The reduction of the oxygen-evolving system in chloroplasts by thylakoid components

Abstract Using thoroughly dark-adapted thylakoids and an unmodulated Joliot-type oxygen electrode, the following results were obtained. (i) At high flash frequency (4 Hz), the oxygen yield at the fourth flash (Y4) is lower compared to Y3 than at lower flash frequency. At 4 Hz, the calculated S0 concentration after thorough dark adaptation is found to approach zero, whereas at 0.5 Hz the apparent S 0 ( S 0 + S 1 ) ratio increases to about 0.2. This is explained by a relatively fast donation ( t 1 2 = 1.0–1.5 s ) of one electron by an electron donor to S2 and S3 in 15–25% of the Photosystem II reaction chains. The one-electron donor to S2 and S3 appears to be rereduced very slowly, and may be identical to the component that, after oxidation, gives rise to ESR signal IIs. (ii) The probability for the fast one-electron donation to S2 and S3 has nearly been the same in triazine-resistant and triazine-susceptible thylakoids. However, most of the slow phase of the S2 decay becomes 10-fold faster ( t 1 2 = 5–6 s ) in the triazine-resistant ones. In a small part of the Photosystem II reaction chains, the S2 decay was extremely slow. The S3 decay in the triazine-resistant thylakoids was not significantly different from that in triazine-susceptible thylakoids. This supports the hypothesis that S2 is reduced mainly by Q−A, whereas S3 is not. (iii) In the absence of CO2/HCO−A and in the presence of formate, the fast one-electron donation to S2 and S3 does not occur. Addition of HCO−3 restores the fast decay of part of S2 and S3 to almost the same extent as in control thylakoids. The slow phase of S2 and S3 decay is not influenced significantly by CO2/HCO−3. The chlorophyll a fluorescence decay kinetics in the presence of DCMU, however, monitoring the Q−A oxidation without interference of QB, were 2.3-fold slower in the absence of CO2/HCO−3 than in its presence. (iv) An almost 3-fold decrease in decay rate of S2 is observed upon lowering the pH from 7.6 to 6.0. The kinetics of chlorophyll a fluorescence decay in the presence of DCMU are slightly accelerated by a pH change from 7.6 to 6.0. This indicates that the equilibrium Q−A concentration after one flash is decreased (by about a factor of 4) upon changing the pH from 7.6 to 6.0. When direct or indirect protonation of Q−B is responsible for this shift of equilibrium Q−A concentration, these data would suggest that the pKa value for Q−B protonation is somewhat higher than 7.6, assuming that the protonated form of Q−B cannot reduce QA.