Genotypic variation within sorghum for transpiration response to drying soil

Sorghum is commonly grown under dryland conditions, where yields are limited by soil water deficits. Yield increase may be possible by selecting genotypes that express traits that are desirable for water-limited conditions. Water conservation is one possible trait for increasing yield since this trait could increase water availability during critical stages of crop development. Water conservation could be achieved by slower transpiration rate with soil drying initiated at a high fraction of transpirable soil water (FTSW) so that the use of soil water is extended over a longer period of time. This water conservation strategy may allow the crop to have water available during the critical phase of grain filling. Therefore, the objective of this study was to compare genotypes of sorghum for possible differences in the threshold for the decline in transpiration. Sixteen sorghum genotypes were selected for this study based on differences in their sensitivity to elevated vapor pressure deficit (VPD). These genotypes were subjected to dry-down experiments in pots in a greenhouse to determine the threshold FTSW for the decrease in transpiration rate as the soil dried. Differences in the FTSW threshold for transpiration decline were observed to range among genotypes from 0.32 to 0.48. The genotypes differed between low and high FTSW thresholds in the same manner as they did for a breakpoint in the VPD or lack of breakpoint, respectively. Those genotypes with high FTSW thresholds exhibited the hypothesized desired trait. However, they did not exhibit the water conserving trait of limited transpiration rate at high VPD. On the other hand, those genotypes with a low FTSW threshold were those selected for limited transpiration rate at high VPD. These genotypes also differed based on their transpiration rate under well-watered conditions with the genotypes with a low FTSW threshold exhibiting a low transpiration rate. These results demonstrated that among the sorghum genotypes there are several alternative traits for enhancing soil water conservation for growth under dry land conditions.