Mechanism of action and selectivity of quinclorac in grass roots

The mechanism of action and selectivity of quinclorac (3,7-dichlaroquinolinecarboxylic acid) were investigated by comparing the inhibitory effect of the herbicide on [C-14]glucose incorporation into the root cell walls of susceptible and resistant grasses and by measuring root accumulation of [C-14]quinclorac. The response to quinclorac in two susceptible grasses, barnyardgrass and smooth crabgrass, was compared to a tolerant species (rice) and a resistant biotype of smooth crabgrass. In whole plant studies, quinclorac completely inhibited root elongation in susceptible grasses at 10 mu M. At the same herbicide concentration, cell wall biosynthesis in the susceptible grasses was reduced by 60 to 73% after a 6-hr treatment, but inhibited by only 20 to 36% in the tolerant grasses. increasing treatment time to 24 hr had little additional response. The effect of quinclorac on cell wall synthesis was dose dependent between 0 and 10 mu M and corresponded closely with the dose-dependent response in root growth of susceptible grasses. All the cell wall constituents in barnyardgrass, including cellulose and several hemicellulose fractions, were inhibited 30 to 50% more than those of rice. Root accumulation of [C-14]quinclorac was greatest in barnyardgrass and rice and lowest in the two biotypes of smooth crabgrass. These results are consistent with a mechanism of action in which quinclorac acts as an inhibitor of cell wall biosynthesis in sensitive grasses. Selectivity among different grasses could be explained by differential sensitivity at the site of action. (C) 1997 Academic Press.