ADAPTATION OF PHOTOSYNTHESIS TO DROUGHT AND HIGH TEMPERATURE STRESS IN SORGHUM

Sorghum cultivars SC90 and 4104 were selected as stress-tolerant genotypes after screening 18 genotypes using heat and water stress combinations. Martin was selected as a control. In subsequent experiments, SC90, 4104, and Martin were subjected to 0, -0.15, and -0.30 MPa osmotic potential in hydroponic nutrient solutions combined with 30, 35, and 40(DEGREES)C. Oxygen evolution by leaf slices in unadjusted assay buffer, and in a medium osmotically adjusted to a minimum of -0.15 MPa to match the leaf water potentials of stressed plants, was used to measure non-stomatal-limited photosynthesis. Water stress appeared to inhibit photosynthesis of the three sorghums mainly through stomatal closure in response to decreased leaf water potential and osmotic potential. Although O(,2) evolution in the osmotically adjusted buffer was lower than in the unadjusted buffer, indicating that the cellular photosynthetic apparatus per se was affected by water stress, no correlation was found between rates of whole leaf CO(,2) uptake and O(,2) evolution, so inhibition of O(,2) evolution could not account for the photosynthetic limitation in whole leaf CO(,2) uptake. At high temperatures, CO(,2) uptake and stomatal conductance were inhibited to the same extent in the three sorghums. With leaf slices in osmotically unadjusted buffer, which allowed leaf slice rehydration, high temperature had almost no effect on O(,2) evolution by SC90. High temperatures inhibited O(,2) evolution by 4104 and Martin, and O(,2) evolution was correlated with CO(,2) uptake for Martin, so inhibition of cellular photosynthetic processes appeared to be the main limiting factor at high temperature. For the three sorghums, the interaction of high temperature and water stress caused CO(,2) uptake and stomatal conductance to decrease more than expected from the combined individual effects of each stress. Oxygen evolution by leaf slices for SC90 was enhanced at each temperature when the osmotic potential of the assay buffer was increased, indicating that dehydration of plant cells protected against high temperature. This was observed for 4104 and Martin at mild heat and water stress combinations, but O(,2) evolution was inhibited for these genotypes under more severe stress combinations