Interaction of γ-Fe(2)O(3) nanoparticles with Citrus maxima leaves and the corresponding physiological effects via foliar application

BACKGROUND: 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-Fe(2)O(3) nanoparticles (NPs), influence the plant growth and health are largely unknown. This study is aimed to assess the effects of foliar-applied γ-Fe(2)O(3) NPs and their ionic counterparts on plant physiology of Citrus maxima and the associated mechanisms. RESULTS: No significant changes of chlorophyll content and root activity were observed upon the exposure of 20–100 mg/L γ-Fe(2)O(3) NPs and Fe(3+). In C. maxima roots, no oxidative stress occurred under all Fe treatments. In the shoots, 20 and 50 mg/L γ-Fe(2)O(3) NPs did not induce oxidative stress while 100 mg/L γ-Fe(2)O(3) NPs did. Furthermore, there was a positive correlation between the dosages of γ-Fe(2)O(3) NPs and Fe(3+) 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-Fe(2)O(3) NPs and Fe(3+) treatments. The gene expression of a Fe(2+) transporter, Nramp3, was down regulated as well under γ-Fe(2)O(3) NPs exposure. Although 100 mg/L γ-Fe(2)O(3) NPs and 20–100 mg/L Fe(3+) led to higher wax content, genes associated with wax formation (WIN1) and transport (ABCG12) were downregulated or unchanged compared to the control. CONCLUSIONS: Our results showed that both γ-Fe(2)O(3) NPs and Fe(3+) exposure via foliar spray had an inconsequential effect on plant growth, but γ-Fe(2)O(3) NPs can reduce nutrient loss due to their the strong adsorption ability. C. maxima plants exposed to γ-Fe(2)O(3) NPs and Fe(3+) 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-Fe(2)O(3) NPs, Fe(3+) 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. [Figure: see text] ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12951-017-0286-1) contains supplementary material, which is available to authorized users.