Influence of soil water status and atmospheric vapor pressure deficit on leaf gas exchange in field-grown winter wheat

Abstract

Soil water status and atmospheric vapor pressure deficit (VPD) are important environmental parameters that influence plant gas exchange. The objective of this study was to investigate the effect of soil water potential ( Ψ s) on the responses of gas exchange to VPD in four field-grown winter wheat ( Triticum aestivum L.) cultivars. The gas exchange parameters: net CO 2 assimilation rate ( A n), stomatal conductance ( g s), transpiration rate ( E), and intercellular CO 2 concentration ( c i) were measured under different VPD and Ψ s conditions during the period from heading to the middle of grain filling. The soil water potential had a significant effect on the responses of gas exchange parameters to VPD. At high Ψ s (−0.09 MPa), A n did not respond to increased VPD. However, A n was sensitive to increased VPD at low Ψ s, and the most pronounced decrease in A n as VPD increased was observed under severe water stress ( Ψ s=−0.7 MPa). Stomatal conductance decreased with increased VPD even at high Ψ s (−0.09 MPa). Transpiration rate increased as VPD increased at high Ψ s, but decreased as VPD increased under low Ψ s. The response of g s to VPD cannot be explained as feedback mechanism, i.e. the decrease in g s is not due to increase in E at high VPD. The response of g s to VPD in this study was consistent with a feedforward mechanism. Among the four cultivars, Karl 92 had the lowest A n, g s, and E, while the other three cultivars (Arapahoe, Cheyenne, and Scout 66) had higher A n, g s, and E than Karl 92 under low Ψ s and high VPD conditions.