Effect of preillumination on the P‐680+reduction kinetics in chloride‐free photosystem II membranes

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Abstract
The re‐reduction course of P‐680+, the photooxidized PS II primary donor, was measured as a function of excitation number in Cl−‐depleted PS II membranes. After the 1st and 2nd excitations the signal amplitude of P‐680+is small, indicating a submicrosecond reduction of P‐680+by Z, the secondary donor of PS II. After the 3rd excitation, however, a larger P‐680+signal with a 40–50 μs half‐life is observed. The slow decay of this signal is attributed to a back‐reaction with a reduced acceptor in the presence of the Z+S2state on the donor side. The state Z+S2has a lifetime longer than 300 ms and its formation was found to depend on the presence of the abnormal S2state created by the 1st excitation. The P‐680 data and thermoluminescence measurements show that the S‐state advancement beyond S2is blocked in the absence of Cl−and that the Cl−‐free abnormal S2state has a lifetime about 10‐times longer than the normal S2state.

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