The weedy annual grass Lamium amplexicaule (L.) originated from the Mediterranean region, spreads quickly, and becomes naturalized in areas it invades. It was assumed that volatile phytotoxic substances are one of the invasive characteristics of L. amplexicaule. However, no volatile compound has been found. Therefore, we re-evaluated the allelopathic potential of L. amplexicaule and investigated the phytotoxic substances with allelopathic activity. An extract of L. amplexicaule inhibited the root and shoot growth of Lepidium sativum, Lactuca sativa, Arctium lappa, Lolium multiflorum Lam., Echinochloa crus-galli, and Vulpia myuros, which indicates that the extract has an allelopathic effect. The extract was purified using bioassay-guided chromatographic separations, and a phytotoxic substance with allelopathic activity was isolated and characterized as methyl caffeate. The compound significantly inhibited the root and shoot growth of L. sativum and L. multiflorum. The present results suggest that methyl caffeate may contribute to the allelopathic effect of the L. amplexicaule extracts and consequently, may be partly responsible for the invasive characteristics of the species.
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