Time course of microsecond-delayed light emission from Scenedesmus obliquus

The time course of microsecond-delayed light emission and that of prompt fluorescence was measured during the period of light adaptation for the wild-type strain and mutants Nos 8 and 11 of Scenedesmus obliquus, Strain D3, exposed t stimulus light in a photon-counting apparatus. As compared to prompt fluorescence, the delayed light emission was generally more susceptible to change in experimental conditions. (1) Illuminating normally grown wild-type cells with higher light intensities (100 mW/cm2 or higher), the intensity of delayed light increases to a maximum in th first few minutes, then decreases to a steady-state level after about 1 h. The maximu is missing at lower exciting light intensities. (2) Illuminating dark-adapted wild-type cells with high light intensities, the time course of delayed light emission is similar to that one described above, apart from a lag period within the very first minutes of light adaptation. That lag period is characterized by a relative minimum of the intensity lying higher than the intensity of the steady-state level. The length and the profile of the lag period depends on the time of dark adaptation. With increasing dark adaptation time the maximum of delayed light intensity appears later. (3) This maximum of delayed light intensity is only present for times shorter than 100 ps after turning off the stimulus light. (4) For mutant No. 8 the intensity of delayed light shows a steady decrease during the time course. (5) For light-adapted wild-type cells the intensity of delayed light and that of prompt fluorescence increases linearly with increasing intensity of exciting light below the saturation point in the light curve (about 10 mW/cm2). The increase is less than linear above the saturation point. The results are discussed in terms of two photosystems coupled by an electron transfer chain.