# Responses of Medicinal and Aromatic Plants to Engineered Nanoparticles +
## 4 Effect of Metal-Based NPs on Medicinal and Aromatic Plants
### Zinc-Based NPs
#### 4.4.2 Elicitor Effect  
Improved production of tropane alkaloids was observed using HRs obtained from two-week-old cotyledon explants of _Hyoscyamus reticulatus_ treated with 100 mg/L of ZnO NPs after 48 and 72 h, when the levels of hyoscyamine and scopolamine reached 37.63% and 37%, respectively. The highest expression of h6h gene in HRs was observed at treatment with 100 mg/L of ZnO NPs after 24 h [67]. Zn-Ag NPs (molar ratio of 19:1) exhibiting photocatalytic and elicitor activity were able to improve the light absorption and increase oxidative stress in _Withania somifera_ plants, resulting in increased photosynthesis and transpiration rates along with an increase in some carbohydrates and enhanced content of withanolide, which was associated with upregulation of genes involved in photosynthesis, Calvin cycle, carbohydrate metabolism, and withanolide biosynthesis. Moreover, cyclic electron flux managing the electron flow around photosystem (PS) I was observed [63]. Spraying of _Capsicum chinense_ Jacq. plants with 1000 ZnO NPs pronouncedly increased TPC, TFC, and content of secondary metabolites, such as dihydrocapsaicin in fruits, suggesting ameliorated nutraceutical quality of fruits [230]. _Hypericum perforatum_ cell suspension cultures elicited with 100 ppm ZnO NPs stimulated production of hypericin and hyperforin, up to 7.87 and 217.45 ug/g d.w., providing 3 and 13-fold higher amounts compared to the control [141]. ZnO NPs capped with PEG and PVP enhanced TPC, TFC, content of steviol glycosides in in shoots of _S. rebaudiana_ [PERSON] cultured in vitro more than upcapped ones [211]. By application of 2 mg/L ZnO NPs, 91% induction of in vitro roots formation in regenerants of _S. rebaudiana_, and increased contents of secondary metabolites of rebaudioside A (2.96% vs. 4.42%) and stevioside (1.01% vs. 1.28%) in leaves, compared to the control, were observed. With application of increasing ZnO NPs, the content of secondary metabolites gradually decreased to 1.22% and 0.21% for rebaudioside A and stevioside, respectively, corresponding to the values observed under stress induced by 2000 mg/L ZnO NPs [135]. In contrast to the beneficial impact of 1 mg/L ZnO NPs on shoot formation of micropropagated _S. rebaudiana_ and steviol glycosides levels, which were practically doubled compared to control, further increase of ZnO NPs was accompanied with a decrease in formation of secondary metabolites and antioxidant activities, and at application of 1000 mg/L ZnO NPs, a strong phytotoxic effect was observed [137]. Toxic impact of ZnNPs (<100 nm) applied at concentrations 400-1000 mg/L accompanied with pronounced reduction of stevioside was also observed in the nodal explants of _Stevia_ cultured in vitro on MS medium [238]. Fertilization of _Stevia rebaudiana_ with 75 mg/L of green synthesized ZnO NPs through roots enhanced the concentrations of TPC and TFC and Zn content up to 406.8% with respect to the control, although the biosynthetic pathway of steviol glycosides was not affected; treatment did not show adverse impact on plant growth [139]. Seedlings of flax (_Linum usitatissimum_) cultivated on MS in the presence of 10 mg/L ZnO NPs showed improved growth compared to control, while application of 500 mg/L ZnO NPs increased the production of seciosolaricinesinolglucoside, lariciresinol diglucoside, dehydrodiconiferyl alcohol glucoside and guiaacylglycerolglycerol-\(\beta\)-coniferyl alcohol ether glucoside. In another experiment, in which the stem explants were cultured on MS media supplemented with plant growth regulator and 25 mg/L ZnO NPs, enhanced rooting effect and effective accumulation of metabolites was observed [138]. In _Thymus kotschyanus_ and _T. daenensis_ micropropagated in the presence of 150 mg/L ZnO NPs thymol and carvacrol contents of 22.8 mg/L and 0.68 mg/L, respectively, were reported by [172].