A photoperiodic response mediated by blue light in the brown alga Scytosiphon lomentaria.

The crustose phase of Scytosiphon lomentaria (Lyngb.) J. Ag. persists indefinitely in 16 h of white light per day, but produces erect, cylindrical thalli vegetatively in 8-h days. The critical daylength for this short-day (SD) response is sharply defined, and, between 12 and 13 h, differences in daylength of only 15 min produce substantial differences in response. A significant response to SD can be induced by as few as 4 SD cycles, but 10-12 cycles are required to saturate the response and induce more than 90% of the plants to form thalli. The response to SD is completely inhibited by a 1-min light-break with a low irradiance of blue light, given in the middle of a 16-h dark period, but is unaffected by longer periods and higher irradiances of red or far-red light. There is good reciprocity between the irradiance and the length of a light-break with blue light, and 50% inhibition of the response to SD is induced by about 2 nE cm(-2) at 449 nm. All attempts to reverse the inhibitory effects of blue light by subsequent irradiation with another wavelength have so far failed. These results indicate that phytochrome is not the photoreceptor pigment for this response, in spite of the similarity of the response in all other respects to the photoperiodic responses of flowering plants and other algae.