EFFECT OF CHLORIMURON AND QUIZALOFOP ON FATTY-ACID BIOSYNTHESIS

Chlorimuron antagonized the activity of quizalofop on broadleaf signalgrass when applied in greenhouse studies as a postemergence tank mix. In vitro leaf disc assays utilizing C-14-acetate or C-14-pyruvate as substrates were conducted to ascertain the effect of clorimuron and quizalofop on fatty acid biosynthesis and to determine if antagonism between the two herbicides occurs at the biochemical sites of action. Incorporation of C-14-acetate in control treatments was linear with time to 120 min. Acetate incorporation in the presence of quizalofop (1.1-mu-M) was also linear but was inhibited 30 min after initialization of the reaction. The concentration of quizalofop that inhibited C-14-acetate incorporation 50% (I50) was 0.54-mu-M. Chlorimuron, up to 155-mu-M, had no effect on C-14-acetate incorporation. A mixture of quizalofop (1.1-mu-M) and chlorimuron (4.8-mu-M) inhibited C-14-acetate incorporation similar to that of quizalofop alone at 1.1-mu-M. Quizalofop I50 for incorporation of C-14-pyruvate was 1.1-mu-M, and clorimuron at 4.8-mu-M decreased incorporation 15%. Excess unlabeled pyruvate (5-mu-M) had no effect on either C-14-acetate or C-14-pyruvate incorporation in the presence of both herbicides. It is believed that antagonism of quizalofop by clorimuron is not due to an excess pool of pyruvate resulting from inhibition of acetolactate synthase by chlorimuron. Nomenclature: Chlorimuron, 2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl)amino]carbonyl]amino] sulfonyl] benzoic acid; quizalofop, (+/-)-2-[4[(6-chloro-2-quinoxalinyl)oxy]phenoxy] propanoic acid; broadleaf signalgrass (Bracharia platyphylla (Griseb.) Nash #3 BRAPP).