Zur Plastochinonoxydation bei der Photosynthese

Abstract

After the reduction of PQ in the light, PQ= is oxidized in the dark. Three types of reoxydation can be distinguished in isolated chloroplasts of spinach: a) Without addition of an electron acceptor: No reoxidation can be observed (fig. 1 a). b) With addition of an electron acceptor: 50% is reoxidized (fig. 1 b). c) With addition of an electron acceptor and permanent far red background light (718 nm) 50% is reoxidized as in b, the other 50% are oxidized in a second phase (see fig. 1 c). This second phase depends on the intensity of the far red background light (figs. 4 and 5). The reoxidation at the time t′ (see fig. 2) follows (d[PQ=]/dt)t´ = — k2´[PQ=] (fig. 3) with k2´=11 sec-1 at 25 °C. The kinetics can be interpreted by assuming a pool of PQ between the two light reactions I and II with a dynamic capacity for ten electrons, and an electron acceptor pool U with a capacity for five electrons (see figs. 7, 8 and 9). Members of U are chlorophyll-al , cytochrome-f and three unknown intermediates. The kinetics should depend on the redox state of these pools. The path and reaction times of the electron flow between the two light reactions are depicted in fig. 13. The phenomena described for isolated chloroplasts appear also for whole chlorella cells (fig. 10). In chlorella the electron acceptor is formed by hvI-light in >0,5 sec. K-ferricyanide has a special function among the electron acceptors. On the addition of K-ferricyanide (>10-4 Mol/l) plastoquinone which is reduced in long flashes is completely oxidized without far red background light (fig. 11). This indicates that the electrons produced in long flashes are removed from the chain by K-ferricyanide at U (see fig. 12).