Physiological responses of sorghum genotypes to environmental stress

Progress in improving sorghum yields under conditions of environmental stress has been limited due to (i) lack of a better understanding of physiological mechanisms of drought tolerance especially as they relate to developmental processes governing grain yield and (ii) nonavailability of a practical drought resistance screening technique. Studies were conducted at Garden City, KS in 1983 and 1984 to address these two limitations. An attempt was made to develop a fast, simple and practical technique to screen drought resistant sorghum genotypes by measuring canopy temperatures on 81 genotypes. Canopy-air temperature (DT) was moderately correlated with grain yield (r = $-0.56)$ in the stress environment. This association was rather strong (r = $-0.81)$ when partial correlation coefficient was computer between yield and DT after removing the variation due to genotypes. The rank correlation of 0.08 between DT under wet and dry treatments showed that DT measurements must be made on the stressed plants if they are to be used for drought screening. Physiological responses of 10 sorghum genotypes to environmental stress during growth stage 2 (GS2) and growth stage 3 (GS3) were studied. Under GS2 stress genotypes differed significantly for leaf temperature, stomatal resistance, photosynthesis, transpiration and water use efficiency. Photosynthesis was associated with stomatal resistance (r$\sp2$ = 0.89). Decrease in water potential of panicle at floret differentiation in B35-6 and MS348 was 56% and 18%, respectively. P/V curves did not show any differential osmoregulation by leaves of these two genotypes under GS2 stress. However, B35-6 is more sensitive to GS2 stress in terms of grain yield losses. Leaf growth was found to be severely inhibited due to GS2 stress. Physiological variables including stomatal resistance were not associated with leaf turgor during GS3 stress. Water vapor and carbon exchange parameters closely followed the stomatal resistance. Panicles under stress lost more water than the ones under no stress. Stressed panicles fixed carbon at a slower rate at 2 weeks after bloom but respired at a faster rate at 4 weeks after bloom as compared to panicles under no stress