Evidence from thermoluminescence for bicarbonate action on the recombination reactions involving the secondary quinone electron acceptor of Photosystem II

In this paper, we present the first measurements on thermoluminescence from isolated thylakoids to probe the recombination reactions of S₂ (or possibly S₃) with Q^?_B or Q^?_A, after bicarbonate depletion and its readdition. The effects of bicarbonate depletion on the S₂Q^?_B (or S₃O^?_B) thermoluminescence band was (1) a 6–10°C shift to a higher temperature; (2) a reduction in its intensity upon prolonged depletion; and (3) elimination after the first few flashes of the characteristic period four oscillations in its intensity as a function of the flash number. On the other hand, addition of diuron (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea), which blocks electron flow from Q^?_A to Q_B, produced the same thermoluminescence band, at about + 20°C, assigned to S₂Q^?_A recombination, in both depleted and reconstituted samples. These results suggest (1) the initial effect of bicarbonate depletion is to increase the activation energy for S₂(S₃)Q^?_B recombination; (2) with further depletion, the incidence of this recombination decreases and the cycling of the S₂Q^?_B and S₃Q^?_B recombination is inhibited through effects at the Q_B apoprotein; and (3) the depletion effects are fully reversible. It is suggested that a conformational change of the PS II complex in the region of the Q_B apoprotein is responsible for these effects.