Slow secondary fluorescence kinetics associated with the onset of photosynthetic carbon assimilation in intact isolated chloroplasts

Experiments with carefully isolated, largely intact chloroplasts, capable of fast rates of CO2-dependent O2 evolution, show that the fall in chlorophyll a fluorescence (from the early maxima reached immediately after illumination) is interrupted by a ‘shoulder’ which is associated with the exponential increase in the rate of O2 evolution. The length of this induction period was increased by storage, by decreased temperature, by increased orthophosphate concentration in the assay medium or by the presence of D, L-glyceraldehyde. It could also be shortened by the addition of 3-phosphoglycerate or dihydroxyacetonephosphate. In each treatment the shoulder in fluorescence shifted so that the association with the period of exponential increase was maintained. When illumination was re-started after a short dark interval, induction was minimal and no shoulder could be discerned, but both the lag in the onset of O2 evolution and the shoulder were restored when the chloroplasts were resuspended in fresh assay medium during the period of darkness. The relation between chlorophyll a fluorescence and the onset of photosynthetic carbon assimilation is discussed.