Effects of CO₂ and NaCl on stomatal traits, photosynthetic performance, and antioxidant systems in soybean

To gain a deeper understanding of the response mechanisms of farmland ecosystems under NaCl stress in the context of future elevated atmospheric CO₂ concentrations, this study employed environmental growth chambers with precisely controlled CO₂ levels to investigate the effects of elevated CO₂ on stomatal morphology and distribution, leaf gas exchange parameters, and the antioxidant enzyme system in soybeans under NaCl stress. The results showed that under ambient CO₂ conditions, NaCl stress reduced stomatal density, length, width, perimeter, and area on the adaxial surface. In contrast, elevated CO₂ increased stomatal density and promoted a more regular stomatal distribution, thereby mitigating the negative effects of NaCl on leaf gas exchange efficiency. Moreover, NaCl stress decreased the net photosynthetic rate (Pn), while elevated CO₂ significantly improved leaf water use efficiency under NaCl stress (S₀, S₅₀, S₁₀₀, and S₁₅₀) by 188%, 243%, 97%, and 85%, respectively, compared to ambient CO₂, indicating that elevated CO₂ can effectively alleviate NaCl-induced physiological stress in soybeans. Elevated CO₂ also enhanced peroxidase and superoxide dismutase activities and increased proline content, while reducing malondialdehyde levels under severe NaCl stress. Furthermore, under NaCl stress, elevated CO₂ downregulated the expression of GmCLC-d1, GmCLC-d2, and GmNHX1, and upregulated GmNcl1. These findings demonstrate that elevated atmospheric CO₂ can alleviate the physiological damage caused by NaCl stress in soybeans by optimizing stomatal traits, enhancing photochemical and biochemical processes, boosting antioxidant defenses, and regulating salt-tolerance gene expression. This study provides a theoretical basis for understanding the physiological and molecular responses of soybeans to elevated CO₂ and NaCl stress under future climate change scenarios.