The mutual effect of extracellular Ca2+, abscisic acid, and pH on the rate of stomatal closure

An analysis of the rate of stomatal closure in epidermal strips of Commelina communis L. induced by different calcium concentrations (10 nM to 1 mM) demonstrated a biphasic dependency, the initial process of closure being more calcium dependent. This dependency was more evident at pH 7 than pH 5 of the bathing medium. Addition of low concentrations of abscisic acid (10 nM) induced a broadening of the Ca-2 dependency, while 10 mu M abscisic acid increased the rate of stomatal closure in the whole Ca2+ range and to a large extent abolished the calcium-induced effects. Such results indicate an interaction of external protons, Ca2+, and abscisic acid, and possible competition for the membrane associated binding sites of the mechanism(s) responsible for the regulation of stomatal closure.