Reactions between primary and secondary acceptors of Photosystem II in Chlorella Pyrenoidosa under anaerobic conditions as studied by chlorophyll a fluorescence

Abstract

The kinetics of the fluorescence yield Ф of chlorophyll a in Chlorella pyrenoidosa were studied under anaerobic conditions in the time range from 50 μs to several minutes after short ( t 1 2 = 30 ns or 5 μs) saturating flashes. The fluorescence yield “in the dark” increased from Ф = 1 at the beginning to Ф ≈ 5 in about 3 h when single flashes separated by dark intervals of about 3 min were given. After one saturating flash, Ф increased to a maximum value (4–5) at 50 μs, then Ф decreased to about 3 with a half time of about 10 ms and to the initial value with a half time of about 2 s. When two flashes separated by 0.2 s were given, the first phase of the decrease after the second flash occurred within 2 ms. After one flash given at high initial fluorescence yield, the 10-ms decay was followed by a 10 s increase to the initial value. After the two flashes 0.2 s apart, the rapid decay was not follewed by a slow increase. These and other experiments provided additional evidence for and extend an earlier hypothesis concerning the acceptor complex of Photosystem II (Bouges-Bocquet, B. (1973) Biochim. Biophys. Acta 314, 250–256; Velthuys, B. R. and Amesz, J. (1974) Biochim. Biophys. Acta 333, 85–94): reaction center 2 contains an acceptor complex QR consisting of an electron-transferring primary acceptor molecule Q, and a secondary electron acceptor R, which can accept two electrons in succession, but transfers two electrons simultaneously to a molecule of the tertiary acceptor pool, containing plastoquinone (A). Furthermore, the kinetics indicate that 2 reactions centers of System I, excited by a short flash, cooperate directly or indirectly in oxidizing a plastohydroquinone molecule (A 2−). If initially all components between photoreaction 1 and 2 are in the reduced state the following sequence of reactions occurs after a flash has oxidised A 2− via System I: Q −R 2− + A → Q −R + A 2− → QR − + A 2−. During anaerobiosis two slow reactions manifest themselves: the reduction of R (and A) within 1 s, presumably by an endogenous electron donor D 1, and the reduction of Q in about 10 s when R is in the state R − and A in the state A 2−. An endogenous electron donor, D 2, and Q − compete in reducing the photooxidized donor complex of System II in reactions with half times of the order of 1 s.