In the unicellular green alga Scenedesmus obliquus the level of photoinhibition and the recovery of the cells after reversal to the initial light conditions in relation to the pre-photoadaptation of the culture to low, medium and high light intensity was determined. The changes in the photosynthetic pigment content and in the intracellular polyamine concentration allowed to distinguish between photoadaptation and photoinhibition. In particular, the level of chlorophylls, xanthophylls and carotenoids decreased inversely proportional to the light intensity applied during photoadaptation, whereas their concentrations remained constant during photoinhibition. The violaxanthin/zeaxanthin and the loroxanthin/lutein cycle work only under photoinhibitory conditions, but not under photoadaptive premises. Changes in the level of these carotenoids in relation to the changes in the photosynthetic apparatus during photoadaptation are discussed. In addition, it was found that the intracellular polyamine level increased only under stress conditions, i. e. during photoinhibition, and decreased during recovery of the cells after reversal to the initial light conditions. The increase of the putrescine level during photoinhibition is inversely proportional to the light intensity used for pre-adaptation. This rise of the polyamine level in the cells photoadapted to high light conditions is an additional indication for the finding that photoadaptation and photoinhibition are different phenomena which are clearly distinguishable from each other. Finally, the changes of the chlorophyll, violaxanthin, zeaxanthin, loroxanthin, lutein and polyamine levels under photoadaptation in high light intensity (50 W m -2) in relation to the range of photoadaptation in Scenedesmus obliquus are discussed.
