In this study, the effects of lead (Pb) on growth, photosynthetic pigments concentration, lipid peroxidation, electrolyte leakage percentage (ELP), protein oxidation, aminolevulinate dehydratase (ALA-D; E.C. 4.2.1.24), ascorbate peroxidase (APX; E.C. 1.11.1.11), catalase (CAT; E.C. 1.11.1.6) and superoxide dismutase (SOD; E.C. 1.15.1.1) activities, and ascorbic acid (AsA), non-protein thiol groups (NPSH) and total soluble protein concentrations in cucumber seedlings (Cucumis sativus L.) were investigated. Seedlings were grown in vitro in an agar-solidified substrate containing three Pb levels as (C2H3O2)Pb.3H2O (0, 100, 400, and 1000 µmol L-1) for 10 d. Increasing Pb concentrations in substrate enhanced Pb concentration in both roots and shoot. Pb accumulated at a higher amount in roots. Root length and total fresh weight were decreased at the two highest Pb concentrations. Cucumber showed no reduction in shoot length and total dry weight at any Pb level. The highest Pb concentration decreased water content and ALA-D activity as well as increased malondialdehyde, carbonyls and total soluble protein concentrations. Carotenoids concentration enhanced at 100 and 400 µmol Pb L-1, while chlorophyll concentration and ELP were not affected by Pb stress. Activity of APX was inhibited while the activities of CAT and SOD were increased at all Pb concentrations. AsA was enhanced at 400 and 1000 µmol Pb L-1 whereas NPSH were increased only at the highest Pb concentration. Therefore, high Pb-exposure caused oxidative stress, and the antioxidant system of the cucumber seedlings was not sufficient to revert it, contributing for growth reduction.
antioxidant enzymes; cucumber; lipid peroxidation; Pb; photosynthetic pigments; protein oxidation
