SORGHUM RESPONSES TO HIGH TEMPERATURE AND WATER STRESS IMPOSED DURING PANICLE DEVELOPMENT

The responses of two sorghum hybrids, C46 and RS671, to water stress imposed for 5 or 6 days starting at the floret differentiation stage, were investigated over a range of temperatures (30/22, 35/22 and 40/22 C, day/night) under controlled environment conditions. Water stress was imposed either by addition of polyethylene glycol 600 to the nutrient solution when plants were grown in hydroponic culture, or by moisture depletion when plants were grown in soil. Both types of water stress inhibited photosynthesis and transpiration in the two sorghum genotypes, as well as caused losses of their functional leaf area, delayed their development rate during panicle expansion, and reduced their grain yield and seed number per head relative to the controls. However, the magnitude of some of these effects of water stress varied with the temperature, the genotype and the culture medium. In hydroponics, the inhibition of photosynthesis due to osmotic stress occurred in the hybrid C46 under all three temperature regimes, but occurred only at 40 C in RS671. In soil, the photosynthetic rates of both genotypes were similarly depressed by water shortage regardless of the temperature. Transpiration rates declined significantly with osmotic stress only at 40 C, but in soil the transpiration rates declined to nearly zero at all three temperatures. Additionally, in hydroponics, grain yield and seed number per panicle of the two sorghum hybrids were reduced by osmotic stress to approximately the same extent; whereas in soil conditions the hybrid C46 was unaffected by water stress at any of the three temperatures, while RS671 was severely restricted in its grain yield when daytime temperature was 30 or 40 C, but not at 35 C. Furthermore, in hydroponics the osmotically stressed plants usually were able to maintain leaf water potentials as high as those of the controls ((TURN) -10 bars), despite the large differences in transpiration rates. Under soil moisture stress the leaf water potentials of both hybrids dropped to about -17 bars below the controls ((TURN) -6 to -10 bars) when daytime temperature was 30 or 40 C. The difference between stressed and unstressed plants was only (TURN) -7 bars at 35 C. Rapid declines in the rates of photosynthesis and transpiration of plants growing in soil occurred when the leaf water potentials dropped below -8 to -10 bars at 35 or 40 C, while at 30 C the threshold leaf water potential was around -17 bars. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM