Interaction of γ-Fe₂O₃ nanoparticles with Citrus maxima leaves and the corresponding physiological effects via foliar...

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. Graphical abstract γ-Fe₂O₃ NPs exposed on plants via foliar spray and genes associated with the absorption and transformation of iron, as well as wax synthesis and secretion in Citrus maxima leaves.