Over the last century human activities have resulted in an unprecedented increase in the level of pollutants, e.g., salinity, ozone levels, pesticides, fertilizers, temperature elevations as well as heavy metals in the environment. Heavy metal pollution of soils is currently a serious environmental problem, because these metals are potential risks to humans. Among the heavy metals, Cd is of increasing scientific interest because of its unique physical and chemical characteristics. Cadmium belongs to group IIB of periodic table, with atomic no 48, atomic weight 112.41, sp. gravity 8.64 g/cm3 and melting point 3210C. Cadmium pigments are found to be unreactive towards chemicals, high temperatures, ultra violent radiation, as well as organic and inorganic solvents. Because of these properties it is widely used in a large number of industrial applications like manufacturing of Cd-Ni batteries, coating of stablizers, paintings, and alloys. This has raised its level in the soil from the normal, 500µg Kg-1 to 500 mg Kg-1. Cadmium is highly toxic to living systems even at very low concentration. It accumulates in the human body with a half-life exceeding more than 10 years. It is responsible for a number of diseases like pulmonary emphysema, demineralization of bones, renal dysfunction, and is categorized as a probable human carcinogen. In plants, it is responsible for a number of physiological breakdowns and even crop failures. Because of these adverse effects of Cd on living system, attempts are being made to minimize its levels in the environment. Traditional methods are expensive, environmentally invasive and labor intensive. So new technology “Phytoremediation” was introduced, which is not only cost effective but also protective to humans as well as to the environment.

          Plants used for phytoremediation must tolerate high levels of metals and translocate the metal in the aboveground part of the plant. To understand the mechanism of Cd tolerance, its accumulation and translocation, we undertook this study with ten genotypes of B. juncea, a high biomass accumulating crop plant belonging to the family Brassicaceae. The study was completed by considering and analyzing the effect of Cd on growth, phytochemical and biochemical processes of these genotypes including enzymatic and non-enzymatic antioxidants. Further, SDS-PAGE and TEM have also been used to characterize/distinguish the tolerance mechanism(s) using resistant, moderate and susceptible genotypes. Our study revealed that accumulation of Cd in B. juncea cv. Pusa Jai Kisan was highest among all the genotypes studied. Cadmium had induced oxidative stress in all the genotypes, but with varying magnitudes. Minimum MDA, accumulation was observed in cv. Pusa Jai Kisan and maximum in cv. Vardhan. Minimum reduction in biomass accumulation, plant height, chlorophyll degradation and maximum levels of protein, proline, ATP-sulphurylase activity, non-protein thiols, phytochelatins, glutathione levels and high activities of antioxidant enzymes like SOD, APX. GR were reported in cv. Pusa Jai Kisan. Transmission electron micrograph of chloroplast and mitochondria isolated from this genotype showed minimum damage to these organelles by Cd-stress as compared to other genotypes. Further, over expression of 66 kDa protein band and induction of two new bands viz, 54kDa and 36kDa were observed in this genotypes. The genotype, B. juncea cv. Pusa Jai Kisan, hence, was resistant, while B. juncea cv.Vardhan susceptible to Cd stress among all the genotypes studied. Our results suggest that high tolerance of B. juncea cv. Pusa Jai Kisan to Cd-induced oxidative stress could be due to the increased level of antioxidant system as well as stress induced proteins in this genotype. The findings of our study thus can find application not only in phytoremediation of heavy metals, but also in developing resistance/tolerance in various crops by transferring these biochemical traits using conventional and non-conventional methods.