HERBICIDAL ACTIVITY OF UCC-C4243 AND ACIFLUORFEN IS DUE TO INHIBITIONOF PROTOPORPHYRINOGEN OXIDASE

Laboratory and greenhouse studies were conducted to determine the mode of action of soil- and foliar-applied UCC-C4243. Experiments demonstr ated that UCC-C4243 required light for phytotoxicity, phytotoxic sympt oms were similar to inhibitors of porphyrin synthesis such as acifluor fen, and UCC-C4243 potently inhibited protoporphyrinogen oxidase. Germ ination and emergence of field pennycress and lentil in the dark were not affected by soil-incorporated UCC-C4243 at rates more than 10 time s greater than like treatments that killed ah plants in the light. Soi l-incorporated UCC-C4243 required light for activity and killed seedli ngs within 1 d after emergence; sublethal doses caused desiccation, ve inal necrosis, and leaf deformation. Field pennycress and lentil were susceptible to soil-incorporated UCC-C4243 and acifluorfen in the ligh t, but were 5 to 93 times less sensitive to the herbicides in the dark . Wheat was not affected by either herbicide in the light or dark. Inj ury symptoms from UCC-C4243 applied POST to redroot pigweed were simil ar to symptoms from diphenyl ether and bipyridinium herbicides: rapid, light-dependent chlorophyll bleaching, desiccation, and necrosis. UCC -C4243, acifluorfen-methyl, and acifluorfen acid caused light- and con centration-dependent chlorophyll bleaching and electrolyte leakage fro m cucumber leaf disks (I-50 = 1.0, 1.8, and 4.3 mu M, respectively). A n inhibitor of the porphyrin synthesis pathway, 4,6-dioxoheptanoic aci d, almost completely inhibited herbicide-induced electrolyte leakage. delta-Aminolevulinic acid, a tetrapyrrole precursor and stimulator of the porphyrin synthesis pathway, caused synergistic effects with each herbicide. Protoporphyrinogen oxidase from barley etioplast preparatio ns was inhibited 50% by 40 nM UCC-C4243. Barley leaf sections treated with 100 mu M UCC-C4243 accumulated protoporphyrin IX in vivo to level s > 75 times nontreated controls. These data indicate the light-requir ing herbicide activity of UCC-C4243, like acifluorfen, is due to inhib ition of protoporphyrinogen oxidase.