Elevated Atmospheric CO2 Concentration Mitigates the Negative Impacts of Soil Water Deficiency on Leaf Photosynthesis of Soybeans

We examined the effects of soil water deficiency and elevated atmospheric CO2 concentration ([CO2]) on plant biomass, leaf gas exchange, stomatal morphology, leaf anatomy, photosynthetic pigments, and Rubisco coding genes expression level of soybeans (Glycine max (L.) Merr.). Our results showed that the net photosynthetic rate (Pn) was significantly decreased by 38% (P = 0.047) and 72% (P = 0.002) under mild and severe water deficiency, while this negative effect of water deficiency on Pn was mitigated by elevated [CO2], which might be associated with the enhanced photosynthetic pigments and increased stomatal density and openness as well as bigger mesophyll cell size. Furthermore, we also found that the stomatal conductance (gs) at severe water deficiency was 80% (P = 0.016) lower than that of gs at full irrigation, indicating that the declined gs might partially contributed to the reduction in photosynthetic under water deficiency. Overall, our results suggest that water deficiency resulted in decreases of leaf photosynthesis, but elevated [CO2] could partially alleviate the impacts of water deficiency on soybeans through the “CO2 fertilization effect”. Our results may have crucial significances on further understanding the potential mechanisms and key processes of agriculture in response to elevated [CO2] and water deficiency in the context of future climate change.