Acacetin is a naturally occurring flavonoid that displays multi-pharmacological activities, as well as phytotoxicity. In this study, seeds of four typical vegetables including lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), onion (Allium cepa L.) and cucumber (Cucumis sativus L.) were selected to evaluate the phytotoxic effects of acacetin, and the model plant lettuce, which is also the most sensitive species to acacetin of the four vegetables, was used to research the phytotoxic mechanism of acacetin. Bioassays showed that the germination rate and germination potential of vegetable seeds were both decreased under a high concentration of acacetin. Acacetin displayed strong inhibitory effects on root growth, shoot growth and fresh weight of vegetable seedlings in a concentration dependent manner. After treatments with acacetin, the levels of O₂⁻, H₂O₂, MDA, free proline and the number of dead cells in lettuce root tips were increased, while the mitosis index (MI) was decreased. These results indicated that acacetin could cause stress on lettuce seedlings and induce the accumulation of reactive oxygen species (ROS) in plant cells, leading to lipid peroxidation and then loss of cell viability and even cell death. Moreover, acacetin influenced the mitosis of the target plant, resulting in a decreased proportion of cells during the division phase. Together, acacetin showed strongly phytotoxic effects on vegetables, and the allelopathic activity mainly depended on the influence of ROS and mitosis of the receptor plant.

Harmful cyanobacterial blooms (CyanoHABs) threaten freshwater ecosystems and human health. Inhibiting cyanobacteria through plant allelopathy is an effective and environmentally friendly approach for CyanoHAB control. In this study, we evaluated the inhibitory activities of several organic solvent extracts from Artemisia argyi against the common bloom-forming cyanobacterium Microcystis aeruginosa, explored the anti-algal mechanism of the active fraction, analyzed its secondary metabolites using liquid chromatography–high-resolution mass spectrometry (LC-HRMS), and screened the potential allelochemicals. The results showed that the crude extract of A. argyi leaves (CE) exhibited significant inhibitory effects on M. aeruginosa. Among several solvent fractions of CE, the dichloromethane extract (DE) demonstrated the strongest inhibitory effect, with a 7-day IC₅₀ of 70.43 mg/L. After treatment with DE, the contents of chlorophyll a (Chl a), carotenoids, and phycobiliproteins (PBPs) in M. aeruginosa were significantly reduced. Meanwhile, an excessive accumulation of reactive oxygen species (ROS), reduction of catalase (CAT) activity, increase in malondialdehyde (MDA) content, and shrinkage of the membrane were found in M. aeruginosa cells under DE treatments. There were 81 secondary metabolites annotated in DE by LC-HRMS. Among them, hispidulin, jaceosidin, 5,7,3′-trihydroxy-6,4′,5′-trimethoxyflavone, and eupatilin possessed strong inhibitory activities, with 7-day IC₅₀ values of 26.23, 27.62, 32.02, and 34.98 mg/L, respectively. These results indicated that the A. argyi extracts possess significant allelopathic activities on M. aeruginosa, and DE was identified as the primary active fraction. It inhibits algae growth by suppressing photosynthesis and inducing peroxidation, ultimately leading to cell death. Flavonoids in DE were the main allelochemicals responsible for the inhibition on algae of A. argyi extracts.