Action Potentials Evoked by Light in Traps of Dionaea muscipula Ellis

At low light intensities (less than 50 /anol m 2 s ') illumination evokes transient depolarization of membrane potential in mesophyll cells of the leaf-trap of Dionaea muscipula Ellis. Darkening causes hyperpolarization approximately symmetric to the response to illumination. The amplitude as well as the rate of potential changes depend on light intensity. After exceeding a definite threshold (usually between 50 and 80/anol m~s~) the depolarization plays the role of a generator potential and an all-ornone action potential (AP) is released. Switching light off in a depolarization phase of an AP does not change its shape and the amplitude. When the light intensity is increased to 80-150 /anol m~ s" a single light stimulus triggers two successive APs. The time interval between the two APs decreases with increasing stimulus strength and reaches the minimum between 300 and 400 /anol m~ s". At higher light intensities the interval increases again, and finally only a single AP is triggered. It was shown that the effect was evoked by light but not by temperature change accompanying illumination. An inhibitor of the photosynthetic electron transport chain, DCMU, blocked the generator potentials mediating between light absorption and APs. Residual responses to light stimuli in plants treated with DCMU had reverse polarity and strongly reduced the amplitudes.