A meta-analysis of crop leaf gas exchange responses to elevated CO2 and water deficits using optimal stomatal theory

Abstract

Elevated atmospheric CO₂ concentrations (eCO₂) and soil water deficits significantly influence gas exchange in plant leaves. However, it remains unclear whether crops optimize carbon assimilation and water dissipation processes in response to eCO₂ and water deficit. Through a comprehensive dataset, we quantified the responses of leaf gas exchange induced by eCO₂ under water deficit, and tested whether the optimal stomatal theory could predict gas exchange responses to elevated atmospheric CO₂ between two typical C3 (wheat) and C4 crops (maize). Our results showed that leaf-scale WUE increased in proportion to increasing eCO₂ for all crops under various water conditions, and there exhibited stronger effects of eCO₂ on reductions in g_s than increases in P_n. A significantly lower stimulatory effect of eCO₂ on maize photosynthesis was observed compared to wheat. This difference is attributed to the distinct physiological characteristics of C4 and C3 plants, with P_n of C4 plants generally showing a less pronounced response to elevated CO₂ due to their different carbon fixation pathways. The eCO₂-induced stimulation of P_n was reduced by the water deficit, and there was a synergistic effect of eCO₂ and water deficit on the g_s and T_r reduction, resulting in further reduction in g_s and T_r under water deficit and eCO₂ condition. The optimal g_s model correctly captured stomatal behavior with eCO₂ across most of datasets in different CO₂ application growth conditions. The stomatal slope parameter (g₁) in optimal stomatal model was lower for maize than wheat, and g₁ exhibited strong species specificity in magnitude and sensitivity to water and CO₂. Under eCO₂ conditions, g₁ increased slightly in wheat but decreased in maize. Incorporating the sensitivity parameters derived from different water levels can avoid significant overestimation of evapotranspiration for possible high-CO₂ scenarios in the future.