## 4 Discussion
In a cell suspension of _Nicotiana tabacum_, ZnONPs reduced the content of phenolic compounds [50], whereas they increased nicotine content in the callus of the related species, _Nicotiana benthamiana_[51]. A higher flavonoid concentration than in the control was obtained in extracts from the callus of _Echinacea purpurea_ from a culture with various concentrations of ZnONPs [52]. The substantial changes in the content of phenolic compounds and other antioxidants measured as TAC (Figures 2 and 3) suggest that oxidative stress takes place in cells in the presence of ZnONPs. Other authors also confirm that ZnONPs can cause changes in parameters such as antioxidant enzyme activity and antioxidant concentrations, as well as damage to cell components. In the callus of _Solanum lycopeneum_, ZnONPs caused an increase in the activity of the main antioxidant enzymes, superoxide dismutase and catalase [31]. In the callus of _Linum usitatissimum_ cv. Barbara, these nanoparticles were shown to increase free radical production, lipid peroxidation, the concentration of 8-oxoguanin--a marker of oxidative damage in DNA, and the activity of antioxidant enzymes [53]. On the other hand, a reduction in the activity of superoxide dismutase and peroxidase and high activity of protease and caspase were noted in a culture of _Nicotiana tabucum_ cells treated with ZnONPs [51]. Cucumber plantlets at lower concentrations of zinc oxide nanoparticles had oxidative stress parameters similar to the control, but at higher concentrations they had increased levels of superoxide dismutase and catalase activity in the leaves [54]. The response of plants to ZnONPs depends largely on the concentration used. Blackberry plantlets on a medium with 10 or 20 mg dm\({}^{-3}\) ZnONPs had similar content of chlorophyll \(a\) and \(b\) and total chlorophyll to the controls (Figure 4). A negative effect of ZnONPs on blackberry plantlets was manifested at 30 and 40 mg dm\({}^{-3}\) as a reduction in the concentration of chlorophyll \(a\) and total chlorophyll, and at 40 mg dm\({}^{-3}\), in chlorophyll \(b\) as well. Zinc oxide nanoparticles also reduced the content of carotenoids in the blackberry plantlets (Figure 4). ZnO nanoparticles--directly or through released zinc ions--enter into interactions with cells and can damage organelles, such as chloroplasts. ZnONPs have also been shown to decrease chlorophyll in mature plants: green pea [55], wheat leaves [56], and the seedlings of mung bean and chickpea [57]. In mesophyll cells of barley [47], zinc oxide nanoparticles caused a reduction in the size of chloroplasts, which had an abnormal structure and reduced photosynthetic activity. The phytotoxic effect of high concentrations of ZnONPs on chloroplast function in tomato plants was manifested as a decreased content of chlorophyll \(a\) and chlorophyll \(b\), reduced photosynthetic efficiency, and reduced chlorophyll fluorescence parameters [58]. Micropropagated strawberry plantlets adapt well to ex vitro conditions (Table 3, Figure 5), as also confirmed by other authors [12]. In the Bacteria\(-\) treatment, only the lowest concentration of ZnONPs had no negative effect on the number and length of shoots (Table 3). However, even this concentration of zinc oxide nanoparticles inhibited root growth and the formation of fresh biomass compared to the control. The results from the Bacteria\(-\) trial clearly indicate that prolonged cultivation enhances the toxicity of nanoparticles for the plantlets, and the effect depends on the concentration of ZnONPs. The reduction in growth parameters and signs of oxidative stress induced by ZnONPs in the present study prompted us to inoculate plantlets with _B. thuringiensis. Bacillus thuringiensis_ is mainly known for its ability to produce toxins with insecticidal properties [22; 23; 24].