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 mu mol & sdot;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.