Photomorphogenesis in Phycomyces: Fluence-response curves and action spectra

Blue light regulates vegetative reproduction in Phycomyces blakesleeanus Bgff. by inhibiting the development of microphores and stimulating that of macrophores. Fluence-response curves were obtained at twelve different wavelengths. Each response exhibits a two-step ("biphasic") dependence on fluence, as if it resulted from the addition of two separate components with different thresholds, midpoints, and amplitudes. The absolute threshold is close to 10 photons·μm². The threshold fluence of the low-intensity component is about 10⁴ times smaller than that of the high-intensity component. The action spectra for each of the two components of the two responses share general similarities, but exhibit significant differences that might be taken to favour four separate photosystems. Additional complexity is indicated by the wavelength dependence of the saturation levels.