ACTION SPECTRUM FOR SUBLIMINAL LIGHT CONTROL OF ADAPTATION IN PHYCOMYCES PHOTOTROPISM

Adaptation processes enable phototropism and other blue light response s of Phycomyces to operate over a 10-decade range of fluence rate. Pho totropic latency, used routinely to monitor the kinetics of sensitivit y recovery after a step down in fluence rate, can be shortened by appl ication of dim light for 35 min during the early part of the latency p eriod. This light is termed subliminal. because it does not elicit pho totropism under these experimental conditions; rather, it exerts its i nfluence on the underlying adaptation kinetics. Fluence rate-response data for this latency reduction, obtained at 17 wavelengths of sublimi nal light from 347 to 742 nm, showed a variety of shapes that could be fit by zero, one, or two sigmoidal components, plus a constant term. At most wavelengths, the fluence-rate threshold for latency reduction by subliminal light tended to be well below the absolute threshold for phototropism, indicating that this effect is highly sensitive. An act ion spectrum for the sensitivity of the subliminal light effect, deriv ed from the fluence rate-response curves, shows major peaks around 400 and 500 nm and a broad band from 570 to 670 nm, followed by a steep a bsorption edge. The sensitivity in the near ultraviolet region is rela tively very low. The magnitude of the latency reduction also depends s trongly on wavelength with a maximum at about 450 nm. The fluence-rate response data and the action spectrum-which is markedly different fro m that for phototropism and other blue-light responses of Phycomyces-i ndicate the participation of multiple pigments, or pigment states, in the photocontrol of adaptation.