Cytochrome b-563 redox changes in intact CO 2-fixing spinach chloroplasts and in developing pea chloroplasts

Abstract

Intact spinach chloroplasts, capable of high rates of photochemical oxygen evolution with CO 2 as electron acceptor (120–350 μmol 0 2 mg chlorophyll −1 · h −1) were examined for cytochrome redox changes. The response of the cytochromes in intact chloroplasts to oxidants and reductants appears to be governed by the permeability of the chloroplast envelope. The low potential cytochromes ( b-559 LP and b-563) were more slowly reduced at 25 °C by dithionite than is the case with broken chloroplasts. At 0 °C, the reduction of the low potential cytochromes in intact chloroplasts was extremely slow. The chloroplast envelope is impermeable to ferricyanide, slowly permeable to ascorbate and rapidly permeable to reduced dichlorophenolindophenol. Light-induced redox changes of cytochrome b-563 in intact chloroplasts were examined both at 0° and 25 °C. A red/far-red antagonism on the redox changes of cytochrome b-563 was observed at 0° C under anaerobic conditions. 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) inhibited the photoreduction of cytochrome b-563 in red light following far-red illumination. The photooxidation of cytochrome b-563 under anaerobic conditions was not influenced by DCMU or 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB). The photoreduction of cytochrome b-563 under aerobic conditions was much less efficient than its photooxidation under anaerobic conditions. Developing pea chloroplasts showed much greater light-induced redox changes of cytochrome b-563 than did intact spinach chloroplasts. Our data are consistent with the view that cytochrome b-563 functions on a cyclic pathway around Photosystem I, but it appears that cyclic flow is sensitive to the relative poising of the redox levels of cytochrome b-563 and the components of the non-cyclic pathway.