9630791 Nilsen The water conducting ability of plant stems must be sufficient to supply water to leaves for growth and other physiological processes. However, thewater conducting ability of the stem can be reduced when air bubbles formwithin the cells that normally conduct water. These bubbles can occur during drought when the tension, which pulls water through the plant from the soil to the leaves, becomes too great and the water column breaks. Also, during the winter, freezing of the water inside these cells causes air dissolved in the water to form bubbles which can expand and block water flow during periods of thaw. We will be studying the impact of the frequency and duration of drought and winter freeze-thaws on the water conducting ability of stems of related Rhododendron species. We will be utilizing the natural variation in winter freeze-thaw frequency and intensity of summer drought provided by two Long Term Ecological Research (LTER) sites to evaluate these following objectives: 1) determine the physical factors which control the maximum potential water flow through these plants, 2) to examine possible limits to water transport in relation to plant response to climatic patterns of water availability, and 3) to determine the structural/physiological adaptations of these species which may prevent or lessen the potential for the development of air blockages in their water conducting cells. This study will provide a cross-LTER comparison of the ecological physiology of closely related taxa, provide information of the controls of water flow through plants, and provide information about the water use of understory plants to be used in models of ecosystem water use budgeting.
