Genetic variability of transpiration response to vapor pressure deficit among sorghum genotypes

Simulation studies have demonstrated that limited maximum transpiration rate (TR) at high air vapor pressure deficit (VPD) in water-limited environments could result in significant increases in sorghum yield. However, such a restriction on TR at high VPD has not been documented in sorghum. The objective of this study was to search within sorghum germplasm for the possibility of restricted TR at high VPD. Twenty six genotypes were selected for measurement of VPD response based on field observations including yield, leaf temperature, and the stay-green phenotype. These genotypes were grown in a greenhouse for about 24-d growth, and then placed into individual chambers in which VPD was varied and TR measured. The results of this study showed marked variation among sorghum genotypes in TR response to VPD. Seventeen genotypes were identified as exhibiting a breakpoint in their VPD response in the range from 1.6 to 2.7 kPa, above which there was little or no further increase in TR. Therefore, these genotypes with a breakpoint have the possibility of soil water conservation when VPD during the midday cycle exceeds the breakpoint VPD. This trait would be desirable in less humid environments for increasing yields in water-deficit seasons. The observed range in the value of the BP among genotypes offers the possibility of developing genotypes with BP appropriate for specific environments.