The seeds of B. juncea(var. RLC 1) were sterilized with 0.01% mercuric chloride (HgCl₂), soaked in distilled water for 2h (h), and germinated in Petri dishes lined with Whatman No. 1 filter paper moistened with 3 ml solutions of sodium selenate (Na₂SeO₄) (2, 4, and 6µM) and potassium chromate (K₂CrO₄) (300µM), in unary and binary treatments. The concentrations of the elements were decided on the basis of preliminary experiments by obtaining the most stimulatory concentration of Se and 50% inhibitory concentration (IC50) of Cr. The treatment solutions were prepared in half strength Hoagland's nutrient medium and the experiment was conducted in triplicates with 16hphotoperiodand 25℃ temperature. The seedlings were harvested after 15days for the estimation of various parameters.

In conclusion, Cr negatively affects the metabolic activities of B. juncea, which were restored by Se application. Se application showed its protective effects on photosynthetic pigments and gene expression studies confirmed the spectrophotometric observations. The Scanning electron microscope (SEM) studies also clearly indicated the positive effects of Se against Cr toxicity on morphology and density of stomata, which further confirmed its imperative role in efficient gas exchange for both photosynthesis and respiration. Exogenous application of Se was also observed to enhance the lipid- and water-soluble antioxidants, which further indicated its role in alleviating oxidative damage. The positive effect of Se in maintaining the osmotic homeostasis as well as activation of thiols in Cr-stressed seedlings also showed its potential to enhance the ability of B. juncea seedlings to combat heavy metal stress effectively. The results, therefore, indicate that Se can be effectively used as a mitigating agent against Cr stress.

Treatments of Se and Cr aided in modulating the gene expression of CHLASE, PSY and CHS. CHLASE showed a significant upregulation of 366.3% in its expression in the Cr treated seedlings with respect to untreated control seedlings. The expression of PSY, however, was observed to be downregulated with Cr application by 28.6%, while CHS expression showed an upregulation by 73.13%. Se application at 4 µM in combination with 300 µM Cr downregulated its expression by 46.92% when compared to Cr-treated seedlings. On the other hand, the same concentration of Se caused an increase of PSY and CHS expression by 425.2 and 209.4%, respectively.