Abstract: Studies on the allelopathy of volatile substances of Ageratum conyzoides, animportant weed in South China, show that the volatiles of its fresh leaves significantly inhibited the seedling growth of all test plants.By means of chromatography, precocene Ⅰ and Ⅱ were isolated and identified, accounting for 30.93% and 51.61% of the volatile oil, respectively. These two substances in aqueous extracts of the weed originated from its volatiles, indicating that volatilization was the main mode by which the weed released allelochemicals. The saturated aqueous solution of the isolated and purified precocene Ⅰ and Ⅱ also significantly inhibited the seedling growth of radish, tomato and ryegrass. Especially, precocene Ⅱ had so strong allelopathic activity that it could inhibit the seedling growth at low concentration of 25μg·ml^-1 . The mixed solution of precocene Ⅰ and Ⅱ did not show a significantly increasing inhibition to plants, indicating that there was no synergistic effect between precocene Ⅰ and Ⅱ.

Key words: Ageratum conyzoides, Allelopathy, Releasing mode, Precocene, nifH gene, community composition, legume-oat intercropping, real-time PCR, high-throughput sequencing