ROLE OF CHLORIDE ION IN PHOTOSYNTHESIS .3. EFFECT OF ARTIFICIAL ELECTRON DONORS UPON ELECTRON TRANSPORT

4- to more than 10-fold acceleration of the Hill reaction (electron acceptor: indophenol dyes, ferricyanide, FMN, etc.) by Cl- is consistently observed over a wide range of pH (5.7-8.3) when EDTA-uncoupled chloroplasts are used as experimental material. Using such highly Cl--sensitive chloroplasts the effect of Cl- was examined on the photooxidation of reducing agents which are capable of donating electrons to Photosystem II. No Cl- effect is observed on the photooxidation of hydroxylamine (at substrate level with indophenol dye as electron acceptor) or of ascorbate (FMN as acceptor). These observations strongly suggest that Cl- acts near the water-splitting end of Photosystem II. High concentrations of ammonia and methylamine, like low concentrations of hydroxylamine, block water splitting on the oxidizing side of Photosystem II. Apparently the unprotonated forms of simple amines act as a specific inhibitor at a point near the site of Cl- involvement. Upon removal of Cl- the steady-state fluorescence yield of chloroplasts is markedly and reversibly depressed. However, the addition of a substrate level of hydroxylamine obliterates the Cl- effect on fluorescence and the yield becomes higher than in the presence of Cl- alone. These observations are discussed in connection with the site of Cl- involvement and with the current model of the chloroplast fluorescence quenching mechanism. © 1969.