Oxidative stress and potential biomarkers in tomato seedlings subjected to soil lead contamination

Oxidative stress and defense response in leaves of tomato seedlings exposed to extraneous lead (Pb) at 0–500 mg kg⁻¹ soil for nearly 2 months were investigated. Superoxide radical (O₂⁻) was quantified by electron paramagnetic resonance (EPR). Results showed that levels of O₂⁻, malondialdehyde (MDA), carbonyl group and superoxide dismutase (SOD) increased with the increase of bioavailable Pb. The O₂⁻ level was well correlated with MDA, carbonyl groups and SOD activities, suggesting that O₂⁻ might be responsible for them. Intensities in two bands of SOD isoenzymes increased along with added Pb in treatments against control, implying that multigenic expression in SOD enzymes were activated to counteract O₂⁻ stress. Heat shock protein 70 (HSP70) was induced sensitively by soil Pb, which was to alleviate oxidative damage (i.e. increased carbonyl groups). The overall results indicated that HSP70 and O₂⁻ were the most sensitive parameters and the combination of them might be potential biomarkers of soil Pb contamination in tomato seedlings.