Allelopathic Effect of Humulus scandens Stem and Leaf Extract on Solidago canadensis

Solidago canadensis is one of the most destructive invasive plants in China, and current physical and chemical control methods are often costly, inefficient, and ecologically risky, highlighting the urgent demand for green and efficient ecological alternatives. The native vine Humulus scandens exhibits strong allelopathic potential. In this study, allelochemicals from its leaves and stems were extracted using petroleum ether, ethyl acetate, n-butanol, and n-hexane. Their allelopathic effects on S. canadensis were systematically evaluated through seed germination, seedling growth, and rhizome propagation assays, combined with widely targeted metabolomics. Leaf extracts showed markedly stronger allelopathic inhibition than stem extracts. Among all fractions, the ethyl acetate extract exerted the most potent suppression on seed germination, significantly reducing germination rate and root elongation in a concentration-dependent manner. At 100 g·L^-1, leaf extracts-especially the ethyl acetate phase-significantly decreased belowground fresh weight, rhizome number, and new seedling emergence. The ethyl acetate extract also significantly elevated root antioxidant enzyme activities at 25-100 g·L^-1, indicating induced oxidative stress. UPLC-MS/MS-based metabolomics identified 186 differential metabolites among extracts. Ten flavonoids were significantly upregulated in the ethyl acetate extract and were predominantly enriched in the kaempferol aglycone biosynthesis I pathway (MetMap 113) and the flavonoid biosynthesis pathway (ko00941), suggesting that these compounds may play a key role in the observed allelopathic inhibition. Collectively, the ethyl acetate extract of H. scandens leaves effectively suppresses S. canadensis growth by inhibiting seed germination and rhizome tillering and by inducing oxidative stress, probably with the 10 candidate flavonoid metabolites, highlighting its strong potential as a biological control agent against this invasive weed.