Can Leaf Gas Exchange Serve as a Reliable Indicator for Predicting Spring Wheat Yield in Response to Drought?

Leaf gas exchange plays a critical role in determining crop final yield, and there is a threshold response of leaf gas exchange to water stress. It is of great significance to quantify crop water stress severity by using the response characteristics of leaf gas exchange to drought. However, it is currently unclear whether leaf gas exchange serve as a reliable indicator for predicting crop final yield in response to drought, which affects the accuracy of monitoring agricultural drought using physiological indicators during the crop growing season. This study determined the response threshold of leaf gas exchange to drought for spring wheat through a serials of soil dry-down experiments and used the threshold characteristics to construct and parameterize a spring wheat growth model. Spring wheat were designed to be irrigated with five treatments (with supplementary irrigation at 230 mm, 165 mm, 115 mm, 50 mm and 0 mm). Crop model were used to simulate and analyze the threshold response characteristics of grain yield to drought and compare them to the thresholds of leaf gas exchange indices for spring wheat. The results showed that the response threshold of stomatal conductance of spring wheat to fraction of transpirable soil water was 0.5, which was greater than that of transpiration rate and net photosynthetic rate, 0.4. The parameterized spring wheat growth model with the response threshold of net photosynthetic rate to fraction of transpirable soil water accurately simulated the aboveground biomass and final yield of spring wheat. The response threshold of spring wheat final yield to fraction of transpirable soil water was significantly smaller than that of leaf gas exchange parameters to fraction of transpirable soil water (0.18 versus 0.4). This indicates that there are certain problems in using physiological indicator such as leaf gas exchange indices during crop growing season to determine the agricultural drought severity and reflect the reduction of final crop yields due to drought.