9604424 Pearcy This research will investigate the interrelationships between the hydraulics of a plant crown, and the amount and arrangement of leaf area on that crown that functions for light capture using the in species of the genus Psychotria, a prominent genus of shrubs. The basic hypothesis is that there is a tradeoff between maximizing light capture and providing sufficient stem conductivity to supply water to the leaves and that this tradeoff differs between species adapted to areas subject to drought and species adapted to areas more or less permanently wet. The species to be utilized occur along habitat gradients of light and fry season severity so that the selective pressures for hydraulics versus light capture will differ among them. A computer simulation model that can reconstruct the three-dimensional architecture of a plant will be used to estimate the light capture capacity and efficiency of the crown architectures of the different species. The stem hydraulic conductances and the vulnerability to cavitation, which is the formation of air bubbles in the xylem that blocks water flow and can lead to death of the shoot, will be determined. Finally, stem hydraulic conductances will be mapped onto the crown architectures and used in the simulations to explore the consequences of species differences in the vulnerability to cavitation under different drought scenarios. This research will provide insight into how the functional morphology of the crowns, the most visible manifestation of species differences, relates to species distribution along light and moisture gradients. Since light and moisture gradients are important determinants of biodiversity in tropical forests the research will ultimately provide insight into the functional controls on biodiversity in these forests.
