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

Elevated atmospheric CO 2 concentrations (eCO 2 ) 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 eCO 2 and water deficit. Through a comprehensive dataset, we quantified the responses of leaf gas exchange induced by eCO 2 under water deficit, and tested whether the optimal stomatal theory could predict gas exchange responses to elevated atmospheric CO 2 between two typical C3 (wheat) and C4 crops (maize). Our results showed that leaf-scale WUE increased in proportion to increasing eCO 2 for all crops under various water conditions, and there exhibited stronger effects of eCO 2 on reductions in g s than increases in P n . A significantly lower stimulatory effect of eCO 2 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 CO 2 due to their different carbon fixation pathways. The eCO 2 -induced stimulation of P n was reduced by the water deficit, and there was a synergistic effect of eCO 2 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 eCO 2 condition. The optimal g s model correctly captured stomatal behavior with eCO 2 across most of datasets in different CO 2 application growth conditions. The stomatal slope parameter (g 1 ) in optimal stomatal model was lower for maize than wheat, and g 1 exhibited strong species specificity in magnitude and sensitivity to water and CO 2 . Under eCO 2 conditions, g 1 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-CO 2 scenarios in the future. • Crop gas exchange response to interactive CO 2 and drought are widespread but poorly understood. • We used the meta-analysis to clarify main drivers of water use efficiency in maize and wheat. • Maize P n responses were weaker than wheat with elevated CO 2 but opposite for water deficit. • The impacts of water deficit and eCO 2 on optimal model parameters g 1 were evaluated. • Crop adaptive strategies to eCO 2 and water deficit were considered.