Zinc homeostasis is critical for optimised antioxidant defense in Faba bean

Faba bean (Vicia faba L.) plants were grown with five levels of Zn supply ranging from 0.01 to 10 μM. Best growth and maximum biomass were obtained with 1 μM Zn. Leaves of plants with sub- and supra- optimal levels of Zn showed accumulation of lipid peroxides and alterations in the concentration of antioxidants (ascorbate, carotenoids). Activities of carbonic anyhydrase (CA) and total and Cu/Zn superoxide dismutase (SOD) increased with increase in Zn concentration. Activities of ascorbate peroxidase (APX) and catalase (CAT) were lower at both sub- and supra- optimal levels of Zn. Leaf tissue concentration of Zn (∼30 μg Zn g⁻¹ dw) was found optimum for biomass yield of faba bean and also provided defense against oxidative damage. Sub-optimal tissue Zn increased sensitivity to oxidative damage because of decreased activity of SOD that implied enhanced accumulation of superoxide. Supra-optimal tissue Zn produced similar, though less pronounced effect because of stimulation of SOD activity associated with inhibition of APX activity, which could impair efficacious scavenging of H₂O₂. The data shows that Zn homeostasis is critical for optimized defense against ROS. Deviation from this induces alterations in antioxidant enzyme activities resulting in weakening of antioxidant defense against the ROS.