Gas Exchange of Three Cool Semi-Desert Species in Relation to Temperature and Water Stress

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Journal of Ecology

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Three plant species prominent in cool shrub-steppe communities of western North America, Artemisia tridentata ssp. tridentata (true shrub), Gutierrezia sarothrae (half-shrub) and Agropyron spicatum var. inerme (perennial bunchgrass), were studied with the aim of defining interspecific differences in gas exchange response to temperature and plant water potential during the course of the year. These differences in patterns and magnitudes of gas exchange in relation to variations in physical factors were taken as evidence of differing adaptive strategies. Different morphological and phenological features of these three species were also correlated with gas exchange response to physical factors. Rates of net assimilation were found to be lowest for the true shrub Artemisia tridentata and much higher in the suffrutescent Gutierrezia sarothrae and the perennial bunchgrass Agropyron spicatum var. inerme. Variations in temperature exerted a pronounced effect on net assimilation rates of all three species with clearly marked optima for photosynthesis of 16-20° C for the two shrub species and 20-25° C for the grass. Dark respiration rates for all three species increased consistently with increases in temperature. On a seasonal basis, net assimilation rates for all three species were highest during spring and early summer, when water supply was least limiting and temperatures were closest to optimal. During periods of drought and higher temperatures in late summer, the Agropyron became photosynthetically dormant, and the shrubs exhibited much lower rates of net photosynthesis; those of Artemisia were far more reduced than those of Gutierrezia, particularly at higher temperatures, when net assimilation for Artemisia frequently became negative during the afternoon periods. Stomatal diffusion resistances to CO2 assimilation (r's) were much higher for Artemisia than for Gutierrezia under all conditions, and r's increased much more rapidly with increases in temperature and water stress for Artemisia. This greater sensitivity of r's of Artemisia is viewed as an adaptation to curb excessive leaf water loss during hot, dry, late summer periods, allowing maintenance of greater quantities of leaves during such periods. Gutierrezia, which exhibited less effective stomatal control of water loss, shed most of its leaves during drought periods. Under conditions of water stress, residual resistances to CO2 assimilation (r'r) for Artemisia were much higher than those of Gutierrezia, although r'r for both species tended to increase with decreasing plant water potentials. One major reason for the higher r'r values of Artemisia at low water potentials is thought to be linked to the fact that decline of respiration rates with decreasing water potential was less rapid than the concurrent decline in photosynthesis. This might also help to explain the negative net assimilation often noted for Artemisia under conditions of high temperatures and low water potentials. Gutierrezia, conversely, exhibited a less pronounced decline in photosynthetic rates and a more rapid decline in respiration rates with decreasing water potential, and no negative net photosynthesis was observed for this species during daylight hours even though plant water potentials were generally lower than those of Artemisia.

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