Effect of pH on the Slow Phase Components of Delayed Luminescence in Chloroplasts

The pH dependence of two, flash-induced slowly decaying delayed lum inescence com ponents — resulting from charge recom bination o f Q ä and Qg with the S2 and S3 states (S2O a + S3Q ä and S2Qb + S ,Q i recom binations) — in u n trea ted and herbicide trea ted chloroplasts was investigated within a pH range o f 5.5 to 8.5. The com ponent associated with Q ä show ed the sam e pH -dependence in u n trea ted and in D C M U -treated chloroplasts: its half-decay tim e increased with increasing pH reflecting an appr. + 5 m eV /pH change in the free energy of activation. C ontrary to this the com ponent associated with Q b exhibited an opposite behaviour: its half-decay time decreased with increasing pH and the free energy of activation changed with about —5 m eV /pH . It is concluded that both acceptor and donor side effects are involved in the pH -dependence of slow phase delayed lum inescence com ponents.