Nutrient-containing nanomaterials have been developed as fertilizers to foster plant growth and agricultural yield through root applications. However, if applied through leaves, how these nanomaterials, e.g. γ-Fe2O3 nanoparticles (NPs), influence the plant growth and health are largely unknown. This study is aimed to assess the effects of foliar-applied γ-Fe2O3 NPs and their ionic counterparts on plant physiology of Citrus maxima and the associated mechanisms. No significant changes of chlorophyll content and root activity were observed upon the exposure of 20–100 mg/L γ-Fe2O3 NPs and Fe3+. In C. maxima roots, no oxidative stress occurred under all Fe treatments. In the shoots, 20 and 50 mg/L γ-Fe2O3 NPs did not induce oxidative stress while 100 mg/L γ-Fe2O3 NPs did. Furthermore, there was a positive correlation between the dosages of γ-Fe2O3 NPs and Fe3+ and iron accumulation in shoots. However, the accumulated iron in shoots was not translocated down to roots. We observed down-regulation of ferric-chelate reductase (FRO2) gene expression exposed to γ-Fe2O3 NPs and Fe3+ treatments. The gene expression of a Fe2+ transporter, Nramp3, was down regulated as well under γ-Fe2O3 NPs exposure. Although 100 mg/L γ-Fe2O3 NPs and 20–100 mg/L Fe3+ led to higher wax content, genes associated with wax formation (WIN1) and transport (ABCG12) were downregulated or unchanged compared to the control. Our results showed that both γ-Fe2O3 NPs and Fe3+ exposure via foliar spray had an inconsequential effect on plant growth, but γ-Fe2O3 NPs can reduce nutrient loss due to their the strong adsorption ability. C. maxima plants exposed to γ-Fe2O3 NPs and Fe3+ were in iron-replete status. Moreover, the biosynthesis and transport of wax is a collaborative and multigene controlled process. This study compared the various effects of γ-Fe2O3 NPs, Fe3+ and Fe chelate and exhibited the advantages of NPs as a foliar fertilizer, laying the foundation for the future applications of nutrient-containing nanomaterials in agriculture and horticulture.

Heavy metal pollution poses great toxicity to plants, which makes methods to mitigate the heavy metal-induced toxicity desirable. This study explored the efficacy of carbon dots (CDs) in reducing the toxicity of cadmium ions (Cd²⁺) to wheat seedlings after 5 days of hydroponic exposure. When treated with Cd²⁺, the addition of CDs (50 and 75 mg L⁻¹) can significantly reduce the cadmium content (33.1–57.7%) in the roots and leaves. Adsorption experiments proved that CDs could adsorb Cd²⁺. The physiological results showed that Cd²⁺ at 50 mg L⁻¹ reduced the shoot length (32.4 ± 5.2%) of wheat, increased the content of soluble sugar and soluble protein, and increased APX, CAT and POD activity in wheat leaves. Comparatively, the joint treatment of 50 mg L⁻¹ Cd²⁺ and 50 mg L⁻¹ CDs significantly recovered the above parameters. Regarding the synthesis of glutathione (an antioxidant), 50 mg L⁻¹ Cd²⁺ decreased the expression of related genes (ATPS) by 36.9 ± 4.4% and indirectly enhanced the expression of APR, SiR, CS and GS. In contrast, in the treatment with both Cd²⁺ and CDs (50 and 75 mg L⁻¹), the expression of ATPS was higher and the expressions of APR and SiR genes were lower than those in the group of 50 mg L⁻¹ Cd²⁺ alone. This finding confirms that CDs can alleviate the abiotic stress induced by Cd²⁺ to plants and lays the foundation for its application in environmental protection and agricultural production.