## 2 Materials and Methods
The H\({}_{2}\)O\({}_{2}\) solutions were prepared from 35% H\({}_{2}\)O\({}_{2}\) (CentralChem, Bratislava, Slovakia) and diluted with tap water to the desired concentration. NO\({}_{3}\)\({}^{-}\) solutions were prepared from NaNO\({}_{3}\) salt (Penta, Praha, Czechia) that was dissolved and diluted with tap water to desired concentrations. The chemical composition and pH of all variants used for lettuce plant irrigation are listed in the Table 1. These variants include the PAW (0.42 mM H\({}_{2}\)O\({}_{2}\) + 0.85 mM NO\({}_{3}\)\({}^{-}\)), solutions of various concentrations of H\({}_{2}\)O\({}_{2}\) (0.4, 1, 10 mM), NO\({}_{3}\)\({}^{-}\) (0.85, 2, 20 mM), H\({}_{2}\)O\({}_{2}\) + NO\({}_{3}\)\({}^{-}\) (0.4 mM + 0.85 mM, and 10 mM + 20 mM), and control. The table defines abbreviations for each solution that are further being used in the text and all figures. The control plants were irrigated with tap water only. The type of soil of low nutrient content (loam-sand soil) was used. The reason why we tested the effect of PAW and H\({}_{2}\)O\({}_{2}\)/NO\({}_{3}\)\({}^{-}\) solutions on pre-grown plants and not on seeds is that the stimulating effect of PAW is usually more pronounced in the later stages of vegetative plant growth, when plants need more nutrients. During the germination, seeds and young seedlings take nutrients mainly from seed supplies, while in later stages, from or soil or water. After 5 weeks of irrigation with the PAW or with solutions of H\({}_{2}\)O\({}_{2}\) and/or NO\({}_{3}\)\({}^{-}\), the complex evaluation of visual appearance (number and quality of leaves), growth parameters (fresh and dry weight), photosynthetic pigment (chlorophyll a + b) content, \begin{table}
\begin{tabular}{c c c c} \hline
**Solution/Variant (Abbrev.)** & **H\({}_{2}\)O\({}_{2}\) [mM]** & **NO\({}_{3}\)\({}^{-}\) [mM]** & **pH** \\ \hline Control (tap water) & - & \(\sim\)0.02 & -7.5 \\ PAW & -0.42 & -0.85 & -7.5 \\ H\({}_{2}\)O\({}_{2}\) (4) & 0.4 & - & -7.6 \\ H\({}_{2}\)O\({}_{2}\) (1) & 1.0 & - & -7.7 \\ H\({}_{2}\)O\({}_{2}\) (10) & 10.0 & - & -7.8 \\ NO\({}_{3}\) (-85) & - & 0.85 & -7.9 \\ NO\({}_{3}\) (2) & - & 2.0 & -7.9 \\ NO\({}_{3}\) (20) & - & 20.0 & -7.9 \\ H\({}_{2}\)O\({}_{2}\) (4) + NO\({}_{3}\)\({}^{-}\) (.85) & 0.4 & 0.85 & -7.9 \\ H\({}_{2}\)O\({}_{2}\) (10) + NO\({}_{3}\)\({}^{-}\) (20) & 10.0 & 20.0 & -7.4 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and pH of plasma activated water (PAW) and various H\({}_{2}\)O\({}_{2}\)/NO\({}_{3}\)\({}^{-}\) solutions. The respective concentrations are indicated in parentheses in abbreviated format that is used in text and figures. photosynthetic rate, and activity of antioxidant enzymes (superoxide dismutase, SOD) in above-ground parts and roots of lettuce plants was performed. Visual appearance was evaluated by counting the number of leaves per plant and qualifying them as healthy (green) or senescent (physiologically aged). The fresh weight of above-ground parts and roots of lettuce were measured separately. The dry weight was also determined separately, after plant parts were dried packed in aluminum foil at 60 \({}^{\circ}\)C for 3 days. The concentration of photosynthetic pigments, i.e., chlorophylls (a + b), in leaves were measured according to Lichenthaler [44]. The method is based on absorbance of pigments in the UV/Vis region. The three repetitions per 1 g of fresh weight of average leaf sample per one variant were homogenized with sand, MgCO3, and 80% acetone (v/v). The homogenized leaves were filtered and filled to a certain volume with acetone. The concentrations of chlorophyll a and chlorophyll b were calculated from the absorbance at 664 and 648 nm, respectively. Net photosynthetic rate was measured by infrared gas analyzer (PP-Systems CIRAS-2, Amesbury, MA, USA). The principle of the measurement is based on the CO2 concentration changes estimated by absorption of infrared radiation.