ABA-mediated stomatal response modulates the effects of drought, salinity and combined stress on tomato plants grown under elevated CO 2

Plants are often exposed to drought and salinity stress concurrently but our knowledge about the effects of combined stress is limited, especially in a CO 2 -enriched environment. This study investigated how these three variables interacted and affected two tomato genotypes with contrasting endogenous abscisic acid (ABA) levels (wild -type AC, ABA -deficient mutant flacca ). Plants were grown at ambient (400 mu mol mol 1 ) ( a [CO 2 ]) or elevated (800 mu mol mol -1 ) CO 2 ( e [CO 2 ]), and exposed to progressive soil drying, salt and combined stress; and plant growth and physiological responses to the treatments were tested. The results showed that, under either CO 2 growth environment, for both genotypes the individual and combined stress significantly decreased plant growth and water status, with the combined stress having the most pronounced effect on water status. Compared with plants grown at a [CO 2 ], e [CO 2 ] significantly enhanced plant aboveground biomass and attenuated the decrease of water status induced by drought and salt, but this only occurred in AC. In addition, salt stress increased the sensitivity of stomatal response to drought in both genotypes, yet it exacerbated the e [CO 2 ]- induced depression of stomatal conductance (g s ) only in the AC plants. On the other hand, drought increased Na + :K + ratio in leaves for both genotypes while decreasing it in roots of AC plants under salt stress. In addition, e [CO 2 ] favored ionic homeostasis in leaves of plants under salt and combined stresses by limiting leaf Na + accumulation and enhancing K + selective transport, but this was only evident for AC, suggesting that e [CO 2 ]- induced plant responses were dependent on plant endogenous ABA levels. The results of this study improve our understanding on the interactive effects of drought and salt stress on tomato plants under e [CO 2 ], which are helpful for improving crop resilience to a future drier, saltier and CO 2 -enriched environment.