Fluorescence induction of Photosystem II membranes shows the steps till reduction and protonation of the quinone pool

Chlorophyll fluorescence induction (Chl-F) was investigated in Photosystem II (PSII)-enriched membranes, which predominantly include active (QB reducing) PSII reaction centres (RCs) and lack Photosystem I (PSI). The Chl-F curve of these preparations show a polyphasic rise from F0, the minimal fluorescence, to FP, the maximal fluorescence, with several intermediate transitions. Analyses of these transitions revealed three exponential rise components with lifetimes of 18 ms, 400 ms and 800 ms. The 18 ms component was assigned to the photoaccumulation of reduced QA. The two slowest components, of 400 ms and 800 ms, were assigned to QB reduction (QB− and QB=) and further QB= protonation (till QBH2), respectively. These assignments were based on the observation of specific quenching of the phases by DCMU or by different oxidized, reduced and protonated quinones. The work is done in low light conditions which are saturating to avoid photoinhibition or PSII inactivation effects. The results suggest that the Chl-F curve observed in PSII-enriched membranes can be attributed to the sequential steps till the photoaccumulation (reduction and protonation) of plastoquinone (PQ) by PSII. These results are in good agreement with the molecular models that show a correspondence between Chl-F and PQ reduction steps, like the models that propose and explain the O-J-I-P transients.