Genotype and harvest time affect the allelopathic activity of Cynara cardunculus L. extracts on Amaranthus retroflexus L. And Portulaca oleracea L

In the last years, due to the environmental impact of synthetic herbicides and the increasing incidence of resistance in weeds, the environmental sustainability of weed management practices has become of central importance. The manipulation of allelopathic mechanisms can offer important tools for an eco-friendly weed management without the adoption of synthetic chemical products. Recently, Cynara cardunculus L. was studied for its allelopathic potential, which is due to the presence of sesquiterpene lactones (e.g., cynaropicrin, grosheimin, aguerin B, etc.), flavones (luteolin- and apigenin derivatives) and caffeoylquinic acids (e.g., chlorogenic acid) in the leaves. Genotype and age of plant organs are widely reported as factors affecting the expression of the allelopathic activity in plants. Therefore, this work aims to study the effect of genotype and harvest time on C. cardunculus phytotoxicity by evaluating the allelopathic activity of its leaf extracts at three concentrations (0, 400 and 800 ppm) on the seedling growth of two weeds: Amaranthus retroflexus L. and Portulaca oleracea L. Leaf ethanolic extracts were prepared from six genotypes (‘Naro 2’, ‘VSB3’, ‘Marsala’, ‘Valparaiso’, ‘Altilis 41’ and ‘Bianco gigante’) belonging to the three C. cardunculus botanical varieties (globe artichoke, wild and cultivated cardoon), harvested three times (November, January and April) during the growing season. Overall, root length was the most affected parameter and allelopathic activity grows with increasing concentration of extracts. Moreover, cultivated cardoon ‘Bianco gigante’ extracts from the April harvest showed the highest inhibitory activity on both weed species, followed by wild cardoon ‘Valparaiso’. In particular, on the average of all genotypes, the April harvest reduced by 39 and 50% the root length of A. retroflexus and P. oleracea, respectively. These results confirmed, for the first time, that both genotype and harvest time strongly influence the allelopathic activity of C. cardunculus.