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.

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.

Watkins, James E., Amber C. Churchill, and Noel Michele Holbrook. 2015. “A site for sori: Ecophysiology of fertile–sterile leaf dimorphy in ferns.” American journal of botany 103 (5): 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–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–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.


Latest News

The Cavitators in action!

The Cavitators won first volleyball game of season!

June 7, 2016

Every year, many science labs at Harvard form volleyball teams and participate in a volleyball tournament known as The Rhino Cup. This year, the Holbrook Lab (along with some friends) felt up to the challenge and formed a team called "The Cavitators". We have been practicing (tirelessly!) and all of our hard work paid off when we won the first game of our season!! Go Cavs!!

Mel Tyree

Mel Tyree

May 11, 2016

On May 11th , Mel Tyree came to visit the lab to talk about his recent cell-water dynamics research and to enjoy the beautiful weather!

Recent Travel of Lab Members

February 18, 2016
Many of our lab members have been out and about recently! John recently came back from a 6 day trip to Nanyang Technological University in Singapore. And Greg recently came back from a 5-day workshop in Manaus, Brazil where he gave a talk entitled: Wood anatomy; basics and tree ring recognition, and was a co-instructor for a course on the basics of tree ring analysis.