Leaflet movements in Samanea saman are driven by the shrinking and swelling of cells in opposing (extensor and flexor) regions of the motor organ (pulvinus). Changes in cell volume, in turn, depend upon large changes in motor cell content of K(+), Cl(-) and other ions. We performed patch-clamp experiments on extensor and flexor protoplasts, to determine whether their plasma membranes contain channels capable of carrying the large K(+) currents that flow during leaflet movement. Recordings in the "whole-cell" mode reveal depolarization-activated K(+) currents in extensor and flexor cells that increase slowly (t((1/2)) = ca. 2 seconds) and remain active for minutes. Recordings from excised patches reveal a single channel conductance of ca. 20 picosiemens in both cell types. The magnitude of the K(+) currents is adequate to account quantitatively for K(+) loss, previously measured in vivo during cell shrinkage. The K(+) channel blockers tetraethylammonium (5 millimolar) or quinine (1 millimolar) blocked channel opening and decreased light- and dark-promoted movements of excised leaflets. These results provide evidence for the role of potassium channels in leaflet movement.