## 2 Materials and Methods  
The experimental setup is depicted in Figure 1a. The plasma reactor of a point-to-plane geometry consisted of a high voltage needle electrode placed above a grounded electrode embedded inside an inclined polytetrafluoroethylene (PTFE) plane. The distance between the electrodes was \(\sim\)1 cm. Tap water was placed in a test tube and, by a peristaltic pump, (Masterflex L/S, Vernon Hills, IL, USA) pumped down the inclined electrode and back to the tube with the constant flow rate of 14 mL\(\cdot\)min\({}^{-1}\). Self-pulsing transient spark (TS) discharge generated in atmospheric pressure air [39] was applied to the tap water circulating through the discharge zone to generate the PAW. The test tube was immersed in an ice bath to prevent the unwanted heating of the produced PAW. The TS discharge was driven by positive DC power supply (Technix RS20-R-1200, Creteil, France) and its electrical characteristics were monitored by a high voltage probe (Tektronix P6015A, Berkshire, UK) and a Rogowski type current probe (Pearson Electronics 2877, Palo Alto, CA, USA) connected to an oscilloscope (Tektronix TDS 2024, Berkshire, UK). The typical electrical characteristics of TS discharge in our experiments was as follows: the amplitude of the applied voltage U \(\sim\) 10-13 kV, amplitude of discharge current pulses I\({}_{\text{max}}\)\(\sim\) 8-10 A, frequency of the discharge current pulses f \(\sim\) 2-3 kHz, and average discharge power was \(\sim\) 6 W. The characteristic voltage and current waveforms of the TS discharge are depicted in Figure 1b. The upper one presents the voltage waveform in ms timescale, while the bottom one presents both voltage and current waveforms in \(\upmu\)s scale. Figure 1: (**a**) The scheme of experimental setup and (**b**) characteristic voltage and current waveforms of self-pulsing transient spark discharge in two different timescales. The PAW was produced from tap water. Unlike deionized or distilled water, it is more physiological for plants and it has a natural ability to preserve constant pH thanks to hydrocarbon buffer system. Therefore, the PAW produced from tap water by TS discharge treatment/activation does not significantly changes its pH. The water activation time by plasma was set to 1 min\(\cdot\)mL\({}^{-1}\) (i.e., every 1 mL of water was activated by plasma for 1 min), and was constant for all experiments. The chemical composition of the PAW was measured by established UV/Vis spectrophotometric methods (Shimadzu UV-1800, Kyoto, Japan) to evaluate the concentrations of hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), nitrites (NO\({}_{2}\)\({}^{-}\)), and nitrates (NO\({}_{3}\)\({}^{-}\)). The H\({}_{2}\)O\({}_{2}\) concentration was determined by its reaction with titanyl ions of TiOSO\({}_{4}\) resulting into a yellow-colored product with maximum absorbance peak at 407 nm [40]. The NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) concentrations were determined by the commercial kit using Griess reagents (Cayman Chemicals, Ann Arbor, MI, USA) forming a pink-colored azo-product with maximum absorbance peak at 540 nm [41; 42]. The pH and temperature were monitored with portable pH meter (WTW 3110, Weilheim, Germany). Additional information on the experimental system, TS discharge and the chemical analysis can be found in our previous papers [18; 43]. As a model plant, we used lettuce (_Lactuca sativa_ L. var. capitata cv. [PERSON]). The plants were pre-cultivated in pots with soil at controlled conditions: 24/18 \({}^{\circ}\)C (light-dark), light intensity 120 mol\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) and 12 h photoperiod. Three week-old pre-grown plants that had 2-3 developed leaves were then separated--one plant per pot, four plants per each variant--and during the next 5 weeks, irrigated with the PAW or with solutions of the same and higher concentrations of H\({}_{2}\)O\({}_{2}\) and/or NO\({}_{3}\)\({}^{-}\) as those in the PAW.