Lead-contaminated soil induced oxidative stress, defense response and its indicative biomarkers in roots of Vicia faba seedlings

Seeds of Vicia faba. L were grown in increasing concentrations of lead (Pb)-added soils (0-2,000 mg/kg). After germination of 25 days, roots were harvested to investigate oxidative stress, defense response and indicative biomarkers based upon chemical analyses and biological measurements. The results showed that higher concentrations of Pb-polluted soils led to seedling growth inhibition, indicative of phytotoxicity. O (2) (aEuro cent a') and lipid peroxidation were increased with the increase of available Pb in soils and Pb contents in roots, displaying a J-shaped dose response curve, whereas H2O2 showed a biphasic dose response curve (a consecutive J-shaped and inverted U-shaped curve). Superoxide dismutase (SOD), guaiacol peroxidase (POD) and ascorbate peroxidase (APX) enzymes were activated by soil Pb, displaying biphasic curves. The upregulated POD and APX enzymes might be major scavengers of excessive H2O2 when CAT activities were drastically reduced with the increasing soil Pb. The enhanced glutathione (GSH) and APX activities suggested that GSH-ascorbate cycle also participated in eliminating H2O2. Moreover, obvious changes were observed in SOD, CAT and POD isoenzyme patterns, but not in APX except increasing intensities of bands. HSP70 synthesis was significantly induced by extraneous Pb from 125 to 1,000 mg/kg and showed a biphasic curve in this experiment. Comparatively, HSP70 and lipid peroxidation might be more sensitive than other parameters in response to Pb stress, suggesting that these two parameters in the roots might be potential biomarkers for early bioassay of Pb-contaminated soils.