Herbicidal potential and quantification of causative allelochemicals from several Compositae weeds

Summary Compositae plants contain biologically active substances that are allelopathic to weed species. Aqueous extracts from leaves of 16 plants were bioassayed against lucerne (Medicago sativa) to determine their allelopathic effects, and the results showed the highest inhibition for the extracts from Lactuca sativa, Xanthium occidentale and Cirsium japonicum. The extracts applied to filter paper in Petri‐dish bioassay tests significantly inhibited root growth of lucerne. Extracts of 40 g dry tissue L−1 from L. sativa, X. occidentale and C. japonicum were completely inhibitory to lucerne root growth, but hypocotyl growth of lucerne was less sensitive. Although allelopathic effects of methanol extracts were much less than those of coumarin or alachlor, early seedling growth of both lucerne and Echinochloa crus‐galli was significantly reduced by methanol extracts. Mixture of L. sativa, X. occidentale and C. japonicum extracts had more inhibitory effects on test plants than each single extract treatment. By means of high‐performance liquid chromatography, responsible causative allelopathic substances present in L. sativa, X. occidentale and C. japonicum were isolated from various fractions and identified as coumarin, trans‐cinnamic acid, o‐coumaric acid and p‐coumaric acid. These results suggest that some Compositae have various herbicidal potentials, and that their activities, types and amount of causative compounds differ, depending on the plant species.