Hormesis effects and implicative application in assessment of lead-contaminated soils in roots of Viciafaba seedlings

Chemical analyses and biological methods were combined to investigate oxidative stress, hormesis effect and concerned mechanism in roots of Viciafaba seedlings grown in 0–2000 mg kg−1 of Pb-treated soils after germination of 20 d. The results showed that U-shaped dose response curves were displayed in superoxide radical (O2-) radicals, guaiacol peroxidase (POD) and ascorbate peroxidase (APX) activities, malondialdehyde (MDA) and carbonyl groups as well as activities of endoproteinase (EP) isoenzymes in the roots at low doses of extraneous Pb, indicating reduced oxidative stress and toxic effect. The inverted U-shaped curves were also exhibited in growth height, superoxide dismutase (SOD) and EP activities as well as inducible heat shock protein70 (HSP70) with the increasing extraneous Pb, indicative of enhanced oxidative stress. The enhancement in HSP70, carbonyl groups and EP activities confirmed intracellular proteotoxicity and proteolytic activity in the roots at higher doses of soil Pb. More interestingly, levels of inducible HSP70 were well correlated with those of growth heights (r = 0.809, p < 0.05), implying that HSP70 induction may be one of the mechanisms underlying the U-shaped growth curve of V. faba seedlings in the experiment. The results suggest that traditional threshold models ought to be combined with hormesis effect in assessment of Pb-polluted soils and the threshold dose range of Pb-treated soils is proposed rudimentally as 25–125 mg kg−1.