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.

To study the allelopathic effect of the extracts of Landoltia punctata , the changes of cell density of Microcystis aeruginosa were measured. The anti-algae allelopathic effect of different organic solvent extracts of L. punctata was evaluated, and the physiological, biochemical indexes were determined to discuss the mechanism of algal inhibition. The results showed that the petroleum ether, dichloromethane and ethyl acetate extracts showed various inhibitory effects on M. aeruginosa . Among them, ethyl acetate extract was the most strongly allelopathic part with the semi-effect concentration(EC 50 ) of 59.6 mg L −1 , the central polarity part of inhibitory activity. The contents of chlorophyll a(Chl a) and phycobiliproteins(PBPs) of M. aeruginosa were decreased under the concentration of 200 mg L −1 ethyl acetate extract, which indicated that the photosynthesis of M. aeruginosa was inhibited. The consent of microcystins was lower compared to control under 200 mg L −1 . The contents of superoxide dismutase(SOD), malondialdehyde(MDA) and hydrogen peroxide(H 2 O 2 ) of cell pellets were firstly increased and then decreased, which suggested that the algal cells were seriously damaged by oxidation. The results indicated that the extracts of L. punctata had inhibitory effect on M. aeruginosa , and the ethyl acetate extract was the central part of the inhibitory substances, which affected photosynthesis and caused peroxidation damage to inhibit cell proliferation. These findings will be helpful for exploration and application of allelopathic effects of L. punctata in harmful algae control.