Abstract

The effects of abscisic acid (ABA) on the size of the aperture of stomata on epidermal strips of Vicia faba were studied in incubation media with different pH values. The osmotic potential of guard cells, as determined by the limiting plasmolysis method, was higher at pH 4.0 than at pH 6.0, although the size of the stomatal apertures was almost identical at both pH values. At pH 4.0, ABA effectively caused stomatal closure but had only a small effect on the osmotic potential, whereas, at pH 6.0, ABA significantly increased the osmotic potential. ABA promoted the efflux of Cl and malate from epidermal strips into the incubation medium, an effect which was more marked at pH 6.0, with a concomitant efflux of K+ to balance the charge on the exported anions. From these results, it is suggested that ABA may cause an increase in the elastic modulus of the cell walls of guard cells.

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