Insights on the responses of Brassica napus cultivars against the cobalt-stress as revealed by carbon assimilation, anatomical changes and secondary metabolites

Cobalt (Co) is a toxin environmental pollutant, and its elevated concentrations in agricultural soils can negatively impact the crop productivity. In the current study, we evaluated the toxic effects of Co levels (0, 100, 400 μM) on the hydroponically grown seedlings of four different cultivars of Brassica napus L. cvs. ZS 758, ZY 50, Zheda 619 and Zheda 622. Results showed that lower Co dose (100 μM) executed significant but less toxic effects as compared to 400 μM Co among all studied cultivars. Excessive Co (400 μM) level had significantly declined the plant growth, biomass, chlorophyll contents, nutrients uptake, activities and transcript levels of antioxidant enzymes in leaf and root tissues of all tested cultivars.The decline in physiochemical traits was more prominent in Zheda 622, and ZS 758 performed better under Co toxicity as compared with other cultivars. Co-induced root impairment led to greater ROS and MDA accumulation and inhibit nutrients uptake. Ultimately, the lower translocation to leaves causes damages in carbon assimilation process.These results were confirmed by root staining with nitro-blue tetrazolium (NBT) and 3,3-diaminobenzidine (DAB) and damages in leaf mesophyll and root tip cells by electron microscopic analysis. More anatomical damages were noted in Zheda 622 and least in ZS 758. A significant induction of stress-related proteins (HSP90-1 and MT-1) and secondary metabolites (PAL, PPO, and CAD) under higher Co level (400 μM) indicated the greater plant-defense system against Co-induced oxidative stress and Co-homeostasis that helps in toleraance to Co-phytotoxicity. The comparison of sensitive (Zheda 622) and tolerant (ZS 758) genotypes showed that root tissues were the major target of Co toxicity. Overall, higher Co-accumulation in leaf and root tissues cause cellular toxicity and overall carbon assimilation process. This phytotoxicity was more pronounced in Zheda 622 and least in ZS 758 as compared with other cultivars.