The interaction of silicomolybdate with the photosystem II herbicide‐binding site

Silicomolybdate and silicotungstate are unique among photosystem II electron acceptors in that they catalyze photosystem II‐dependent electron transfer in thylakoid membranes that is resistant to inhibition by diuron. On this basis it is generally accepted that these compounds interact with the photosystem II electron transfer sequence prior to the site of diuron inhibition thereby creating an even more abbreviated partial reaction than is found with other photosystem II electron acceptors. The data presented here demonstrate that low concentrations of the silicomolybdate anion (10–30 μM) inhibit the binding of [ 14 C]diuron to spinach thylakoid membranes. This effect of silicomolybdate is reversible and is a function of the redox state of silicomolybdate. Reduction of silicomolybdate leads to a loss of its ability to prevent herbicide binding. Thus, there is no reason to believe that silicomolybdate intercepts electrons from photosystem II prior to the site of diuron intervention or at a site different from any other photosystem II electron acceptor.