Abscisic acid-induced elevation of guard cell cytosolic Ca super(2+) precedes stomatal closure

It has been proposed that an abscisic acid-dependent increase in the concentration of guard cell cytosolic free calcium triggers the intracellular machinery responsible for stomatal closure, but attempts to test this hypothesis by measuring ( super(45)Ca)fluxes have produced equivocal results. Using the fluorescent calcium indicator fura-2, we report that abscisic acid induces a rapid increase in guard cell cytosolic free Ca super(2+) in Commelina communis L., and that this increase precedes stomatal closure. These results strongly support the suggestion that Ca super(2+) is an intracellular second messenger in this response.