CO2 elevation and N fertilizer supply modulate leaf physiology, crop growth and water use efficiency of maize in response to progressive soil drought

Elevated atmospheric CO2 concentration (eCO2) and varied nitrogen (N) fertilization levels may mediate the different responses of C4 crops to progressive soil drought. In this study, the effects of reduced N (N1, 0.8 g pot−1) and adequate N (N2, 1.6 g pot−1) supply on leaf physiology, plant growth and water use efficiency (WUE) of maize (C4 crop) exposed to progressive soil drought grown at ambient CO2 (aCO2, 400 ppm) and elevated CO2 (eCO2, 800 ppm) concentration were investigated. The results indicated that compared with aCO2, net photosynthetic rate (An) and leaf water potential (Ψl) at eCO2 were maintained in maize leaves, while stomatal conductance (gs), transpiration rate and leaf hydraulic conductance were decreased, leading to enhanced WUE from stomatal to leaf scale. Despite An and Ψl of eCO2 plants were more sensitive to progressive soil drought under both N fertilization levels, eCO2 would increase leaf ABA concentration (ABAleaf) but decline the gs response to ABAleaf under N1 supply. eCO2 coupled with N1 fertilization was conducive to enlarging leaf area, promoting specific leaf area, root and total dry mass, whereas reduced stomatal aperture and plant water use under progressive drought stress, contributing to an improvement in plant WUE, implying a better modulation of maize leaf stomata and water status under reduced N supply combined with eCO2 responding to progressive soil drought. These findings in the current study would provide valuable advice for N management on maize (C4) crop efficient water use in a drier and CO2-enriched environment.