Interaction of γ-FeO 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-FeO nanoparticles (NPs), influence the plant growth and health are largely unknown. This study is aimed to assess the effects of foliar-applied γ-FeO 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-FeO NPs and Fe. In C. maxima roots, no oxidative stress occurred under all Fe treatments. In the shoots, 20 and 50 mg/L γ-FeO NPs did not induce oxidative stress while 100 mg/L γ-FeO NPs did. Furthermore, there was a positive correlation between the dosages of γ-FeO NPs and Fe 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-FeO NPs and Fe treatments. The gene expression of a Fe transporter, Nramp3, was down regulated as well under γ-FeO NPs exposure. Although 100 mg/L γ-FeO NPs and 20-100 mg/L Fe 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-FeO NPs and Fe exposure via foliar spray had an inconsequential effect on plant growth, but γ-FeO NPs can reduce nutrient loss due to their the strong adsorption ability. C. maxima plants exposed to γ-FeO NPs and Fe 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-FeO NPs, Fe 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-FeO 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.