Fulvic acid-functionalized Fe3O4 nanoparticles enhance Chinese kale tolerance to saline-alkali stress via antioxidant and metabolic reprogramming.

BACKGROUND: In recent years, iron oxide nanoparticles (Fe3O4 NP) have emerged as a promising eco-friendly approach to mitigate various abiotic stresses. However, the role of surface-coated Fe3O4 NP with bio-derived organic compounds, particularly fulvic acid, in mitigating saline-alkali stress in Chinese kale plants has not yet been explored. In this study, we investigated for the first time the effects of green-synthesized Fe3O4 NP, both uncoated (UFe) and fulvic acid-coated (FFA), on mitigating saline-alkali stress in Chinese kale, with a specific focus on modulating physiological responses and biochemical parameters.

RESULTS: Both UFe and FFA alleviated growth inhibition caused by saline-alkali stress; however, FFA consistently outperformed UFe across all measured parameters. UFe and FFA treatments restored shoot and root biomass, improved relative water content, and stabilized the membrane integrity by reducing electrolyte leakage and lipid peroxidation. Photosynthetic capacity was strongly enhanced, as reflected by higher net photosynthetic rate, stomatal conductance, Rubisco activity, chlorophyll index, and PSII efficiency. Oxidative stress was alleviated through significant reductions in H2O2 and MDA, supported by activation of SOD, CAT, and APX enzymes and strong recovery of the ascorbate-glutathione cycle, particularly under FFA treatment. At the transcriptional level, both treatments upregulated genes associated with antioxidant defense (SOD, CAT, APX, GR, DHAR), osmolyte biosynthesis (P5CS1), stress tolerance (LEA), photosynthesis (RBCS, LHCB1), and secondary metabolism (PAL, CYP79F1); however, this response was more pronounced in FFA-treated plants. These transcriptional changes were consistent with enhanced accumulation of osmolytes, including soluble sugars, free amino acids, and proline. Untargeted LC-MS/MS metabolomics identified 46 differentially regulated metabolites spanning sugars, amino acids, organic acids, glucosinolates, and phenolics. Multivariate analyses (PLS-DA and VIP scores) further highlighted proline, glucobrassicin, sinigrin, tryptophan, citric acid, and caffeoylquinic derivatives as key discriminators, enriched in both treatments but more substantially in stress + FFA, reflecting enhanced osmoprotection, redox stability, and secondary metabolism.

CONCLUSION: Fulvic acid functionalization substantially enhanced the stress-mitigation potential of Fe3O4 NP, with FFA conferring significant improvements in growth, photosynthesis, antioxidant defense, gene regulation, and metabolic reprogramming, suggesting its potential as a nano-biofertilizer for saline-alkali soils.