XRD-characterized SiO₂ and CeO₂–nanoparticles synergize with earthworms to mitigate chromium toxicity in wheat (Triticum aestivum L.): insights into plant physiology and health risk assessment

Soil contamination with toxic heavy metals such as chromium (Cr) is becoming a serious global problem due to rapid industrial and agricultural activities. Nanoparticles and earthworms (Eisenia fetida) are efficient, environmentally friendly, and biodegradable and they enhance the solubility, absorption, and stability of metals. Therefore, the present study investigated the individual and combined effects of a nanobio strategy integrating X-ray diffraction-verified silica (SiO₂) and cerium dioxide (CeO₂) nanoparticles (50 µM L⁻¹) with earthworms (Eisenia fetida) on wheat (Triticum aestivum L.) grown in chromium-spiked soil (100 mg kg⁻¹), focusing on plant growth and biomass, photosynthetic performance, oxidative stress regulation, antioxidant defense mechanisms, metabolic and nutritional status, chromium accumulation, molecular responses, and associated health risks. Results from the present study revealed that the Cr stress markedly reduced plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugar metabolism, and mineral nutrient uptake, while inducing excessive oxidative stress, as indicated by elevated malondialdehyde and hydrogen peroxide levels. Cr exposure also disrupted antioxidant homeostasis, cellular compartmentalization, and stress-responsive gene expression. In contrast, individual and combined application of NPs and E. fetida significantly improved plant growth, photosynthetic performance, antioxidant defense capacity, and nutritional status. These treatments enhanced enzymatic and non-enzymatic antioxidants, stimulated the ascorbate–glutathione cycle and proline metabolism, and reduced oxidative damage. Moreover, NPs and E. fetida effectively restricted Cr accumulation in plant tissues, leading to a notable reduction in estimated daily Cr intake and associated health risk indices. Gene expression analysis further supported the activation of antioxidant and detoxification pathways under these treatments. Overall, the findings demonstrate that NPs and E. fetida, particularly in combination, are effective in mitigating Cr toxicity, improving wheat growth and physiological stability, and enhancing food safety in Cr-contaminated soils.