Improved efficacy of foliar application of zinc oxide nanoparticles on zinc biofortification, primary productivity and secondary metabolite production in dragonhead

Improving medicinal plants' performance in semi-arid regions facing climate change could support sustainable food and agriculture production. Nano-fertilization is considered an emerging strategy for increasing plant production, while avoiding agroecosystem contamination. The objective of this study was to evaluate potential beneficial effects of foliar applications of two sources of zinc (Zn oxide nanoparticles [ZnONPs] as a novel nano-fertilizer and Zn sulfide [ZnS] as a conventional fertilizer) on Zn biofortification, primary productivity (biomass) and secondary metabolite production in a medicinally important dragonhead ( Dracocephalum moldavica L.) under semi-arid conditions. After 60- and 74-day post-planting, the desired fertilizers were sprayed on to plant foliage. The results showed that the leaf Zn bioaccumulation, total chlorophyll, and total flavonoids increased in a dose-dependent manner for both Zn compounds. Sub-toxic levels of Zn-based NPs uptake (up to on average 152.6 mg Zn/kg upon foliar treatment with 160 mg ZnONPs treatment) led to stimulatory effects on total chlorophyll (0.93 g/kg FW), shoot biomass (2168 kg/ha), and essential oil content (3.4 g/kg) and yield (7.4 kg/ha) compared to ZnS treatment or control. The levels of secondary metabolites including neral, geraniol and geranial increased significantly with 160 mg/L ZnONPs compared to equivalent concentration of ZnS or control. The TEM analyses of leaf ultrastructure revealed changes in cellular organelles at 400 mg/L ZnONPs, likely a result of higher oxidative stress (malondialdehyde and H 2 O 2 ). Higher efficacy of ZnONPs will support greater economic return for dragonhead growers and curtail environmental contamination. Foliar application of 160 mg/L ZnONPs is recommended for optimal micronutrient Zn biofortification, biomass production and essential oil yield in dragonhead, and may serve as a sustainable strategy to promote agricultural yield in (semi-) arid regions. • Foliar nano-fertilization tested for sustainable agriculture in semi-arid condition. • Foliar zinc oxide nanoparticle (ZnONP) promoted dragonhead biomass than Zn sulfide (ZnS). • ZnONP significantly enriched leaf Zn accumulation and oil yield than ZnS. • ZnONP yielded more medicinally valuable secondary metabolites than ZnS. • Foliar ZnONP may enhance sustainable food and agriculture production.