Nitrogen Sources Mitigate Cadmium Phytotoxicity Differentially by Modulating Cellular Buffers, N-assimilation, Non-protein Thiols, and Phytochelatins in Mustard ( Brassica juncea L.)

This study evaluated the extent and the mechanisms underlying cadmium (Cd; 200 mg kg −1 soil) impacts and their minimization via four soil-applied nitrogen (N) sources, namely potassium nitrate, urea, ammonium nitrate, and calcium nitrate, at 100 mg N kg −1 soil (100 N) and 200 mg N kg −1 soil (200 N) in mustard ( Brassica juncea L. cv. Giriraj). The experiment was conducted in a randomized block design, and various growth, photosynthetic, oxidative damage, histochemical, reactive oxygen species (ROS) localization, antioxidant defense system, cell viability, nitrogenous osmolyte (proline), and N-assimilation were assessed at 30 days after sowing (DAS), whereas plant yield was measured at 120 DAS. Compared to 100 N, the other level of N (200 N) was found more efficient based on the observed growth and photosynthetic responses. Cd 200 in the soil exacerbated oxidative stress indicators by enhancing the content of superoxide (2.9 times), hydrogen peroxide (1.8 times), lipid peroxidation (3.3 times), and electrolyte leakage (1.9 times), thereby impairing nitrate assimilation, growth and photosynthetic characteristics, and plant yield when compared with control plants. Soil amendment with different sources of N moderated the detrimental impacts of Cd on plant health by enhancing the antioxidant metabolism, the contents of non-protein thiols (NPTs), phytochelatins (PCs), and proline. However, among the N-sources tested, ammonium nitrate maximally protected B. juncea growth, yield, and photosynthesis by decreasing Cd accumulation and root-shoot translocation, improving cellular buffers, further enhancing the activity of antioxidants (superoxide dismutase, ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase), the contents of NPTs, PCs, and proline, N-assimilation enzymes (nitrate reductase, nitrite reductase glutamine synthase and glutamine-2-oxoglutarate aminotransferase), and thereby diminishing oxidative damage. The soil amendment with ammonium nitrate increased plant dry biomass by 97.5%, leaf area by 37.4%, net photosynthesis by 57.2%, chlorophyll content by 85.4%, Rubisco activity by 129.1%, PCs by 76%, the activity of NR by 285.5%, and NiR by 215.6% when compared to only Cd-treated plants. This study suggests that different forms of N mitigate Cd stress differentially, and ammonium nitrate augments plant growth and yields maximally under Cd stress. The results of the study might be beneficial in constructing an efficient agronomic approach for the optimal amendment of N source and level in Cd-contaminated agricultural soils. Graphical abstract