Doubled CO ₂ 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 C 4 crops to atmospheric doubled CO 2 ( d [CO 2 ]) environment during progressive drought remains largely elusive. Here, C 4 maize plants were grown in two [CO 2 ] concentrations (ambient CO 2 ( a [CO 2 ], 400 ppm) and d [CO 2 ], 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 [CO 2 ] maintained photosynthetic rate ( A n ) and leaf water potential ( Ψ 1 ), but enhanced leaf WUE. Salt stress advanced the decline in A n , but retarded g s , Ψ 1 and K leaf , whereas d [CO 2 ] advanced leaf WUE decline during progressive drought. The g s decline at d [CO 2 ] was less sensitive to the enhanced ABA leaf and declined Ψ 1 related to a [CO 2 ] under salt stress. Hydraulic ( Ψ 1 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 [CO 2 ] in response to progressive drought. Moreover, d [CO 2 ] mitigated the adverse effects of drought and salt stress on maize plant growth and WUE. The beneficial modulation of d [CO 2 ] on maize stomata under salt stress in response to progressive soil drought provides some novel insights on improving C 4 crop WUE in a future drier and saltier soil in a CO 2 ‐enriched scenario.