Allelopathic Activity of Three Wild Mediterranean Asteraceae: Silybum marianum, Cynara cardunculus var. sylvestris, Galactites tomentosus

The manipulation of allelopathic mechanisms, such as the isolation of plant allelochemicals for bioherbicide production, is currently providing a new tool for weed management methods of reducing or potentially eliminating the use of synthetic herbicides. In Mediterranean agroecosystems, wild Asteraceae are the prevalent taxa, likely due to their allelopathic activity. Hence, the objective of this study was to evaluate the allelopathic effects of the aqueous extracts obtained from milk thistle [Silybum marianum (L.) Gaertn], wild cardoon (Cynara cardunculus L. var. sylvestris) and purple milk thistle (Galactites tomentosus Moench) on the seed germination, mean germination time, and seedling growth of three target weeds: Portulaca oleracea L., Taraxacum officinale (Weber) ex Wiggers and Anagallis arvensis L. The total polyphenol (TP), flavonoid (TF), flavonol (TFL), and phenolic acid (TPA) content in the aqueous extracts was also evaluated. Overall, the allelopathic effects were species-dependent and root length was the most affected parameter. All extracts completely inhibited root development in P. oleracea. Averaged over target weeds, C. cardunculus extract had the greatest allelopathic activity, followed by G. tomentosus and by S. marianum. In particular, C. cardunculus reduced seed germination by over 50% and increased the mean germination time by 154%, likely due to the highest TP (13.2 g kg−1 DM) and TPA (11.4 g kg−1 DM) content, compared to the other Asteraceae species. These results provide evidence of the phytotoxic activity of the three wild Asteraceae members and suggest their possible future exploitation as potential bioherbicides for sustainable weed management.