Welcome to the Holbrook lab! We study the physics and physiology of vascular transport in plants with the goal of understanding how constraints on the movement of water and solutes between soil and leaves influences ecological and evolutionary processes. Currently, we are working on questions relating to cavitation, stomatal mechanics, leaf hydraulic design, and xylem evolution.

Holbrook Lab 2016
Lab photo 2016. Top row: Teressa Alexander, Cynthia Gerlein-Safdi, Greg Ceccantini, Jess Gersony, Alexandre "Pono" Ponomarenko, Walton "Tinker" Green. Bottom row: Nathan Mueller, Kadeem Gilbert, Fulton "Tony" Rockwell, Noel Michele "Missy" Holbrook, Uri Hochberg, Clement Quintard, Yongjiang "John" Zhang. Not pictured: Laura Clerx, Robinson "Wally" Fulweiler, Juan Losada, Bridget Power and Jessica Savage

Recent Publications

Knoblauch, Michael, Jan Knoblauch, Daniel L Mullendore, Jessica A Savage, Benjamin A Babst, Sierra D Beecher, Adam C Dodgen, Kaare H Jensen, and Noel Michele Holbrook. 2016. “Testing the Münch hypothesis of long distance phloem transport in plants.” eLife 5. Abstract

Long distance transport in plants occurs in sieve tubes of the phloem. The pressure flow hypothesis introduced by Ernst Münch 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 Münch hypothesis, while providing new tools for the investigation of one of the least understood plant tissues.

Rolland, V., D.M. Bergstrom, T. Lenne, G. Bryant, H.Chen, J. Wolfe, M. N Holbrook, D. E. Stanton, and M.C. Ball. 2015. “Easy Come, Easy Go: Capillary Forces Enable Rapid Refilling of Embolized Primary Xylem Vessels.” Plant Physiology American Society of Plant Biologists , 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.

Ronellenfitsch, H, J Liesche, KH Jensen, NM Holbrook, A Schulz, and E Katifori. 2015. “Scaling of phloem structure and optimality of photoassimilate transport in conifer needles.” Proc Biol Sci 282: 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.


Latest News

Molly and Jess riding in the bucket truck at the Harvard Forest

Harvard grad students explore the tree canopy of the Harvard Forest in an episode of the YouTube series called ScienceIRL

February 15, 2018

The Earth's climate is rapidly changing- how will its inhabitants be affected? Harvard University PhD student and Holbrook Lab member, Jess Gersony, is studying how trees will respond to drought, a consequence of climate change. A key component of her science toolbox is the *pressure bomb,* an instrument that measures how dry trees are- joins us as we venture into the canopy of the Harvard Forest to find out!



Read more about Harvard grad students explore the tree canopy of the Harvard Forest in an episode of the YouTube series called ScienceIRL
Cavitation or deformation? Our new study is out in Plant Physiology!

Cavitation or deformation? Our new study is out in Plant Physiology!

October 12, 2016

Check out our new study by John, Tony, Adam, Teressa and Missy! Reversible leaf xylem collapse: a potential 'circuit breaker' against cavitation.


We report a novel form of xylem dysfunction in angiosperms: reversible collapse of the xylem conduits of the smallest vein orders that demarcate and intrusively irrigate the areoles of Quercus rubra leaves. Cryo-scanning electron microscopy revealed gradual increases in collapse from ~ -2 MPa down to ~ -3 MPa, saturating thereafter (to -4 MPa). Over this...

Read more about Cavitation or deformation? Our new study is out in Plant Physiology!
Times they are a changin' (departures) (cont.)

Times they are a changin' (departures) (cont.)

September 21, 2016

For the past year we were very lucky to host Wally Fulweiler, a biogeochemist and ecosystem ecologist, (with a focus on the marine world!) (and just generally a cheerful, positive person!) from BU while she was on sabbatical! This summer she spent time sampling in Harvard forest, and also at the bottom of the sea in Alvin, the submarine! Now, with the new semester beginning, we will be seeing less of her - but will always treasure our time with a marine biologist in the lab!!!!! 

We welcome baby Lucy into our lab!!!

We welcome baby Lucy into our lab!!!

September 16, 2016

Tinker Green, a dear post-doc!, recently welcomed a new baby, Lucy, into the world!! Congratulations! and we can't wait for her to become the next paleo-botanist of the group!!