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PLANT PHYSIOLOGY , Vol 113, Issue 2 559-573, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
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
S. Wilkinson and W. J. Davies
Division of Biological Sciences, Institute of Environmental and Biological Sciences, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, United Kingdom
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.
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