Doctor of Philosophy

dissertationWater potential was measured at various times throughout spring, summer, and fall of 1968 and 1969 in important plant species of four different plant communities located in the salt desert near Great Salt Lake, Utah. In all species studied (greasewood, Sarcobatus vermiculatus ; perennial seepweed, Suaeda fruticoas; water potential decreased from spring through summer, although plants of the different communities expericenced different water potentials. Salt grass and grease-wood had the highest water potentials, while annual and perennial seepweed experienced considerably lower water potentials. Rates of net photosynthesis declined in each of these halophytes as plant water potential decreased, but they were able to maintain positive net photosynthesis over a much wider range of sub-optimal water potentials than reported for most glycophytes from more mesic habitats. This suggests a physiological adaptation of these halophytes to their saline desert environments in which they experience low water potentials during much of the growing season. The photosynthetic response of these species to low water potentials appeared to paralled the water potentials observed for each species in the field. Perennial seepweed, which experienced lowest water potentials in the field, maintained positive net photosynthesis at lower water potentials, and had maximum net photosynthetic rates at lower water potentials, then greasewood and salt grass. This suggests that distribution of these species may be attributed in part to their relative tolerances to low water potential. Respiratory rates of the three halophytes declined either slowly or not at all with decreasing water potential. They would be able therefore to continue cell metabolic processes but not to deplete food reserves rapidly during periods of severs moisture stress. This suggests a further physiological adaptation of these plants to their enviroment. Multiple regression analyses of soil moisture, soil salinity, and leaf salinity against net photosynthesis and dark respiration reveal that non of these factors probably affects photosynthesis and respiration to a greater extent than they contribute to plant water potential. Greasewood and perennial seepweed, both succulent-leaved plants, are able to tolerate high internal concentrations of salt. Salt greass, a non-succulent halophyte, had a considerably lower internal salt concentration, suggesting that this species survives in saline areas by excluding salt from the plants. Other physiological responses to low water potential, and other aspects of the ecology of these species should be studied before definite conclusions regarding factors influencing their distribution can be made