# Responses of Medicinal and Aromatic Plants to Engineered Nanoparticles +
## 4 Effect of Metal-Based NPs on Medicinal and Aromatic Plants
### TiO\({}_{2}\)-Based NPs
#### 4.5.2 Elicitor Effect  
Spraying of _S. rebuudiana_ Bertoni plants with 400 ppm TiO\({}_{2}\) NPs showed pronounced beneficial effect on fresh and dry weights of shoots, while application of 200 ppm mostly enhanced stevioside glycoside and reduced MDA levels [147]. In addition, application of 100 mg/L TiO\({}_{2}\) NPs also increased EO content and contents of important secondary metabolites. Moreover, TiO\({}_{2}\) NPs ameliorated agronomic traits and increased antioxidant enzyme activities in \(D\). _moldavica_ L. plants grown under salt stress, and considerably reduced H\({}_{2}\)O\({}_{2}\) levels [149].  
The beneficial impact of 150 mg/L TiO\({}_{2}\) NPs on folily treated \(M\). _piperita_ plants was reflected in an increase of EO yield up to 105.1% and application of 150 mg/L TiO\({}_{2}\) NPs increased the yields of menthol, menthone, and menthol acetate by 124.1%, 169.4%, and 130.5%, respectively, over the control, whereby contents of these secondary metabolites were enhanced as well [144]. Concentration of 0.06% TiO\({}_{2}\) NPs was reported as most favorable for foliar spraying of _Cuminumuminum_ L. to achieve the highest EO yield of cumin [256]. Treatment of black cumin plants in the early flowering stage with 100 mg/L TiO\({}_{2}\) NPs promoted the geranyl diphosphate synthase (GPPS) expression 24 h after treatment more than application of a half dose, which resulted in improved production of thymquinone [257]. TiO\({}_{2}\) NPs applied folily at a dose 100 mg/L acted as an elicitor by improving secondary metabolism in _S. officinalis_ plants resulting in enhanced biosynthesis of natural antioxidants and at application of 100 mg/L TiO\({}_{2}\) NPs highest dry weights of roots and shoots were observed as well [146]. Spraying of rosemary leaves with TiO\({}_{2}\) NPs (7-times, using doses 20-400 ppm) pronouncedly affected quantity of EO, mostly increasing the content of secondary metabolites, although at concentrations >200 ppm some of these compounds (e.g., \(\beta\)-pinene, 3-pinanol, myrcene, 1,8-cineol) showed a decrease [258].  
[PERSON] et al. [154] compared the effect of TiO\({}_{2}\)/perlite nanocomposites (15.50-24.61 nm) applied at doses 25-200 mg/L on production of secondary metabolites in the calli obtained from in vitro, as well as from field grown plants of _Hypericum perforatum_. They found that, at application of 25 mg/L TiO\({}_{2}\)/perlite NCs, the enhancement of alkaloids production in the calli obtained from in vitro grown plants related to control was almost by one order higher than in calli obtained from field grown plants. Exposure of _Tunactum parthenium_ L. grown in greenhouse to TiO\({}_{2}\) NPs affected the expression of genes involved in biosynthesis pathway of parthenolide and \(\beta\)-caryophyllene; TiO\({}_{2}\) NPs upregulated the costunolide synthase (COST) and fewerfew (_T. parthetinium_) germacrene A synthase (TpGAS) but downregulated the fewerfew (_T. parthetinium_) (_E_)-\(\beta\)-caryophyllene synthase (TpCarS) with increasing time from 6 to 24 h. Exposure to salinity increased the expression of these three genes with increasing time, suggesting that TiO\({}_{2}\) NPs and salinity can be used as elicitors for achieving more effective production of secondary metabolites [259].  
Khusimol content and yield of _Vetveria zizanioides_ plants sprayed with 90 mg/L TiO\({}_{2}\) NPs 300 d after transplantation increased by 24.5% and 93.2%, respectively, compared to control [145]. In hydroponically cultivated _Nigella arrensis_ plants treated for 21 days with TiO\({}_{2}\) NPs increased amounts of secondary metabolites, such as glaucoma and quercetin, as well as higher TPC levels were observed compared to the control, although the concentrations of the elicitor providing the highest levels of individual bioactive components differed each from other [148].