Abstract
The effect of interaction of 50 and 100 µM concentrations of Cd, a non-essential element and 50, 100, 150, and 200 µM concentrations of Zn; an essential micronutrient on growth, physiological, biochemical and molecular processes in Vicia faba L. were studied to evaluate the possible protective role of Zn against Cd toxicity. Treatment with 100 µM Zn and Cd alone caused significant increase in electrolyte leakage percent (ELP), lipid peroxidation and hydrogen peroxide content. Cadmium at 100 μM concentration increased the activities of superoxide dismutase, catalase, peroxidase, ascorbate peroxidase but decreased those of glutathione reductase and glutathione S-transferase. On the other hand, under these conditions, thiol content enhanced oxidation of ascorbate (AsA) to dehydroascorbate (DHA) and glutathione (GSH) to glutathione disulfide (GSSG); a clear indication of oxidative stress. The 100 µM Cd treatments, reduced total phenolic content in the roots and shoots of the plant, but Zn application on the contrary enhanced the accumulation of phenolic compounds in the tested organs. In addition, α-tocopherol increased in the roots and shoots at 100 µM Cd and 100 µM Zn. Supplementation of zinc at (200 μM) to Cd at (100 μM) effectively restored thiols, inhibited oxidation of AsA and GSH thus maintained the redox molecules in reduced form. The effect of Cd at100μM cause a slight induction of ascorbate peroxidase (APX, E.C. 1.11.1.11) but inhibition of glutathione reductase (GR, E.C. 1.6.4.2), enzymes of the ascorbate–glutathione cycle (AGC). Zn supplementation not only restored but enhanced the functional activity of all the AGC enzymes (APX and GR). Zn application increased Glutathione-S- transferase (GST, E.C.2.5.1.18) activity. Treatments of Zn-alone did not change the above investigated parameters, but, total phenolic content and α-tocopherol increased in the roots and shoots at 100 µM Cd. These results clearly indicated a protective role of Zn in modulating the redox status of the plant system through the antioxidant pathway AGC, AsA and GSH enzymes to combating Cd induced oxidative stress.

Key words: Antioxidants, Cadmium, Zinc, Vicia faba