Foliar co-application of zinc oxide and copper oxide nanoparticles promotes phytochemicals and essential oil production in dragonhead (Dracocephalum moldavica)

Individual nanoparticle application has been documented to promote plant production; however, whether co-application of two nanoparticles (NPs) is more sustainable and significantly promotes plant production is unclear. Herein, foliar co-applications of two NPs or their conventional fertilizer forms on the growth, micronutrient (copper and zinc) enrichment, primary productivity, and essential oil (EO) production in a medicinal annual, dragonhead (Dracocephalum moldavica L.), were investigated. Treatments included 1:1 ratio of zinc oxide nanoparticles (ZnONPs):copper oxide nanoparticles (CuONPs) (40-400 mg/L), and compared with individual NPs, individual zinc suspension (ZnS) and chelated copper (chelated-Cu), and their combination, at equivalent concentrations. Results showed that the highest bioenrichment of Zn and Cu was observed with 80-160 mg/L ZnS+chelated-Cu, 400 mg/L ZnONPs+CuONPs, or ionic combination treatments. A dose-dependent increase in hydrogen peroxide and malondialdehyde was observed with co-treatment of NPs or ions, and oxidative stress responses were higher with NPs or ions co-treatment than individual treatment. With 160 mg/L ZnONPs+CuONPs treatment, total chlorophyll, aboveground biomass, and essential oil production increased significantly compared to control, 160 mg/L CuONPs, and 160 mg/L ZnONPs (227, 157 and 823 %; 58, 79, and 51 %; and 46, 80, and 3 %, respectively). Flavonoid and anthocyanin content also increased significantly (58 and 50 %, respectively) with ZnONPs+CuONPs compared to ZnS+chelated-Cu and were higher than ZnONPs or CuONPs alone by 10 and 25 %, and 37 and 36 %, respectively. More importantly, EO production and quality improved with 160 mg/L ZnONPs+CuONPs treatment compared to control. Taken together, our findings showed that foliar co-treatment of 160 mg/L ZnONPs+CuONPs could significantly improve primary productivity, aboveground biomass, and EO quality and yield in dragonhead grown in semi-arid field conditions; and thus, 160 mg/L ZnONPs+CuONPs is recommended as an optimal foliar co-treatment strategy for promoting sustainable plant production in semi-arid regions where soil nutrients and water are limiting factors inhibiting crop yield.

Foliar enrichment of copper oxide nanoparticles promotes biomass, photosynthetic pigments, and commercially valuable secondary metabolites and essential oils in dragonhead (Dracocephalum moldavica L.) under semi-arid conditions

High alkaline and low organic carbon hinder micronutrients, such as copper (Cu), bioavailability in (semi-) arid soils, affecting plant nutrient quality and productivity. This study aimed at investigating the potential beneficial effects of foliar Cu oxide nanoparticles (CuONPs) and conventional chelated-Cu applications (0-400 mg Cu/L) on the biomass, physiological biomarkers of plant productivity and oxidative stress, Cu bioaugmentation, and essential oils and secondary metabolites in dragonhead (Dracocephalum moldavica [L.]) grown in Cu-limited alkaline soil in semi-arid condition. Employing a randomized complete block design with three replicates, two different sources of Cu (CuONPs and chelated-Cu), and a wide range of Cu concentrations (0, 40, 80, 160, and 400 mg Cu/L), plants were foliarly treated at day-60 and day-74. At day-120, plants were harvested at the end of the flowering stage. Results showed shoot Cu bioaccumulation, flavonoids and anthocyanin increased in a dose-dependent manner for both Cu compounds, but the beneficial effects were significantly higher with CuONPs compared to chelated-Cu treatments. Further, shoot biomass (23 %), photosynthetic pigments (chlorophyll-a and chlorophyll-b; 77 and 123 %, respectively), and essential oil content and yield (70 and 104 %, respectively) increased significantly with foliar application of 80 mg/L CuONPs compared to equivalent concentration of chelated-Cu, suggesting an optimal threshold beyond which toxicity was observed. Likewise, commercially important secondary metabolites' yield (such as geranyl acetate, geranial, neral, and geraniol) was higher with 80 mg/L CuONPs compared to 160 mg/L chelated-Cu (2.3, 0.5, 2.5, and 7.1 %, respectively). TEM analyses of leaf ultrastructure revealed altered cellular organelles for both compounds at 400 mg/L, corroborating the results of oxidative stress response (malondialdehyde and H2O2). In conclusion, these findings indicate significantly higher efficacy of CuONPs, with an optimal threshold of 80 mg/L, in promoting essential oil and bioactive compound yield in dragonhead and may pave a path for the use of nano-Cu as a sustainable fertilizer promoting agricultural production in semi-arid soils that are micronutrient Cu deficient.