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 (e[CO₂]) 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 (a[CO₂], 400 ppm) and elevated CO₂ (e[CO₂], 800 ppm) concentration were investigated. The results indicated that compared with a[CO₂], net photosynthetic rate (A_n) and leaf water potential (Ψ_l) at e[CO₂] were maintained in maize leaves, while stomatal conductance (g_s), transpiration rate and leaf hydraulic conductance were decreased, leading to enhanced WUE from stomatal to leaf scale. Despite A_n and Ψ_l of e[CO₂] plants were more sensitive to progressive soil drought under both N fertilization levels, e[CO₂] would increase leaf ABA concentration ([ABA]_leaf) but decline the g_s response to [ABA]_leaf under N1 supply. e[CO₂] 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 e[CO₂] 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.