| Summary: | Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species <i>Orobanche cumana</i> and <i>Phelipanche ramosa</i> was studied. Buckwheat root exudates induced a significant growth inhibition in <i>P. ramosa</i> radicles but radicles of <i>O. cumana</i> were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene <i>p</i>-coumaric acid methyl ester with only quercetin showing inhibitory effect on <i>P. ramosa</i>. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-<i>O</i>-acetylpadmatin extracted respectively from <i>Lavandula stoechas</i> and <i>Dittrichia viscosa</i> plants. In this comparative assay only 3-<i>O</i>-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two <i>ortho</i>-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of <i>P. ramosa</i> radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in <i>P. ramosa</i> was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-<i>p</i>-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that <i>P. ramosa</i> haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM.
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