A specific and sensitive assay to quantify the herbicidal activity of chloroacetamides

Based on a recently discovered property of chloroacetamide herbicides—the inhibition of the incorporation of oleic acid into sporopollenin of Scenedesmus acutus—a rapid quantitative test was developed for chloroacetamide‐type herbicidal activity. In this test, algal cells are incubated for 3 h with [14C]oleic acid, saponified and the lipids (including non‐saponifiable ones) extracted and discarded. The radioactivity incorporated into the residual non‐lipid fraction is determined, and inhibition of this incorporation is used as a marker of chloroacetamide‐type activity. Twenty‐two agrochemical compounds were screened in this assay, which was found to be very sensitive, a 50% inhibition being reached with submicromolar herbicide concentrations. It is specific to chloroacetamides and related amides, since all these herbicides tested were potent inhibitors, while other herbicides were not. Highest inhibition was shown by cafenstrole followed by butachlor, fluthiamid, metazachlor, alachlor, dimethenamid, metolachlor and mefenacet. For these herbicides (with the exception of butachlor) sensitivity in the test was positively correlated (r=0·984) with their phytotoxic effect on the alga. © 1998 SCI