Determination of Antioxidant Activities in Melon (Cucumis melo L.) Seedlings Exposed to Arsenic Stress

Arsenic (As) is a toxic metalloid for all living organisms. Uptake of As in plant tissues affects plant metabolism and causes to several physiological and structural disorders. In this study, antioxidant profile of melon (Cucumis melo L.) seedlings exposed to different arsenate [As(V)] treatments was investigated. Four seeds were sowed in each magenta vessel including filter paper and following ten days after sowing, melon seedlings were treated with Hoagland solution containing 0, 50, 100, 150 and 200 mg L-1 disodium hydrogen arsenate heptahydrate (Na2HAsO4.7H2O) for 10 days. The experiment was set up in a plant growth cabinet as randomized plots design with 3 replications. Superoxide dismutase (SOD) (EC 1.15.1.1) and catalase (CAT) (EC 1.11.1.6) enzyme activities which are the key enzymes of antioxidant system, total antioxidant and lipid peroxidation levels, the contents of photosynthetic pigments (total chlorophyll and carotenoids) and free proline were determined in leaf and root tissues of melon seedlings. In leaves, high concentration of As(V) (200 mg L-1) treatment caused a significant decrease in total chlorophyll and carotenoid content by 26% and 33%, respectively. SOD and CAT enzyme activities and total antioxidant level increased in response to 100, 150 and 200 mg L-1 As(V) treatments in roots. In leaves, while SOD and CAT activities increased at 50 and 100 mg L-1 As(V) treatments, enzyme activities decreased as a result of 100 and 150 mg L-1 As(V) treatments compared with other As(V) treatments. Lipid peroxidation, one of the indicators of oxidative stress, increased in leaf and root tissues under As(V) stress when compared control. In addition, all As(V) treatments caused an increase in free proline content in both tissues significantly