Cytosolic calcium concentrations (Ca1) of barley aleurone protoplasts after stimulation with the plant hormone abscisic acid (ABA) were measured by using the calcium-sensitive flourescent dye Indo.1. The measured basal Ca1 is about 200 nM. Stimulation with ABA induces a strong dose-dependent decrease in Ca1 to a minimal value of about 50nM. The decrease occurs within 5 s. The Ca2+ antagonists La3+ and Cd2+ inhibit the ABA-induced Ca1 decrease in a dose-dependent manner, while the Ca2+ channel blockers verapamil and nifedipine give no inhibition. The induction of Ca1 decrease by ABA is consistent with activation of the plasma membrane Ca2+- ATPase by ABA. The possible role of this ABA-induced Ca1 decrease in ABA signal transduction and in counteracting the effects of gibberellic acid are discussed.
