Principles of leaf hydraulics
Leaves are complex structures
charged with the task of balancing CO2 uptake and water loss.
Substantial effort has been directed towards understanding leaves as photosynthetic machines that
evolved to capture light, while their underlying hydraulic design has received much less attention.
This work will address the
micro-hydrology of leaves, focusing on two goals: (1) understand the
biophysics of water movement within leaves and (2) determine the physiological significance of
separating photosynthetic cells from the high demands for water imposed by the transpiration.
The research is motivated by the finding that leaves consist of two hydraulically isolated water
pools linked to the water supply path (xylem) via low and high resistance pathways. It is not
known how these hydraulically distinct pools of water are related to the structure and physiology
of leaves. We hypothesize that photosynthetic cells are hydraulically isolated from the transpiration
stream and that this isolation is under biological control. The hypothesis is that separation
of the transpirational path from the photosynthetic tissue will allow better protection of the
chlorophyll bearing cells from sudden changes in micro-environmental conditions.
The basic approach to study the hydraulic design of leaves includes the use of
rehydration kinetics to quantify the relative sizes of the two compartments and cell pressure
probe to determine hydraulic linkages between different “compartments” as well as the identity
of these pools. These methods will be combined with experimental treatments (temperature, metabolic
inhibitors) and morphological studies to resolve both the identity and the nature of the hydraulic
separation of the two water pools. Mathematical modeling will be used to explore the ways in which
compartmentalization impacts leaf performance; measurements of stomatal behavior in relation to
xylem vulnerability will be used to test the hypothesis that the existence of a fast compartment
allows plants to operate close to their cavitation limit. The goals are to understand the principles
of leaf hydraulic design, explore the biological basis for hydraulic compartmentalization, and
determine the physiological significance of the internal hydraulic architecture of leaves.
Expectations are that research results will have significant impact in three major areas:
1. Plant biology where recognition of leaf hydraulic compartmentalization will fundamentally alter understanding of the structure and physiology of leaves.
2. Plant diversity where comparison of the degree of compartmentalization, morphology, and photosynthetic performance across a wide range of taxa representing diverse evolutionary lineages and a diversity of growth forms and ecologies will enhance our understanding of the functional role of leaf hydraulic design.
3. Water relations methodology where the presence of two hydraulic compartments in leaves has important implications for the interpretation of results from basic techniques used to determine leaf water status.