ABSTRACT
Few herbicides are known to inhibit a synthetic process by directly inhibiting the activity of an enzyme involved in that synthetic process. Thus, most literature dealing with herbicidal effects on plant constituent synthesis describes secondary or tertiary effects due to herbicidal effects on other processes, such as photosynthesis or respiration. Few commercially used herbicides are known to directly affect nonaromatic amino acid metabolism. Herbicides which cause photobleaching of chloroplasts also secondarily cause photodestruction of 70S ribosomes. Although many herbicides cause rapid and profound chlorosis, few are known to have any direct effects on chlorophyll synthesis. Many of the so-called "bleaching" herbicides cause their effects by inhibiting carotenoid synthesis or inducing carotenoid destruction. Few herbicides are known to exert their major phytotoxic effects through initially inhibiting a nonphotosynthetic, enzyme-controlled synthetic process. The sulfonylurea herbicides chlorsulfuron and sulfometuron methyl have been shown to block the synthesis of valine and isoleucine by inhibiting acetolactate synthase.
