Imidazolinone inhibition of acetohydroxyacid synthase in vitro and in vivo

Imidazolinone herbicides inhibit the first enzyme of branched‐chain amino acid biosynthesis, acetohydroxyacid synthase. The inhibition of the enzyme in vitro by these herbicides increases with incubation time, but is not irreversible, as deduced by reaction progress curves. In contrast to this result is the apparent irreversible inhibition of the enzyme by these imidazolinones that occurs when the herbicide is applied to intact corn plants. Plants treated with imidazolinone herbicide and then extracted showed dramatically reduced enzyme activity. This effect on extractable enzyme level occurred with several different imidazolinone herbicides in either foliar or soil application. The decrease in extractable enzyme activity could be observed within four hours after treatment. Herbicides other than imidazolinones did not reduce the extractable enzyme level. These findings suggest that the enzyme‐ imidazolinone interaction in vivo may be different from the interaction observed in vitro.