Phenolic acids are one kind of important allelochemicals,which generally exist in crops and vegetables,as well as their cultivated soils.These metabolites are involved in plant allelopathic effect and continuous cropping obstacles.In this paper,we investigated the allelopathic effect of chlorogenic acid and its phytotoxic mechanism on lettuce seedlings by plant cell and physiology methods,in order to reveal the allelopathy and continuous cropping obstacles mediated by chlorogenic acid.The results showed that:(1)Root length,shoot length and fresh weight of lettuce seedlings were promoted by chlorogenic acid at low concentrations(0.1 and 1 μmol/L) and were inhibited at high concentrations(10,100 and 1 000 μmol/L).Chlorogenic acid displayed significant inhibitory effects on root length and fresh weight of lettuce at concentrations of 10 μmol/L and more.However,shoot growth of lettuce was not significantly inhibited by chlorogenic acid at each concentrations used.(2)After treated by chlorogenic acid,the mitotic index of lettuce root tips was significantly decreased,and with the concentration of chlorogenic acid increasing,the proportion of cells in each mitotic phase was also significantly reduced,resulted in arrest of cell division.(3)Under low concentrations(0.1,1.0 and 10 μmol/L),chlorogenic acid had no significant effect on cell viability in lettuce root tips.However,high concentrations of chlorogenic acid(100 and 1 000 μmol/L) significantly increased the number of dead cells,and caused losing of cell vitality in lettuce root tips.(4)Fluorescence staining of DHE showed that low concentrations(0.1 and 1.0 μmol/L) of chlorogenic acid had no significant effects on the accumulation of reactive oxygen species(ROS) compared with the control.However,when the concentration of chlorogenic acid reached 10 μmol/L,a lot of ROS accumulated in lettuce roots with concentration increasing.These results indicated that under the stress of chlorogenic acid,ROS in lettuce was induced and then accumulated.Furthermore,the cell division and viability were inhibited,resulting in growth and development arrest in lettuce seedlings.