Two Types of Primary Acceptors in Chloroplasts Photosystem II. II. Reduction in Two Successive Photo Acts

This paper proposes a further characterization of two types (“B” and “non-B”) of photosystem II (PS-II) primary acceptors, and attempts to correlate this partition with other heterogeneous models (Joliot, P. and A. Joliot (1980) in: Proceedings 5th Int. Congr. Photosynthesis, Halkidiki (in press), and Melis, A. and P. Homann (1976) Photochem. Photobiol. 23, 343—350). B-type acceptors are those which take part in the DCMU-induced rise of fluorescence after a preillumination according to the reaction QB - → DCMU Q - B. Non-B-type acceptors are responsible for the complemen tary part of the variable fluorescence. 1. In the presence of hydroxylamine, the efficiency of reduction of B-type acceptors by a flash was found unaffected, although the efficiency of the global quenching destruction is markedly decreased (in connection with a slowing down of the re-reduction of the center chlorophyll, which prevents double hitting in PS-II centers during a μ s flash). 2. The reduced B-type acceptors were found to be stable in the presence of DCMU and hydroxylamine on a few minutes time range, whereas a significant reoxidation of the non-B-type acceptors was observed. Points 1 and 2 suggest that B-type acceptors belong to the “Q 1 ” type defined by Joliot and Joliot. 3. In experiments involving a mixing with DCMU of chloroplasts preilluminated by a sequence of flashes (F 1 ) in the presence of hydroxylamine, the effect of a flash (F 2 ), triggered after the mixing, was studied. The fluorescence level reached after F 2 is still modulated by the period 2 oscillation of the DCMU-induced transient. This suggests that in the presence of hydroxylamine the reduction of non-B acceptors by a flash can only take place after prior reduction of B-type acceptors. This finding supports the hypothesis that B- and non-B-type acceptors are parallel systems successively reduced (in this order) by the same center chlorophyll in two photoacts. 4. This point is consistent with the results of variable intensity experiments which show that (in the absence of hydroxylamine) B-type acceptors are preferentially reduced at low flash intensity. 5. The slow phase of the fluorescence induction curve in the presence of DCMU (or the “tail” of the saturation curve with a flash of variable intensity), attributed by Melis to Q β centers, is shown to correspond to a part of the acceptors Q 1 as defined by Joliot. It is concluded that B-type acceptors are a subset of Joliot’s Qi acceptors, occurring in centers with two parallel acceptor systems, and, possibly, two parallel donor systems as well. Melis’ Q β acceptors might be identified to the non-B type subset of Q 1.