## 4 Conclusions  
The effect of PAW generated by transient spark discharge on visual appearance, growth parameters, and important physiological and biochemical parameters of lettuce plants were investigated. The effect of irrigation with the PAW was compared and correlated with the effect of various H2O2 and/or NO3\({}^{-}\) solutions.  
Lettuce plants irrigated with the PAW in comparison to lettuce irrigated with chemically equivalent solution of H2O2 (.4) + NO3\({}^{-}\) (.85) had similar dry weight of above-ground parts and roots. However, the PAW induced higher photosynthetic pigment content (chlorophyll a + b), higher photosynthetic rate, and lower activity of antioxidant enzymes (superoxide dismutase, SOD). The NO3\({}^{-}\) in the solution mainly contributed to the increase of dry weight, photosynthetic pigment content, photosynthetic rate, decrease of SOD  
Figure 6: Activity of superoxide dismutase (SOD) in the 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 (.04, 1, 10 mM), NO3\({}^{-}\) (.085, 2, 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. The lowercase letters represent statistically significant difference at probability \(p\) < 0.05.  
activity, and overall better visual appearance of plants. The H\({}_{2}\)O\({}_{2}\) in the solutions had usually negative effect on the development of plants. Although it induced an increase of dry weight, it did not contribute to photosynthetic pigment content and photosynthetic rate and even increased SOD activity. The NO\({}_{3}\)\({}^{-}\) and H\({}_{2}\)O\({}_{2}\) are, without a doubt, the most important RONS in the PAW that affects the development and quality of the plants. Their joined effect usually results in an improvement of the plant growth and their physiological parameters; however, their ratio must be optimized for further improvement. Based on the results obtained in this study, we can conclude that PAW may positively affect several physiological parameters of plants. However, the overall effect of PAW on plant growth in this study was rather limited. Thus, optimizing the PAW composition must be performed prior to its eventual applications in agriculture.  
Conceptualization, K.K., M.H., and K.H.; methodology, K.K., M.H., L'S., and M.B.; investigation, K.K., M.B., and K.H.; data processing, K.K.; writing--original draft preparation, K.K. and K.H.; writing--review and editing, M.H., and M.B.; visualization, K.K. and K.H.; supervision, K.H.; project administration, K.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Slovak Research and Development Agency APVV 17-0382 and SK-PL-18-0090 grants and Slovak Grant Agency VEGA 1/0419/18.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors would like to thank [PERSON] for the help with an interpretation of selected results.  
The authors declare no conflict of interest.