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Phosphoryiat~o~ of proteins within pea thylako~d membranes decreases photosystem II (PSII) mediated electron transfer at saturating Iight intensities which, according to changes in room temperature chlorophyll fluorescence transients, is due to a modi~~atio~ in the electron transfer from QA to QB. However, a previously reported increase in the ability of DCMU to inhibit PS2 efectron Sow to DCPIP, as a consequence of protein phosphoryiation, was not observed, although changes in DCMU efficacy were found to depend upon the redox state of the plastoquinone (PQ) pool.

Phosphorylation of thylakoid membrane proteins results in a partial inhibition (approximately 15-20%) of the light-saturated rate of oxygen evolution. The site of inhibition is thought to be located on the acceptor side of photosystem 2 (PS2) between the primary, QA, and secondary, QB, plastoquinone acceptors (Hodges et al. 1985, 1987). In this paper we report that thylakoid membrane phosphorylation increases the damping of the quaternary oscillation in the flash oxygen yield and increases the extent of the fast component in the deactivation of the S2 oxidation state. These results support the proposal that thylakoid membrane protein phosphorylation decreases the equilibrium constant for the exchange of an electron between QA and QB. An analysis of the oxygen release patterns using the recurrence matrix model of Lavorel (1976) indicates that thylakoid membrane phosphorylation increases the probability that PS2 miss a S-state transition by 20%. This is equivalent, however, to an insignificant inhibition (approximately 2.4%) of the light-saturated oxygen evolution rate. If a double miss in the S-state transitions is included when the PS2 centres are in S2 the fit between the experimental and theoretical oxygen yield sequences is better, and sufficient to account for the 15-20% inhibition in the steady-state oxygen yield. A double miss in the S-state transition is a consequence of an increased population of PS2 centres retaining QA (-): not only will these PS2 centres fail to catalyse photochemical charge transfer until QA (-) is reoxidized, but the re-oxidation reaction will also result in the deactivation of S2 to S1.