## 3 Nano-Elicitation of Flavonoids
### Various Metallic Oxide NPs as an Elicitor of Flavonoids  
There are various other metallic oxide NPs reported as elicitors of flavonoids (Table 4). The effects of thidiazuron (TDZ) growth regulator and magnesium oxide NPs (MgONPs) on radish seeds under lead stress were evaluated. There was a significant increase in plant growth, phenolic and flavonoid contents, free radical scavenging activity, and lead phytoaccumulation in response to combined TDZ and MgONP treatment. Moreover, there was a positive correlation of flavonoids with dry biomass and lead accumulation [69; 74]. Nickle oxide NPs (NiONPs), CuO and ZnO significantly enhanced anthocyanins, TPC and TFC in in vitro grown eggplant seeds in a dose-dependent manner [75].  
_Dracoephalum kotschyi_ is a valuable source of rosmarinic acid and methoxylated hydroxyflavones which shows antioxidative and antiproliferative potential against various cancer cells [76]. The application of Fe\({}_{3}\)O\({}_{4}\)NPs induced secondary metabolic gene expression as well as enhanced flavonoid content in a hairy root culture of _D. kotschyi_[77]. Fe\({}_{3}\)O\({}_{4}\)NPs led to a superior TFC (4.65 mg g\({}^{-1}\) DW) in the hairy roots of chicory [78]. Iron NPs (FeNPs) showed growth promoting effects on in vitro grown _S. rebaudiana_ at lower concentrations (45 mg/L). However, higher doses (90 and 135 mg/L) of FeNPs were found detrimental to growth characteristics and development. Moreover, a higher concentration of FeNPs (135 mg/L) induced stress, which produced a higher TFC [79]. Recently, magnetic Fe\({}_{2}\)O\({}_{3}\)NPs in the presence of a static magnetic field (SMF) resulted in the enhanced production of rosmarinic acid, naringin, apigenin, thymol, carvacrol, quercetin and rutin, TPC, TFC, anthocyanins, lignin and malondialdehydedehyde in a cell suspension culture of _D. polychateum_[80]. The use of magnetic fields has recently been proposed for plant growth and the production of plant secondary metabolites since they are less hazardous for the environment [81].  
The role of cerium oxide NPs (CeO\({}_{2}\)NPs) on food quality is not well known yet. Therefore, the impact of CeO\({}_{2}\)NPs on the secondary metabolism of _R. sativus_ seed was analyzed. The results were not as promising, as NPs increased the level of antioxidant compounds and showed no effect on TPC and TFC [82]. _Nigella arreensis_ was treated with different concentrations of Al\({}_{2}\)O\({}_{3}\)NPs, NiONPs, and TiO\({}_{2}\)NPs for 21 days. A significant increase in TFC and TPC was found. Moreover, the maximum content of quercetin was observed in the shoot and root parts under 50 mg/L NiONPs with a 2.2- and 1.8-fold increase compared to the control group. However, the kaempfer content was significantly decreased in all treatments, except for 1000 mg/L NiONP treatment in the root parts, which was 2.9-fold higher than the control group. These findings suggest the potential use of NPs as elicitors, considering their effective concentration [83]. TiO\({}_{2}\)NPs have also been reported to enhance phenolic and flavonoid content in embryonic callus culture of _Cicer arietinum_[84].  
The stimulation of _D. kotschyi_ hairy roots by silicon dioxide NPs (SiO\({}_{2}\)NPs) led to a significant increase in the induction and production of important pharmaceutical compounds including anticancer flavonoids, including crisimaritin, xanthomicrol and isokaempfer [85]. SiO\({}_{2}\)NPs also elicited in vitro hairy root cultures of _H. reticulatus_ and _H. pusillus_[86]. Manganese oxide NPs (Mn\({}_{2}\)O\({}_{3}\)NPs) at a lower concentration promoted plantlet growth and enhanced the biosynthesis of the regulation of pharmacologically active metabolites by the activation of specific antioxidant enzymes. However, at higher concentrations (50-200 mg/L), Mn\({}_{2}\)O\({}_{3}\)NPs exhibited oxidative stress and toxicity in an vitro plant culture of deadly nightshade [87].