Photochemical and thermal phases in the short-term chlorophyll fluorescence induction kinetics of Chlorella fusca

Abstract

The yield of chlorophyll fluorescence from photosystem II of Chlorella fusca was measured during a short xenon flash (half-width, 2 μs) and rectangular laser pulse (duration, 100 μs) of variable intensity. At the highest light intensity (above 10 5Wm −2), the photochemical rise and the decay attributed to the carotenoid triplet quenching could be well separated and a non-photochemical phase was observed which completed within 60 μs. If the algae are pre-excited with a saturating nitrogen laser pulse, the yield of fluorescence can exceed the maximum value measured routinely (stationary conditions and longer time range). It is argued that the unusually high fluorescence state can arise from a model of the reaction centre of two photoreactions separated by a thermal step. A numerical simulation of the model, based on the freely moving exciton approximation and competition between the carotenoid triplet traps and the different states of the reaction centre for exciton capture, gave reasonably good agreement with the experimental results.