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The flash-induced oscillation of a slowly decaying component of delayed luminescence (half time ~ 42 s) was investigated in spinach chloroplasts for various redox states of the secondary acceptor pool (QB). In preilluminated chloroplasts (30 s light followed by 5 min dark) the slow component exhibited a period-4 oscillation as a function of flash number. Upon oxidation of a major part of the QB pool by dark-adaptation or by ferricyanide treatment of chloroplasts, the period-4 oscillation was converted into a period-2 oscillation, providing direct experimental evi­ dence of the participation of QB - in the charge recombination reaction. The measured oscillatory patterns could be simulated in model calculations by assuming that the slow component originates from charge recombination of the redox couples S2QB - and S3QB -.

The thermoluminescence and delayed luminescence characteristics of PS IIα and PS IIβ centers were investigated in BBY particles and stroma thylakoids, respectively. The BBY particles exhibited a thermoluminescence band at 25 °C (B band) which was associated with the charge recombination of the S2Qв - redox couple and underwent period-2 oscillation in a sequence of flashes. In the flash-induced decay of delayed luminescence of BBY particles a component with a half-time of 34 s corresponded to the B thermoluminescence band and was also assigned to S2Oв - charge recombination. No corresponding thermoluminescence or delayed luminescence components associated with the secondary acceptor Qв could be observed in the glow curve or delayed luminescence decay of stroma thylakoids. These observations indicate that unlike PS IIα the PS IIβ centers are not associated with the two-electron gate, Qв.

The pH dependence of two, flash-induced slowly decaying delayed luminescence components - resulting from charge recombination of QA - and QB - with the S2 and S3 states (S2QA - + S3QA - and S2QB - + S3QB - recombinations) - in untreated and herbicide treated chloroplasts was investigated within a pH range of 5.5 to 8.5. The component associated with QA - showed the same pH-dependence in untreated and in DCMU-treated chloroplasts: its half-decay time increased with increasing pH reflecting an appr. +5 meV/pH change in the free energy of activation. Contrary to this the component associated with Ob exhibited an opposite behaviour: its half-decay time decreased with increasing pH and the free energy of activation changed with about -5 meV/pH. It is concluded that both acceptor and donor side effects are involved in the pH-dependence of slow phase delayed luminescence components.