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

Binding of radioactively labelled DCMU, ioxynil and terbutryn to spinach chloroplasts is determined following preillumination by single-turnover, saturating light flashes. With all of the three herbicides binary oscillations of binding are observed. Dark-adapted samples, or those preilluminated by an even number of flashes, bind more inhibitor than samples preilluminated by an odd number of flashes. Binding oscillations depend on inhibitor incubation time and on the seasonal adaptation of the plants. The binding kinetics following a single flash display three phases, the last two of which can be correlated with reoxidation kinetics of the secondary photo- system II acceptor Q - в , as determined by fluorescence measurements. Analysis of the binding at DCMU concentrations up to 10 -7 m free DCMU yields identical binding constants for dark- and 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 do not contribute significantly to total binding. Displacement of Q - в from the binding site is half-saturated at about 10 -6 M DCMU, as monitored via fluorescence induction. The data are considered strong support for the Velthuys ‘inhibitor- Q - в competition model’.