Xylem Sap pH Increase: A Drought Signal Received at the Apoplastic Face of the Guard Cell That Involves the Suppression of Saturable Abscisic Acid Uptake by the Epidermal Symplast.

Drought increased the pH of Commelina communis xylem sap from 6.1 to 6.7. Conductances of transpiring leaves were 50% lower in pH 7.0 than in pH 6.0 buffers, but bulk leaf abscisic acid (ABA) concentration and shoot water status were unaffected by pH. Stomatal apertures of isolated abaxial epidermis incubated on simple buffers increased with external pH, so in vivo this must be overridden by alternative pH effects. Reductions in leaf transpiration rate at pH 7.0 were dependent on the presence of 10-8 mol dm-3 ABA in the xylem stream. We inferred that at pH 7.0 leaf apoplastic ABA concentrations increased: pH did not affect distributions of ABA among leaf tissues, but isolated epidermis and mesophyll tissue took up more 3H-ABA from pH 6.0 than from pH 7.0 buffers. The apoplastic ABA increase at pH 7.0 may result from reduced symplastic sequestration. A portion of 3H-ABA uptake by the epidermis was saturable at pH 6.0 but not at pH 7.0. An ABA uptake carrier may contribute to ABA sequestration by the leaf symplast of well-watered plants, and its inactivity at pH 7.0 may favor apoplastic ABA accumulation in draughted plants. Effects of external pH on stomatal apertures in the isolated epidermis indicate that published data supporting a role for internal guard cell ABA receptors should be reassessed.