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Understanding how growth and development of grain sorghum Sorghum bicolor (L.) Moench genotypes respond to water limitation would provide a basis to assess the value of such responses in crop production and crop improvement. We conducted a greenhouse experiment to quantify responses of six genotypes to five levels of water limitation between floral initiation and flowering achieved by daily watering to weight. At flowering, the thermal time, biomass, height, water transpired, and transpiration efficiency (biomass/water transpired) were recorded. Results for water‐limited treatments were expressed relative to the well‐watered treatment. Degree of water limitation was indexed by calculating the ratio of the amount of water transpired in water‐limited treatments to that in the well‐watered treatment. This transpiration ratio decreases from a value of one as degree of water limitation increases. Under well‐watered conditions, genotypes differed significantly in thermal time and biomass, but not in height. Differences in biomass were related to differences in both water transpired and transpiration efficiency. The least efficient genotype had a value of transpiration efficiency 25% lower than that of the most efficient. Under water‐limiting conditions, relative thermal time increased at transpiration ratios < 0.55; genotypes did not differ. Relative height decreased linearly with transpiration ratio, and genotypes did not differ. Relative biomass decreased with increasing water limitation, but genotypes differed. This was related to genotypic differences in relative transpiration efficiency for transpiration ratios < 1. Some genotypes increased transpiration efficiency 28% under water limitation. The value of expressing water limitation as transpiration ratio and possible mechanisms explaining these findings are discussed.
Donatelli et al. (Fri,) studied this question.