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Photoreduction of pheophytin as a result of electron donation from the water-splitting system to Photosystem-II reaction centers

1985; Elsevier BV; Volume: 809; Issue: 3 Linguagem: Inglês

10.1016/0005-2728(85)90184-7

1879-2650

Vyacheslav V. Klimov, В. А. Шувалов, U. Heber,

Photoreceptor and optogenetics research

Reversible photoreduction of pheophytin (Pheo) accompanied by a decrease of chlorophyll-fluorescence yield is observed in subchloroplast oxygen-evolving preparations of Photosystem II (PS II) under anaerobic conditions. This photoreaction is activated at addition of CCCP, inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and reactivated upon subsequent addition of ascorbate. Benzyl viologen as well as methyl viologen accelerates dark oxidation of reduced pheophytin, indicating that they are able to accept an electron from Pheo⨪. The data on both the photoreduction of pheophytin in the absence of exogenous reductants - when electron donation to reaction centers of PS II occurs only from water - and the inhibition of this photoreaction by DCMU show that the pheophytin photoreduction is sensitized by reaction centers of PS II, and it probably occurs as a result of electron donation from the water-splitting system being in the sate S3 to P⨥-680Pheo⨪Q−, producing the long-lived state S0 P-680Pheo⨪Q− and O2. Photoreduction of pheophytin in the presence of ascorbate (and dithionite) evidently occurs as a result of donation of its electrons to P⨥-680Pheo⨪Q− by means of the S-states of the water-oxidizing system. It is shown that the photoinduced decrease of fluorescence in chloroplasts under anaerobic conditions is due to two processes: photoreduction of pheophytin in Photosystem II and photooxidation of Q− by Photosystem I. It is suggested that photoreduction of pheophytin takes also place under aerobic conditions when Q is reduced. It may contribute to the P−S fluorescence decrease during fluorescence induction in leaves.