Incorporating the temperature responses of stomatal and non-stomatal limitations to photosynthesis improves the predictability of the unified stomatal optimization model for wheat under heat stress

Drought and heat stress often occur simultaneously causing detrimental impacts on crop growth and physiology. Stomata behave differently when plants are exposed to drought and heat stress, which may change the coupling relationship of stomatal conductance (g_s) and photosynthesis (A_n) and thereby influence the capability of the Ball-Berry (BB)-based g_s model. To examine the prevalence of this g_s-A_n decoupling and its influence on the predictability of g_s model, three pot experiments in climate-controlled greenhouses or climate chambers were conducted where leaf gas exchange of four wheat genotypes with varied sensitivity to drought or heat stress was measured, and the performance of the unified stomatal optimization model (USO model) in simulating g_s under individual or combined stress was evaluated. Data obtained from 2019 were used for model parameterization and from 2020 to 2023 were used for model validation. Results showed that the g_s-A_n decoupling only occurred in well-watered plants under heat regardless of genotype, where the original USO model underestimated the g_s. To improve the model prediction, a new slope parameter, which based on the differential effect of the relative stomatal (l_s) and non-stomatal limitations (l_ns) on A_n, with respect to leaf temperature was incorporated to modify the USO model. Compared with the original USO model, the modified USO model showed lower Akaike's information criterion and improved predictability for g_s with higher R² (> 0.90), lower RMSE (< 0.08) and MAE (< 0.06). These findings underscore the critical importance of integrating the effect of leaf temperature on the l_s and l_ns into the USO model for accurately predicting g_s in wheat plants subjected to heat stress.