Selectivity and mode of action of carfentrazone-ethyl, a novel phenyl triazolinone herbicide

Post-emergence application of carfentrazone-ethyl at rates as low as 2·2 g ha-1 caused greater leaf injury and growth reduction in ivyleaf morningglory (Ipomoea hederacea) and velvetleaf (Abutilon theophrasti) than in soybean (Glycine max). The herbicide was more rapidly metabolized in the crop than in the weed species, with 26·7, 54·3 and 60·6% of the parent compound remaining in soybean, ivyleaf morningglory and velvetleaf, respectively, 24 h after exposure. The free acid metabolite, carfentrazone, was present in all species and accounted for 21·2–27·4% of the total radioactivity. Unknown metabolites (Rf 0 and 0·22) were four to five times more abundant in soybean than in the weed species. Carfentrazone-ethyl induced more leakage from leaf discs from the weeds than those from soybean and the degree of injury correlated with the amount of protoporphyrin IX (Proto IX) present in the treated tissues. Both carfentrazone-ethyl and carfentrazone were potent inhibitors of protoporphyrinogen oxidase (Protox). Therefore, the selectivity of this herbicide may, at least in part, be attributed to the lower accumulation of Proto IX in soybean than in the weeds, probably because of the ability of soybean to metabolize more carfentrazone into unknown metabolites than the weeds. © 1997 SCI.