Potential involvement of alkoxyl and hydroxyl radicals in the peroxidative action of oxyfluorfen

The potential involvement of hydroxyl and alkoxyl radicals in the peroxidative action of the p-nitro diphenyl ether herbicides acifluorfen (5-2-chloro-4-(trifluoromethyl)phenoxyl]-2-nitrobenzoic acid), acifluorfen-methyl (methyl ester of acifluorfen), nitrofen 2,4-dichloro-1-(4-nitrophenoxy)benzene], nitrofluorfen 2-chloro-1-(4-nitrophenoxy)-4-(trifluoromethyl)benzene], and oxyfluorfen 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] was evaluated under laboratory conditions. Methional was added to illuminated thylakoids from peas (Pisum sativum L., cv Little Marvel) and its oxidation to ethylene was used as an indicator of hydroxyl and alkoxyl radical production. Oxyfluorfen stimulated the rate of methional oxidation by 138% at 10 μM and 175% at 1 mM. This oxyfluorfen-induced stimulation of the rate of methional oxidation was dependent on light, photosynthetic electron transport, and hydrogen peroxide since it was not observed under dark conditions or in the presence of DCMU and catalase. Addition of Fe-EDTA, a catalyst of the Fenton reaction, stimulated the oxyfluorfen-induced enhancement of methional oxidation sixfold, suggesting that hydroxyl radicals are synthesized through a Fenton reaction. Acifluorfen, nitrofen, and nitrofluorfen inhibited the rate of methional oxidation whereas acifluorfen-methyl had no effect on the rate of methional oxidation, even at high concentrations (1 mM). Nitrofluorfen at 1 mM was the only p-nitro diphenyl ether herbicide tested to inhibit photosynthetic electron transport of pea thylakoids. In experiments with pea leaf disks, acifluorfen at low concentrations stimulated the rate of methional oxidation, whereas acifluorfen-methyl, nitrofen, and nitrofluorfen had no effect. These data indicate that hydroxyl and alkoxyl radicals could be involved in the mechanism of cellular damage caused by oxyfluorfen but they are not important for the activity of the diphenyl ether herbicides acifluorfen, acifluorfen-methyl, nitrofen, and nitrofluorfen.