Light-adaptation in the photophobic response by Stentor coeruleus.

Effects of preillumination on photophobic response (light-adaptation) and recovery of the photophobic sensitivity in the dark (dark-adaptation) in Stentor coeruleus were examined. When the cells were preilluminated with white light of 7.80 W/m2 for 2 min, the fluence-rate response curve of photophobic response was shifted toward higher light intensities by half an order of magnitude compared to the one without preillumination. Preillumination with a higher light intensity resulted in a further shift of the fluence-rate response curve. An action spectrum for light-adaptation showed a primary peak at 610 nm and secondary peaks at 540 and 480 nm which are almost identical to the peaks observed in the photophobic action spectrum. The light-adapted cells showed a recovery of their photophobic sensing ability following dark treatment. Dark-adaptation resulted in total recovery of photophobic sensing ability in 8 minutes for the most cases examined.