The dual effect of abscisic acid on stomatal conductance

International audienceThe classical view that the drought-related hormone abscisic acid (ABA) simply acts at the guard cell level to induce stomatal closure is questioned by discrepancies in stomatal response to ABA between isolated epidermis and intact plants. We tested the hypothesis that ABA mediates an additional effect in planta by changing hydraulic regulation in the leaf upstream the stomata. By gravimetry, porometry to water vapour and argon, and psychrometry, we investigated the effect of exogenous ABA on transpiration, stomatal conductance and leaf hydraulic conductance of mutants described as ABAinsensitive in epidermal peels. We show that stomatal transpiration of ABA-insensitive mutants does respond to ABA in folio. We then demonstrate that ABA decreases stomatal conductance by downregulating leaf hydraulic conductance in both the wild-type Col-0 and the ABA-insensitive ost2. We propose that ABA promotes stomatal closure both via its biochemical effect on guard cells and via an indirect hydraulic effect through a decrease in leaf water permeability, triggered within vascular tissues by distinct signalling components. Variability in sensitivity of leaf hydraulic conductance to ABA among species could provide a physiological basis to the isohydric or anisohydric behaviour