## 3 Results and Discussion
### Antioxidant Enzyme Activity  
Fumigation with gaseous ozone applied to _Kalanchoe daigremontiana_ plants led to an increase in reactive oxygen species (ROS), such as superoxide anions (\(\mathrm{O_{2}}^{\bullet-}\)) and hydrogen peroxide (\(\mathrm{H_{2}O_{2}}\)) in plant cells (Figure 4). The increased production of ROS coincided with the activation of the antioxidant system in the plant cells, consisting of a number of  
Figure 4: ROS generation in _Kalanchoe daigremontiana_ leaves depending on the duration of ozone treatment (n = 20): differences between the results for duration of ozone treatment on specific days are marked with small letters, and differences between dose of ozone (ppm) measurements are marked with capital letters; significance level is defined as \(p<0.05\).  
enzymes responsible for the decomposition of ROS, as well as small-molecule compounds easily reacting with free radicals. The enzymatic antioxidant system includes enzymes such as superoxide dismutase (SOD) and catalase (CAT) [32; 43].  
The findings of the study show a visible increase in the activity of SOD (Figure 5A) and CAT (Figure 5B) after 1 min of ozone treatment, irrespective of the ozone concentration applied; it is likely that this effect, in combination with the increased level of low-molecular weight antioxidants (Figure 2), contributed to the decreased production of ROS in cells of _Kalanchea diagramotiana_ plants (Figure 4). The decrease in the production of ROS in cells of plants treated with ozone for a short time (1 min) possibly resulted from the fact that superoxide anion radicals were transformed by SOD into hydrogen peroxide and oxygen [44], which is a natural mechanism of initial defence against oxidative stress. Subsequently, hydrogen peroxide is decomposed by CAT to H\({}_{2}\)O and O\({}_{2}\), which consequently leads to a greater tolerance in _Kalanchea diagramotiana_ plants to stress induced by gaseous ozone [45]. It should also be emphasised that the activation of these enzymes in the chloroplasts of plants treated with ozone contributed to the lack of defects in these plants due to the activity of this reactive gas. The increased activity of SOD and CAT, resulting from the rapid production of ROS in cells affected by a stressor (ozone, drought, pathogen attack), was also observed in other studies [46]. The researchers found that the exposure of three varieties of _P. vulgaris_ to ozone led to a significantly increased activity of CAT, compared to the value identified in the control plants. The authors, however, showed that the activity of CAT was different, relative to the variety of _P. vulgaris_, which means that in the case of the two other varieties, seedlings exposed to O\({}_{3}\) did not exhibit significant changes in the activity of CAT, compared to their respective control samples [46].  
A longer duration of ozone treatment applied to _Kalanchea diagramotiana_, in particular at a rate of 10 ppm, produced phytotoxic effects, because the activity of SOD and CAT was gradually decreased (Figure 5A,B), whereas the level of ROS was increased (Figure 4). Phytotoxic effects of high doses of ozone, resulting from the decreased expression of CAT and SOD, were also demonstrated by other researchers. [PERSON] et al. [37] showed that as a result of ozone-induced oxidative stress there was a decrease in the expression of enzymes, mainly SOD, in the leaves of _Oryza sativa_ L.  
Figure 5: SOD (**A**) and CAT (**B**) activity in _Kalanchea diagramotiana_ leaves depending on the duration of ozone treatment (n = 20): differences between the results for duration of ozone treatment on specific days are marked with small letters, and differences between dose of ozone (ppm) measurements are marked with capital letters; significance level is defined as \(p\) < 0.05.