In vitro sensitivity of wheat and oat mitochondria to the selective herbicide,diclofop-methyl

Compared to diclofop-methyl (methyl 2-4-(2′,4′-dichlorophenoxy)phenoxy]propanoate), diclofop (the demethylated derivative) was a more potent inhibitor of polarographically monitored state 3 respiration of mitochondrial preparations isolated from shoots of dark-grown wheat (Triticum aestivum L. cv. Neepawa) and oat (Avena sativa L. cv. Terra) seedlings. Wheat and oat mitochondria demonstrated essentially similar concentration-response patterns for the uncoupler-like stimulation of state 4 respiration and the inhibition of state 3 respiration by diclofop, thereby intimating that differential mitochondrial sensitivity was not a selectivity factor between these species. Diclofop suppression of unconstrained oxygen utilization elicited by the respiratory uncoupler FCCP indicated that inhibition of state 3 respiration involved interference with some site(s) on the mitochondrial electron transport chain and not with energy transfer directly. Cytochrome c oxidase activity was not affected by diclofop, but succinate- and malate-PMS oxidoreductase activities were inhibited by diclofop. Enhanced rates of passive mitochondrial swelling in isotonic KCl medium in the presence of diclofop pointed to a direct influence on the permeability properties of the inner mitochondrial membrane and indicated that membrane disruption could have been a factor in the effects elicited by diclofop on mitochondrial respiration. However, it does not appear that specific interference with mitochondrial functionality is the primary mechanism of phytotoxicity in susceptible plants.