Leaf Conductance and Osmoregulation of Field‐grown Sorghum Genotypes 1

The relationship between stomatal activity and leaf water potential components was examined for several sorghum [ Sorghum bicolor (L.) Moench] genotypes as a function of growth stage under semi‐arid field conditions. During vegetative development, prior to flowering, genotypic differences were observed in the leaf water potential required to initiate stomatal control of transpirational water loss and the rate of change in conductance per unit change in leaf water potential. No significant genotypic differences existed in conductance when leaf water potential was high. After flowering and attainment of maximum leaf size, conductance remained high even when the leaf water potentials declined to −24 to −26 bars. The relationships between leaf water potential components were examined to help explain the leaf conductance observations. Genotypic differences were observed in the leaf water potential‐leaf turgor potential relationships at each growth stage. Prior to flowering, as leaf water potential declined, leaf turgor declined accordingly with little evidence of osmoregulation, and all genotypes reached calculated 0 turgor at essentially the same leaf water potential (−18 to −20 bars). Genotypic differences were also observed in the leaf turgor‐stomatal conductance relationships. After flowering, during the grain filling stage, significant genotypic differences were observed in the degree of osmotic adjustment. The genetic range in osmotic adjustment represents a potentially important adaptive mechanism to water stress which allows for maintenance of turgor and thus of metabolic functions dependent on turgor. Apparently, water conservation through stomatal control is practiced to some extent in sorghum prior to the onset of reproductive development; however, after flowering photosynthetic productivity is apparently maintained at the expense of water conservation.