Phenolic herbicides and their methylethers: Binding characteristics and inhibition of photosynthetic electron transport and photophosphorylation

Abstract

Phenolic herbicides exhibit triphasic binding characteristics which can be attributed to high-, middle-, and low-affinity binding. The middle-affinity binding constant corresponds to the inhibition constant of photosystem II electron transport. Inhibitory activity of phenolic herbicides drops by more than two orders of magnitude if the hydroxyl group is methylated. After covalent attachment of the photoreactive probe 2-azido-4-ethylamino-6-isopropylamino-1,3,5-triazine (azido-atrazine) to the thylakoid membrane, in a subsequent binding experiment with diuron no change in its binding affinity but a decrease in its number of binding sites is observed. Conversely, a subsequent middle-affinity binding of phenolic herbicides is characterized by a decrease in the binding constant but identical numbers of binding sites. This suggests different binding sites for both types of herbicides. Phenols, and to a lesser extent their methylethers, uncouple photophosphorylation. For phenol, this is in part due to their protonophoric properties which lead to a dissipation of the light-induced proton gradient across the thylakoid membrane. In addition, phenolic herbicides, but not their methylethers, inhibit exchange of tightly bound nucleotides at the chloroplast ATP synthase complex.