Allelopathic potential of diffuse knapweed (Centaurea diffusa) extracts

An in vitro survey was conducted to determine the allelopathic potential of diffuse knapweed, a noxious weed invading the Pacific Northwest. Knapweed extracts were prepared using various solvents and inhibitory activity was bioassayed by measuring ryegrass seed germination and seedling growth. Inhibitory activity was found mainly in chloroform- and water-soluble extracts of knapweed. The impact of the latter was not due to osmotic effects as shown by osmolarity measurements or total cation concentration. When the chloroform extract of shoots was purified by column chromatography, inhibition was associated with fractions containing sesquiterpene lactones. Inhibitor levels were lower in knapweed roots than in shoots. In addition to different solubilities, the polar and nonpolar inhibitors showed different elution profiles on Sephadex G-10.Key words: Centaurea diffusa, knapweed, allelopathy, seed bioassays, ryegrass