# Responses of Medicinal and Aromatic Plants to Engineered Nanoparticles +
## 2 Effects of Metal-, Metalloid-, and Carbon-Based NPs on Vascular Plants  
Metal-, carbon-, silicon-, and selenium-based compounds, applied in bulk or in nanoform, affect growth and development of vascular plants. In general, metal ions and metal NPs, including some essential metals (e.g., Cu, Zn, Fe. Ni, etc.) applied at higher concentrations, are phytotoxic, generate oxidative stress, adversely affect germination, photosynthetic processes, inhibit activities of enzymes and biosynthesis of photosynthetic pigments and non-enzymatic antioxidants, and accumulate high metal amounts in their plant organs, which is reflected in impaired plant growth and development [55; 56; 57; 76; 77; 78; 79; 80; 81; 82; 83]. On the other hand, some medicinal plants (e.g., _Hypericum perforatum_ L. or _Matricia recititiatia_ L.) were found to be tolerant metal excess and accumulate in their shoots/leaves high concentrations of toxic metal, such as cadmium [83; 84]. Therefore, to ensure food safety,cultivation of medicinal plants for commercial purposes under natural conditions must be performed on soils that are not contaminated with toxic metals [85].  
Moreover, bulk and nanoscale metals and metal oxides can also be cytotoxic and genotoxic, cause cell-cycle arrest and apoptotic induction, as well as serious damage to DNA [86; 87; 88; 89; 90]. On the other hand, their low doses may stimulate plant growth and can be used as effective fertilizers for achieving higher yields [55; 56; 91; 92; 93; 94; 95; 96]. Therefore, for individual plant species, it is necessary to known which concentrations have plant growth simulating effects and those with plant growth inhibiting impact.  
Carbon-based nanomaterials, such as multi-walled carbon nanotubes (MWCNTs), can penetrate seed coats and, when used in low concentrations, improve water absorption, water transport, seed germination, nitrogenase, photosystem and antioxidant activities, activate water channel proteins, and stimulate nutritional absorption [97; 98]. The entry of carbon-based nanomaterials into roots in an apoplastic way is thought to cause elongation of cell membrane pores [99] and the increased ability of plants to absorb water and essential nutrients when exposed to carbon nanotubes leads to stimulation of plant growth [98; 100; 101]. They can also affect the plant phenotype and composition of soil microbiota [102]. On the other hand, at higher concentrations, CNTs may have an adverse effect on plants due to the reactive oxygen species (ROS) generation, leading to cell death [103].  
Silicon is a metalloid, which is beneficial for plant growth and development, particularly under abiotic stress conditions (e.g., high salinity, water deficit, high temperature, presence of toxic metals) because it can alleviate damages in stressed plants; it can activate some defense mechanisms and regulate some physiological processes related to defense mechanisms in plants [50; 54; 104; 105]. [PERSON] and [PERSON] [106] stated that mitigation of abiotic stresses in vascular plants by Si is associated with SiO\({}_{2}\) deposition inside the plant tissues. Se acts as antioxidant and protects plants from oxidative damages and, depending on applied concentration; it can have both beneficial and adverse impact on plants, and modulate negative effects of abiotic stresses on plants, e.g., [107; 108].  
NPs affecting medicinal and aromatic plants discussed in this review are presented in Figure 1. Beneficial and adverse effects of NPs on medicinal and aromatic plants are shown in Figure 2.  
Figure 1: Types of nanoparticles (NPs) affecting medicinal and aromatic plants discussed in this paper.