In this project,the mechanisms and consequences of a major plant response will be determined that is increasingly recognized to play a central role in processes at cell, tissue and whole-plant level - the vulnerability of the leaf water transport (hydraulic) system to dehydration. Equally, the implications of leaf water transport for stomatal control and urban forest-level transpiration, a major component of city water use, will be clarified. For diverse urban trees in Los Angeles, the project will combine new experimental and nano-microscopy techniques with modeling to determine the relative roles in the dynamics of leaf hydraulic conductance (Kleaf) of mechanisms including air blockage within the leaf veins, cell collapse, and changes in the activity of membrane proteins (aquaporins). Second, the project will study the role of Kleaf in the determination of the stomatal response to atmospheric drought. A novel computer simulation model of the water transport and gas exchange systems will be developed, to be parameterized with data from experiments and from anatomical analyses. This model will predict the effect of xylem anatomy and its dynamics during transpiration or drought on the responses of stomata. The project will test and refine this model against experimental measurements of gas exchange responses for the urban trees. The third objective is to combine the measurements and model with additional functional trait data to predict tree water use and water use efficiency, and to test these predictions with sapflow and growth measurements for urban trees. Overall,this research will break new scientific ground in clarifying the dynamics of plant water transport, and improving prediction of urban forest water use, especially critical for conservation of water resources. The broader educational impacts include collaborative data collection within a mentoring program for under-represented undergraduates in research, to improve participation and appreciation of urban ecophysiology and ecology.
