Change in the photoresponse action spectrum of the dinoflagellate Gyrodinium dorsum Kofoid by red and far-red light.

Using cessation of movement (stop-response) as an index for light reception by the dinoflagellate Gyrodinium dorsum Kofoid, an association was shown between the blue-light action-spectrum maximum for this response and prior exposure of the organism to red and far-red light. When the light energy necessary to produce a positive stop-response (above 50%) was used as a criterion for the threshold intensity, the cells were most sensitive to light of 470 nm following an exposure to red (620 nm) light; after a far-red (700 nm) light exposure, the threshold was lowest for light of 490 nm. A second response criterion was the time in darkness until a positive response could no longer be initiated upon stimulation. The response persists longer for 490 nm than for 470 nm after both red and far-red light exposure. This result can theoretically be attributed to thermal reversion of the proposed phytochrome from the Pfr to the Pr form. A two-pigment system in which a phytochrome works in combination with a blue-absorbing pigment may be involved in the photoresponse.