## 3 Nano-Elicitation of Flavonoids
### CuNPs and CuONPs as a Novel Elicitor of Flavonoids  
CuNPs possess unique physical resistance, electrical conductivity and chemical reactivity linked to their small size and high specific surface area [50]. The foliar spray of CuNPs in different concentrations (50, 125, 250, 500 mg\(\cdot\)L\({}^{-1}\)) on tomato showed an increase in flavonoid contents in tomato fruit. The highest content of flavonoids in the tomato fruits was obtained with the application of 50 mg\(\cdot\)L\({}^{-1}\) of CuNPs, exceeding the control by 36.14%. This suggests that low doses of CuNPs generate metabolic effects due to the Cu accumulation and ROS generation [51]. _Ocinum bsilicum_ or basil is an important medic \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Plant Species**} & **Type of In Vitro Culture** & \multirow{2}{*}{**Growth Media**} & **NP Size** & **Effective** & \multirow{2}{*}{**Effect on Flavonoid Production**} & \multirow{2}{*}{**References**} \\  & **Culture** & & **(nm)** & & **of NPs** & & \\ \hline \multirow{4}{*}{_M. charantia_} & Cell suspension & \multirow{2}{*}{MS media} & \multirow{2}{*}{50} & \multirow{2}{*}{5 mg/L} & Enhanced production of & \multirow{2}{*}{} \\  & culture & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Role of silver NPs (AgNPs) as an elicitor of flavonoids. inal herb cultivated around the world which is enriched with bioactive compounds. A comparative study suggested the elevated regeneration capacity of basil plant via somatic embryogenesis using CuNPs as compared to copper sulfate solution [52]. Moreover, copper sulfate NPs (CuSO4 NPs) enhanced the flavonoid content in \(O\). _basilicum_ seeds [53]. CuONPs are also reported to elicit in vitro cultures for enhanced flavonoid production (Table 2). Nano-CuO stress on Srian Barley (_Hordeum vulgare_) leaves exhibited a significantly increased flavonoid level (-1.2-fold over the control) on day 20 after 20 days; however, chlorophyll content decreased significantly [54]. CuONPs upregulated the transcription of genes associated with oxidative stress, therefore enhancing the phytochemical profile of \(B\). _rapa_ seedlings [55]. The cell suspension culture of _Gymema sylvestre_ when elicited with CuONPs (25-55 nm) resulted in a 2-fold increase in TPC and TFC [56]. Similarly, a higher TFC (9.450 and 12.878 mg QE g\({}^{-1}\) extract) in the shoots and roots of _Cichorium thybus_ was observed upon nano-elicitation with CuONPs 20 days after treatment [57]. The highest flavonoid content, 23.076 \(\pm\) 5.128 mg QE/g extract, was observed in the roots of CuONP elicited _Withania somifera_ plant 20 days after treatment [58]. The influence of CuONPs and polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) impregnated CuONPs on seed germination, callus induction and the production of secondary metabolites in the shoots and roots of _Trigonella founm-gracum_ was studied in a comparative manner. It was found that PVP and PEG impregnated CuONPs were less toxic for calli induction. Furthermore, secondary metabolism was also enhanced as a result of nano-elicitation in a concentration dependent manner [59]. Despite the enhancement in bioactive compounds, a negative impact of nano-elicitation on biomass has also been reported. The influence of CuONPs on _A. thaliana_ seedlings showed a significant reduction in plant biomass and total chlorophyll content, while anthocyanin content enhanced in response to all concentrations [60]. _Stevia rebuudiana_, called candy leaf, is a perennial herb which naturally protects from diabetes mellitus, hypertension and obesity [61]. Callus culture of _S. rebuudiana_ was treated with CuONPs and ZnONPs in various concentrations.