Redox-State Dependent Changes of Inhibitor-Binding to the Photosystem II Acceptor Complex

Inhibitor Binding, Photosystem II, Binary Oscillation, Chlorophyll Fluorescence Binding o f radioactively labelled DCM U, ioxynil and terbutryn to spinach chloroplasts is determined following preillumination by single-turnover, saturating light flashes. W ith all o f the three herbicides binary oscillations o f binding are observed. Dark-adapted samples, or those pre­ illuminated by an even number o f flashes, bind more inhibitor than samples preilluminated by an odd number o f flashes. Binding oscillations depend on inhibitor incubation tim e and on the seasonal adaptation o f the plants. The binding kinetics following a single flash display three phases, the last two of which can be correlated with reoxidation kinetics o f the secondary photo­ system II acceptor Qb, as determined by fluorescence measurements. Analysis o f the binding at DCMU concentrations up to 10"7 M free DCM U yields identical binding constants for darkand flash preilluminated samples, but much less binding sites following one flash. It is concluded that up to 10~7 M free DCMU, centers with bound Q b do not contribute significantly to total binding. Displacement o f Q b from the binding site is half-saturated at about 10-6 M DC M U , as monitored via fluorescence induction. The data are considered strong support for the Velthuys ‘inhibitorQ i competition model’.