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

Abstract

Drought increased the p H of Commelina communis xylem sap from 6.1 t o 6.7. Conductances of transpiring leaves were 50% lower i n p H 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 p H effects. Reductions in leaf transpiration rate at p H 7.0 were dependent on the presence of 1OP8 mo1 dm-3 ABA in the xylem stream. We inferred that at p H 7.0 leaf apoplastic ABA concentrations increased: p H did not affect distributions of ABA among leaf tissues, but isolated epidermis and mesophyll tissue took up more 3H-ABA from p H 6.0 than from p H 7.0 buffers. The apoplastic ABA increase at p H 7.0 may result from reduced symplastic sequestration. A portion of 3H-ABA uptake by the epidermis was saturable at p H 6.0 but not at p H 7.0. An ABA uptake carrier may contribute t o ABA sequestration by the leaf symplast of well-watered plants, and its inactivity at p H 7.0 may favor apoplastic ABA accumulation i n droughted plants. Effects of external p H on stomatal apertures in the isolated epidermis indicate that published data supporting a role for interna1 guard cell ABA receptors should be reassessed.In droughted plants an increase in the ABA concentration of the apoplastic compartment of the leaf (Vanrensburg et al., 1996) is an important determinant of stomatal behavior (Gowing et al., 1993), and there is some evidence that a group of ABA receptors is located at the external surface of the plasmalemma of the guard cell (Hartung 1983;Hornberg and Weiler, 1984). When bound, the receptors induce changes in membrane ion transport and reduce the osmolarity of the guard cell such that it loses turgor pressure, which leads to stomatal closure (see Assmann, 1993).ABA carried by the transpiration stream from the xylem vessels arrives in the leaf apoplast as a dominant source (Weyers and Hillman, 1979), giving rise to a concentration sufficient to close stomata (Loveys, 1984). There is much evidence that some of the ABA carried by the xylem stream (1993,1995) have shown that nmol dmP3 concentrations of ABA supplied directly to isolated epidermal strips of the same species were sufficient to close stomata. These results demonstrate that the leaf itself influences the response of the stomata in the epidermis to the constituents of the transpiration stream. Trejo et al. (1993) concluded that, in whole leaves, the accumulation of ABA by the epidermis was limited by the presence of the mesophyll. Cells of this tissue rapidly metabolized ABA (see also Gowing et al., 1993), thus maintaining a concentration gradient for continuous ABA sequestration into the symplast. Daeter and Hartung (1995) demonstrated that the epidermal symplast of barley leaves is an even better sink for apoplastic ABA than the mesophyll, because it has a catabolic rate that is 5-fold greater.A physicochemical basis for ABA distribution betw...

Assessing effects of deficit irrigation techniques on water productivity of tomato for subsurface drip irrigation system

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A possible stress physiological role of abscisic acid conjugates in root‐to‐shoot signalling

“…This enzyme activity increased seven-fold when plants were salt stressed. An increase of apoplastic pH as it occurs in stressed leaves (Wilkinson & Davies 1997) also enhances the activity of this enzyme. Thus ABA-GE xyl in combination with the extracellular enzyme activity in leaves (Fig.…”

Systemic Signaling Under Water Deficit Condition and Its Exploitation in Water Saving Agriculture

“…In the Commelina communis plant, it was reported that in response to soil drying, the pH of xylem sap raised from 6.1 to 6.7, and when artificial xylem sap buffered to different pH was fed to detached leaves of Commelina communis, and it was found that an increase in pH from 6.0 to 7.0 caused a reduction of transpiration rate by about 50% in the presence of low concentrations of ABA. This indicated that a change in xylem pH may be able to act as a signal mediating the root shoot signaling (Wilkinson and Davies, 1997). The stomatal movement in relation to the pH functioning is complex.…”