Water stress changes the relationship between photosynthesis and stomatal conductance

Understanding the relationship between stomatal conductance (g_s) and photosynthesis (A_n) under water stress conditions can improve the accuracy of land surface models for estimating the gas exchange of crop canopies with the atmosphere. However, little is known about the effect of water stress on this relationship in crops. A glasshouse experiment was, therefore, conducted to investigate changes in the linear relationship between g_s and A_n owing to water stress in rice and the association with soil moisture content. Severe (SWS), mild (MWS), and no water stress (NWS) conditions were applied from flowering onwards and the gas exchange in fully developed flag leaves was assessed weekly. The Ball-Woodrow-Berry linear model was used to assess the relationship between g_s and A_n under different treatments. SWS had a significant effect and reduced the slope of the linear relationship between g_s and A_n by 30 % compared with NWS. Only in SWS were A_n and g_s strongly correlated with soil water content. Our study revealed that changes in the linear relationship through a reduction in the slope imply a conservative water-use strategy for rice under intense water stress. We propose that crop models that use the linear relationship should consider the impact of water stress conditions when simulating yields and estimating CO₂ and H₂O fluxes from crop canopies.