Difference of response thresholds between leaf gas exchange and yield to drought for spring wheat

Leaf gas exchange is the basis for crop biomass and yield formation. During drought development, the leaf gas exchange exhibits a threshold response to water stress, and many related physiological indicators are based on this response to monitor drought severity in crops. However, the focus of agricultural production is crop yield, and it is unclear whether the response threshold of leaf gas exchange indicators used to monitor drought is synchronous with that of crop yield to drought. To some extent, this affects the accuracy of agricultural drought monitoring using physiological indicators related to leaf gas exchange. In this study, based on drying experiments, changes in leaf gas exchange in spring wheat during the drought development process were observed and analyzed. The response threshold of leaf gas exchange in spring wheat to drought was determined and used to parameterize the crop model for spring wheat. Drought stress simulation experiments were designed to analyze the response threshold characteristics of spring wheat yield to drought and the differences in the threshold of leaf gas exchange. The results showed that the response threshold of stomatal conductance for spring wheat to available soil water content was 0.50, which was higher than that of transpiration rate and net photosynthetic rate (0.40). The aboveground biomass and yield of spring wheat were simulated by parameterizing the crop model for spring wheat with the response threshold of the net photosynthetic rate to the available soil water content. The model simulation values explained more than 70% of the observed variation, and the results were highly significant (P<0.01). The relative root mean square error between the model simulation and observed values was less than 30%, indicating a high overall simulation accuracy of the model. The consistency index was greater than 0.85, and the relationship slope between the simulated and observed values was between 1.00 and 1.50. This indicates that the proposed crop model can accurately simulate changes in the aboveground biomass and yield of spring wheat. Using the validated model, this study analyzed the formation process of spring wheat soil moisture, leaf area index, aboveground biomass, and yield under different drought stress scenarios. The response threshold of the aboveground biomass and yield of spring wheat to available soil water content was 0.18, which was significantly lower than that of the leaf gas exchange indicators. These results demonstrate that using physiological indicators, such as leaf gas exchange, during the crop growth period to characterize drought severity and reflect the degree of crop yield reduction may have certain issues. When using physiological indicators, such as leaf gas exchange, obtained during the crop growth period to characterize drought severity, the crop’s own drought resistance characteristics and the impact of drought duration on crop yield might be overlooked, which can lead to an overestimation of the severity of crop drought and underestimation of the final crop yield. The results of this study provide a reference for agricultural drought monitoring, prediction, and impact assessment.