ABA-mediated regulation of leaf and root hydraulic conductance in tomato grown at elevated CO(2) 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.