Graphical abstract Sukinda chromite mine of Odisha is a heavily polluted site, generating huge overburden dumps. The present experiment was designed to evaluate the potential of two native nodule endophytic bacterial strains, viz. Bacillus aryabhattai AS03 (MT645244) and Rhizobium pusense AS05 (MT645243), isolated from contaminated sites to be considered remediation tool to minimize the effect of Cr toxicity on Macrotyloma uniflorum var. Madhu. The two nodule endophytic bacterial strains AS03 and AS05 exhibited tolerance to 1800 and 3000 ppm of Cr(VI) respectively in vitro when cultured alone. AAS analysis confirmed higher accumulation of Cr(VI) in roots and less accumulation in shoots which is dose-specific (bio-inoculant) either treated alone or combined. Complete absence of Cr accumulation approximately 99% in shoots of Macrotyloma was observed owing to synergistic effect of both the strains (biochar-based formulation). This study also suggests increased shoot and root length, nodule nos., and leghemoglobin content of the plant at 60 days indicating the plant growth-promoting effects of both the strains. ROS and antioxidant enzymes of the plant recorded decreasing trend in inoculated plants. However, a significant increment in transpiration rate, total photosynthetic rate, intracellular CO 2 conc., and stomatal conductance in leaves was observed owing to dual inoculation. Our findings corroborate the supremacy of synergistic effect of both the strains applied in the form of biochar-based biofertilizer in enhancing growth and tolerance index of M. uniflorum cultivated in Cr(VI)-stressed soil. This investigation depicts the efficiency of the two nodule bacteria as a mixed inoculant to alleviate Cr toxicity and making the seeds safe for consumption. Supplementary Information The online version contains supplementary material available at 10.1007/s11356-021-13009-2.

The discharge of contaminated mining and industrial residues has become a major cause of environmental pollution. Seed is highly protective against external metal stresses. But they are highly sensitive during vegetative developmental process. In this study, germination was conducted in Macrotyloma uniflorum var. Madhu, in order to find out the effect of chromium toxicity on germination, growth and biochemical variations. The seeds were germinated in five different concentrations of chromium (25ìM, 50ìM, 100ìM, 150ìM and 200 ìM) solution. The hydroponically grown plants revealed the toxic phenotypic expression (root length, shoot length, seedling length, shoot vigor index, shoot fresh weight, shoot dry weight, root fresh weight and root dry weight) of plant after 3 growth stage (15 d, 30 d and 45 d). Similarly biochemical characteristics as well as antioxidant activities (Chlorophyll, carotenoid, carbohydrate, reducing sugar, catalase, proline content and peroxidase) were studied to see the toxic effect of chromium.

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