Interferences expressed by herbicides classified as inhibitory uncouplers were measured on the induction and maintenance of ΔpH and ΔΨ, the chemical and electrical components, respectively. of the proton motive force (pmf) generated by light-induced cyclic electron transport in spinach thylakoids. Maintenance of the pmf is required for the synthesis of ATP. The inhibitory uncouplers arc known to inhibit photophosphorylation, but the mechanisms involved remain to be identified. The dinoseb types (dinitrophenols. benzimidazoles. benzonitriles. bromophenoxim. perfluidone. thiadiazoles) of inhibitory uncouplers, most of which contain dissociable protons, were found to discharge ΔpΗ at low concentrations and to collapse ΔΨ at high concentrations. Collapse of ΔpΗ can be attributed to the protonophoric (proton shuttling) action of the herbicides. However, collapse of ΔΨ can be caused by alterations induced to the integrity and loss of semipermeability of the thylakoid membrane. As a result the membrane bccomes permeable to protons and other cations, and the electrical charges across the membrane are neutralized. The non-ionic dicryl types of inhibitory uncouplers (acylanilides. dinitroanilines. diphenylethcrs. bis-carbamates) collapsed ΔΨ at concentrations that were somewhat lower than those required for the collapse of ΔpΗ. These herbicides appear only to alter the integrity and permeability of the thylakoid membrane. Inhibition of photophosphorylation by the inhibitory uncouplcrs correlated with their ability to dissipate the pmf.
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