Abstract: Since the tolerance of alligator weed to glyphosate under high manganese (Mn) stress was increased,the response of shikimate accumulation and antioxidant enzymes activity to glyphosate under high concentration of Mn in alligator weed (Alternanthera philoxeroides) was investigated in hydroponic culture systems.The activity of catalase (CAT) in alligator weed leaves in medium concentration of Mn (0.31 mmol/L) was higher than that of the low concentration of Mn (0.009 1 mmol/L, control) 120 days after treatment.The activity of superoxide dismutase (SOD) was higher but that of CAT was lower in high concentration of Mn(2.45 mmol/L) treatment than those of the control.Within 6 days after glyphosate application,the shikimate concentrations in alligator weed leaves that treated with low concentration of Mn was increased by 31.9%~226.0%,which were significantly higher than those in medium and high concentration of Mn treatments.The activity of SOD,CAT and peroxidase (POD) in the leaves of the weed were increased and then declined in the three concentration of Mn treatments after glyphosate application.However,the activity of the three antioxidant enzymes in alligator weed leaves in medium concentration of Mn was always higher than those of the control,and the activity of SOD and POD in high concentration of Mn was higher than those of the control,but the activity of CAT was similar to those of the control.The results indicated that the antioxidant enzymatic systems might have been initiated in alligator weeds in response to high Mn stress.The shikimate pathway was less inhibited and activity of antioxidant enzymes was relatively high in the weed under high Mn condition,these might be part of the mechanism of alligator weed tolerates to high Mn level and tolerates to glyphosate under high Mn stress.