## 2 Materials and Methods
### Activity of Antioxidant Enzymes  
The activity of antioxidant enzymes reflects the level of oxidative stress that a plant is exposed to and depends on many factors, e.g., direct or indirect plasma treatment, application on seeds, type of seedling or plant, etc. Here, we present the results on superoxide dismutase (SOD) activity as a representative enzyme that is a part of a complex specialized system protecting plant cells against oxidation. SOD is an intracellular antioxidant that decomposes superoxide to hydrogen peroxide in a reaction 2O2\({}^{\bullet-}\) + 2H\({}^{+}\) \(\rightarrow\) H2O2 + O2. The PAW can be a potential source of oxidation as it contains various reactive oxygen species and free radicals. However, in our experiments, we found a decrease of activity of antioxidant enzymes in the PAW irrigated plants by 15% when compared to control (Figure 6). It indicates that the PAW did not increase the oxidative stress in plant cells. However, it can be also an evidence of elevated oxidation stress in plants irrigated with tap water (control) with a deficit of nutrients. In comparison to the effect of PAW, the effect of various H2O2/ NO3\({}^{-}\) solutions on the activity of antioxidant enzymes was a little more complicated. The SOD activity in above-ground parts of plants decreased with concentration of NO3\({}^{-}\) (20) by 42% and increased with concentration of H2O2 (10) by 60% (Figure 6). The H2O2 can contribute to elevated oxidative stress by its oxidation-reduction potential, but also as the result of nutrient deficit. On the other hand, NO3\({}^{-}\) works well  
Figure 5: Net photosynthetic rate in above-ground parts of lettuce plants after 5 weeks of soil cultivation irrigated with the plasma activated water (PAW), solutions of various concentrations of H2O2 (10 mM), NO3\({}^{-}\) (20 mM), H2O2 + NO3\({}^{-}\) (0.4 mM + 0.85 mM, 10 mM + 20 mM), and control irrigated with tap water. The data are presented as mean values \(\pm\) standard deviation from 3 independent repetitions.  
against the antioxidant enzymes activity through its nutrient function and the increase of NO3\({}^{-}\) (.85, 2, 20) led to a decrease of SOD activity, as also shown in our previous paper [18].  
The results reported by various groups are disparate. [PERSON] et al. found increased concentration of radicals and activation of antioxidant enzymes in seeds exposed to air plasma [13; 56]. [PERSON] et al. reported that PAW applied on mung bean seedlings possessed higher SOD activity indicating reduction of oxidative damage [17]. [PERSON] et al. also found the antioxidant enzyme activity level to be higher after the PAW treatment in fresh-cut kiwi fruit than when treated by HNO3 or H2O2 solution [57]. [PERSON] et al. found that dielectric-barrier discharge plasma treatment enhanced the antioxidant activity of wheat seeds under drought stress by regulating the functional and regulatory genes [58]. On the other hand, our current as well as previous results [18] indicate a decrease of SOD activity with an increase of PAW activity. [PERSON] et al. studied the effect of PAW generated by various air discharges in about-ground parts of barley and maize plants and found similar effect. They also observed a decrease of SOD, especially for PAW, but at the same time an increase of other antioxidants enzymes, e.g., guajacol peroxidase and catalase [20]. Overall, the activity of antioxidant enzymes can be affected by various parameters, e.g., growth phase of plants, soil type, or activity and composition of PAW. Our results showed that H2O2 stimulate SOD activity, while NO3\({}^{-}\) suppresses it. Thus, total SOD activity depends on their actual balance in the PAW and may decrease despite the fact that the PAW contains RONS that are eventual oxidative stressors.