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Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II ¹

Northern Regional Research Center, Agricultural Research Service, United States Department of Agriculture, Peoria, Illinois 61604, Department of Botany, University of California, Davis, California 95616

One hypothesis of triazine-type herbicide action in photosynthetic material is that the herbicide molecule competes with a secondary quinone acceptor, B, for a binding site at the reaction center of photosystem II. The binding affinity of B has been suggested to change with its level of reduction, being most strongly bound in its semiquinone form. To test this hypothesis, [¹⁴C]atrazine binding studies have been carried out under different photochemically induced levels of B reduction in Pisum sativum. It is found that herbicide binding is reduced in continuously illuminated samples compared to dark-adapted samples. Decreased binding of atrazine corresponds to an increase in the semiquinone form of B. With flash excitation, the herbicide binding oscillates with a cycle of two, being low on odd-numbered flashes when the amount of semiquinone form of B is greatest. Treatment with NH₂OH was found to significantly decrease the strength of herbicide binding in the dark as well as stop the ability of p-benzoquinone to oxidize the semiquinone form of B. It is suggested that the mode of action of NH₂OH is disruption of quinones or their environment on both the oxidizing and reducing sides of photosystem II. Herbicide binding was found to be unaltered under conditions when p-benzosemiquinone oxidation of the reduced primary acceptor, Q^–, is herbicide insensitive; weak herbicide binding cannot explain this herbicide insensitivity. It is concluded that the quinone-herbicide competition theory of herbicide action is correct. Also, since quinones are lipophilic the importance of the lipid composition of the thylakoid membrane in herbicide interactions is stressed.