The fluence-rate and wavelength dependence of chlorophyll synthesis and cell growth of Scenedesmus obliquus were measured under heterotrophic conditions. Cultures were first grown autotrophically under white light from fluorescent lamps (20 W 9 m -2) and were then transferred to a glucose-containing medium in which they were exposed to light of different fluence rates and wavelengths. Fluence rate-response curves, which were obtained with monochromatic blue (453 nm) and red (644 nm) light were complex and indicated the existence of a very low-and a low-irradiance photoreceptor-system with antagonistic modes of interaction. In the very lowirradiance region (10-1~ -6 mol.m-2-s-1), blue light partially inhibited chlorophyll synthesis. In the lowirradiance region (> 10 -6 mol 9 m -2 " S-1), chlorophyll synthesis and cell growth were stimulated. At these elevated fluence rates the curve was bell shaped. Red light inhibited chlorophyll synthesis in the very low-as well as in the low-irradiance region. The effects of blue and red light on chlorophyll synthesis and cell growth were unaffected by the photosynthesis inhibitor 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) showing that photosynthesis per se did not mediate these photoresponses. Based on detailed fluence rate-response curves and also on equal quantum-flux measurements, action spectra were obtained for chlorophyll synthesis and cell growth in the region below 10 -6 mol. m -2. s -1. The action spectra for the very low-irradiance region (inhibition of chlorophyll synthesis and stimulation of cell growth) showed two sharp peaks near 408 and 645 nm. The action spectrum for the low-irradiance region showed stimulation of chlorophyll synthesis between 450 and 480-nm. Below and above this wavelength range an inhibitory action was found. We propose that two photoreceptor systems regulate chlorophyll synthesis, * Dedicated to Professor Dr. Hans Mohr on the occasion of his 60th birthday Abbreviations: PCV=packed cell volume; Chl=total amount of chlorophyll; Chl a, b = chlorophyll a, b; DCMU = 3-(3,4-dichlorphenyl)-1,1 -dimethylurea
and that these can be distinguished by different thresholds, different spectral sensitivities and also by their antagonistic modes of action. The implications of these findings with respect to autotrophic conditions are discussed.