EVIDENCE FOR A CAUSATIVE ROLE OF CYANIDE, DERIVED FROM ETHYLENE BIOSYNTHESIS, IN THE HERBICIDAL MODE OF ACTION OF QUINCLORAC IN BARNYARD GRASS

Root application of the quinolinecarboxylic acid herbicide quinclorac to barnyard grass ((Echinochloa crus-galli (L.) P. Beauv.) caused inhi bition of shoot growth accompanied by chlorosis and necrosis. After 1 day of exposure, cyanide levels in the shoot tissue increased accordin g to the concentration of the herbicide and the time of its applicatio n and closely correlated with the reduction in shoot fresh weight. Fou r days after the application of 10 and 100 mu M quinclorac, the respec tive cyanide levels in the shoot tissue were approximately two and thr ee times higher than controls and reached a maximum of 40 mu M. Increa ses in beta-cyanoalanine synthase activity, the main HCN detoxifying e nzyme, ethylene production, and in the endogenous levels of 1-aminocyc lopropane-1-carboxylic acid (ACC) preceded cyanide accumulation, When ACC was exogenously supplied to detached shoots of barnyard grass via the vascular system, its accumulation within the tissue coincided with increases in ethylene formation and cyanide content. Both metabolites of ACC appeared to be formed in stoichiometrically equivalent amounts . Root treatment of intact plants with KCN caused concentration-depend ent increases in the levels of cyanide found in the shoot tissue which inversely correlated with the changes in fresh weight. The concentrat ions of quinclorac or KCN applied via the roots which led to twofold i ncreases in the cyanide levels of the shoots had similar effects on th e shoot fresh weight, causing reductions of nearly 30%. The phytotoxic symptoms, chlorosis and necrosis, were also similar. In contrast, phy totoxic tissue sensitivity to ethylene released by applied 2-chloroeth ylphosphonic acid (ethephon) was only very low. It is concluded that c yanide, derived ultimately from quinchlorac-stimulated ACC synthesis, is a causative factor in the herbicidal effects observed in the shoots of barnyard grass. (C) 1995 Academic Press, Inc.