CO2 elevation modulates the growth and physiological responses of soybean (Glycine max L. Merr.) to progressive soil drying

Understanding the impact of future climates on crop performance is essential for sustainable agricultural production. In the current research, the development and biological behavior of soybean plants during gradual desiccation of the soil (from the 100% of pot water holding capacity to the g_s of plant decreased to 10% of that of the control plants) at ambient [CO₂] (a[CO₂], 400 ppm) and elevated [CO₂] (e[CO₂], 800 ppm) were investigated. The results showed that plants grown under e[CO₂] conditions had remarkably higher photosynthetic rate (A_n) but lower stomatal conductance (g_s) and transpiration rate (E) compared to plants at a[CO₂] conditions, which led to an enhanced water use efficiency at both stomatal (WUE_i) and leaf levels (WUE_leaf). In addition, the e[CO₂]-grown soybeans showed a stunted g_s response to progressive soil drying, coinciding with a decrease in the susceptibility of g_s to the ABA signaling, though they tended to maintain a better leaf water status under drought than the a[CO₂]-grown plants. Although the leaf nitrogen concentration (N_leaf) and the total plant N content were notably lower at the e[CO₂] condition, the specific leaf N content (SLN) was similar at different [CO₂] conditions. Compared to soybean grown under e[CO₂], the greater number of nodules at e[CO₂] treatment would lead to an enhanced N-fixation, yet, it did not improve the N nutrition of the plants. Nevertheless, by sustaining the SLN, the soybean plants enhanced A_n when growing at e[CO₂], particularly under dry conditions. This knowledge is essential for sustaining soybean production in future climate change scenarios.