Elevated CO2 concentration increases maize growth under water deficit or soil salinity but with a higher risk of hydraulic failure.

The aim of this study was to investigate the capacity for acclimation in water relations characteristics under changing environmental conditions and quantify the potential hydraulic risks in maize. Potted maize was acclimated to varying CO2 concentration ([CO2]) levels (400 and 700 ppm) while under water stress (WS) or soil salinity (SS) treatments. Plant growth and hydraulic traits following acclimation were comprehensively assessed. We found that WS and SS inhibited maize growth and showed remarkable impacts on hydraulic traits. In particular, the water potential at 50% loss of stem hydraulic conductance (P50) decreased by 1 MPa. When subjected to elevated [CO2], the plants under WS and SS improved their growth by 7-23%. Furthermore, elevated [CO2] significantly increased xylem vulnerability, resulting in smaller hydraulic safety margins. According to the plant desiccation model, the critical desiccation degree (time*VPD) that plants can tolerate under drought was reduced by 43-64% under elevated [CO2]. In addition, the sensitivity analysis showed that P50 is the most important trait in determining the critical desiccation degree. Our findings demonstrated that elevated [CO2] benefits plant growth under WS or SS. Nevertheless, it interfered with plant hydraulic acclimation, potentially placing them at a higher risk of hydraulic failure and mortality.