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.