Alteration of the stromal side of photosystem II by diphenylcarbazide

Abstract

In the thylakoid membranes, the photosystem II electron donor diphenylcarbazide decreases the inhibitory effect of quinone-type herbicides like atrazine (Purcell et al., 1990, Pest. Biochem. Physiol. 37, 83–89). This behavior is studied in more detail and it is shown that the donor strongly inhibits the binding of radiolabelled atrazine to the Q B protein. The electron acceptor activity of 2,6-dichlorobenzoquinone (DCBQ) is also impaired but not that of potassium ferricyanide or 2,6-dichlorophenolindophenol (DCIP). Electron transport from photosystem II to photosystem I, which involves the reduction of plastoquinone at the Q B site, is only slightly affected. This data indicated that at concentrations where it is commonly used as an electron donor ( 0.5 ∼ 1.0 mM ), diphenylcarbazide can alter the binding of herbicides and artificial electron acceptors having overlapping-binding domains in the Q B pocket. However, the effect of diphenyl-carbazide on the Q B site can affect plastoquinone reduction only at relatively high concentrations ( > 2 mM ).