The extraction procedure improves the allelopathic activity of cardoon (Cynara cardunculus var. altilis) leaf allelochemicals

Cultivated cardoon [Cynara cardunculus L. var. altilis (DC)] has been found to manifest phytotoxic activity on weeds and standard target species. The sesquiterpene lactones cynaripicrin, groshemin and aguerin B, as well as chlorogenic acid, luteolin and apigenin derivatives, have been indicated as the phytotoxic compounds. However, different extraction methodologies have been carried out in previous studies, resulting in different extract yields and biological activities. This work aimed to select the most efficient extraction method for the analysis of cultivated cardoon phytotoxic compounds and to broad the spectrum of its allelochemicals. Methanol, ethanol, ethyl acetate and water were used as extracting solvents and tested on wheat coleoptile bioassay. All the extracts had inhibitory effects (more than 50%), and the ethyl acetate extract was the most active (close to 100%). Extracts with the highest allelopathic potential were tested on germination rate, root length, and shoot length of six weed species (Amaranthus retroflexus L., Portulaca oleracea L., Stellaria media L., Anagallis arvensis (L.) Vil., Echinochloa crus-galli L. and Lolium perenne L.). Root length was the most affected parameter, and ethanolic and ethyl acetate extracts were the most actives. Nevertheless, as the ethyl acetate extract was previously isolated and with the aim to increase the knowledge of the allelochemicals present in the extracts of C. cardunculus, the aqueous extract was fractioned by using liquid-liquid partitions, column chromatography and HPLC analysis. Four sesquiterpene lactones (cynaropicrin, cynaratriol, desacylcynaropicrin and 11,13-dihydro-desacylcynaropicrin) and a lignan (pinoresinol) were isolated from the ethyl acetate fraction of the aqueous extract, and three of them are described in cultivated cardoon for the first time. Their general phytotoxicity was evaluated using the wheat coleoptile bioassay. Four of them evidenced allelopathic effects, with cynaropicrin, desacylcynaropicrin and pinoresinol showed the highest activity, inhibiting the wheat coleoptile by 93, 96 and 83% at 10–3 M.