Phototropic Responses of Vaucheria geminata to Intermittent Blue Light Stimuli.

Phototropic responses of a tip-growing coenocytic alga Vaucheria geminata to intermittent blue light pulses were analyzed. Curvatures caused by repeated light pulses separated by dark intervals of various lengths were as large as those obtained by continuous light of the same incident energy, unless the length of the dark interval exceeded a critical value of about 30 to 40 seconds. If the dark interval was longer than the critical length, bending no longer took place. Another response to intermittent irradiation was found. In a narrow range of light pulse and dark interval lengths, i.e. light pulses longer than 10 milliseconds and dark intervals shorter than a value ranging from 15 to 150 milliseconds, intermittent irradiation was more effective than continuous irradiation of the same total energy. In this region, the maximum response to repeated pulses was equal to the curvature caused by a continuous irradiation of the same total elapsed time. It is evident that in this condition the dark interval is not perceived and the alga responds as if it were in light. The results suggest that two different dark reactions may be involved in the phototropic response of this algal cell.