Comparative Binding of Photosystem II — Herbicides to Isolated Thylakoid Membranes and Intact Green Algae

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Abstract
The binding of the photosystem II herbicides diuron (DCMU), atrazine (s-triazine), ioxynil and dinoseb (substituted phenols) to isolated spinach thylakoids was saturated in less than 2 min in the dark. In intact cells of the green alga Ankistrodesmus b. it took 10 to 20 min to reach the binding equilibrium. Binding affinity of diuron, atrazine, dinoseb, measured as equilibrium binding constants, was found to be comparable in isolated thylakoids and intact algal cells. For ioxynil, reduced binding affinity was observed in algae. The concentration of binding sites in thylakoids and intact cells was determined to be 300-500 chl/inhibitor binding site, suggesting a 1:1 stoichiometry between bound herbicide and electron transport chains. In intact cells only the phenol herbicides ioxynil and dinoseb showed increased concentrations of binding sites. Strong correlation of herbicide binding and inhibition of electron transport was found for diuron in isolated thylakoids and intact cells. In thylakoids this is valid also for atrazine and dinoseb. For ioxynil a difference between the amount of binding and inhibition was found. This correlation of herbicide binding and inhibition proves that binding specifically occurs at the inhibition site at photosystem H. In addition to the specific binding, for all four herbicides studied, (except for ioxynil in thylakoids) unspecific binding was observed in thylakoids as well as in algae, which was not related to inhibition.

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