Phytotoxic Effects of Allelochemical Acacetin on Seed Germination and Seedling Growth of Selected Vegetables and Its Potential Physiological Mechanism

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 O-2(-), 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.