Doubled CO₂ Mediated Stomatal Behaviour and Water Use Efficiency in Saline Maize Plants Responding to Progressive...

Chemical abscisic acid (ABA) and/or hydraulic signals' role in modulating stomatal and water use efficiency (WUE) response of saline C₄ crops to atmospheric doubled CO₂ (d[CO₂]) environment during progressive drought remains largely elusive. Here, C₄ maize plants were grown in two [CO₂] concentrations (ambient CO₂ (a[CO₂], 400 ppm) and d[CO₂], 800 ppm) and subjected to two salt stress levels (0 and 100 mM NaCl pot^-1) during progressive soil drought. The results indicated that maize leaf ABA concentration (ABA_leaf) was increased and stomatal conductance (g_s) and leaf hydraulic conductivity (K_leaf) were decreased under salt stress, while d[CO₂] maintained photosynthetic rate (A_n) and leaf water potential (Ψ₁), but enhanced leaf WUE. Salt stress advanced the decline in A_n, but retarded g_s, Ψ₁ and K_leaf, whereas d[CO₂] advanced leaf WUE decline during progressive drought. The g_s decline at d[CO₂] was less sensitive to the enhanced ABA_leaf and declined Ψ₁ related to a[CO₂] under salt stress. Hydraulic (Ψ₁ and K_leaf) signals were more crucial than chemical (ABA_leaf) signals to mediate leaf g_s and WUE upon both soil salinity and d[CO₂] in response to progressive drought. Moreover, d[CO₂] mitigated the adverse effects of drought and salt stress on maize plant growth and WUE. The beneficial modulation of d[CO₂] on maize stomata under salt stress in response to progressive soil drought provides some novel insights on improving C₄ crop WUE in a future drier and saltier soil in a CO₂-enriched scenario.