Physiological basis for antagonism of clethodim by CGA 362622

Greenhouse and laboratory experiments were conducted to determine the effect of CGA 362622 on the herbicidal activity of clethodim on goosegrass. CGA 362622 did not affect absorption and translocation of C-14-clethodim by goosegrass. Averaged across the two treatments of clethodim alone and clethodim plus CGA 362622, absorption was 27 and 85% of the applied C-14-clethodim at 0.5 and 96 h, respectively. By 96 HXF, only 0.8% of applied C-14 had translocated to the shoot below the treated leaf Metabolism of clethodim was not affected by the presence of CGA 362622. Three metabolites of clethodim were detected in treated tissue at all harvest intervals. By 96 HAT, 56% of absorbed C-14 converted to a relatively polar form when clethodim was applied alone or in the presence of CGA 362622. One day after treatment, the photosynthetic rate in plants treated with CGA 362622 had decreased below the rate in the nontreated check and remained lower until 6 d after treatment. These data suggest that the antagonism of clethodim by CGA 362622 may result from CGA 362622 altering the photosynthetic rate of goosegrass and therefore the sensitivity of acetyl-coenzyme A carboxylase to clethodim.