Studies on chlorophyll fluorescence in chloroplasts. I. Effects of dithionite on fluorescence of chlorophyll A in isolated spinach chloroplasts

Effects of dithionite on the time-course of fluorescence emitted from chlorophyll a in isolated spinach chloroplasts were studied. Addition of dithionite markedly shortened the induction period of fluorescence and increased the steady-state level of fluorescence. However, a small but distinct induction, comparable to that observed in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea, was always observed in the presence of dithionite. When the fluorescence change was determined in the presence of DCMU, preincubation of the chloroplasts with dithionite for a prolonged period further shortened, but only slowly, the induction period. However, addition of DCMU during the incubation period abolished most of the effects of dithionite in reducing the induction period. The results obtained were interpreted in terms of the reduction by dithionite of endogenous electron carriers associated with photosystem 2.