Allelopathic Effects of Aqueous Extracts from Alternanthera philoxeroides (Mart.) Griseb on Seed Germination and Seedling Growth of Zinnia elegans

As a global environmental problem, biological invasion poses a serious threat to natural ecosystems. To explore the influence mechanism of Alternanthera philoxeroides (Mart.) Griseb on the growth and development of landscape plants, this study systematically analyzed the effects of extracts from different organs (stems, leaves, and roots) of A. philoxeroides on the seed germination and seedling growth of Zinnia elegans Jacq. by combining the Petri dish filter paper method with a pot experiment to reveal the potential mechanism of allelopathy. The results showed that the aqueous extract of A. philoxeroides inhibited the seed germination and seedling growth of Z. elegans. The high concentration (100 mg·mL−1) of stem and leaf extracts significantly reduced the germination rate (by 99.10% and 90.65%) and seedling morphological parameters. The allelopathic inhibition increased with an increase in concentration, and the inhibitory effect of stem and leaf extracts was significantly stronger than that of root extracts. Aqueous extracts from the roots, stems, and leaves of A. philoxeroides at three concentrations (25, 50, and 100 mg·mL−1) induced oxidative stress in seedlings, as evidenced by the elevated malondialdehyde (MDA) content and dysregulated activities of antioxidant enzymes. Specifically, superoxide dismutase (SOD) and catalase (CAT) activities exhibited a concentration-dependent trend of initial induction followed by subsequent inhibition, while root activity was significantly suppressed (p < 0.05), ultimately impairing seedling growth. The aqueous extracts of A. philoxeroides showed a concentration-dependent inhibitory effect on the seed germination and seedling growth of Z. elegans. High concentrations of stem and leaf extracts exerted a significant inhibitory effect on seedling growth, and this growth suppression was attributed to the induction of oxidative stress by the extracts. This study elucidated the phytotoxicity degree and physiological response mechanisms underlying the biochemical allelopathy of A. philoxeroides on Z. elegans. The findings provide a theoretical foundation for the selection of horticultural plant cultivars resistant to allelopathic stress and the development of management strategies for invasive plants.