Xylem cavitation and tolerance to freezing and water stress in sagebrush

Hydraulic transport is essential for a plant to maintain gas exchange, yet the ability to do so can be compromised by the occurrence of xylem cavitation. This dissertation considers the occurrence and physiological implications of water stress and freezing-induced cavitation in Artemisia tridentata, and the potential for ecotypic differentiation in susceptibility to cavitation.; To investigate the influence of cavitation on hydraulic conductance, it was necessary to first develop a method that would work with A. tridentata. Branches of A. tridentata have low hydraulic conductance which made conventional methods inappropriate. The method developed for A. tridentata allows measurement of hydraulic conductance for both root and shoot systems, and has facilitated research on other systems that were previously difficult.; Hydraulic limitations to gas exchange in A. tridentata showed that a decrease in plant water use was necessary to prevent catastrophic cavitation. Soil-leaf hydraulic conductance, transpiration, and leaf area were found to decline in concert with decreasing soil and plant water status. The decline in whole plant hydraulic conductance was likely due to changes in the transport capacity of the root system, as roots were more vulnerable than shoots to water stress-induced cavitation.; A comparative study of the subspecies of A. tridentata showed that vulnerability to water stress-induced cavitation is likely an important determinant of their distribution within the Great Basin. Vulnerability to cavitation paralleled the reputed habitat segregation of the subspecies. Furthermore, differences were maintained when the plants were grown in a common environment, suggesting a genetic basis for intraspecific variation in vulnerability to cavitation.; In addition to susceptibility to water stress-induced cavitation, vulnerability to cavitation caused by freezing may also influence distribution of A. tridentata subspecies. Populations of A. tridentata differed in resistance to freezing-induced cavitation with increased resistance in plants from high elevations. Thus, freezing-induced cavitation likely has contributed to the elevational zonation of the subspecies.; These studies suggest that hydraulic limits imposed by cavitation influence plant distribution by restricting the habitats where a plant can persist. Furthermore, intraspecific variation in cavitation resistance appears to be an important adaptive feature of A. tridentata.