Mitochondrial involvement in the mode of action of acifluorfen

The herbicidal action of acifluorfen {5-2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} was studied with greened and expanded discs from cotyledons of cucumber (Cucumis sativus L.). Discs were floated on various treatment solutions for 20 hr in darkness before exposure to 400 μE m^?2 sec^?1 of white light. Herbicide damage, as measured by electrolyte leakage, began in the light after a 1- to 2-hr lag period. Cytochemical methods at the ultrastructural level indicated that acifluorfen caused marked increases in production of superoxide radical and hydrogen peroxide in the mitochondrion, but not in the plastid. The mitochondrial inhibitors antimycin A, rotenone, CCCP, and DNP antagonized the action of acifluorfen, lengthening the lag period and reducing the rate of electrolyte leakage. Ethanol, α-tocopherol, N-2-(2-oxo-1-imidazolidinyl)ethyl]-N′-phenylurea, and copper-penicillin also lengthened the lag phase and slowed the rate of damage, indicating that acifluorfen damage involves toxic oxygen species. PS II-inhibiting levels of DCMU, atrazine, or bentazon did not affect acifluorfen-induced ion leakage. Yellow tissue produced by treatment with tentoxin was supersensitive to acifluorfen, but white tissue produced by treatment with norflurazon was relatively insensitive. These data indicate that, after an initial carotenoid-acifluorfen interaction, the mitochondrion is involved in production of toxic oxygen species and that this process is closely tied to the mechanism of action of this herbicide.