ABA-mediated regulation of leaf and root hydraulic conductance in tomato grown at elevated CO2 is associated with altered gene expression of aquaporins

Elevated CO 2 concentration in the air ( e [CO 2 ]) decreases stomatal density (SD) and stomatal conductance ( g s ) where abscisic acid (ABA) may play a role, yet the underlying mechanism remains largely elusive. We investigated the effects of e [CO 2 ] (800 ppm) on leaf gas exchange and water relations of two tomato ( Solanum lycopersicum ) genotypes, Ailsa Craig (WT) and its ABA-deficient mutant ( flacca ). Compared to plants grown at ambient CO 2 (400 ppm), e [CO 2 ] stimulated photosynthetic rate in both genotypes, while depressed the g s only in WT. SD showed a similar response to e [CO 2 ] as g s , although the change was not significant. e [CO 2 ] increased leaf and xylem ABA concentrations and xylem sap pH, where the increases were larger in WT than in flacca . Although leaf water potential was unaffected by CO 2 growth environment, e [CO 2 ] lowered osmotic potential, hence tended to increase turgor pressure particularly for WT. e [CO 2 ] reduced hydraulic conductance of leaf and root in WT but not in flacca , which was associated with downregulation of gene expression of aquaporins. It is concluded that ABA-mediated regulation of g s , SD, and gene expression of aquaporins coordinates the whole-plant hydraulics of tomato grown at different CO 2 environments.