Yoshiji Okazaki

Internodal cells of a brackish water charophyte, Lamprothamnium succinctum, regulate turgor pressure in response to changes in external osmotic pressure by modifying vacuolar concentrations of KCl. An increase in cytosolic concentration of free Ca²⁺ ([Ca²⁺]_c) is necessary for the progress of turgor regulation induced by hypotonic treatment. Initial changes in membrane potential and [Ca²⁺]_c upon hypotonic treatment were measured to examine the temporal relationship between the two parameters. Fura-dextran (potassium salt, M_r 10,000, anionic) that had been injected into the cytosol was used to measure [Ca²⁺]_c. Membrane potential and membrane conductance under a current-clamp condition were also measured. Decrease in external osmotic pressure by 0.16 Osm induced a simultaneous increase in [Ca²⁺]_c with both depolarization of the membrane and increase in the membrane conductance. Decrease in external osmotic pressure by 0.05 Osm induced a simultaneous increase in [Ca²⁺]_c with membrane depolarization but the increase in membrane conductance started later than the other two processes. There was a close temporal relationship between the increase in [Ca²⁺]_c and membrane depolarization on the initial response of turgor regulation induced by hypotonic treatment.

Unilateral blue light irradiation induces bending of pulvini of Phaseolus vulgaris towards the source of light. The pulvinar bending is caused by a decrease in turgor pressure of motor cells that are irradiated with blue light. Decrease in the turgor pressure is caused by the net efflux of K⁺ and counter anions, accompanying membrane depolarization. In the present study the effect of blue light on the activity of plasma membrane H⁺-ATPase was studied in relation to the membrane depolarization. The activity of the plasma membrane H⁺-ATPase was measured using protoplast suspensions prepared from laminar pulvini from primary leaves. A pulse of blue light under continuous red light irradiation induced both a transient increase in the external pH and transient inhibition of the vanadate-sensitive ATPase. Continuous blue light irradiation under continuous red light irradiation induced both a sustained increase in the external pH and sustained inhibition of the vanadate-sensitive ATPase. These results show that blue light inhibits the activity of the plasma membrane H⁺-ATPase. Inactivation of the plasma membrane H⁺-ATPase supports the membrane depolarization induced by the blue light irradiation.

A pulse of blue light induced both a transient increase in activity of apoplastic K⁺ and membrane depolarization in laminar pulvinus of Phaseolus vulgaris L. This shows that blue-light-induced net efflux of K⁺ from motor cells is closely related to membrane depolarization.