MECHANISM OF ACTION AND SELECTIVITY OF QUINCLORAC IN GRASS-ROOTS

The mechanism of action and selectivity of quinclorac (3,7-dichlaroqui nolinecarboxylic acid) were investigated by comparing the inhibitory e ffect of the herbicide on [C-14]glucose incorporation into the root ce ll walls of susceptible and resistant grasses and by measuring root ac cumulation of [C-14]quinclorac. The response to quinclorac in two susc eptible 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 elongatio n in susceptible grasses at 10 mu M. At the same herbicide concentrati on, cell wall biosynthesis in the susceptible grasses was reduced by 6 0 to 73% after a 6-hr treatment, but inhibited by only 20 to 36% in th e tolerant grasses. increasing treatment time to 24 hr had little addi tional response. The effect of quinclorac on cell wall synthesis was d ose 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 seve ral hemicellulose fractions, were inhibited 30 to 50% more than those of rice. Root accumulation of [C-14]quinclorac was greatest in barnyar dgrass and rice and lowest in the two biotypes of smooth crabgrass. Th ese results are consistent with a mechanism of action in which quinclo rac acts as an inhibitor of cell wall biosynthesis in sensitive grasse s. Selectivity among different grasses could be explained by different ial sensitivity at the site of action. (C) 1997 Academic Press.