The involvement of a high potential acceptor in the acid-base induced reduction of the acceptor Q in chloroplasts.

Study of the acid-base induced fluorescence transient, the so-called 'reverse electron flow', in chloroplasts revealed the following new properties: (1) Experiments in which the acid-base transition was performed in the absence of the measuring beam showed that the high fluorescence state induced by a pH-jump was attained and decayed even in complete darkness. These results indicated that a non-photochemical electron transfer was affected by the pH transition. A pH-induced reverse electron transfer from some secondary electron acceptor to Q probably occurred during that process. (2) This conclusion was supported by the effect of the Photosystem I electron acceptor methyl viologen. Methyl viologen accelerated the decay phase of the transient showing that this phase was controlled by the rate of electron flow to Photosystem I, but this acceptor did not diminish the size of the transient's initial rise, probably because this rise reflected a pH effect on a non-photochemical step located between Q and P-700. (3) The size of the fluorescence transient was dependent upon the reduction state of both parts of the secondary pool of electron acceptors, A2 and A1. (4) Redox potential measurements using ferricyanide-ferrocyanide mixtures showed that the size of the transient was directly dependent on a midpoint potential of +385 mV at pH 6.9 and with n = 1. This suggested the involvement of a high potential secondary electron acceptor in the acid-base induced reduction of Q.