Is the Stay‐Green Trait in Sorghum a Result of Transpiration Sensitivity to Either Soil Drying or Vapor Pressure Deficit?

Persistence of green leaves during seed fill, referred to as a stay‐green trait, has been investigated in sorghum [Sorghum bicolor (L.) Moench] as an approach to increasing yields under water‐limited conditions. An hypothesis to explain the observation of stay green in some sorghum genotypes and not in others is that the genotypes expressing the trait employ mechanisms to increase availability of soil water during seed fill. In this study, the expression of two mechanisms resulting in soil water conservation to allow greater water availability during seed fill was explored among 12 sorghum genotypes. One mechanism is an earlier decrease in transpiration with soil drying so that the rate of soil water loss is decreased earlier in the soil drying cycle. The second mechanism is a limitation on transpiration rate at high vapor pressure deficit (VPD) so that soil water is conserved on days when midday VPD is high. Field studies were undertaken to identify seven genotypes that consistently expressed the stay‐green trait and five genotypes that did not exhibit this trait. The range of the threshold for the decrease in transpiration rate with soil drying was similar among the two sets of genotypes. Similarly, the expression of the limited transpiration rate under high VPD was found for both sets of genotypes. There was no evidence in these studies that the stay‐green trait was closely linked with either mechanism of water conservation.