# Responses of Medicinal and Aromatic Plants to Engineered Nanoparticles +
## 4 Effect of Metal-Based NPs on Medicinal and Aromatic Plants
### Silver-Based NPs
#### 4.1.3 Effect on Antioxidant Enzymes  
Significantly enhanced callus induction of _Citrus reticulata_ L., as well as high TPC, TFC, and antioxidant activity were observed using co-treatment with 20 ppm AgNPs and 1 mg/L thidiauron; considerable increase of SOD, peroxidase (POD), and catalase (CAT) levels were recorded on MS medium supplemented with 30 ppm AgNPs and thidiauron [189]. Hormetic effects of Argovit on in vitro regeneration of vanilla using a temporary immersion bioreactor system were reported by [PERSON] et al. [127]. Whereas doses 25 and 50 mg/L of Argovit increased _Vanilla planifolia_ growth, application of 100 and 200 mg/L showed strong inhibitory effect; a dose-dependent effect on generation of ROS formation, and increasing TPC, antioxidant capacity, and lipid peroxidation was observed. AgNPs in combination with plant growth regulators added to MS medium pronouncedly enhanced callus biomass of _Caralluma tuberculata_. Stimulated production of phenolics, flavonoids, and increased PAL, SOD, POD, CAT, ascorbate peroxidase (APX) activities, as well as antioxidant activity were observed in the callus cultures treated with 90 mg/L AgNPs [124]. AgNPs green synthesized from leaf extracts of _Swertia chirata_ applied on regenerating shoot cultures of _S. chirata_ were found to generate ROS. However, due to induced antioxidant defense by antioxidant enzymes, ROS content was balanced and shoot regeneration increased, suggesting that AgNPs applied to plant tissue cultures can act as growth stimulant for improved shoot regeneration [190].  
_Calendula officinalis_ L. plants exposed to 50 ppm AgNPs and magnetic fields (3 mT), either alone or in combination, pronouncedly increased phenolic content, activities of several enzymes, such as PAL, polyphenol oxidase (PPO), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), and radical scavenging in plants, suggesting positive effect of AgNPs on the defense mechanism of _C. officinalis_ plants, combined treatment being the most effective [191]. Treatment of _Calendula officinalis_ with AgNPs also reduced its DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging activity and increased formation of ROS and membrane peroxidation [192]. In the extracts of _Calendula officinalis_ plants cultivated in hydroponic solution containing 0.4 mM AgNPs (30-50 nm), the extracts contained lower contents of chlorophyll (Chl), carotenoids, and anthocyanins than the control. Only content of flavonoids showed an increase by 14.6%; however, at higher AgNPsN concentrations, their content decreased as well, and saponin content increased by ca. 77% [192].  
It was reported that AgNPs pronouncedly suppressed oxidative stress triggered by naphthalene toxicity in _Moringa oleifera_ called the \"Miracle Tree\" [193]. AgNPs applied foliarly to the _Salvia officinalis_ plants, penetrated via the leaf epidermis into the parenchyma cells, and after application of 50 and 1000 mg/L AgNPs, reduced levels of assimilation pigments and increased activities of antioxidants (enzymatic and non-enzymatic) were able to cope with the oxidation stress, compared to the control plants [128]. The cetyltrimethylammonium bromide (CTAB) coated AgNPs were found to be toxic to _A. cepa_ roots, causing robust inhibition of roots and pronounced oxidative damage, whereas much bigger negatively charged citrate coated AgNPs aggregating to larger particles were least toxic. However, it could be mentioned that the toxicity of Ag\({}^{+}\) ions was higher than that of both coated AgNPsPs [194].