Effect of organic ligands on lead-induced oxidative damage and enhanced antioxidant defense in the leaves of Vicia faba plants

• The biogeochemical behavior of Pb depends on its speciation.
• Organic ligands modify Pb speciation & in turn, its biogeochemical behavior.
• Role of organic ligands towards Pb toxicity varies with metal binding constant.

The biogeochemical behavior of metals varies with their chemical speciation. The present study evaluated the effect of metal speciation on lead-induced oxidative stress to Vicia faba leaves. Lead was applied to V. faba plants in the form of lead nitrate alone or chelated by citric acid (CA) and ethylenediaminetetraacetic acid (EDTA) in hydroponic conditions. The exposure time for all the treatments was 1, 4, 8, 12 and 24 h. The activation of antioxidant enzymes in V. faba leaves varies with Pb speciation and duration of exposure. Application of Pb alone increased the activities of superoxide dismutases (SOD), guaiacol peroxidase (GPOX), ascorbate peroxidase (APX) and glutathione reductase (GR) while reducing that of catalase (CAT) in V. faba leaves. Application of EDTA dose dependently alleviated the Pb-induced activation of SOD, APX, GPOX and GR, and depression of CAT by decreasing Pb accumulation in V. faba leaves. By contrast, CA had no effect on Pb accumulation and antioxidant enzyme activities except that the activation of SOD, APX, GPOX and GR, and reduction of CAT was delayed. The results showed that the physiological responses of V. faba leaves to Pb toxicity vary with its chemical speciation. Therefore, it is proposed that metal speciation must be given consideration in risk assessment and remediation studies.