Foliar-applied nanoscale zero-valent iron (nZVI) and iron oxide (Fe3O4) induce differential responses in growth, physiology, antioxidative defense and biochemical indices in Leonurus cardiaca L.

The impacts of nZVI and iron oxides on growth, physiology and elicitation of bioactive antioxidant metabolites in medicinal aromatic plants must be critically assessed to ensure their safe utilization within the food chain and achieve nutritional gains. The present study investigated and compared the morpho-physiological and biochemical changes of Leonurus cardiaca L. plants as affected by various concentrations (0, 250, 500 and 1000 mg L-1) of nZVI and Fe3O4. The foliar uptake of nZVI was verified through Scanning Electron Microscopy (SEM) images and Energy Dispersive X-ray (EDX) analytical spectra. Plants exposed to nZVI at low concentration showed comparatively monotonic deposition of NPs on the surface of leaves, however, the agglomerate size of nZVI was raised as their doses increased, leading to remarkable changes in anatomical and biochemical traits. 250 mg L-1 nZVI and 500 mg L-1 Fe3O4 significantly (P < 0.05) increased plant dry matter accumulation by 37.8 and 27% over the control, respectively. The treatments of nZVI and Fe3O4 at 250 mg L-1 significantly (P < 0.01) improved chlorophyll a content by 22.4% and 15.3% as compared to the control, and then a rapid decrease (by 14.8% and 4.1%) followed at 1000 mg L-1, respectively. Both nZVI and Fe3O4 at 250 mg L-1 had no significant impact on malondialdehyde (MDA) formation, however, at an exposure of 500-1000 mg L-1, the MDA levels and cellular electrolyte leakage were increased. Although nZVI particles could be utilized by plants and enhanced the synthesis of chlorophylls and secondary metabolites, they appeared to be more toxic than Fe3O4 at 1000 mg L-1. Exposure to nZVI levels showed positive, negative and or neutral impacts on leaf water content compared to control, while no significant difference was observed with Fe3O4 treatments. Soluble sugar, total phenolics and hyperoside content were significantly increased upon optimum concentrations of employed treatments-with 250 mg L-1 nZVI being most superior. Among the extracts, those obtained from plants treated with 250-500 mg L-1 nZVI revealed the strong antioxidant activity in terms of scavenging free radical (DPPH) and chelating ferrous ions. These results suggest that nZVI (at lower concentration) has alternative and additional benefits both as nano-fertilizer and nano-elicitor for biosynthesis of antioxidant metabolites in plants, but at high concentrations is more toxic than Fe3O4.