Principles of leaf hydraulic design

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Leaves are complex structures that balance CO2 uptake and water loss. This work addresses 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. Earlier studies show that leaves consist of two hydraulically isolated water pools linked to the water supply path (xylem) via low and high resistance pathways. How these hydraulically distinct pools of water are related to the structure and physiology of leaves is not known. One hypothesis is that photosynthetic cells are hydraulically isolated from the transpiration stream and that this isolation is under biological control. The separation of the transpirational path from the photosynthetic tissue may protect chlorophyll bearing cells from sudden changes in micro-environmental conditions. 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. The research will include analysis of rehydration kinetics of leaves to quantify the relative sizes of the two compartments. In addition, cell pressure probe measurements will 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 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. Stomatal behavior and xylem vulnerability will be measured to determine whether a fast hydraulic compartment exists that allows plants to operate close to their cavitation limit. The research will fundamentally alter the understanding of the structure and physiology of leaves. In addition, the research will improve understanding of plant diversity by extending the studies of foliar water compartmentalization across a wide range of taxa representing diverse evolutionary lineages, growth forms, and ecologies. Activities leading to broader impacts are designed to extend ideas from this research to a broader audience. Such activities include: (1) mentoring and supporting younger scientists at all levels and (2) outreach activities directed to the public. Efforts will include creation and maintenance of a webpage on leaf hydraulics that is linked to the Arnold Arboretum website. The website will explain the goals and current findings of this research and will outline activities relevant to this topic for school classes visiting the Arboretum. In addition this research will be popularized via public lectures at The Arnold Arboretum as a part of their ongoing public program and an article in Arnoldia magazine.