Studies into the action of the diphenyl ether herbicides acifluorfen and oxyfluorfen. Part II: The interaction with photosynthetic electron transport reactions

The effects of acifluorfen and oxyfluorfen on photosynthetic electron transport reactions of pea chloroplasts were compared with those induced by paraquat and monuron. Monuron inhibited electron flow between photosystems I and II, and paraquat acted as an electron acceptor for photosystem I, promoting superoxide formation by illuminated chloroplasts. Neither acifluorfen nor oxyfluorfen at concentrations up to 50 μM affected non‐cyclic electron flow or promoted superoxide formation. Both herbicides were shown to repress ferredoxin‐dependent NADP+ reduction by illuminated chloroplasts. Further experiments showed that, in the presence of ferredoxin‐NADP+ reductase and chloroplast membranes maintained in the dark, p‐nitro diphenyl ether (DPE) herbicides promoted the rate of ferredoxin‐dependent oxidation of NADPH, implying that these herbicides can accept electrons from reduced ferredoxin. The interaction between acifluorfen, ferredoxin and chloroplast membranes was examined further by following the effect of this herbicide on the peroxidation of illuminated thylakoids. Lipid peroxidation was promoted by acifluorfen, although this effect was abolished if thylakoids were washed prior to use. The effect of washing could be reversed by adding exogenous ferredoxin. These data demonstrate that interaction of DPE herbicides with photosynthetic electron transport in the vicinity of ferredoxin is necessary for light‐dependent herbicide activation.