Oxidative stress, defense response, and early biomarkers for lead-contaminated soil in Vicia faba seedlings

Chemical analyses and biological measurements were investigated in leaves of Vicia faba seedlings exposed to extraneous lead (Ph) at 0 to 2,000 mg/kg of soil for a month. The results showed that superoxide radical (O-2(center dot-)) production, increased along with total Ph in leaves and available Pb in soil, resulted in enhancement of malondialdehyde and carbonyl groups. Antioxidant enzymes. including corresponding isoenzymes and heat shock protein 70 (hsp 70), were also enhanced to some extent. Significant changes were detected in the patterns and intensities of guaiacol peroxidase isoenzymes, while superoxide dismutase, catalase, and ascorbate peroxidase isoenzymes only changed intensities. Superoxide dismutase activities increased with the increase of extraneous Ph at 0 to 500 mg/kg of soil and tended to decline thereafter, which might be responsible for the decrease of hydrogen peroxide and accumulation of O-2(center dot-). Guaiacol peroxidase and ascorbate peroxidase enzymes were upregulated to become major scavengers of excess hydrogen peroxide on the condition of decreased catalase activities. Levels of hsp 70 were well correlated with Pb contents in leaves (r = 0.777) O-2(center dot-) accumulation (r = 0.985 p < 0.01), and carbonyl groups (r = 0.920, p < 0.01) under extraneous Ph at 0 to 250 mg/kg of soil, suggesting that hsp 70 induced by O-2(center dot-) was possibly involved in disposal of denatured proteins. The results showed that O-2(center dot-), hsp 70. and guaiacol peroxidase isoenzymes had the most sensitive responses in the seedlings and these parameters could be potential early biomarkers of soil Pb contamination.