Light-promoted changes in apoplastic K(+) activity in the Samanea saman pulvinus, monitored with liquid membrane microelectrodes.

The movement of Samanea saman (Jacq.) Merrill leaflets is a consequence of the re-distribution of K+ and anions between motor cells on opposite sides of the pulvinus. We used a K+-sensitive microelectrode to study dynamic changes in K+ transport through motor-cell membranes during and immediately after change in illumination. Potassium-ion-sensitive and reference microelectrodes were inserted into extensor or flexor tissue of a whole pulvinus in white light (WL). A brief pulse of red light (RL) followed by darkness (D) (a) increased K+ activity in the extensor apoplast, indicating K+ release by the protoplast; and (b) decreased K+ activity in the flexor apoplast, indicating K+ uptake by the protoplast. White light after 35–40 min D reversed K+ activity in the extensor apoplast to approximately its original value. Blue light substituted partially for WL in this regard. Potassium-ion activity in the flexor apoplast reverted to approximately its original value after 2 h, with or without white illumination. Our data support the hypothesis that K+ efflux from extensor cells and K+ uptake by flexor cells following a WL→RL→D transition occurs by way of K+ channels.