Weed phytotoxicity bioassay.
The light stress and the interaction “stress × harvest time × concentration” were not significant for all parameters under study in both species (Table 3). Germination of both species was not affected by light stress and harvest time, while the extract concentration was highly significant. In particular, all the concentrations were significantly lower than to the negative control (0 ppm) (Fig. 5). In both A. retroflexus and P. oleracea , 800 ppm resulted the most efficient in reducing seed germination, even if not statistical differences were observed with respect to the other concentrations.
Regarding root length, the interaction “harvest time × concentration” was significant in both weed species, with a greater contribution to variance of concentration. In both weeds the allelopathic activity significantly increases with increasing concentrations (Fig. 5). Moreover, regardless of extract concentration, April confirmed to be the most appropriate period for the harvest time of cultivated cardoon leaf extracts since it significantly stimulates the production of STLs and their phytotoxicity. A reduction of 86 and 62% in A. retroflexusroot length was observed going from January to April at 800 and 400 ppm, respectively. A similar trend was observed for P. oleracea .
Harvest time was the major factor affecting the analysis of variance forA. retroflexus and P. oleracea shoot length (Table 3). As observed for root length, the interaction “harvest time x concentration” was highly significant in both species. Contrary to the April harvest, no statistical differences were observed among concentrations for the January one (Fig. 5). April harvest exerted a greater allelopathic activity compared to January. In particular, the highest inhibitory activity was observed in 800 ppm of A. retroflexus with a reduction of 93% between the two harvest periods, followed by 64% in 800 ppm of P. oleracea .