Studies on the mechanism of selectivity of the auxin herbicide quinmerac

Investigations were conducted to elucidate the mechanism of selectivity of the auxin herbicide, quinmerac, in cleavers (Galium aparine) and the tolerant crops sugarbeet (Beta vulgaris), oilseed rape (Brassica napus) and wheat (Triticum aestivum). After root treatment with the herbicide, the selectivity has been quantified as approximately 400-fold between oilseed rape and Galium and 1000-fold between sugarbeet or wheat and the weed species. When 1 and 10 μM [ 14 C]quinmerac were applied for 4 h, no significant differences between root absorption and translocation of 14 C by Galium and the crop species were found. After 16 h, metabolism of [ 14 C]quinmerac to the biologically inactive hydroxymethyl and dicarboxylic acid derivatives was more rapid in wheat and sugarbeet than in Galium. In oilseed rape, a lower rate of herbicide metabolism 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 of application. Within 1 h of treatment, quinmerac stimulated 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and ACC concentration specifically in Galium shoot tissue. During the next 4 h, ACC synthase activity was increased up to 50-fold, relative to the control. Within 3 h of exposure to quinmerac, increased ethylene formation followed by higher ABA levels was detected. In sugarbeet, oilseed rape and wheat, quinmerac did not stimulate ACC synthase activity and ACC and ABA levels. It is suggested that (i) the selectivity of quinmerac is primarily based upon the lower sensitivity to the herbicide of the tissue/target in the crop species, (ii) the induction process of the ACC synthase activity in the shoot tissue is the primary target of herbicidal interference. In wheat and sugarbeet, tolerance to quinmerac is additionally increased by a more rapid metabolism.