Abscisic Acid Down-Regulates Hydraulic Conductance of Grapevine Leaves in Isohydric Genotypes Only

Plants evolved different strategies to cope with water stress. While isohydric species maintain their midday leaf water potential (Ψ_M) under soil water deficit by closing their stomata, anisohydric species maintain higher stomatal aperture and exhibit substantial reductions in Ψ_M It was hypothesized that isohydry is related to a locally higher sensitivity of stomata to the drought-hormone abscisic acid (ABA). Interestingly, recent lines of evidence in Arabidopsis (Arabidopsis thaliana) suggested that stomatal responsiveness is also controlled by an ABA action on leaf water supply upstream from stomata. Here, we tested the possibility in grapevine (Vitis vinifera) that different genotypes ranging from near isohydric to more anisohydric may have different sensitivities in these ABA responses. Measurements on whole plants in drought conditions were combined with assays on detached leaves fed with ABA. Two different methods consistently showed that leaf hydraulic conductance (K_leaf) was down-regulated by exogenous ABA, with strong variations depending on the genotype. Importantly, variation between isohydry and anisohydry correlated with K_leaf sensitivity to ABA, with K_leaf in the most anisohydric genotypes being unresponsive to the hormone. We propose that the observed response of K_leaf to ABA may be part of the overall ABA regulation of leaf water status.