Effects of vanadate, N2 and light on the membrane potential of motor cells and the lateral leaflet movements of Desmodium motorium

The lateral leaflets of Desmodium motorium (Houtt.) Merr. exhibit ultradian up- and down movements, which are paralleled by oscillations of the membrane potential of motor cells in the pulvinus. By different treatments we have tested the hypothesis that both that both oscillation-types are causally related. The reactions of the leaflet movement and the membrane potential were evaluated by the following approaches. (1) Application of vanadate. an inhibitor of the proton pump in the plasmalemma. and N₂ suppressed leaflet movements and finally arrested the leaflet in the lower position. Before the oscillations damped out, a strong lengthening in period was found. This indicates that the pump is part of the ultradian clock. A period lenthening and a final suppression of the rhythm by vanadate was also seen in the extracellular electric potential of the pulvinus. Intracellular recordings in situ showed that vanadate application depolarized the motor cells. (2) Light of high fluence rates diminished the amplitude of the oscillations of the membrane potential of single motor cells and shortened the period. The same effects were observed when monitoring the lateral leaflet movement. The leaflet always moved towards the direction of the light. whether it was applied from the abaxial or from the adaxial part of the pulvinus. (3) When light was applied to the pulvinus of lateral leaflets. which had spontancously stopped moving in an upper position. oscillations were induced transiently. This effect was also found for the membrane potential of motor cells in the pulvinus. - Our results thus provide further evidence that the membrane potential controls the volume state of the motor cells in the pulvinus of lateral leaflets of Desmodium motorium .