Elevated CO2 concentration enhances plant growth, photosynthesis, and ion homeostasis of soybean under salt-alkaline stress

Salt-alkaline stress adversely affects growth and productivity of soybean. In the event of global climate change, the effects of elevated CO₂ concentration (eCO₂) and salt-alkaline stress on soybean remain unclear. This study investigated the combined effects of elevated CO₂ concentration (700 μmol·moL⁻¹) and salt-alkaline stress on soybean growth, gas exchange, pigments profiles, antioxidative enzyme activities, osmolyte accumulation, Na⁺ and K⁺ contents, and genes involved in ion homeostasis. This study suggested that eCO₂ improved plant physiological performance due to the greater net photosynthetic rate (+212.49 %) and water use efficiency (+92.86 %). Both salt-alkaline stress and eCO₂ significantly increased catalase (CAT) activity in leaves and stems, significantly increased superoxide dismutase (SOD) activity in stems, and significantly increased peroxidase (POD) activity in whole plants of soybean. eCO₂ significantly inhibited Na⁺ absorption as indicated by decreased Na⁺ contents in whole plants under salt-alkaline stress accompanied by lower relative electrical conductivity, thus reducing osmotic and ionic stress. eCO₂ induced enhancement of expressions of gene encoding the ion transporter of GmHKT1;2, GmHKT1;5, GmHKT1;6, GmNHX5, and GmSOS1 in stems mediated Na⁺ and K⁺ transport, thus benefiting to keep ions homeostasis. These results suggest that eCO₂ contributes to enhancing soybean tolerance to saline-alkaline stress.