STUDIES ON THE MECHANISM OF SELECTIVITY OF THE AUXIN HERBICIDE QUINMERAC

Investigations were conducted to elucidate the mechanism of selectivit y of the auxin herbicide, quinmerac, in cleavers (Galium aparine) and the tolerant crops sugarbeet (Beta vulgaris), oilseed rape (Brassica n apus) and wheat (Triticum aestivum). After root treatment with the her bicide, the selectivity has been quantified as approximately 400-fold between oilseed rape and Galium and 1000-fold between sugarbeet or whe at and the weed species. When 1 and 10 mu M [C-14]quinmerac were appli ed for 4 h, no significant differences between root absorption and tra nslocation of C-14 by Galium and the crop species were found. After 16 h, metabolism of [C-14]quinmerac to the biologically inactive hydroxy methyl and dicarboxylic acid derivatives was more rapid in wheat and s ugarbeet than in Galium. In oilseed rape, a lower rate of herbicide me tabolism was observed. In Galium, accumulations of abscisic acid (ABA) , triggered by quinmerac-stimulated ethylene biosynthesis, were found to cause the herbicidal growth inhibition which develops during 24 h o f application. Within 1 h of treatment, quinmerac stimulated 1-aminocy clopropane-1-carboxylic acid (ACC) synthase activity and ACC concentra tion specifically in Galium shoot tissue. During the next 4 h, ACC syn thase activity was increased up to 50-fold, relative to the control. W ithin 3 h of exposure to quinmerac, increased ethylene formation follo wed by higher ABA levels was detected. In sugarbeet, oilseed rape and wheat, quinmerac did not stimulate ACC synthase activity and ACC and A BA levels. It is suggested that (i) the selectivity of quinmerac is pr imarily based upon the lower sensitivity to the herbicide of the tissu e/target in the crop species, (ii) the induction process of the ACC sy nthase activity in the shoot tissue is the primary target of herbicida l interference. In wheat and sugarbeet, tolerance to quinmerac is addi tionally increased by a more rapid metabolism (C) 1998 SCI.