Abstract: Effects of high level of Mn on oxidative stress and antioxidant enzymes in cucumber roots under different pH (pH 4.5 and pH 6.5) were studied. The results indicated that lipid peroxidation increased seriously with the increasing concentration of Mn, especially under low pH. As a results, the growth of cucumber roots and shoots was significantly inhibited. High level of Mn led to an elevation in activities of Mn-SOD and Fe-SOD, which played an important role in removing O2?· CAT was sensitive to high level of Mn, and its activity significantly decreased under high level of Mn and lower pH，while the activities of GPX, APX, DHAR and GR increased under low pH and some extents of high level of Mn, which indicate their important role in scavenging reactive oxygen in cucumber roots tolerance to low pH and high level of Mn.

Key words: Cucumber(Cucumis sativus L.), Manganese, Stress, pH, Antioxidant enzyme