A Physiological Approach to Study the Interaction of Cadmium and Zinc in Groundnut ( Arachis hypogaea L.) Seedlings

Background: Heavy metal toxicity affects plant growth and alters physiological processes. Soils in many areas are often contaminated by cadmium and zinc which show varied response on plants by their interactive effects. The experiment was done to study the effect of cadmium and zinc as sole presence and in combination in groundnut seedlings. Methods: The laboratory experiment was conducted on groundnut cultivar TG 51 in sand culture using modified Hoagland solution. After initial screening, three concentrations of cadmium (Cd 100, Cd 300 and Cd 500 mu M) and two concentrations of zinc (Zn 50 and Zn 150 mu M) were selected for studying their effects individually and in combination on physiological and biochemical parameters. Result: The reduction in root length increased over control as the concentration of cadmium in the medium increased. Cadmium or zinc alone led to a decrease in chlorophyll a, b and relative water content of the leaf. Zinc supplement at 150 pM not only mitigated the negative effect of Cd 100 pM and 300 pM, but also increased the chlorophyll content above control level. Zinc supplement not only increased the protein content over the control but also mitigated to some extent the adverse effects of cadmium in protein content when applied in combination. Under both cadmium and zinc treatment, the inhibition of nitrate reductase (NR) activity over unstressed control was found. Different treatment combinations, however, reduced the negative effects of cadmium, although zinc could not completely override such damage, change the level of toxicity. Treatment with Cd 100 pM and 300 pM induced an increase in phenol content over the control, while higher concentration (500 pM) of the metal led to a decrease in this potent antioxidant compound. Presence of Zinc in the growing medium significantly enhanced the accumulation of phenolic compounds highlighting its protective role against oxidative damage.