Action spectrum of light pulse-induced membrane depolarization in pulvinar motor cells of Phaseolus

In addition to circadian changes in the membrane potential and leaf movement, light applied to the pulvinus causes changes in both the membrane potential and the pulvinar movement in Phaseolus vulgaris L. Even after a short pulse of light, a transient depolarization of the membrane occurs and leaf movement is observed. Decreases of turgor pressure of the motor cells are always preceded by the depolarization. The direction of the leaf movement can be explained by the decrease of turgor pressure in the motor cells on the irradiated side of the pulvinus. Using the Okazaki Large Spectrograph at the National Institute for Basic Biology, we determined the action spectrum of the membrane depolarization induced by light pulses (30 s) in motor cells of Phaseolus. The pulvinus was left exposed to air during measurement of the membrane potential with microelectrodes. The action spectrum obtained was in the range of 300 to 730 nm. It had the highest peak at 460 nm with lower peaks at 380 nm and 420 nm. Almost no sensitivity was observed at wavelengths shorter than 360 nm and longer than 520 nm. Red and far-red light had no effect on the depolarization of the motor cell. The features of the action spectrum are almost the same as those of the Blue-Type response in plants.