EFFECT OF COMPOST ON THE ANTIOXIDANT DEFENSE SYSTEMS OF CUCUMBER (Cucumis sativus L.) AGAINST CADMIUM TOXICITY

The effects of compost on plant growth, photosynthetic pigments, activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR), as well as concentrations of . ascorbate (ASC) and glutathione (GSH) were investigated in cucumber (Cucumis sativus l.) plants treated with cadmium (Cd2). Four different treatments were applied: control without any treatment (T1), compost at 10% rate by weight (T2), cadmium (Cd^2+) at 100 μM (T3) and combined treatment of compost + Cd^2+ (T4). The leaves showed apparent symptoms of Cd^2+ toxicity, I.e. chlorosis, and the plant growth, as well as chlorophylls and carotenoids were significantly inhibited in Cd^2+-treated plants. In addition, compared with the control, 100 μM Cd^2+ considerably ·Increased H202 concentration and lipid peroxidation indicated by accumulation of thiobarbituric acid reactive substances (TBARS), pointing out to cellular oxidative stress. The addition of 10% compost to the sandy soil decreased the contents of TSARS and the production of H2O2, but increased the levels of photosynthetic pigments, and improved the plant growth. These results suggest that compost protects plants from oxidative damage, and this protection is performed via significant increase in the activities of the antioxidant enzymes namely SOD, APX, DHAR, GR, as well as the levels of antioxidants, ASC and GSH. The present results coin-tided with the conclusion that compost components, i.e. macro- and mlcronutrients, organic matters, humic substances, hormone-like substances, biotic agents, emission of carbon dioxide and nitric oxide and many others, are effectively contributed in the physiological metabolism to counteract oxidative stress Induced by Cd2^+ contaminated soil. Abbreviations: APX, ascorbate peroxidase; ASC, reduced ascorbate; Cd^2+, cadmium; Co, compost; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; EDT A, ethylene diamine tetraacetic acid; GR, glutathione reductase ; GSH, reduced gultathione; GSSG, oxidized glutathione; ROS, reactive oxygen species; SOD, superoxide dismutase; TBARS, thiobarbituric acid reactive sub-stances