The effect of abscisic acid on epidermal cells: protoplast swelling and ATPase activity

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Abstract
Isolated epidermal protoplasts of Commelina communis L. increase in volume in the presence of KCl. Since this swelling is an osmotic phenomenon it reflects K+ influx. ATP slightly decreased the volume of the protoplasts, pointing towards the possibility that K+ uptake is passive. On the other hand abscisic acid (ABA) and sodium orthovanadate increased the swelling, and their effect was reversed by ATP. This may support the suggestion that ABA inhibits the active and ATPase‐mediated relase of K+ from epidermal cells. Mg2+‐dependent, K+‐stimulated ATPase activity was found in the microsomal fraction from epidermal cells. This activity was vandadate sensitive. ABA increased the basal activity in the presence of Mg2+ but inhibited the K+ stimulation.