The relation between the photochemical yield and variable fluorescence of photosystem II in the green alga Scenedesmus obliquus

The redox state of the primary quinone acceptor (Q(A)) of photosystem II (PS II) and specific non-photochemical effects determine the relation between PS II photochemistry and chlorophyll fluorescence. For the unicellular green alga Scenedesmus obliquus, the fluorescence yield (F), the fluorescence yield in the presence of reduced Q(A) (F-M') and the yield of respiration-corrected oxygen evolution (Phi(ox), evolution rate divided by light intensity) were determined during exposure to actinic light of various intensities (0.1-1600 mu E m(-2) s(-1)). Irrespective of the growth conditions, the parameter Phi(P) = (F-M'-F)/F-M' was found to be proportional to Phi(ox) for each algal culture. This finding is in full agreement with non-photochemical fluorescence quenching by increased thermal deactivation of excited antenna states, but is difficult to reconcile with non-photochemical quenching due to modified PS II electron transfer reactions. In particular, no indications of cyclic PS II electron flow were found.