Abstract: To explore the toxic effects of herbicides on different species of Lemnaceae, the median inhibition concentration (IC₅₀) and chronic value (ChV) of paraquat to Spirodela polyrhiza, Landoltia punctata, and Lemna minor were measured, with the acute and chronic toxic effects of paraquat on these three Lemnaceae species compared. After treated for 96 hours, the IC₅₀ value of paraquat to S. polyrhiza, L. punctata, and L. minor was 17.37, 6.16, and 4.75 μg·L^-1, respectively, and there existed significant differences among the acute toxicity (P<0.05). The chlorophyll content of the three species all decreased significantly with increasing paraquat concentration, and after treated for four days, a significant negative correlation was observed between the chlorophyll content and paraquat concentration (P<0.05). The ChV of paraquat to S. polyrhiza was 3.050 μg·L^-1, and while that to both L. punctata and L. minor was 0.964 μg·L^-1. However, in light of frond features, the ChV of paraquat to S. polyrhiza was 10.780 μg·L^-1, while that to L. minor was only 3.410 μg·L^-1, implying that the chronic toxic effect of paraquat to L. minor, whether from the viewpoint of frond features or chlorophyll content, was greater than that to S. polyrhiza. Therefore, under the treatment of paraquat, the three species of Lemnaceae demonstrated obvious plant taxonomy effect, dose effect, and time lag effect.

Key words: Chongming Dongtan, inter-specific association, ecological species group, species protection., salt marsh plant