Replacement of photosynthetic electron transport inhibitors by silicomolybdate

KCN-treated spinach chloroplasts, their photosystem I being ineffective, exhibit a single reaction site for silicomolybdate. Using this heteropolyanion as electron acceptor, photosynthetic oxygen evolution is partially inhibited by ureas, triazines, or phenylpyridazinone herbicides, their inhibitory effect depending on the concentration of silicomolybate. Labelled atrazine attached to isolated chloroplast material is competitively replaced by silicomolybdate in the same manner as e.g. ureas complete with a triazine herbicide. – It is concluded (1) that silicomolybdate is bound and reduced at the herbicide-binding protein, and (2) that the inhibition of silicomolybdate reduction by herbicides such as DCMU is due to loss of reaction sites for silicomolybdate.