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

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 (ABAleaf) was increased and stomatal conductance (gs) and leaf hydraulic conductivity (Kleaf) were decreased under salt stress, while d[CO2] maintained photosynthetic rate (An) and leaf water potential (Ψ1), but enhanced leaf WUE. Salt stress advanced the decline in An, but retarded gs, Ψ1 and Kleaf, whereas d[CO2] advanced leaf WUE decline during progressive drought. The gs decline at d[CO2] was less sensitive to the enhanced ABAleaf and declined Ψ1 related to a[CO2] under salt stress. Hydraulic (Ψ1 and Kleaf) signals were more crucial than chemical (ABAleaf) signals to mediate leaf gs 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.