Regulation of chromium toxicity tolerance in tomato and brinjal by calcium and sulfur through nitric oxide: Involvement of enzymes of sulfur assimilation and the ascorbate-glutathione cycle

In soils, toxic metals pose negative impact on crop productivity worldwide. This warrants need of some cost effective and eco-friendly strategies which could mitigate/manage metal toxicity in crop plants for sustained and increased crop productivity. In the past years, exogenous application of nutrient elements has attracted wider attention in mitigating negative consequences of varied abiotic stresses with a much focus on Cd toxicity. However, potential of nutrients like calcium (Ca) and sulfur (S) in mitigating hexavalent chromium [Cr(VI)] toxicity in vegetable crops together with their mechanisms of action are still not known. Therefore, in this study potential of Ca and S in mitigating Cr(VI) toxicity in tomato and brinjal along with their mode of action have been explored. Cr(VI) significantly decreased leaf area, photosynthetic rate, sub-stomatal CO₂, stomatal conductance and transpiration rate and cell viability due to enhanced intracellular accumulation of Cr(VI). Cr(VI) enhanced generation of reactive oxygen species (ROS) i.e. superoxide radical (O₂•̄) and hydrogen peroxide (H₂O₂) that leads to the enhanced protein oxidation and decreased membrane stability index (MSI) as a result of down-regulation in S assimilation and ascorbate-glutathione cycle (AsA-GSH cycle; enzymes activity as well as relative gene expression). The results also show that Cr(VI)-mediated negative impacts were accompanied by declined endogenous nitric oxide (NO). However, additional supply of either Ca or S with Cr(VI) significantly ameliorated Cr(VI) toxicity by down-regulating accumulation of Cr(VI) and ROS as a result of up-regulation in S assimilation and AsA-GSH cycle (enzymes activity as well as relative gene expression) which collectively protect photosynthetic characteristics and cell viability. Interestingly addition of Nω-nitro-L-arginine methylester (L-NAME, an inhibitor of nitric oxide synthase) with Cr(VI) further increased Cr(VI) toxicity in both vegetables while addition of sodium nitroprusside (a NO donor) reversed this response. Thus, overall results showed that NO is an essential component in Ca and S-mediated mitigation of Cr(VI) toxicity in tomato and brinjal seedlings.