Subliminal light control of dark adaptation kinetics in Phycomyces phototropism

The dark adaptation kinetics of Phycomyces phototropism depend critically on the experimental protocol. When sporangiophores that had been light-adapted to a fluence rate of 1 W m-2 at 447 nm were exposed to dim unilateral light, the adaptation kinetics showed exponential decay (6 min time constant). However, when light-adapted sporangiophores were kept for variable intervals in darkness (i.e. in presence of traditional red safelight) and then exposed to dim unilateral test light, the decay kinetics of adaptation were biexponential with a rapid decay during the first minute (1 min time constant), followed by a slow recovery (11 min time constant). Thus, the dim subliminal light given after the sporangiophores had been adapted to 1 W m-2, was actually perceived, and exerted control over the dark-adaptation process. The observed acceleration of dark-adaptation kinetics constitutes a novel light effect of the sporangiophore. At wavelength 383 nm this effect was not observed. Because a beta-carotene lacking mutant, L91 (genotype carB), was unmodified in dark-adaptation kinetics measured in the presence or absence of subliminal light, it appears that beta-carotene is not involved in the photocontrol of adaptation.