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 mo vement, light applied to the pulvinus causes changes in both the membr ane potential and the pulvinar movement in Phaseolus vulgaris L. Even after a short pulse of light, a transient depolarization of the membra ne occurs and leaf movement is observed. Decreases of turgor pressure of the motor cells are always preceded by the depolarization. The dire ction of the leaf movement can be explained by the decrease of turgor pressure in the motor cells on the irradiated side of the pulvinus. Us ing the Okazaki Large Spectrograph at the National Institute for Basic Biology, we determined the action spectrum of the membrane depolariza tion induced by light pulses (30 s) in motor cells of Phaseolus. The p ulvinus was left exposed to air during measurement of the membrane pot ential with microelectrodes. The action spectrum obtained was in the r ange of 300 to 730 nm. It had the highest peak at 460 nm with lower pe aks at 380 nm and 420 nm. Almost no sensitivity was observed at wavele ngths shorter than 360 nm and longer than 520 nm. Red and far-red ligh t 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 r esponse in plants.