EFFECTS OF HEAT AND WATER STRESS AND THEIR INTERACTIONS ON GRAIN SORGHUM (SORGHUM BICOLOR (L.) MOENCH)

A series of experiments were conducted to evaluate the interactions of water stress and high temperatures on the growth and development of grain sorghum {Sorghum bicolor (L.) Moench}. Effects of combinations of water stress and high temperatures on intact plants and of heat and desiccation on cut leaf tissue were also examined. Potted plants were grown in temperature controlled greenhouses. Water stress reduced leaf emergence rates and the final leaf area while elevated temperatures, under non water stressed conditions and under mild water stress, enhanced leaf emergence rates and increased leaf area. When plants were well watered or only mildly water stressed, exposure to temperatures of 35(DEGREES)C during panicle development resulted in higher grain yields, but under 8 and 12 days of water stress plants grown in a 30(DEGREES)C environment gave higher yields. Elevated temperatures during boot stage reduced panicle weight, seed numbers, and grain yield, possibly due to floret abortion. Leaf photosynthetic rates were greater for plants that had been water stressed at -0.3 and -0.45 MPa than for the controls, but dry matter accumulation rates were greater for the controls. Exposure to temperatures of 38(DEGREES)C increased photosynthetic rates but 40(DEGREES)C caused photosynthetic rates to decline. Under non water stressed conditions, exposure to elevated temperatures hastened bloom and physiological maturity but bloom was delayed when elevated temperatures were coupled with water stress. Eight and 12 days without irrigation delayed bloom by 2 to 9 days, and physiological maturity by 3 to 12 days. Eight and 12 days without irrigation induced tillering in plants grown in a 30(DEGREES)C environment but even 4 days without irrigation induced tillering in plants grown under 35(DEGREES)C. Both water stress and elevated temperatures increased heat and desiccation tolerance of sorghum leaf tissue. There were genotypic differences in the degree of heat and desiccation tolerance but these differences disappeared with plant age, with increased water stress, and with extended exposure to elevated temperatures. Significant interactions between water stress and temperature were observed with 4 to 12 days of water stress, but not when osmotically controlled water stress levels were -0.3 to -0.6 MPa. Under well watered conditions and under 4 days of water stress, elevated temperatures accelerated plant growth and increased grain yield and total dry matter. When water stress periods were 8 or 12 days, elevated temperatures delayed plant development and reduced grain yield and total dry matter