Plastoquinone compartmentation in chloroplasts. I. Evidence for domains with different rates of photo-reduction

The photo-reduction of plastoquinones by Photosystem II reaction centers was investigated using fluorescence and oxygen-evolution measurements in thylakoids deprived of Photosystem I acceptors. The process appears biphasic under limiting, as well as saturating, illumination. The 'fast' pool fraction (about 6 PQ molecules per PS II center) represents 50-70%, of the total. Its half reduction time under saturating light was found about 25-60 ms, while that of the 'slow' pool was 0.8-1 s. When the photo-reduction process is interrupted after reduction of the fast pool and resumed after a dark period, a redistribution of the reduced plastoquinones towards the slow pool is observed, with t1/2 approximately 6 s. We interpret these results as expressing a limitation of PQ diffusion in the membrane and propose that the fast pool reflects the fraction present in the grana region where most PS II centers are located, while the slow pool corresponds to quinones from the stromal region. The relationship between the redox states of PQ and of the primary acceptor QA during photo-reduction of the fast pool expresses marked discrepancies with respect to a quasi-equilibrium relationship. This failure to achieve equilibrium on a rapid time-scale and the slow diffusion rate of quinones over long distances are accounted for by small size domains bounded by, membrane proteins. In agreement with this view, we found that the amount of fast photo-reducible quinones is decreased when a fraction of PS II centers is inhibited, indicating that the domains contain, on average, about 3-4 PS II centers. We conclude that PQ cannot be responsible for the long range diffusion involved in rapid electron transfer from granal (PS II) to stromal (PS I) regions, a role that must be fulfilled by plastocyanin.