Fate of quinclorac in resistant Echinochloa crus-galli

Barnyardgrass (Echinochloa crus-galli) is one of the most troublesome weeds in rice and causes serious yield losses. One of the main herbicides used in this crop is quinclorac. Quinclorac, a quinolinecarboxylic acid, causes inhibition of shoot growth accompanied by chlorosis and necrosis. These effects, observed in Echinochloa shoots, are closely correlated with the accumulation of endogenous HCN formed as a co-product during quinclorac-stimulated ethylene biosynthesis. One population off. crus-galli (R) that exhibited resistance to quinclorac was harvested from a rice monoculture in Southern Spain. A susceptible (S) population off. oryzoides was collected from a location which had never been treated with herbicides. The ED50 value (dose causing 50% reduction in shoot flesh weight) for the R population was 26-fold greater than for the susceptible Echinochloa. Usually, herbicide resistance mechanisms include differences in absorption and/or translocation of the herbicide, mutation of the target site, or metabolism of herbicide to non-toxic products. Laboratory studies conducted on the R and S Echinochloa and rice plants using C-14-quinclorac applied to leaves indicated that foliar absorption was not different between the Echinochloa populations and rice 72 h after herbicide application. In all cases, translocation from the treated leaves to lower and upper leaves was minimal, and no C-14-quinclorac translocation into the roots was observed. Metabolic studies using C-14-quinclorac applied to the leaves of R and S Echinochloa and rice indicated that the herbicide was not metabolized after 72 h of application. Studies carried out on ethylene production indicated that the application of 10(-4) M quinclorac via the roots induced ethylene production in the S population but did not have any effect on ethylene biosynthesis in R Echinochloa or rice.