Soil Sulfur Sources Differentially Enhance Cadmium Tolerance in Indian Mustard (Brassica juncea L.)

The effect of four soil-applied sulfur (100 mg S kg−1 soil (100S) and 200 mg S kg−1 soil (200S)) in different sources (elemental S, ammonium sulfate, gypsum or magnesium sulfate) in protecting mustard (Brassica juncea L. (Czern & Coss.)) from cadmium effects was studied. Based on the observed reduction in growth and photosynthesis in plants subjected to 100 and 200 mg Cd kg−1 soil, B. juncea cv. Giriraj was selected as the most Cd-tolerant among five cultivars (namely, Giriraj, RH-0749, Pusa Agrani, RH-406, and Pusa Tarak). Sulfur applied to soil mitigated the negative impact of Cd on sulfur assimilation, cell viability, and photosynthetic functions, with a lower lipid peroxidation, electrolyte leakage, and contents of reactive oxygen species (ROS: hydrogen peroxide, H2O2, and superoxide anion, O2•−). Generally, added S caused higher activity of antioxidant enzymes (ascorbate peroxidase, catalase and superoxide dismutase), contents of ascorbate (AsA) and reduced glutathione (GSH); increases in the activities of their regenerating enzymes (dehydroascorbate reductase and glutathione reductase); as well as rises in S assimilation, biosynthesis of non-protein thiols (NPTs), and phytochelatins (PCs). Compared to the other S-sources tested, elemental S more prominently protected B. juncea cv. Giriraj against Cd-impacts by minimizing Cd-accumulation and its root-to-shoot translocation; decreasing cellular ROS and membrane damage, and improving Cd-chelation (NPTs and PCs), so strengthening the defense machinery against Cd. The results suggest the use of elemental S for favoring the growth and development of cultivated plants also in Cd-contaminated agricultural soils.