Various industrial and other anthropogenic activities have considerably increased the levels of heavy metals in the environment. These metals restrict the plant growth and pose a threat to human life. Experiments were conducted to assess the effects of heavy metals on various physiological parameters in Brassica juncea cvs. Further, whether hardening treatment with nitrate salt mitigates some physiological parameters under controlled and heavy metal HgCl₂ treatment. The results show the percentage germination progressively declined with increasing concentrations of heavy metal. However, in Hardened Seeds (HS) and Non-Hardened Seeds (NHS) germination was 100%. At lower concentrations of heavy metals, i.e., 0.5-1 mM, germination was reduced and at 10 mM concentration, seeds failed to germinate. HS showed enhanced germination percentage over NHS in both cvs. With increasing concentration of heavymetals the length of root and shoot seedlings progressively declined in both HS and NHS. In HS, deleterious effect of HgCl₂ was mitigated. Dry matter of seedlings of HS in comparison to NHS control was more in both the cvs. There was a considerable decrease in the total chlorophyll content with metal treatment. Metal-stressed seedlings retained less water content and showed a greater damage to membranes as Electrolyte Leakage (EL) and proline accumulated in seedlings in response to heavy metal treatment. Relatively, HS showed increase in the proline content compared to NHS. The present investigation suggests the nitrate pretreatment possibly induces withstanding capacity in Brassica seedlings to grow in soils affected by heavy metal mercury. The data are compared in the two cvs, i.e., P Bold and Vasundhra.

The contamination of the environment by heavy metals is currently a major global environmental problem threatening the health of vegetation, wildlife and humans (Salt et al., 1995). Various industrial activities, including battery production, metals products, metal smelting and cable industries, mining, vehicular emissions, agricultural waste disposal technologies and atmospheric emissions contribute to the increasing levels of heavy metals in the environment. Besides anthropogenic activities, geothermal streams and volcanic eruptions also contribute to heavy metal accumulation in the environment (Foy et al., 1978). Eventually, these metals find entry into the food chain and tend to biomagnify at subsequent trophic levels (Woolhouse, 1983). Heavy metals like arsenic (As), cadmium (Cd), lead (Pb) and mercury (Hg), which form a part of the present study, have no role to play in plant metabolic activities and thus altogether are nonessential.

Brassica is an important oil seed crop which is being grown in heavy metal-contaminated sites (Tickoo et al., 2007). Earlier studies have reported the positive effects of toxic metals on Brassica (Inoue and Saeki, 2004; Qadir et al., 2004; and Tickoo et al., 2007). The relative sensitivity of Brassica cv to Cd, Pb and Hg was reported previously. In this study, we have assessed the effects of various concentrations of Hg on seed germination, seedling growth and related physiological changes in the Indian mustard genotypes for their response to Hg accumulation and tolerance.

The seeds of Brassica juncea genotype P Bold and Vasundhra were procured from Mustard Rape Research Institute. The healthy seeds were surface-sterilized with 1% sodium hypochloride solution for 2 min and washed thoroughly with distilled water. Seeds were germinated in petridishes of 15 cm diameter having blotting papers moistened with distilled water and respective metal solutions. For imposing heavy metal stress, mercuric chloride (HgCl₂) at 0.5, 1.5 and 10 mM concentrations were used. The seeds were allowed to grow into seedlings under these conditions. The experiments were terminated on the 10^th day and data on germination and growth of seedlings were recorded on the 5^th and 10^th day as length of shoots and roots. Root Growth Rate (RGR) was determined by dividing the difference of root length on the 10^th day and 5^th day by number of days. Similarly, the Shoot Growth Rate (SGR) was determined. The 10^th day-old seedlings were analyzed for chlorophyll content, Relative Leaf Water Content (RLWC), Electrolyte Leakage (EL) and proline that were estimated at 5 mM concentration of this metal.

Life Sciences Journal, Nitrate-Hardened Brassica Seeds, Electrolyte Leakage, Geothermal Streams, Battery Production, Volcanic Eruptions, Relative Leaf Water Content, Brassica Seedlings, Root Cell Metabolism, Hardened Seeds, Seed Germination.