The Mechanism of Quinclorac Selectivity in Grasses

Abstract

The mechanism of selectivity of the auxin herbicide quinclorac was studied in susceptible and resistant biotypes of Echinochloa hispidula and E. crus-galli; in grass species of different susceptibility like Brachiaria platyphylla, Setaria viridis, E. colonum, and Digitaria sanguinalis; and in tolerant crop rice (Oryza sativa cv. Thaibonnet). No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, the mechanism of selectivity is target-site-based. It has been demonstrated that cyanide, which accumulates as a coproduct during stimulation of ethylene biosynthesis, is the primary phytotoxic principle in the herbicidal mode of action of quinclorac in E. crus-galli (K. Grossmann and J. Kwiatkowski, 1995, Pestic. Biochem. Physiol. 51, 150). Following root treatment of plants at the third leaf stage with 100 M quinclorac in hydroponics, the reductions in shoot growth of plants were closely correlated with the stimulation of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and with subsequent increases in endogenous concentrations of ACC and cyanide. When compared quinclorac with other auxin herbicides including quinmerac, α-naphthaleneacetic acid and dicamba, lower ACC and cyanide levels and correlated lower growth effects were found after treatment of D. sanguinalis. In quinclorac-resistant grass species and biotypes, ACC synthase activity was not induced and plants exhibited no significant changes in ACC and cyanide levels. Thus, the selective induction of ACC synthase activity, which ultimately leads to cyanide accumulation in the tissues of susceptible plants, plays the primary role for the selectivity of grasses toward quinclorac. This selectivity could be at the level of quinclorac perception or subsequent signal transduction via the auxin pathway leading to regulation of gene expression or posttran scriptional de novo enzyme synthesis. In rice, tolerance to quinclorac could be additionally favored by a higher β-cyanoalanine synthase activity, the main HCN-detoxifying enzyme.