The effect of dithionite on fluorescence and luminescence of chloroplasts

Abstract

The kinetics of dithionite-induced changes in fluorescence and luminescence of isolated chloroplasts were studied as function of dithionite concentration and number of preilluminating short saturating flashes. 1. 1. The kinetics of the dithionite-induced fluorescence increase were found to be of an apparently biphasic n ature. A relatively small, rapid, increase (accounting for about 15% of the maximum increase that could be obtained with light) was followed by a “slow” increase. This second, “slow”, phase, in contrast to the first phase, was strongly dependent on the number of preilluminating flashes. It was slowest after two flashes and most rapid after three. 2. 2. The first phase of the dithionite-induced fluorescence increase was approximately correlated with reduction of the largest part of the total pool of primary and secondary acceptors of Photosystem II. 3. 3. The kinetics of luminescence induced by dithionite were correlated with the rapid phase of fluorescence increase. The highest emission intensity of luminescence was observed after two flashes, but apart from a proportionality factor the kinetics were almost the same after different numbers of flashes. The results indicate that the luminescence is produced by a reaction between an oxidized product, which is the same after different numbers of preilluminating flashes, and reduced quencher, formed mainly during the “rapid” phase of the dithionite-induced fluorescence increase. 4. 4. The results are discussed in relation to existing hypotheses, involving the existence of two primary acceptors of Photosystem II. It is argued that our results do not support these hypotheses and that the effects of dithionite may be explained in terms of a single primary acceptor (and quencher), reduced by dithionite via the secondary acceptor pool.