Biosynthesized hematite nanoparticles mitigate drought stress by regulating nitrogen metabolism and biological nitrogen fixation in Trigonella foenum-graecum

•The waste leaves of Polyalthia longifolia, first time synthesized highly stable hematite (Fe2O3) nanoparticles.•50 mgL−1 hematite nanoparticles concentration was found most efficient in osmolyte regulation and nitrogen metabolism under drought stress also improving the micronutrient deficiency caused due to alkalinity of the droughted soil..•Biological nitrogen fixation (nifH gene expression) was enhanced in both drought tolerant (Afg 1) and drought sensitive (Afg 3) varieties of Trigonella foenum-graecum by hematite nanoparticles. The widespread and increasing use of biosynthetic hematite nanoparticles (HNPs) has led to their application in agriculture. Drought poses a significant threat to crop development and production in agricultural regions around the world. Two varieties of Trigonella foenum-graecum, Afg 1 (drought tolerant) and Afg 3 (drought sensitive) were treated with 20 and 50 mgL−1 HNPs concentrations tested under 5 and 10 days drought stress with control. Both Afg 1 and Afg 3 varieties responded most efficiently at a concentration of 50 mgL−1 HNPs in mitigating drought stress. HNPs reduced proline, nitrate, and ammonia concentrations by 22.4%, 25%, and 37% respectively. They are involved in nitrogen regulation and accumulate in dose-dependent manner during drought stress, causing toxicity. HNPs were found to be important in positively regulating the activity of nitrogen metabolizing enzymes (p ≤ 0.05). Nitrate reductase, glutamine synthetase, and glutamate synthase are increased by 41%, 80%, and 63% respectively. This study suggests that HNPs enhance the ability of Trigonella foenum-graecum to maintain high nitrogen metabolism and biological nitrogen fixation. nifH gene expression increased by 21.7% under drought stress by alleviating the Fe micronutrient deficiency in plants caused by dry soil alkalinity. [Display omitted]