Translocation and Metabolism of Pyridinyloxyphenoxypropionate Herbicides in Rhizomatous Quackgrass (Agropyron repens)

Abstract

Radiolabeled (14C) 2-[4-(3-chloro-5-trifluoromethyl-2-pyridinyloxy)phenoxy] propionic acid (Compound C); its methyl-,n-butyl-, and ethoxyethyl-esters; and fluazifop-butyl {(±)-butyl 2-[4-[(5-(trifluoromethyl)-2-pyridinyl)oxy] phenoxy] propionate} were applied to the primary shoots of young rhizomatous quackgrass [Agropyron repens(L.) Beauv. # AGRRE]. Plants were sampled from 0.5 to 24 days after treatment (DAT) and analyzed for radiochemical. All treatments caused phytotoxic symptoms in primary shoots, rhizomes, and tillers and significantly reduced growth of primary shoots and rhizomes. Treatment with compound C or its methyl and butyl esters eliminated regrowth from all rhizomes excised from treated plants. The ethoxyethyl-ester and fluazifop-butyl controlled regrowth from rhizomes from 50% of the plants and substantially reduced shoot regrowth from the remainder. Rhizomes that did produce shoots contained significantly less radiochemical than those from which no regrowth occurred. At 24 DAT, a maximum of only 1% of each radiochemical applied was translocated to the first tiller and rhizomes and had been sufficient to cause marked phytotoxic symptoms. Soon after application (1 DAT), the major metabolites in the treated leaves and remainder of the plant for all compounds were the free acid and substantial amounts of polar conjugates that were hydrolyzable to the free acids. Radiochromatography of extracts from rhizomes and first tillers from all treatments gave similar chemical profiles, with the free acids and their conjugates as the predominant components present. These results provide evidence that esters of the pyridinyloxyphenoxypropionic acids are rapidly hydrolyzed after absorption by quackgrass; the free acids are then translocated and are the active form of these herbicides.