9307196 Comstock Transpiration is the unavoidable water loss from a plant incurred during photosynthesis. Several methods are employed in this study to better understand the ways in which very high rates of transpiration limit plant performance, and what specializations can enable plants to endure them. The study focuses on the performance of arid-land plants, and examines both physiological mechanisms and population-level adaptation. A new climatic index is proposed, which ranks sites in terms of the evaporative demand of the environment during growing season. Preliminary evidence suggests that this index explains population-level genetic differentiation in key water-use characteristics of warm-desert shrubs. Special attention is given to the importance of the capacity for water transport between root and shoot. It is suggested that this is often a key limiting factor constraining maximum tolerable transpiration rate, and therefore limits photosynthetic activity under conditions of high temperature and low humidity. Specific mechanistic models of how transpiration rate is sensed and controlled will be tested. Water-use characters under study include leaf gas-exchange with the atmosphere, functional measures of liquid water transport capacity, transport tissue anatomy, and humidity treatments during growth, drought studies, and pressurization of the soil volume to control leaf water status. These experiments will contribute directly to understanding the environmental physiology of arid-land plants i particular, but the principles under study are general constraints experienced by all terrestrial plants. The results will be pertinent to all environmental biologists interested in either natural or cultivated plant systems ***
