Stomatal response drives between-species difference in predicted leaf water-use efficiency under elevated ozone.

Ozone-induced changes in the relationship between photosynthesis (A) and stomatal conductance (g) vary among species, leading to inconsistent water use efficiency (WUE) responses to elevated ozone (O). Thus, few vegetation models can accurately simulate the effects of O on WUE. Here, we conducted an experiment exposing two differently O-sensitive species (Cotinus coggygria and Magnolia denudata) to five O concentrations and investigated the impact of O exposure on predicted WUE using a coupled A-g model. We found that increases in stomatal O uptake caused linear reductions in the maximum rates of Rubisco carboxylation (V) and electron transport (J) in both species. In addition, a negative linear correlation between O-induced changes in the minimal g of the stomatal model (g) derived from the theory of optimal stomatal behavior and light-saturated photosynthesis was found in the O-sensitive M. denudata. When the O dose-based responses of V and J were included in a coupled A-g model, simulated A under elevated O were in good agreement with observations in both species. For M. denudata, incorporating the O response of g into the coupled model further improved the accuracy of the simulated g and WUE. In conclusion, the modified V, J and g method presented here provides a foundation for improving the prediction for O-induced changes in A, g and WUE.