New results on the properties of photosystem II centers blocked by 3-(3,4-dichlorophenyl)-1,1-dimethylurea in their different photoactive states.

We have studied the 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) action on the different S states by oxygen, fluorescence and luminescence measurements. We show that no oxygen is evolved during a flash following the addition of DCMU to centers in their S3 state. This suggests that oxygen inhibition cannot be attributed solely to a blocking between Q and A. For all the photoinactive states, the only remaining pathway for the quencher reoxidation, in the presence of DCMU, appears to proceed through a back reaction. Therefore, the complete quencher regeneration still occurring when the fourth positive charge is formed in the presence of DCMU is also an indication of an action by DCMU at the donor side. The data well fit the model in which the oscillations of the fluorescence yield and their damping are attributed to a fast equilibrium between two forms of the centers: a photoactive and a photoinactive form, both of which are quenchers. The equilibrium constant depends on the number of positive charges stored and DCMU changes the characteristics of this equilibrium.