Control of the maximal chlorophyll fluorescence yield by the Q B binding site

Differences in maximal yields of chlorophyll variable fluorescence (F m ) induced by single turnover (ST) and multiple turnover (MT) excitation are as great as 40%. Using mutants of Chlamydomonas reinhardtii we investigated potential mechanisms controlling F m above and beyond the Q A redox level. F m was low when the Q B binding site was occupied by PQ and high when the Q B binding site was empty or occupied by a PSII herbicide. Furthermore, in mutants with impaired rates of plastoquinol reoxidation, F m was reached rapidly during MT excitation. In PSII particles with no mobile PQ pool, F m was virtually identical to that obtained in the presence of PSII herbicides. We have developed a model to account for the variations in maximal fluorescence yields based on the occupancy of the Q B binding site. The model predicts that the variations in maximal fluorescence yields are caused by the capacity of secondary electron acceptors to reoxidize Q A – .