms), to a design with good hydraulic connection between epidermis and veins that largely bypasses the mesophyll (four of six angiosperms), to a design in which almost the entire leaf appears to function as a single pool.

}, keywords = {Ginkgo biloba/anatomy \& histology/metabolism/*physiology, Gnetum/anatomy \& histology/metabolism/*physiology, Kinetics, Plant Leaves/anatomy \& histology/metabolism/physiology, Plant Transpiration/physiology, Water/*metabolism}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Zwieniecki, M. A. and Brodribb, T. J. and Holbrook, N. M.} } @article {552516, title = {Parenchyma cell respiration and survival in secondary xylem: does metabolic activity decline with cell age?}, journal = {Plant Cell Environ}, volume = {30}, number = {8}, year = {2007}, note = {

Spicer, RHolbrook, N MengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2007/07/10 09:00Plant Cell Environ. 2007 Aug;30(8):934-43.

}, month = {Aug}, pages = {934-43}, abstract = {

Sapwood respiration often declines towards the sapwood/heartwood boundary, but it is not known if parenchyma metabolic activity declines with cell age. We measured sapwood respiration in five temperate species (sapwood age range of 5-64 years) and expressed respiration on a live cell basis by quantifying living parenchyma. We found no effect of parenchyma age on respiration in two conifers (Pinus strobus, Tsuga canadensis), both of which had significant amounts of dead parenchyma in the sapwood. In angiosperms (Acer rubrum, Fraxinus americana, Quercus rubra), both bulk tissue and live cell respiration were reduced by about one-half in the oldest relative to the youngest sapwood, and all sapwood parenchyma remained alive. Conifers and angiosperms had similar bulk tissue respiration despite a smaller proportion of parenchyma in conifers (5\% versus 15-25\% in angiosperms), such that conifer parenchyma respired at rates about three times those of angiosperms. The fact that 5-year-old parenchyma cells respired at the same rate as 25-year-old cells in conifers suggests that there is no inherent or intrinsic decline in respiration as a result of cellular ageing. In contrast, it is not known whether differences observed in cellular respiration rates of angiosperms are a function of age per se, or whether active regulation of metabolic rate or positional effects (e.g. proximity to resources and/or hormones) could be the cause of reduced respiration in older sapwood.

}, keywords = {Acer/cytology/metabolism, Cell Aging/*physiology, Fraxinus/cytology/metabolism, Oxygen/*metabolism, Pinus/cytology/metabolism, Quercus/cytology/metabolism, Time Factors, Trees/cytology/*metabolism, Tsuga/cytology/metabolism, Xylem/cytology/*metabolism}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Spicer, R. and Holbrook, N. M.} } @article {552526, title = {The role of freezing in setting the latitudinal limits of mangrove forests}, journal = {New Phytol}, volume = {173}, number = {3}, year = {2007}, note = {

Stuart, S AChoat, BMartin, K CHolbrook, N MBall, M CengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2007/01/25 09:00New Phytol. 2007;173(3):576-83.

}, pages = {576-83}, abstract = {

Mangrove trees dominate coastal vegetation in tropical regions, but are completely replaced by herbaceous salt marshes at latitudes above 32 degrees N and 40 degrees S. Because water deficit can increase damage caused by freezing, we hypothesized that mangroves, which experience large deficits as a result of saline substrates, would suffer freeze-induced xylem failure. Vulnerability to freeze-induced xylem embolism was examined in the most poleward mangrove species in North America, in an area where freezing is rare but severe, and in Australia, in an area where freezing is frequent but mild. Percentage loss in hydraulic conductivity was measured following manipulations of xylem tension; xylem sap ion concentration was determined using X-ray microanalysis. Species with wider vessels suffered 60-100\% loss of hydraulic conductivity after freezing and thawing under tension, while species with narrower vessels lost as little as 13-40\% of conductivity. These results indicate that freeze-induced embolism may play a role in setting the latitudinal limits of distribution in mangroves, either through massive embolism following freezing, or through constraints on water transport as a result of vessel size.

}, keywords = {Avicennia/anatomy \& histology/*physiology, Freezing, Plant Exudates/chemistry, Plant Stems/physiology, Rhizophoraceae/anatomy \& histology/*physiology, Time Factors, Trees/*physiology, Xylem/chemistry/physiology}, isbn = {0028-646X (Print)0028-646X (Linking)}, author = {Stuart, S. A. and Choat, B. and Martin, K. C. and Holbrook, N. M. and Ball, M. C.} } @article {552211, title = {Baobab trees (Adansonia) in Madagascar use stored water to flush new leaves but not to support stomatal opening before the rainy season}, journal = {New Phytol}, volume = {169}, number = {3}, year = {2006}, note = {

Chapotin, Saharah MoonRazanameharizaka, Juvet HHolbrook, N MicheleengComparative StudyResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2006/01/18 09:00New Phytol. 2006;169(3):549-59.

}, pages = {549-59}, abstract = {

Baobab trees (Adansonia, Bombacaceae) are widely thought to store water in their stems for use when water availability is low. We tested this hypothesis by assessing the role of stored water during the dry season in three baobab species in Madagascar. In the dry season, leaves are present only during and after leaf flush. We quantified the relative contributions of stem and soil water during this period through measures of stem water content, sap flow and stomatal conductance. Rates of sap flow at the base of the trunk were near zero, indicating that leaf flushing was almost entirely dependent on stem water. Stem water content declined by up to 12\% during this period, yet stomatal conductance and branch sap flow rates remained very low. Stem water reserves were used to support new leaf growth and cuticular transpiration, but not to support stomatal opening before the rainy season. Stomatal opening coincided with the onset of sap flow at the base of the trunk and occurred only after significant rainfall.

}, keywords = {*Rain, *Seasons, Adansonia/anatomy \& histology/*metabolism, Madagascar, Plant Leaves/anatomy \& histology/metabolism, Plant Stems/metabolism, trees, Water/*metabolism}, isbn = {0028-646X (Print)0028-646X (Linking)}, author = {Chapotin, S. M. and Razanameharizaka, J. H. and Holbrook, N. M.} } @article {552206, title = {A biomechanical perspective on the role of large stem volume and high water content in baobab trees (Adansonia spp.; Bombacaceae)}, journal = {Am J Bot}, volume = {93}, number = {9}, year = {2006}, note = {

Chapotin, Saharah MoonRazanameharizaka, Juvet HHolbrook, N Micheleeng2006/09/01 00:00Am J Bot. 2006 Sep;93(9):1251-64. doi: 10.3732/ajb.93.9.1251.

}, month = {Sep}, pages = {1251-64}, abstract = {

The stems of large trees serve in transport, storage, and support; however, the degree to which these roles are reflected in their morphology is not always apparent. The large, water-filled stems of baobab trees (Adansonia spp.) are generally assumed to serve a water storage function, yet recent studies indicate limited use of stored water. Through an analysis of wood structure and composition, we examined whether baobab morphology reflects biomechanical constraints rather than water storage capacity in the six Madagascar baobab species. Baobab wood has a high water content (up to 79\%), low wood density (0.09-0.17 g . cm(-3)), high parenchyma content (69-88\%), and living cells beyond 35 cm into the xylem from the cambium. Volumetric construction cost of the wood is several times lower than in more typical trees, and the elastic modulus approaches that of parenchyma tissue. Safety factors calculated from estimated elastic buckling heights were low, indicating that baobabs are not more overbuilt than other temperate and tropical trees, yet the energy investment in stem material is comparable to that in temperate deciduous trees. Furthermore, the elastic modulus of the wood decreases with water content, such that excessive water withdrawal from the stem could affect mechanical stability.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Chapotin, S. M. and Razanameharizaka, J. H. and Holbrook, N. M.} } @article {552181, title = {Declining hydraulic efficiency as transpiring leaves desiccate: two types of response}, journal = {Plant Cell Environ}, volume = {29}, number = {12}, year = {2006}, note = {

Brodribb, Tim JHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2006/11/04 09:00Plant Cell Environ. 2006 Dec;29(12):2205-15.

}, month = {Dec}, pages = {2205-15}, abstract = {

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Psileaf). Manipulating the transpiration rate in excised leaves enabled us to vary Psileaf in the range -0.1 MPa to less than -1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Psileaf remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Psileaf led to a proportional decrease in Kleaf, such that midday Psileaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37\%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Psileaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Psileaf at turgor loss and Psileaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero.

}, keywords = {*Desiccation, Carbon Dioxide/metabolism, Plant Leaves/*physiology, Plant Transpiration/*physiology, Water/physiology}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Brodribb, T. J. and Holbrook, N. M.} } @article {552216, title = {Direct measurements of intervessel pit membrane hydraulic resistance in two angiosperm tree species}, journal = {Am J Bot}, volume = {93}, number = {7}, year = {2006}, note = {

Choat, BrendanBrodie, Tyler WCobb, Alexander RZwieniecki, Maciej AHolbrook, N Micheleeng2006/07/01 00:00Am J Bot. 2006 Jul;93(7):993-1000. doi: 10.3732/ajb.93.7.993.

}, month = {Jul}, pages = {993-1000}, abstract = {

The hydraulic resistance of pit membranes was measured directly in earlywood vessels of Fraxinus americana and Ulmus americana. The area-specific resistance of pit membranes (r(mem)) was higher than modeled or measured values obtained previously for hardwood species, with r(mem) of 5.24 x 10(3) MPa.s.m(-1) for Fraxinus and 2.56 x 10(3) MPa.s.m(-1) for Ulmus. The calculated resistance of pit canals was three orders of magnitude below total pit resistance indicating that pit membranes contributed the majority of resistance. Scanning electron microscopy indicated that pit membranes of Ulmus were thinner and more porous than those of Fraxinus, consistent with the difference in r(mem) between the species. Measurements of average vessel diameter and length and area of wall overlap with neighboring vessels were used to partition the vascular resistance between vessel lumen and pit membrane components. Pit membrane resistance accounted for 80\% of the total resistance in Fraxinus and 87\% in Ulmus in 2-yr-old branch sections. However, measurements of vessel dimensions in the trunk suggest that the division of resistance between pit membrane and lumen components would be closer to co-limiting in older regions of the tree. Thus, pit membrane resistance may be of greater relative importance in small branches than in older regions of mature trees.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Choat, B. and Brodie, T. W. and Cobb, A. R. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552606, title = {Hydraulic design of pine needles: one-dimensional optimization for single-vein leaves}, journal = {Plant Cell Environ}, volume = {29}, number = {5}, year = {2006}, note = {

Zwieniecki, Maciej AStone, Howard ALeigh, AndreaBoyce, C KevinHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2006/11/08 09:00Plant Cell Environ. 2006 May;29(5):803-9.

}, month = {May}, pages = {803-9}, abstract = {

Single-vein leaves have the simplest hydraulic design possible, yet even this linear water delivery system can be modulated to improve physiological performance. We determined the optimal distribution of transport capacity that minimizes pressure drop per given investment in xylem permeability along the needle for a given length without a change in total water delivery, or maximizes needle length for a given pressure difference between petiole and needle tip. This theory was tested by comparative analysis of the hydraulic design of three pine species that differ in the length of their needles [Pinus palustris (Engl.) Miller, approximately 50 cm; Pinus ponderosa Lawson \& Lawson, approximately 20 cm and Pinus rigida Miller, approximately 5 cm]. In all three species, the distribution of hydraulic permeability was similar to that predicted by the optimum solution. The needles of P. palustris showed an almost perfect match between predicted and actual hydraulic optimum solution, providing evidence that vein design is a significant factor in the hydraulic design of pine leaves.

}, keywords = {Pinus/*anatomy \& histology/physiology, Plant Leaves/*anatomy \& histology/physiology, Water}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Zwieniecki, M. A. and Stone, H. A. and Leigh, A. and Boyce, C. K. and Holbrook, N. M.} } @article {552446, title = {Leaf hydraulics}, journal = {Annu Rev Plant Biol}, volume = {57}, year = {2006}, note = {

Sack, LawrenHolbrook, N MicheleengReview2006/05/04 09:00Annu Rev Plant Biol. 2006;57:361-81.

}, pages = {361-81}, abstract = {

Leaves are extraordinarily variable in form, longevity, venation architecture, and capacity for photosynthetic gas exchange. Much of this diversity is linked with water transport capacity. The pathways through the leaf constitute a substantial (\>or=30\%) part of the resistance to water flow through plants, and thus influence rates of transpiration and photosynthesis. Leaf hydraulic conductance (K(leaf)) varies more than 65-fold across species, reflecting differences in the anatomy of the petiole and the venation architecture, as well as pathways beyond the xylem through living tissues to sites of evaporation. K(leaf) is highly dynamic over a range of time scales, showing circadian and developmental trajectories, and responds rapidly, often reversibly, to changes in temperature, irradiance, and water supply. This review addresses how leaf structure and physiology influence K(leaf), and the mechanisms by which K(leaf) contributes to dynamic functional responses at the level of both individual leaves and the whole plant.

}, keywords = {Gases/metabolism, photosynthesis, Plant Leaves/*physiology}, isbn = {1543-5008 (Print)1543-5008 (Linking)}, author = {Sack, L. and Holbrook, N. M.} } @article {552371, title = {Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle}, journal = {Plant Cell Environ}, volume = {29}, number = {5}, year = {2006}, note = {

Lovelock, Catherine EBall, Marilyn CChoat, BrendanEngelbrecht, Bettina M JHolbrook, N MichelleFeller, Ilka CengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2006/11/08 09:00Plant Cell Environ. 2006 May;29(5):793-802.

}, month = {May}, pages = {793-802}, abstract = {

Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity (Ks), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stem hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced mid-day loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle.

}, keywords = {*Photosynthesis, Phosphorus/*metabolism, Rhizophoraceae/growth \& development/metabolism/*physiology, Xylem/metabolism}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Lovelock, C. E. and Ball, M. C. and Choat, B. and Engelbrecht, B. M. and Holbrook, N. M. and Feller, I. C.} } @article {552201, title = {Water relations of baobab trees (Adansonia spp. L.) during the rainy season: does stem water buffer daily water deficits?}, journal = {Plant Cell Environ}, volume = {29}, number = {6}, year = {2006}, note = {

Chapotin, Saharah MoonRazanameharizaka, Juvet HHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2006/11/04 09:00Plant Cell Environ. 2006 Jun;29(6):1021-32.

}, month = {Jun}, pages = {1021-32}, abstract = {

Baobab trees are often cited in the literature as water-storing trees, yet few studies have examined this assumption. We assessed the role of stored water in buffering daily water deficits in two species of baobabs (Adansonia rubrostipa Jum. and H. Perrier and Adansonia za Baill.) in a tropical dry forest in Madagascar. We found no lag in the daily onset of sap flow between the base and the crown of the tree. Some night-time sap flow occurred, but this was more consistent with a pattern of seasonal stem water replenishment than with diurnal usage. Intrinsic capacitance of both leaf and stem tissue (0.07-0.08 and 1.1-1.43 MPa(-1), respectively) was high, yet the amount of water that could be withdrawn before turgor loss was small because midday leaf and stem water potentials (WPs) were near the turgor-loss points. Stomatal conductance was high in the daytime but then declined rapidly, suggesting an embolism-avoidance strategy. Although the xylem of distal branches was relatively vulnerable to cavitation (P50: 1.1-1.7 MPa), tight stomatal control and minimum WPs near--1.0 MPa maintained native embolism levels at 30-65\%. Stem morphology and anatomy restrict water movement between storage tissues and the conductive pathway, making stored-water usage more appropriate to longer-term water deficits than as a buffer against daily water deficits.

}, keywords = {*Rain, *Seasons, Adansonia/*metabolism, Biological Transport, Plant Leaves/physiology, Plant Stems/metabolism, Water/metabolism}, isbn = {0140-7791 (Print)0140-7791 (Linking)}, author = {Chapotin, S. M. and Razanameharizaka, J. H. and Holbrook, N. M.} } @article {931681, title = {From wet to dry: tropical trees in relation to water availability}, journal = {Trees}, volume = {19}, year = {2005}, month = {February 8, 2005}, pages = {280-281}, author = {N. Michele Holbrook and Augusto C. Franco} } @article {931711, title = {Leaf physiology does not predict leaf habit; examples from tropical dry forest}, journal = {Trees}, year = {2005}, month = {December 9, 2004}, pages = {290-295}, abstract = {

Leaf structure and physiology are thought to be closely linked to leaf longevity and leaf habit. Here we com- pare the seasonal variation in leaf hydraulic conductance (kleaf ) and water potential of two evergreen tree species with contrasting leaf life spans, and two species with similar leaf longevity but contrasting leaf habit, one being decidu- ous and the other evergreen. One of the evergreen species, Simarouba glauca, produced relatively short-lived leaves that maintained high hydraulic conductance year round by periodic flushing. The other evergreen species, Quercus oleoides, produced longer-lived leaves with lower kleaf and as a result minimum leaf water potential was much lower than in S. glauca (-2.8 MPa vs -1.6 MPa). Associated with exposure to lower water potentials, Q. oleoides leaves were harder, had a higher modulus of elasticity, and were less vulnerable to cavitation than S. glauca leaves. Both species operate at water potentials capable of inducing 20 (S. glauca) to 50\% (Q. oleoides) loss of kleaf during the dry season although no evidence of cumulative losses in kleaf were observed in either species suggesting regular re- pair of embolisms. Leaf longevity in the deciduous species Rhedera trinervis is similar to that of S. glauca, although maximum kleaf was lower. Furthermore, a decline in leaf water potential at the onset of the dry season led to cumu- lative losses in kleaf in R. trinervis that culminated in leaf shedding.

}, author = {Timothy J. Brodribb and N. Michele Holbrook} } @article {931671, title = {The spatial pattern of air seeding thresholds in mature sugar maple trees}, journal = {Plant, Cell and Environment}, volume = {28}, year = {2005}, month = {2005}, pages = {1082-1089}, abstract = {

Air seeding threshold (Pa) of xylem vessels from current year growth rings were measured along the vertical axis of mature sugar maple trees (Acer saccharum Marsh.), with sampling points in primary leaf veins, petioles, 1-, 3-, and 7-year-old branches, large branches, the trunk and roots. The air seeding threshold was taken as the pressure required to force nitrogen gas through intervessel pit mem- branes. Although all measurements were made on wood produced in the same year, Pa varied between different regions of A. saccharum, with distal organs such as leaves and petioles having lower Pa than basal regions. Mean (SE) Pa ranged from 1.0 ({\textpm} 0.1) MPa in primary leaf veins to 4.8 ({\textpm} 0.1) MPa in the main trunk. Roots exhibited a Pa of 2.8 ({\textpm} 0.2) MPa, lower than all other regions of the tree except leaf veins and petioles. Mean xylem vessel diameter increased basipetally, with the widest vessels occurring in the trunk and roots. Within the shoot, wider vessels had greater air seeding thresholds, contrasting with trends pre- viously reported. However, further experimentation revealed that differences in Pa between regions of the stem were driven by the presence of primary xylem conduits, rather than differences in vessel diameter. In 1-year-old branches, Pa was significantly lower in primary xylem ves- sels than in adjacent secondary xylem vessels. This explained the lower values of Pa measured in petioles and leaf veins, which possessed a greater ratio of primary xylem to secondary xylem than other regions. The difference in Pa between primary and secondary xylem was attributed to the greater area of primary cell wall (pit membrane) exposed in primary xylem conduits with helical or annular thickening.

}, author = {Brendan Choat and Eleanor C. Lahr and Peter J. Melcher and Maciej A. Zwieniecki and N. Michele Holbrook} } @article {931716, title = {Within-stem oxygen concentration and sap flow in four temperate tree species: does long-lived xylem parenchyma experience hypoxia?}, journal = {Plant, Cell and Environment }, year = {2005}, month = {2005}, pages = {192-201}, abstract = {

Oxygen levels as low as 1{\textendash}5\% (gaseous mole fraction) occur in secondary xylem, but it is not known if there is a consistent pattern of decline in O2 from the cambium toward the pith, or whether parenchyma cells experience hypoxic conditions deep within the stem. We developed a system for repeated in situ measurement of O2 at different depths within stems of Acer rubrum, Fraxinus americana, Tsuga canadensis, and Quercus rubra. In summer during active transpiration, O2 declined from the cambium toward the heartwood boundary in F. americana, T. canadensis and Q. rubra, but remained constant in A. rubrum. Average sapwood O2 was about 10\%, with the lowest values observed in the innermost sapwood around 3{\textendash}5\%. Before spring leaf flush, O2 content in the outer sapwood was reduced in Q. rubra and T. canadensis relative to summer, and was occasionally lower than in the inner sapwood. Sap- wood respiration in T. canadensis was constant above 5\% O2, but reduced by about 65\% at 1\% O2. In F. americana, sapwood respiration was constant above 10\% O2 but reduced by 25\% at 5\% O2, and by 75\% at 1\% O2, the most extreme inhibition observed. However, when prolonged (72 h) exposure to 1\%, 5\% and 10\% O2 was followed by re-equilibration to 10\% O2, no inhibition was found. Given the minor (and reversible) effect of low O2 on parenchyma metabolism at levels common in the inner sapwood, it is unlikely that O2 content severely limits parenchyma respi- ration or leads to parenchyma cell death during sapwood senescence. Within-stem O2 levels may instead be most rel- evant to metabolism in the cambial zone and phloem, for which sapwood could serve as a significant source of O2.

}, author = {R. Spicer and N. M. Holbrook} } @article {931201, title = {Seedling growth in conifers and angiosperms: impacts of contrasting xylem structure}, journal = {Australian Journal of Botany}, year = {2005}, month = {2005}, pages = {749-755}, author = {T. J. Brodribb and N. M. Holbrook} } @article {552501, title = {Daily transpiration rates of woody species on drying soil}, journal = {Tree Physiol}, volume = {25}, number = {11}, year = {2005}, note = {

Sinclair, Thomas RHolbrook, N MichelleZwieniecki, Maciej AengResearch Support, Non-U.S. Gov{\textquoteright}tCanada2005/08/18 09:00Tree Physiol. 2005 Nov;25(11):1469-72.

}, month = {Nov}, pages = {1469-72}, abstract = {

Among annual plants, daily transpiration rates, expressed as a fraction of volumetric soil water content available for transpiration, show a common pattern in response to soil drying. Initially, as soil dries, there is little decrease in transpiration rate until water availability has fallen to about one third that at field capacity. With further soil drying, relative transpiration rate decreases in a more-or-less linear fashion until all available water has been used. Data previously obtained for perennial woody species have often been confounded by different methods for determining available soil water. In this study, we investigated the daily transpiration response to soil drying in five woody perennial species: Thuja plicata Donn ex D. Don, Acer rubrum L., Robinia pseudoacacia L., Hibiscus sp. and Ibex aquifolium L. Transpiration was unaffected by soil drying until the initial estimated transpirable soil water fraction had decreased to between 0.23 and 0.32 of that at field capacity. Beyond this point, transpiration rate declined linearly with available soil water fraction until reaching one fifth the rate observed in well-watered plants. With further soil drying, the relative transpiration rates remained between 10 and 20\% of that observed in well-watered plants. Maintenance of transpiration at these rates with further soil drying was hypothesized to result from contributions to transpiration of water stored in plant tissues. After taking tissue water storage into account, it was estimated that transpiration was curtailed as the available soil water fraction fell to between 0.26 and 0.37 of that at field capacity, which is comparable to values reported for annual crop plants.

}, keywords = {Acer/physiology, Hibiscus/physiology, Ilex/physiology, Plant Transpiration/*physiology, Robinia/physiology, Soil, Thuja/physiology, Trees/*physiology}, isbn = {0829-318X (Print)0829-318X (Linking)}, author = {Sinclair, T. R. and Holbrook, N. M. and Zwieniecki, M. A.} } @article {552496, title = {The importance of frictional interactions in maintaining the stability of the twining habit}, journal = {Am J Bot}, volume = {92}, number = {11}, year = {2005}, note = {

Silk, Wendy KHolbrook, N Micheleeng2005/11/01 00:00Am J Bot. 2005 Nov;92(11):1820-6. doi: 10.3732/ajb.92.11.1820.

}, month = {Nov}, pages = {1820-6}, abstract = {

The stability of twining vines under gravitational loads suggests an important role for friction. The coefficient of friction, mu, between vine stems and wood is high, often five times greater than between leather and wood, as determined by slip tests on an inclined plane. Stem trichomes function like ratchets to facilitate climbing upward (or to facilitate slipping if the stem is inverted). A mathematical model predicts large masses (kg) must be applied to the base of a twining vine to cause slipping. Vines slip as predicted when mu is low and arc length on the pole is short, and they break before slipping when mu is large or arc length is long. In contrast, twining vines are unstable in compression, collapsing when small masses (\<10 g) are hung from the top of the vine. However, if the loads are applied below the uppermost gyre, the stabilizing tensional effect dominates. Therefore, in nature vines twining on a cylindrical support are stable under gravitational loads, unless these loads occur near the apex. A corollary is that a short apical coil can hold up large masses of maturing shoot.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Silk, W. K. and Holbrook, N. M.} } @article {552461, title = {Leaf hydraulic architecture correlates with regeneration irradiance in tropical rainforest trees}, journal = {New Phytol}, volume = {167}, number = {2}, year = {2005}, note = {

Sack, LawrenTyree, Melvin THolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2005/07/07 09:00New Phytol. 2005 Aug;167(2):403-13.

}, month = {Aug}, pages = {403-13}, abstract = {

The leaf hydraulic conductance (K(leaf)) is a major determinant of plant water transport capacity. Here, we measured K(leaf), and its basis in the resistances of leaf components, for fully illuminated leaves of five tree species that regenerate in deep shade, and five that regenerate in gaps or clearings, in Panamanian lowland tropical rainforest. We also determined coordination with stomatal characters and leaf mass per area. K(leaf) varied 10-fold across species, and was 3-fold higher in sun- than in shade-establishing species. On average, 12\% of leaf hydraulic resistance (= 1/K(leaf)) was located in the petiole, 25\% in the major veins, 25\% in the minor veins, and 39\% outside the xylem. Sun-establishing species had a higher proportion of leaf resistance in the xylem. Across species, component resistances correlated linearly with total leaf resistance. K(leaf) correlated tightly with indices of stomatal pore area, indicating a coordination of liquid- and vapor-phase conductances shifted relative to that of temperate woody species. Leaf hydraulic properties are integrally linked in the complex of traits that define differences in water use and carbon economy across habitats and vegetation zones.

}, keywords = {Photobiology, Plant Leaves/anatomy \& histology/physiology/radiation effects, Regeneration/radiation effects, Trees/anatomy \& histology/*physiology/radiation effects, Tropical Climate}, isbn = {0028-646X (Print)0028-646X (Linking)}, author = {Sack, L. and Tyree, M. T. and Holbrook, N. M.} } @article {552186, title = {Leaf hydraulic capacity in ferns, conifers and angiosperms: impacts on photosynthetic maxima}, journal = {New Phytol}, volume = {165}, number = {3}, year = {2005}, note = {

Brodribb, Tim JHolbrook, N MicheleZwieniecki, Maciej APalma, BeatrizengComparative StudyResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2005/02/22 09:00New Phytol. 2005 Mar;165(3):839-46.

}, month = {Mar}, pages = {839-46}, abstract = {

* The hydraulic plumbing of vascular plant leaves varies considerably between major plant groups both in the spatial organization of veins, as well as their anatomical structure. * Five conifers, three ferns and 12 angiosperm trees were selected from tropical and temperate forests to investigate whether the profound differences in foliar morphology of these groups lead to correspondingly profound differences in leaf hydraulic efficiency. * We found that angiosperm leaves spanned a range of leaf hydraulic conductance from 3.9 to 36 mmol m2 s-1 MPa-1, whereas ferns (5.9-11.4 mmol m-2 s-1 MPa-1) and conifers (1.6-9.0 mmol m-2 s-1 MPa-1) were uniformly less conductive to liquid water. Leaf hydraulic conductance (Kleaf) correlated strongly with stomatal conductance indicating an internal leaf-level regulation of liquid and vapour conductances. Photosynthetic capacity also increased with Kleaf, however, it became saturated at values of Kleaf over 20 mmol m-2 s-1 MPa-1. * The data suggest that vessels in the leaves of the angiosperms studied provide them with the flexibility to produce highly conductive leaves with correspondingly high photosynthetic capacities relative to tracheid-bearing species.

}, keywords = {Angiosperms/*physiology, Coniferophyta/*physiology, Ecosystem, Ferns/*physiology, Photosynthesis/*physiology, Plant Leaves/*physiology, Water/*metabolism}, isbn = {0028-646X (Print)0028-646X (Linking)}, author = {Brodribb, T. J. and Holbrook, N. M. and Zwieniecki, M. A. and Palma, B.} } @article {552176, title = {Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer}, journal = {Plant Physiol}, volume = {137}, number = {3}, year = {2005}, note = {

Brodribb, Tim JHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2005/03/01 09:00Plant Physiol. 2005 Mar;137(3):1139-46. Epub 2005 Feb 25.

}, month = {Mar}, pages = {1139-46}, abstract = {

Just as a soggy paper straw is prone to yielding under the applied suction of a thirsty drinker, the xylem tracheids in leaves seem prone to collapse as water potential declines, impeding their function. Here we describe the collapse, under tension, of lignified cells peripheral to the leaf vein of a broad-leaved rainforest conifer, Podocarpus grayi de Laub. Leaves of Podocarpus are characterized by an array of cylindrical tracheids aligned perpendicular to the leaf vein, apparently involved in the distribution of water radially through the mesophyll. During leaf desiccation the majority of these tracheids collapsed from circular to flat over the water potential range -1.5 to -2.8 MPa. An increase in the percentage of tracheids collapsed during imposed water stress was mirrored by declining leaf hydraulic conductivity (K(leaf)), implying a direct effect on water transport efficiency. Stomata responded to water stress by closing at -2.0 MPa when 45\% of cells were collapsed and K(leaf) had declined by 25\%. This was still substantially before the initial indications of cavitation-induced loss of hydraulic conductance in the leaf vein, at -3 MPa. Plants droughted until 49\% of tracheids had collapsed were found to fully recover tracheid shape and leaf function 1 week after rewatering. A simple mechanical model of tracheid collapse, derived from the theoretical buckling pressure for pipes, accurately predicted the collapse dynamics observed in P. grayi, substantiating estimates of cell wall elasticity and measured leaf water potential. The possible adaptive advantages of collapsible vascular tissue are discussed.

}, keywords = {Biophysical Phenomena, Biophysics, Coniferophyta/anatomy \& histology/*physiology, Plant Leaves/anatomy \& histology/*physiology, Pressure, Water/*physiology}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Brodribb, T. J. and Holbrook, N. M.} } @article {931741, title = {Scaling phloem transport: information transmission}, journal = {Plant, Cell and Environment}, year = {2004}, pages = {509-519}, abstract = {

Sieve tubes are primarily responsible for the movement of solutes over long distances, but they also conduct informa- tion about the osmotic state of the system. Using a previ- ously developed dimensionless model of phloem transport, the mechanism behind the sieve tube{\textquoteright}s capacity to rapidly transmit pressure/concentration waves in response to local changes in either membrane solute exchange or the magni- tude and axial gradient of apoplastic water potential is demonstrated. These wave fronts can move several orders of magnitude faster than the solution itself when the sieve tube{\textquoteright}s axial pressure drop is relatively small. Unlike the axial concentration drop, the axial pressure drop at steady state is independent of the apoplastic water potential gra- dient. As such, the regulation of whole-sieve tube turgor could play a vital role in controlling membrane solute exchange throughout the translocation pathway, making turgor a reliable source of information for communicating change in system state.

}, author = {M. V. Thompson and N. M. Holbrook} } @article {931731, title = {Water relations under root chilling in a sensitive and tolerant tomato species}, journal = {Plant, Cell and Environment}, year = {2004}, month = {2004}, pages = {971-979}, abstract = {

The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling tempera- tures of around 5 $\infty$C. Under the same treatment, a chilling- tolerant congener (Lycopersicon hirsutum LA 1778) main- tains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure cham- bers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, how- ever, in stomatal behaviour: in L. hirsutum, stomatal con- ductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 $\infty$C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differen- tial stomatal behaviour during root chilling has distinct shoot and root components.

}, author = {A.J. Bloom and M. A. Zwieniecki and J. B. Passioura and L. B. Randall and N. M. Holbrook and D. A. St. Clair} } @article {552221, title = {Changes in pit membrane porosity due to deflection and stretching: the role of vestured pits}, journal = {J Exp Bot}, volume = {55}, number = {402}, year = {2004}, note = {

Choat, BrendanJansen, StevenZwieniecki, Maciej ASmets, ErikHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2004/06/08 05:00J Exp Bot. 2004 Jul;55(402):1569-75. Epub 2004 Jun 4.

}, month = {Jul}, pages = {1569-75}, abstract = {

The effect of increasing pressure difference (DeltaP) on intervessel pit membrane porosity was studied in two angiosperm tree species with differing pit architecture. Fraxinus americana L. possesses typical angiosperm bordered pit structure while Sophora japonica L. exhibits well-developed vestures in intervessel pit chambers. It was hypothesized (a) that large DeltaP across intervessel pits would cause the deflection of pit membranes in the stems of F. americana resulting in significant increases in porosity and thus lower cavitation thresholds, and (b) that the presence of vestures would prevent the deflection of pit membranes in S. japonica. To determine if the porosity of pit membranes increased under mechanical stress, suspensions of colloidal gold, 5 nm and 20 nm in diameter, were perfused across intervessel pit membranes at DeltaP ranging from 0.25 MPa to 6.0 MPa. The effect of increasing DeltaP on membrane porosity was also tested by comparing air seeding thresholds (Pa) in stems perfused with water or a solution with lower surface tension. Air seeding and colloidal gold experiments indicated that pit membrane porosity increased significantly with DeltaP in F. americana. In S. japonica, increases in permeability to colloidal gold with DeltaP were small and maximum pore diameters predicted from Pa were independent of DeltaP, suggesting that vestures limited the degree to which the membrane can be deflected from the centre of the pit cavity. This provides the first experimental evidence that vestures reduce the probability of air seeding through pit membranes.

}, keywords = {Air, Angiosperms/physiology/ultrastructure, Cell Membrane Permeability, Coated Pits, Cell-Membrane/*physiology/ultrastructure, Cryoelectron Microscopy, Fraxinus/*physiology, Microscopy, Electron, Scanning, Porosity, Sophora/*physiology, Stress, Mechanical}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Choat, B. and Jansen, S. and Zwieniecki, M. A. and Smets, E. and Holbrook, N. M.} } @article {552401, title = {Developmental and physiological correlates of leaf size in Hyeronima alchorneoides (Euphorbiaceae)}, journal = {Am J Bot}, volume = {91}, number = {4}, year = {2004}, note = {

Reich, AlexandraHolbrook, N MicheleEwel, John Jeng2004/04/01 00:00Am J Bot. 2004 Apr;91(4):582-9. doi: 10.3732/ajb.91.4.582.

}, month = {Apr}, pages = {582-9}, abstract = {

The tropical emergent tree Hyeronima alchorneoides has large decreases in leaf size with tree age: 1200 cm(2) at 1 yr, 900 cm(2) at 3 yr, 200 cm(2) at 11 yr, and 80 cm(2) in old (\>30 yr) individuals. We tracked leaf growth and physiological attributes on trees of three different ages (1, 3, and 11 yr) to determine the developmental basis and functional consequences of this variation. Leaves on young trees grew faster and sustained maximum rates of leaf expansion longer than leaves on older trees. Leaf mass per area (LMA) did not differ among age classes. Maximum photosynthetic rates reflected differences in leaf nitrogen concentration, in which leaves from the lower crown of younger trees outperformed those at a comparable crown position in older trees. One-year-old trees had the lowest stomatal conductance and the greatest instantaneous water use efficiency. Ontogenetic plasticity in mature leaf size, structure, and physiology may be a balance between the advantages conferred by rapid height growth when trees are young and the benefits derived from producing branches that increase light harvesting ability as trees reach the canopy.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Reich, A. and Holbrook, N. M. and Ewel, J. J.} } @article {552166, title = {Evolution of xylem lignification and hydrogel transport regulation}, journal = {Proc Natl Acad Sci U S A}, volume = {101}, number = {50}, year = {2004}, note = {

Boyce, C KevinZwieniecki, Maciej ACody, George DJacobsen, ChrisWirick, SueKnoll, Andrew HHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2004/12/03 09:00Proc Natl Acad Sci U S A. 2004 Dec 14;101(50):17555-8. Epub 2004 Dec 1.

}, month = {Dec 14}, pages = {17555-8}, abstract = {

In vascular plants, the polysaccharide-based walls of water-conducting cells are strengthened by impregnation with the polyphenolic polymer lignin. The fine-scale patterning of lignin deposition in water-conducting cells is shown here to vary phylogenetically across vascular plants. The extent to which water transport in xylem cells can be modified in response to changes in the ionic content of xylem sap also is shown to vary in correlation with variation in lignification patterns, consistent with the proposed mechanism for hydraulic response through size change of middle-lamella pectins. This covariation suggests that the fine-scale distribution of hydrophilic polysaccharides and hydrophobic lignin can affect hydraulic as well as mechanical properties, and that the evolutionary diversification of vascular cells thus reflects biochemical as well as morphological innovations evolved to fulfill opposing cell functions of transport and structural support.

}, keywords = {*Biological Evolution, Biological Transport, Ions/metabolism, Lignin/*metabolism, Phylogeny, Plant Leaves/anatomy \& histology/cytology/growth \& development/metabolism, Plant Stems/anatomy \& histology/*cytology/growth \& development/*metabolism}, isbn = {0027-8424 (Print)0027-8424 (Linking)}, author = {Boyce, C. K. and Zwieniecki, M. A. and Cody, G. D. and Jacobsen, C. and Wirick, S. and Knoll, A. H. and Holbrook, N. M.} } @article {552571, title = {Functional design space of single-veined leaves: role of tissue hydraulic properties in constraining leaf size and shape}, journal = {Ann Bot}, volume = {94}, number = {4}, year = {2004}, note = {

Zwieniecki, Maciej ABoyce, C KevinHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2004/08/21 05:00Ann Bot. 2004 Oct;94(4):507-13. Epub 2004 Aug 19.

}, month = {Oct}, pages = {507-13}, abstract = {

BACKGROUND AND AIMS: Morphological diversity of leaves is usually quantified with geometrical characters, while in many cases a simple set of biophysical parameters are involved in constraining size and shape. One of the main physiological functions of the leaf is transpiration and thus one can expect that leaf hydraulic parameters can be used to predict potential morphologies, although with the caveat that morphology in turn influences physiological parameters including light interception and boundary layer thickness and thereby heat transfer and net photosynthesis. METHODS: An iterative model was used to determine the relative sizes and shapes that are functionally possible for single-veined leaves as defined by their ability to supply the entire leaf lamina with sufficient water to prevent stomatal closure. The model variables include the hydraulic resistances associated with vein axial and radial transport, as well as with water movement through the mesophyll and the leaf surface. KEY RESULTS: The four parameters included in the model are sufficient to define a hydraulic functional design space that includes all single-veined leaf shapes found in nature, including scale-, awl- and needle-like morphologies. This exercise demonstrates that hydraulic parameters have dissimilar effects: surface resistance primarily affects leaf size, while radial and mesophyll resistances primarily affect leaf shape. CONCLUSIONS: These distinctions between hydraulic parameters, as well as the differential accessibility of different morphologies, might relate to the convergent evolutionary patterns seen in a variety of fossil lineages concerning overall morphology and anatomical detail that frequently have evolved in linear and simple multi-veined leaves.

}, keywords = {Models, Biological, Plant Development, Plant Leaves/*anatomy \& histology/growth \& development, Plant Transpiration/physiology}, isbn = {0305-7364 (Print)0305-7364 (Linking)}, author = {Zwieniecki, M. A. and Boyce, C. K. and Holbrook, N. M.} } @article {552456, title = {Hydraulic analysis of water flow through leaves of sugar maple and red oak}, journal = {Plant Physiol}, volume = {134}, number = {4}, year = {2004}, note = {

Sack, LawrenStreeter, Christopher MHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2004/04/06 05:00Plant Physiol. 2004 Apr;134(4):1824-33. Epub 2004 Apr 2.

}, month = {Apr}, pages = {1824-33}, abstract = {

Leaves constitute a substantial fraction of the total resistance to water flow through plants. A key question is how hydraulic resistance within the leaf is distributed among petiole, major veins, minor veins, and the pathways downstream of the veins. We partitioned the leaf hydraulic resistance (R(leaf)) for sugar maple (Acer saccharum) and red oak (Quercus rubra) by measuring the resistance to water flow through leaves before and after cutting specific vein orders. Simulations using an electronic circuit analog with resistors arranged in a hierarchical reticulate network justified the partitioning of total R(leaf) into component additive resistances. On average 64\% and 74\% of the R(leaf) was situated within the leaf xylem for sugar maple and red oak, respectively. Substantial resistance-32\% and 49\%- was in the minor venation, 18\% and 21\% in the major venation, and 14\% and 4\% in the petiole. The large number of parallel paths (i.e. a large transfer surface) for water leaving the minor veins through the bundle sheath and out of the leaf resulted in the pathways outside the venation comprising only 36\% and 26\% of R(leaf). Changing leaf temperature during measurement of R(leaf) for intact leaves resulted in a temperature response beyond that expected from changes in viscosity. The extra response was not found for leaves with veins cut, indicating that water crosses cell membranes after it leaves the xylem. The large proportion of resistance in the venation can explain why stomata respond to leaf xylem damage and cavitation. The hydraulic importance of the leaf vein system suggests that the diversity of vein system architectures observed in angiosperms may reflect variation in whole-leaf hydraulic capacity.

}, keywords = {Acer/*physiology, Biological Transport/physiology, Plant Leaves/*physiology, Quercus/*physiology, Temperature, Water/*physiology}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Sack, L. and Streeter, C. M. and Holbrook, N. M.} } @article {552591, title = {A potential role for xylem-phloem interactions in the hydraulic architecture of trees: effects of phloem girdling on xylem hydraulic conductance}, journal = {Tree Physiol}, volume = {24}, number = {8}, year = {2004}, note = {

Zwieniecki, Maciej AMelcher, Peter JFeild, Taylor SHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Canada2004/06/03 05:00Tree Physiol. 2004 Aug;24(8):911-7.

}, month = {Aug}, pages = {911-7}, abstract = {

We investigated phloem-xylem interactions in Acer rubrum L. and Acer saccharum Marsh. Our experimental method allowed us to determine xylem conductance of an intact branch by measuring the flow rate of water supplied at two delivery pressures to the cut end of a small side branch. We found that removal of bark tissue (phloem girdling) upstream of the point at which deionized water was delivered to the branch resulted in a decrease (24\% for A. rubrum and 15\% for A. saccharum) in branch xylem hydraulic conductance. Declines in hydraulic conductance with girdling were accompanied by a decrease in the osmotic concentration of xylem sap. The decrease in xylem sap concentration following phloem girdling suggests that ion redistribution from the phloem was responsible for the observed decline in hydraulic conductance. When the same measurements were made on branches perfused with KCl solution (approximately 140 mOsm kg(-1)), phloem girdling had no effect on xylem hydraulic conductance. These results suggest a functional link between phloem and xylem hydraulic systems that is mediated by changes in the ionic content of the cell sap.

}, keywords = {Acer/anatomy \& histology/physiology, Models, Biological, Plant Stems/anatomy \& histology/*physiology, Plant Transpiration/physiology, Trees/*physiology, Water/physiology}, isbn = {0829-318X (Print)0829-318X (Linking)}, author = {Zwieniecki, M. A. and Melcher, P. J. and Feild, T. S. and Holbrook, N. M.} } @article {552536, title = {Root-gel interactions and the root waving behavior of Arabidopsis}, journal = {Plant, Cell and Environment}, volume = {135}, number = {3}, year = {2004}, note = {

Thompson, Matthew VHolbrook, N MicheleengResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2004/07/13 05:00Plant Physiol. 2004 Jul;135(3):1822-37. Epub 2004 Jul 9.

}, month = {Jul}, pages = {1822-37}, abstract = {

Arabidopsis roots grown on inclined agarose gels exhibit a sinusoidal growth pattern known as root waving. While root waving has been attributed to both intrinsic factors (e.g. circumnutation) and growth responses to external signals such as gravity, the potential for physical interactions between the root and its substrate to influence the development of this complex phenotype has been generally ignored. Using a rotating stage microscope and time-lapse digital imaging, we show that (1) root tip mobility is impeded by the gel surface, (2) this impedance causes root tip deflections by amplifying curvature in the elongation zone in a way that is distinctly nontropic, and (3) root tip impedance is augmented by normal gravitropic pressure applied by the root tip against the gel surface. Thus, both lateral corrective bending near the root apex and root tip impedance could be due to different vector components of the same graviresponse. Furthermore, we speculate that coupling between root twisting and bending is a mechanical effect resulting from root tip impedance.

}, keywords = {Arabidopsis/*growth \& development, Gels, Gravitropism/*physiology, Plant Roots/*growth \& development, Regression Analysis, Surface Properties, Temperature}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {M. V. Thompson and N. M. Holbrook} } @article {552531, title = {Application of a single-solute non-steady-state phloem model to the study of long-distance assimilate transport}, journal = {J Theor Bio}, volume = {220}, number = {4}, year = {2003}, note = {

Thompson, Matthew VHolbrook, N MichelleengResearch Support, Non-U.S. Gov{\textquoteright}tEngland2003/03/08 04:00J Theor Biol. 2003 Feb 21;220(4):419-55.

}, month = {Feb 21}, pages = {419-55}, abstract = {

A mass-balanced, finite-difference solution to Munch{\textquoteright}s osmotically generated pressure-flow hypothesis is developed for the study of non-steady-state sucrose transport in the phloem tissue of plants. Major improvements over previous modeling efforts are the inclusion of wall elasticity, nonlinear functions of viscosity and solute potential, an enhanced calculation of sieve pore resistance, and the introduction of a slope-limiting total variation diminishing method for determining the concentration of sucrose at node boundaries. The numerical properties of the model are discussed, as is the history of the modeling of pressure-driven phloem transport. Idealized results are presented for a sharp, fast-moving concentration front, and the effect of changing sieve tube length on the transport of sucrose in both the steady-state and non-steady-state cases is examined. Most of the resistance to transport is found to be axial, rather than radial (via membrane transport), and most of the axial resistance is due to the sieve plates. Because of the sieve plates, sieve tube elasticity does not provide a significant enhancement to conductivity at high pressure, as previously suspected. The transit time of sucrose through a sieve tube is found to be inversely proportional to the square of the sieve tube{\textquoteright}s length; following that observation, it is suggested that 20 1-m-long sieve tubes could transport sucrose 20 times faster than a single 20 m sieve tube. Short sieve tubes would be highly sensitive to differentials between loading and unloading rate, and would require close cooperation with adjacent companion cells for proper function.

}, keywords = {*Models, Biological, Biological Transport/physiology, Elasticity, Osmotic Pressure, Plant Structures/*metabolism, Sucrose/pharmacokinetics, Viscosity}, isbn = {0022-5193 (Print)0022-5193 (Linking)}, author = {Thompson, M. V. and Holbrook, N. M.} } @article {552601, title = {Ionic control of the lateral exchange of water between vascular bundles in tomato}, journal = {J Exp Bot}, volume = {54}, number = {386}, year = {2003}, note = {

Zwieniecki, Maciej AOrians, Colin MMelcher, Peter JHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2003/04/24 05:00J Exp Bot. 2003 May;54(386):1399-405.

}, month = {May}, pages = {1399-405}, abstract = {

Ions can enhance water flow through the xylem via changes in the hydraulic resistance at border pit membranes. Because flow between adjacent xylem vessels occurs primarily via bordered pit fields, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal flow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water flow between adjacent xylem bundles increased by approximately 50\% in the presence of ions while longitudinal flow only increased by approximately 20\%. However, the enhancement of lateral exchange due to ions was magnified by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone.

}, keywords = {Biological Transport/drug effects/physiology, Ion Transport/drug effects/physiology, Lycopersicon esculentum/*physiology, Plant Stems/*physiology, Potassium Chloride/metabolism/pharmacology, Water/*metabolism/pharmacology}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Zwieniecki, M. A. and Orians, C. M. and Melcher, P. J. and Holbrook, N. M.} } @article {552436, title = {The major veins of mesomorphic leaves revisited: tests for conductive overload in Acer saccharum (Aceraceae) and Quercus rubra (Fagaceae)}, journal = {Am J Bot}, volume = {90}, number = {1}, year = {2003}, note = {

Sack, LawrenCowan, Peter DHolbrook, N Micheleeng2003/01/01 00:00Am J Bot. 2003 Jan;90(1):32-9. doi: 10.3732/ajb.90.1.32.

}, month = {Jan}, pages = {32-9}, abstract = {

Many leaves survive the severing of their major veins in apparently excellent health. According to the classical explanation, the leaf minor veins provide "conductive overload," an excess of parallel conductive paths, rendering the major veins hydraulically dispensable. Whether such an excess of conductive paths exists has important implications for vascular design and for leaf response to vascular damage. We subjected leaves of Acer saccharum and Quercus rubra to cutting treatments that disrupted the major vein system and determined leaf survival, stomatal conductance (g), quantum yield of photosystem II (Phi(PSII)), and leaf hydraulic conductance (K(leaf)). For A. saccharum, the cuts led to the death of distal lamina. For Q. rubra, however, the treated leaves typically remained apparently healthy. Despite their appearance, the treated Q. rubra leaves had a strongly reduced K(leaf), relative to control leaves, and g and Phi(PSII) were reduced distal to the cuts, respectively, by 75-97\% and 48-76\%. Gas exchange proximal to the cuts was unaffected, indicating the independence of lamina regions and their local stomata. Analogous results were obtained with excised Q. rubra leaves. These studies demonstrate an indispensable, vital role of the major veins in conducting water throughout the lamina.

}, isbn = {0002-9122 (Print)0002-9122 (Linking)}, author = {Sack, L. and Cowan, P. D. and Holbrook, N. M.} } @article {552316, title = {Plant biology: water gate}, journal = {Nature}, volume = {425}, number = {6956}, year = {2003}, note = {

Holbrook, N MicheleZwieniecki, Maciej AengCommentNewsEngland2003/09/26 05:00Nature. 2003 Sep 25;425(6956):361.

}, month = {Sep 25}, pages = {361}, keywords = {*Ion Channel Gating, Aquaporins/*metabolism, Arabidopsis/cytology/*metabolism, Biological Transport, Cytosol/metabolism, Diffusion, Disasters, Hydrogen-Ion Concentration, Oxygen/*metabolism, Permeability, Plant Roots/cytology/*metabolism, Water/*metabolism}, isbn = {1476-4687 (Electronic)0028-0836 (Linking)}, author = {Holbrook, N. M. and Zwieniecki, M. A.} } @article {552196, title = {Restoration of genetic diversity in the dry forest tree Swietenia macrophylla (Meliaceae) after pasture abandonment in Costa Rica}, journal = {Mol Ecol}, volume = {12}, number = {12}, year = {2003}, note = {

Cespedes, MGutierrez, M VHolbrook, N MJ Rocha, OengComparative StudyResearch Support, Non-U.S. Gov{\textquoteright}tEngland2003/11/25 05:00Mol Ecol. 2003 Dec;12(12):3201-12.

}, month = {Dec}, pages = {3201-12}, abstract = {

We studied the levels of genetic diversity of Swietenia macrophylla (big leaf mahogany) in five successional plots in the Santa Rosa National Park, Guanacaste, Costa Rica. We selected sites with different lengths of time since the last major disturbance (typically fire): 6, 9, 15 and 20 years. In addition, we also included a patch of mature forest that had experienced selective logging and other human activity in the past 100 years. Genetic diversity was assessed using five polymorphic DNA microsatellite loci. We found a total of 21 alleles in the five loci examined, in which the number of alleles present varied among the five sites studied. Allelic diversity varied between sites ranging from 20 to 14 alleles, and our data revealed that earlier successional sites have more alleles than older sites. There was significant heterogeneity in allele frequencies between sites; however, genetic differentiation between populations was low (FST = 0.063) indicating that most of the variation was found within sites and extensive gene flow between sites. In addition, our analysis also showed that genetic diversity of adult trees does not solely determine the diversity of seedlings and saplings found around them, also supporting the existence of extensive gene flow. The impact of these findings for the design of conservation strategies for tropical dry forests trees is discussed.

}, keywords = {*Genetic Variation, *Trees, Cluster Analysis, Conservation of Natural Resources, costa rica, Electrophoresis, Polyacrylamide Gel, Gene Frequency, Genetics, Population, Meliaceae/*genetics, Microsatellite Repeats/genetics, Time Factors}, isbn = {0962-1083 (Print)0962-1083 (Linking)}, author = {Cespedes, M. and Gutierrez, M. V. and Holbrook, N. M. and J. Rocha O} } @article {552171, title = {Stomatal closure during leaf dehydration, correlation with other leaf physiological traits}, journal = {Plant Physiol}, volume = {132}, number = {4}, year = {2003}, note = {

Brodribb, Tim JHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2003/08/13 05:00Plant Physiol. 2003 Aug;132(4):2166-73.

}, month = {Aug}, pages = {2166-73}, abstract = {

The question as to what triggers stomatal closure during leaf desiccation remains controversial. This paper examines characteristics of the vascular and photosynthetic functions of the leaf to determine which responds most similarly to stomata during desiccation. Leaf hydraulic conductance (K(leaf)) was measured from the relaxation kinetics of leaf water potential (Psi(l)), and a novel application of this technique allowed the response of K(leaf) to Psi(l) to be determined. These "vulnerability curves" show that K(leaf) is highly sensitive to Psi(l) and that the response of stomatal conductance to Psi(l) is closely correlated with the response of K(leaf) to Psi(l). The turgor loss point of leaves was also correlated with K(leaf) and stomatal closure, whereas the decline in PSII quantum yield during leaf drying occurred at a lower Psi(l) than stomatal closure. These results indicate that stomatal closure is primarily coordinated with K(leaf). However, the close proximity of Psi(l) at initial stomatal closure and initial loss of K(leaf) suggest that partial loss of K(leaf) might occur regularly, presumably necessitating repair of embolisms.

}, keywords = {*Desiccation, Angiosperms/*cytology/*metabolism, photosynthesis, Plant Leaves/*cytology/*metabolism, Water/*metabolism}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Brodribb, T. J. and Holbrook, N. M.} } @article {552376, title = {Vulnerability of xylem vessels to cavitation in sugar maple. Scaling from individual vessels to whole branches}, journal = {Plant Physiol}, volume = {131}, number = {4}, year = {2003}, note = {

Melcher, Peter JZwieniecki, Maciej AHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2003/04/15 05:00Plant Physiol. 2003 Apr;131(4):1775-80.

}, month = {Apr}, pages = {1775-80}, abstract = {

The relation between xylem vessel age and vulnerability to cavitation of sugar maple (Acer saccharum Marsh.) was quantified by measuring the pressure required to force air across bordered pit membranes separating individual xylem vessels. We found that the bordered pit membranes of vessels located in current year xylem could withstand greater applied gas pressures (3.8 MPa) compared with bordered pit membranes in vessels located in older annular rings (2.0 MPa). A longitudinal transect along 6-year-old branches indicated that the pressure required to push gas across bordered pit membranes of current year xylem did not vary with distance from the growing tip. To understand the contribution of age-related changes in vulnerability to the overall resistance to cavitation, we combined data on the pressure thresholds of individual xylem vessels with measurements of the relative flow rate through each annual ring. The annual ring of the current year contributed only 16\% of the total flow measured on 10-cm-long segments cut from 6-year-old branches, but it contributed more than 70\% of the total flow when measured through 6-year-old branches to the point of leaf attachment. The vulnerability curve calculated using relative flow rates measured on branch segments were similar to vulnerability curves measured on 6-year-old branches (pressure that reduces hydraulic conductance by 50\% = 1.6-2.4 MPa), whereas the vulnerability curve calculated using relative flow rates measured on 6-year-old branches were similar to ones measured on the extension growth of the current year (pressure that reduces hydraulic conductance by 50\% = 3.8 MPa). These data suggest that, in sugar maple, the xylem of the current year can withstand larger xylem tensions than older wood and dominates water delivery to leaves.

}, keywords = {Acer/*anatomy \& histology/*physiology, Air, Dehydration, Pressure, Water/*metabolism}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Melcher, P. J. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552321, title = {The dynamics of "dead wood": maintenance of water transport through plant stems}, journal = {Integr Comp Biol}, volume = {42}, number = {3}, year = {2002}, note = {

Holbrook, N MicheleZwieniecki, Maciej AMelcher, Peter JengEngland2002/07/01 00:00Integr Comp Biol. 2002 Jul;42(3):492-6. doi: 10.1093/icb/42.3.492.

}, month = {Jul}, pages = {492-6}, abstract = {

The lack of mobility in plants is often interpreted as a sign of their passivity in the face of environmental variation. This view is perhaps most firmly entrenched with regard to water transport through the xylem in which water flows through the lumen of cells that are "dead" (i.e., lack any cytoplasm or nucleus) at maturity. However, recent work demonstrates that a number of active, physiological processes may be involved in maintaining the transport capacity of this essential pathway. Here we review work relating to both embolism repair and the effect of ion concentrations on xylem hydraulic properties as examples of such dynamic processes.

}, isbn = {1540-7063 (Print)1540-7063 (Linking)}, author = {Holbrook, N. M. and Zwieniecki, M. A. and Melcher, P. J.} } @article {552241, title = {Hardly a relict: freezing and the evolution of vesselless wood in Winteraceae}, journal = {Evolution}, volume = {56}, number = {3}, year = {2002}, note = {

Feild, Taylor SBrodribb, TimHolbrook, N MicheleengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2002/05/07 10:00Evolution. 2002 Mar;56(3):464-78.

}, month = {Mar}, pages = {464-78}, abstract = {

The Winteraceae are traditionally regarded as the least-specialized descendents of the first flowering plants, based largely on their lack of xylem vessels. Since vessels have been viewed as a key innovation for angiosperm diversification, Winteraceae have been portrayed as declining relicts, limited to wet forest habitats where their tracheid-based wood does not impose a significant hydraulic constraints. In contrast, phylogenetic analyses place Winteraceae among angiosperm clades with vessels, indicating that their vesselless wood is derived rather than primitive, whereas extension of the Winteraceae fossil record into the Early Cretaceous suggests a more complex ecological history than has been deduced from their current distribution. However, the selective regime and ecological events underlying the possible loss of vessels in Winteraceae have remained enigmatic. Here we examine the hypothesis that vessels were lost as an adaptation to freezing-prone environments in Winteraceae by measuring the responses of xylem water transport to freezing for a diverse group of Winteraceae taxa as compared to Canella winterana (Canellaceae, a close relative with vessels) and sympatric conifer taxa. We found that mean percent loss of xylem water transport capacity following freeze-thaw varied from 0\% to 6\% for Winteraceae species from freezing-prone temperate climates and approximately 20\% in those taxa from tropical (nonfreezing) climates. Similarly, conifers exhibit almost no decrease in xylem hydraulic conductivity following freezing. In contrast, water transport in Canella stems is nearly 85\% blocked after freeze-thaw. Although vessel-bearing wood of Canella possesses considerably greaterhydraulic capacity than Winteraceae, nearly 20\% of xylem hydraulic conductance remains, a value that is comparable to the hydraulic capacity of vesselless Winteraceae xylem, if the proportion of hydraulic flow through vessels (modeled as ideal capillaries) is removed. Thus, the evolutionary removal of vessels may not necessarily require a deleterious shift to an ineffective vascular system. By integrating Winteraceae{\textquoteright}s phylogenetic relationships and fossil history with physiological and ecological observations, we suggest that, as ancestors of modern Winteraceae passed through temperate conditions present in Southern Gondwana during the Early Cretaceous, they were exposed to selective pressures against vessel-possession and returned to a vascular system relying on tracheids. These results suggest that the vesselless condition is advantageous in freezing-prone areas, which is supported by the strong bias in the ecological abundance of Winteraceae to wet temperate and tropical alpine habitats, rather than a retained feature from the first vesselless angiosperms. We believe that vesselless wood plays an important role in the ecological abundance of Winteraceae in Southern Hemisphere temperate environments by enabling the retention of leaves and photosynthesis in the face of frequent freeze-thaw events.

}, keywords = {*Biological Evolution, Angiosperms/classification/*genetics, Freezing, Models, Biological, Phylogeny, Seasons}, isbn = {0014-3820 (Print)0014-3820 (Linking)}, author = {Feild, T. S. and Brodribb, T. and Holbrook, N. M.} } @article {552451, title = {The hydraulic conductance of the angiosperm leaf lamina: a comparison of three measurement methods}, journal = {J Exp Bot}, volume = {53}, number = {378}, year = {2002}, note = {

Sack, LawrenMelcher, Peter JZwieniecki, Maciej AHolbrook, N MicheleengComparative StudyResearch Support, Non-U.S. Gov{\textquoteright}tEngland2002/10/16 04:00J Exp Bot. 2002 Nov;53(378):2177-84.

}, month = {Nov}, pages = {2177-84}, abstract = {

A comparison was made of three methods for measuring the leaf lamina hydraulic conductance (K(lamina)) for detached mature leaves of six woody temperate angiosperm species. The high-pressure method, the evaporative flux method and the vacuum pump method involve, respectively, pushing, evaporating and pulling water out of the lamina while determining the flow rate into the petiole and the water potential drop across the leaf. Tests were made of whether the high-pressure method and vacuum pump method measurements of K(lamina) on single leaves were affected by irradiance. In Quercus rubra, the high pressure method was sensitive to irradiance; K(lamina) measured under high irradiance (\>1200 micro mol m(-2) s(-1 )photosynthetically active radiation) was 4.6-8.8 times larger than under ambient laboratory lighting (approximately 6 micro mol m(-2) s(-1 )photosynthetically active radiation). By constrast, the vacuum pump method was theoretically expected to be insensitive to irradiance, and this expectation was confirmed in experiments on Hedera helix. When used in the ways recommended here, the three methods produced measurements that agreed typically within 10\%. There were significant differences in species{\textquoteright} K(lamina); values ranged from 1.24x10(-4) kg s(-1) m(-2) MPa(-1) for Acer saccharum to 2.89x10(-4) kg s(-1) m(-2) MPa(-1) for Vitis labrusca. Accurate, rapid determination of K(lamina) will allow testing of the links between K(lamina), water-use, drought tolerance, and the enormous diversity of leaf form, structure and composition.

}, keywords = {Acer/physiology, Angiosperms/*physiology, Chemistry, Physical/methods, Equipment Design, Hedera/physiology, Light, Photosynthesis/physiology, Plant Leaves/*physiology, Plant Transpiration/physiology, Quercus/physiology, Vitis/physiology, Water/*physiology}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Sack, L. and Melcher, P. J. and Zwieniecki, M. A. and Holbrook, N. M.} } @article {552306, title = {Stomatal control in tomato with ABA-deficient roots: response of grafted plants to soil drying}, journal = {J Exp Bot}, volume = {53}, number = {373}, year = {2002}, note = {

Holbrook, N MicheleShashidhar, V RJames, Richard AMunns, RanaengComparative StudyEngland2002/05/22 10:00J Exp Bot. 2002 Jun;53(373):1503-14.

}, month = {Jun}, pages = {1503-14}, abstract = {

The hypothesis that ABA produced by roots in drying soil is responsible for stomatal closure was tested with grafted plants constructed from the ABA-deficient tomato mutants, sitiens and flacca and their near-isogenic wild-type parent. Three types of experiments were conducted. In the first type, reciprocal grafts were made between the wild type and sitiens or flacca. Stomatal conductance accorded with the genotype of the shoot, not the root. Stomates closed in all of the grafted plants in response to soil drying, regardless of the root genotype, i.e. regardless of the ability of the roots to produce ABA. In the second type of experiment, wild-type shoots were grafted onto a split-root system consisting of one wild-type root grafted to one mutant (flacca or sitiens) root. Water was withheld from one root system, while the other was watered well so that the shoots did not experience any decline in water potential or loss of turgor. Stomates closed to a similar extent when water was withheld from the mutant roots or the wild-type roots. In the third type of experiment, grafted plants with wild-type shoots and either wild-type or sitiens roots were established in pots that could be placed inside a pressure chamber, and the pressure increased as the soil dried so that the shoots remained fully turgid throughout. Stomates closed as the soil dried, regardless of whether the roots were wild type or sitiens. These experiments demonstrate that stomatal closure in response to soil drying can occur in the absence of leaf water deficit, and does not require ABA production by roots. A chemical signal from roots leading to a change in apoplastic ABA levels in leaves may be responsible for the stomatal closure.

}, keywords = {Abscisic Acid/*metabolism, Acclimatization/physiology, Desiccation, Disasters, Lycopersicon esculentum/genetics/*physiology, Mutation, Plant Epidermis/*physiology, Plant Roots/*physiology, Plant Shoots/physiology, Signal Transduction/physiology, Soil/analysis, Water/metabolism}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Holbrook, N. M. and Shashidhar, V. R. and James, R. A. and Munns, R.} } @article {552596, title = {Hydraulic properties of individual xylem vessels of Fraxinus americana}, journal = {J Exp Bot}, volume = {52}, number = {355}, year = {2001}, note = {

Zwieniecki, M AMelcher, P JHolbrook, N MengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.England2001/04/03 10:00J Exp Bot. 2001 Feb;52(355):257-64.

}, month = {Feb}, pages = {257-64}, abstract = {

Studies of the hydraulic properties of xylem vessels have been limited to measurements of whole plant or whole stem segments. This approach allows the longitudinal transport properties of the ensemble of vessels within a stem to be determined, but provides little information on radial transport. Here the xylem of Fraxinus americana L. has been examined using a new method that allows the transport properties of individual vessels to be examined. One goal of this study was to quantify transport parameters relevant to embolism repair. The longitudinal conductivity of vessel segments open at both ends (i.e. no end walls) agreed with values predicted by the Poiseuille equation. Radial specific conductance (conductance per unit area) was approximately six orders of magnitude lower than the longitudinal conductance of the vessel segment normalized by the cross-sectional area of the vessel lumen. There was a step increase in the radial specific conductance of previously gas-filled vessels when the delivery pressure exceeded 0.4 MPa. This is consistent with the idea that positive pressure, required for embolism repair, can be compartmentalized within a vessel if the bordered pit chambers are gas-filled. The diffusion coefficient for the movement of gas from a pressurized air-filled vessel was of the same order of magnitude as that for air diffusing through water (1.95 e(-9) m(2) s(-1)). Estimates of the time needed to displace all of the gas from an air-filled vessel were in the order of 20 min, suggesting that gas removal may not be a major limitation in embolism repair.

}, keywords = {Angiosperms/anatomy \& histology/*physiology, Biological Transport, Cell Wall/physiology, Diffusion, Gases, Models, Biological, Plant Stems/*physiology, Pressure, Trees/physiology, Water/*physiology}, isbn = {0022-0957 (Print)0022-0957 (Linking)}, author = {Zwieniecki, M. A. and Melcher, P. J. and Holbrook, N. M.} } @article {552301, title = {In vivo observation of cavitation and embolism repair using magnetic resonance imaging}, journal = {Plant Physiol}, volume = {126}, number = {1}, year = {2001}, note = {

Holbrook, N MAhrens, E TBurns, M JZwieniecki, M AengDA08944/DA/NIDA NIH HHS/MH61223/MH/NIMH NIH HHS/RR13625/RR/NCRR NIH HHS/Research Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Research Support, U.S. Gov{\textquoteright}t, P.H.S.2001/05/15 10:00Plant Physiol. 2001 May;126(1):27-31.

}, month = {May}, pages = {27-31}, abstract = {

Magnetic resonance imaging (MRI) was used to noninvasively monitor the status of individual xylem vessels in the stem of an intact, transpiring grape (Vitis vinifera) plant over a period of approximately 40 h. Proton density-weighted MRI was used to visualize the distribution of mobile water in the stem and individual xylem vessels were scored as either water or gas filled (i.e. embolized). The number of water-filled vessels decreased during the first 24 h of the experiment, indicating that approximately 10 vessels had cavitated during this time. Leaf water potentials decreased from -1.25 to -2.1 MPa during the same period. Watering increased leaf water potentials to -0.25 MPa and prevented any further cavitation. Refilling of xylem vessels occurred as soon as the lights were switched off, with the majority of vessels becoming refilled with water during the first 2 to 3 h in darkness. These measurements demonstrate that MRI can be used to monitor the functional status of individual xylem vessels, providing the first method to study the process of cavitation and embolism repair in intact plants.

}, keywords = {*Angiosperms, *Plant Stems, Magnetic Resonance Imaging, Water}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Holbrook, N. M. and Ahrens, E. T. and Burns, M. J. and Zwieniecki, M. A.} } @article {552481, title = {Temporal and spatial patterns of twining force and lignification in stems of Ipomoea purpurea}, journal = {Planta}, volume = {213}, number = {2}, year = {2001}, note = {

Scher, J LHolbrook, N MSilk, W KengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.Germany2001/07/27 10:00Planta. 2001 Jun;213(2):192-8.

}, month = {Jun}, pages = {192-8}, abstract = {

Using the TWIFOR, an electronic device for continuous, in vivo measurement of the forces exerted by twining vines, we examined the forces generated by vines growing on cylindrical poles of slender (6.35 mm) and thicker (19.05 mm) diameter. In stems of Ipomoea purpurea (L.) Roth. magnitudes of twining force (axial tensions) were, on average, less at a particular time and location on the more slender poles; while twining loads (normal force per unit length of vine) were much greater on the slender poles because of the greater curvature of the vines. Thus, the geometry of the helix formed by the vine on the pole affects the ability of the vine to maintain a frictional interaction with its support. In addition, the plant-to-plant variation in twining force was twice as great on the thicker support poles. Metaxylem and fibers developed closer to the plant apex in vines on the slender poles. On the thicker poles, a significant fraction of the maximum twining force developed during the establishment of the first gyre, before fibers were lignified, indicating that primary growth can be sufficient to establish high twining forces. On the slender poles, however, twining force increased with developmental stage until the gyre was at least 1.5 m from the apex. Thus, twining force can increase after cessation of primary growth. No simple relationship was found between the site of fiber differentiation and twining force.

}, keywords = {Ipomoea/cytology/growth \& development/*physiology, Lignin/*metabolism, Models, Biological, Plant Stems/cytology/growth \& development, Stress, Mechanical, Tensile Strength}, isbn = {0032-0935 (Print)0032-0935 (Linking)}, author = {Scher, J. L. and Holbrook, N. M. and Silk, W. K.} } @article {552161, title = {United kingdoms}, journal = {Plant Physiol}, volume = {126}, number = {3}, year = {2001}, note = {

Bloom, A JHolbrook, N MengCongresses2001/07/18 10:00Plant Physiol. 2001 Jul;126(3):952-5.

}, month = {Jul}, pages = {952-5}, keywords = {*Symbiosis, Animals, Biological Evolution, Plants}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Bloom, A. J. and Holbrook, N. M.} } @article {552246, title = {Why leaves turn red in autumn. The role of anthocyanins in senescing leaves of red-osier dogwood}, journal = {Plant Physiol}, volume = {127}, number = {2}, year = {2001}, note = {

Feild, T SLee, D WHolbrook, N MengResearch Support, Non-U.S. Gov{\textquoteright}t2001/10/13 10:00Plant Physiol. 2001 Oct;127(2):566-74.

}, month = {Oct}, pages = {566-74}, abstract = {

Why the leaves of many woody species accumulate anthocyanins prior to being shed has long puzzled biologists because it is unclear what effects anthocyanins may have on leaf function. Here, we provide evidence for red-osier dogwood (Cornus stolonifera) that anthocyanins form a pigment layer in the palisade mesophyll layer that decreases light capture by chloroplasts. Measurements of leaf absorbance demonstrated that red-senescing leaves absorbed more light of blue-green to orange wavelengths (495-644 nm) compared with yellow-senescing leaves. Using chlorophyll a fluorescence measurements, we observed that maximum photosystem II (PSII) photon yield of red-senescing leaves recovered from a high-light stress treatment, whereas yellow-senescing leaves failed to recover after 6 h of dark adaptation, which suggests photo-oxidative damage. Because no differences were observed in light response curves of effective PSII photon yield for red- and yellow-senescing leaves, differences between red- and yellow-senescing cannot be explained by differences in the capacities for photochemical and non-photochemical light energy dissipation. A role of anthocyanins as screening pigments was explored further by measuring the responses PSII photon yield to blue light, which is preferentially absorbed by anthocyanins, versus red light, which is poorly absorbed. We found that dark-adapted PSII photon yield of red-senescing leaves recovered rapidly following illumination with blue light. However, red light induced a similar, prolonged decrease in PSII photon yield in both red- and yellow-senescing leaves. We suggest that optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.

}, keywords = {*Photosynthesis/physiology/radiation effects, Adaptation, Physiological, Anthocyanins/*metabolism/radiation effects, Apoptosis, Carotenoids/*metabolism/radiation effects, Chlorophyll/*metabolism/radiation effects, Chloroplasts/physiology/radiation effects, Cornus/*physiology/radiation effects, Darkness, Fluorescence, Light, Light-Harvesting Protein Complexes, Nitrogen/metabolism, oxidative stress, Photosynthetic Reaction Center Complex Proteins/radiation effects, Photosystem II Protein Complex, Plant Epidermis/physiology/radiation effects, Plant Leaves/physiology/radiation effects, Seasons}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Feild, T. S. and Lee, D. W. and Holbrook, N. M.} } @article {552581, title = {Bordered pit structure and vessel wall surface properties. Implications for embolism repair}, journal = {Plant Physiol}, volume = {123}, number = {3}, year = {2000}, note = {

Zwieniecki, M AHolbrook, N MengResearch Support, Non-U.S. Gov{\textquoteright}tResearch Support, U.S. Gov{\textquoteright}t, Non-P.H.S.2000/07/13 11:00Plant Physiol. 2000 Jul;123(3):1015-20.

}, month = {Jul}, pages = {1015-20}, abstract = {

The idea that embolized xylem vessels can be refilled while adjacent vessels remain under tension is difficult to accept if the cavitated vessels remain hydraulically connected to vessels under tension. A mechanism by which embolized conduits could be hydraulically isolated from adjacent conduits requires the existence of a non-zero contact angle and a flared opening into the bordered pit chamber such that a convex air-water interface forms at the entrance into the pit chamber. We measured the contact angle and pit chamber geometry for six species. The contact angle measured in the vessel lumen ranged between 42 degrees to 55 degrees, whereas the opening into the pit chamber ranged between 144 degrees and 157 degrees. If the surface properties within the pit chamber are similar to those in the lumen, a convex meniscus will form at the flared opening into the pit chamber. The maximum pressure difference between water in the lumen and gas in the pit chamber that could be stabilized by this interface was calculated to be within the range of 0.07 to 0.30 MPa.

}, keywords = {Biological Transport, Cell Wall/physiology, Plant Structures/anatomy \& histology/*physiology, Plants/*anatomy \& histology}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Zwieniecki, M. A. and Holbrook, N. M.} } @article {552311, title = {Embolism repair and xylem tension: Do We need a miracle?}, journal = {Plant PhysiolPlant PhysiolPlant Physiol}, volume = {120}, number = {1}, year = {1999}, note = {HolbrookZwienieckiEng1999/05/11 02:03Plant Physiol. 1999 May;120(1):7-10.}, month = {May}, pages = {7-10}, isbn = {1532-2548 (Electronic)0032-0889 (Linking)}, author = {Holbrook, N. M. and Zwieniecki, M. A.} } @article {552231, title = {Potential errors in measurement of nonuniform sap flow using heat dissipation probes}, journal = {Tree PhysiolTree PhysiolTree Physiol}, volume = {19}, number = {10}, year = {1999}, note = {Clearwater, Michael J.Meinzer, Frederick C.Andrade, Jose LuisGoldstein, GuillermoHolbrook, N. MichelleENG2003/03/26 04:00Tree Physiol. 1999 Aug;19(10):681-687.}, month = {Aug}, pages = {681-687}, abstract = {The empirical calibration of Granier-type heat dissipation sap flow probes that relate temperature difference (DeltaT) to sap velocity (v) was reevaluated in stems of three tropical tree species. The original calibration was confirmed when the entire heated probe was in contact with conducting xylem, but mean v was underestimated when part of the probe was in contact with nonconducting xylem or bark. Analysis of the effects of nonuniform sap velocity profiles on heat dissipation estimates showed that errors increased as v and the proportion of the probe in nonconducting wood increased. If half of a 20-mm probe is in sapwood with a v of 0.15 mm s(-1) and the other half is in nonconducting wood, then mean v for the whole probe can be underestimated by as much as 50\%. A correction was developed that can be used if the proportion of the probe in nonconducting wood is known. Even with the entire heated probe in contact with conducting xylem, v would be underestimated when radial velocity gradients are present. In this case, the error would be smaller except when velocity gradients are very steep, as can occur in species with ring-porous wood anatomy. Errors occur because the relationship between DeltaT and v is nonlinear. Mean DeltaT along the probe is therefore not a measure of mean v, and users of heat dissipation probes should not assume that v is integrated along the length of the probe. The same type of error can occur when DeltaT is averaged through time while v is changing, but the error is small unless there are sudden, step changes between zero and high sap velocity. It is recommended that relatively short probes (20 mm or less) be used and that probes longer than the depth of conducting sapwood be avoided. Multiple probes inserted to a range of depths should be used in situations where steep gradients in v are expected. If these conditions are met, heat dissipation probes remain useful and widely applicable for measuring sap flow in woody stems.}, isbn = {1758-4469 (Electronic)0829-318X (Linking)}, author = {Clearwater, M. J. and Meinzer, F. C. and Andrade, J. L. and Goldstein, G. and Holbrook, N. M.} } @article {552251, title = {Stomatal plugs of Drimys winteri (Winteraceae) protect leaves from mist but not drought}, journal = {Proc Natl Acad Sci U S AProc Natl Acad Sci U S AProc Natl Acad Sci U S A}, volume = {95}, number = {24}, year = {1998}, note = {Feild, T SZwieniecki, M ADonoghue, M JHolbrook, N Meng1998/11/25 03:01Proc Natl Acad Sci U S A. 1998 Nov 24;95(24):14256-9.}, month = {Nov 24}, pages = {14256-9}, abstract = {Two outstanding features of the flowering plant family Winteraceae are the occlusion of their stomatal pores by cutin plugs and the absence of water-conducting xylem vessels. An adaptive relationship between these two unusual features has been suggested whereby stomatal plugs restrict gas exchange to compensate for the presumed poor conductivity of their vesselless wood. This hypothesized connection fueled evolutionary arguments that the vesselless condition is ancestral in angiosperms. Here we show that in Drimys winteri, a tree common to wet forests, these stomatal occlusions pose only a small fixed resistance to water loss. In addition, they modify the humidity response of guard cells such that under high evaporative demand, leaves with plugs lose water at a faster rate than leaves from which the plugs have been experimentally removed. Instead of being adaptations for drought, we present evidence that these cuticular structures function to maintain photosynthetic activity under conditions of excess water on the leaf surface. Stomatal plugs decrease leaf wettability by preventing the formation of a continuous water film that would impede diffusion of CO2 into the leaf. Misting of leaves had no effect on photosynthetic rate of leaves with plugs, but resulted in a marked decrease ( approximately 40\%) in leaves from which the plugs had been removed. These findings do not support a functional association between stomatal plugs and hydraulic competence and provide a new perspective on debates surrounding the evolution of vessels in angiosperms.}, isbn = {0027-8424 (Print)0027-8424 (Linking)}, author = {Feild, T. S. and Zwieniecki, M. A. and Donoghue, M. J. and Holbrook, N. M.} } @article {552366, title = {Tropical deciduous forest: Death of a biome}, journal = {Trends Ecol EvolTrends Ecol EvolTrends Ecol Evol}, volume = {6}, number = {7}, year = {1991}, note = {Lerdau, MWhitbeck, JHolbrook, N MengEngland1991/07/01 00:00Trends Ecol Evol. 1991 Jul;6(7):201-2. doi: 10.1016/0169-5347(91)90019-T.}, month = {Jul}, pages = {201-2}, isbn = {0169-5347 (Print)0169-5347 (Linking)}, author = {Lerdau, M. and Whitbeck, J. and Holbrook, N. M.} } @article {552296, title = {Small plants in high places: The conservation and biology of epiphytes}, journal = {Trends Ecol EvolTrends Ecol EvolTrends Ecol Evol}, volume = {6}, number = {10}, year = {1991}, note = {Holbrook, N MengEngland1991/10/01 00:00Trends Ecol Evol. 1991 Oct;6(10):314-5. doi: 10.1016/0169-5347(91)90037-X.}, month = {Oct}, pages = {314-5}, isbn = {0169-5347 (Print)0169-5347 (Linking)}, author = {Holbrook, N. M.} } @article {552511, title = {Spring filling of xylem vessels in wild grapevine}, journal = {Plant PhysiolPlant PhysiolPlant Physiol}, volume = {83}, number = {2}, year = {1987}, note = {Sperry, J SHolbrook, N MZimmermann, M HTyree, M Teng1987/02/01 00:00Plant Physiol. 1987 Feb;83(2):414-7.}, month = {Feb}, pages = {414-7}, abstract = {Xylem vessels in grapevines Vitis labrusca L. and Vitis riparia Michx. growing in New England contained air over winter and yet filled with xylem sap and recovered their maximum hydraulic conductance during the month before leaf expansion in late May. During this period root pressures between 10 and 100 kilopascals were measured. Although some air in vessels apparently dissolved in ascending xylem sap, results indicated that some is pushed out of vessels and then out of the vine. Air in the vessel network distal to advancing xylem sap was compressed at about 3 kilopascals; independent measurements indicated this was sufficient to push air across vessel ends, and from vessels to the exterior through dead vine tips, inflorescence scars, and points on the bark. Once wetted, vessel ends previously air-permeable at 3 kilopascals remained sealed against air at pressures up to 2 and 3 megapascals. Permeability at 3 kilopascals was restored by dehydrating vines below -2.4 megapascals. We suggest that the decrease in permeability with hydration is due to formation of water films across pores in intervascular pit membranes; this water seal can maintain a pressure difference of roughly 2 megapascals, and prevents cavitation by aspirated air at xylem pressures less negative than -2.4 megapascals.}, isbn = {0032-0889 (Print)0032-0889 (Linking)}, author = {Sperry, J. S. and Holbrook, N. M. and Zimmermann, M. H. and Tyree, M. T.} }