EFFECT OF FENOXAPROP, HALOXYFOP AND SETHOXYDIM ON ACETYL COENZYME-A CARBOXYLASE FROM RICE, BARNYARDGRASS AND SPRANGLETOP

Acetyl coenzyme A carboxylase (ACCase), the target site inhibited by two groups of herbicides, the aryloxyphenoxypropionates and cyclohexanediones, was extracted and partially purified from rice (Oryza sativa LeMont L.), barnyardgrass (Echinchloa crusgalli (L.) Beauv.) and sprangletop (Leptochloa fascicularis (Lam.) Gray). This was to determine whether the lack of inhibition of rice to three grass toxic herbicides, haloxyfop, fenoxaprop and sethoxydim is due to a differential selectivity at the enzyme level. Incorporation rates of NaH14CO3 into malonyl coenzyme A (CoA) were linear up to 8 min and maximum specific activity was obtained at 0.1 -0.2 mg protein. Inhibition of ACCase from rice and the two weed species by haloxyfop and fenoxaprop showed differences that disappeared with higher concentrations of herbicide. I50 values for haloxyfop inhibition of ACCase from rice, barnyardgrass and sprangletop were 3.50, 2.50 and 2.40-mu-M, respectively. I50 values for fenoxaprop inhibition of enzyme from rice, barnyardgrass and sprangletop were 0.63, 0.33 and 0.33-mu-M, respectively. ACCase from rice was more sensitive to sethoxydim than from the two grass weeds. I50 values for rice, barnyardgrass and sprangletop were 6.18, 6.76 and 7.23-mu-M, respectively. These results suggest that selectivity of fenoxaprop and haloxyfop in rice may be in part attributable to a decreased sensitivity of ACCase, but this cannot totally account for the margin of tolerance exhibited in the field.