| Summary: | Cold plasma generated by atmospheric pressure air discharge is a source of various gaseous reactive oxygen and nitrogen species (RONS). When the plasma is generated in a contact with water, the RONS dissolve into water, change its chemical composition, while producing so-called plasma activated water (PAW). The PAW has the potential to be effectively used in various agricultural applications, as the long lived liquid RONS (H<sub>2</sub>O<sub>2</sub>, NO<sub>2</sub><sup>−</sup>, NO<sub>3</sub><sup>−</sup>) may act like signaling molecules in plant metabolism or serve as nutrients. We studied the effect of the PAW on lettuce plants and compared it with the effect of H<sub>2</sub>O<sub>2</sub> and/or NO<sub>3</sub><sup>−</sup> solutions of various concentrations to assess their role in the PAW. The PAW was generated from tap water by DC driven self-pulsing transient spark discharge. Pre-grown lettuce plants were cultivated in pots with soil and irrigated with the PAW or solutions of H<sub>2</sub>O<sub>2</sub> and/or NO<sub>3</sub><sup>−</sup>. After 5 weeks the growth parameters, number and quality of leaves, fresh and dry weight of plants, photosynthetic pigment (chlorophyll a + b) content, photosynthetic rate, and activity of antioxidant enzymes (superoxide dismutase, SOD) were evaluated. Lettuce plants irrigated with the PAW in comparison with chemically equivalent solution of H<sub>2</sub>O<sub>2</sub> and NO<sub>3</sub><sup>−</sup> had similar dry weight; however, the PAW induced higher photosynthetic pigment content, higher photosynthetic rate, and lower activity of SOD. The NO<sub>3</sub><sup>−</sup> mainly contributed to the increase of dry weight, photosynthetic pigment content, photosynthetic rate, and overall better appearance of plants. The H<sub>2</sub>O<sub>2</sub> contributed to an increase of dry weight and induced SOD activity. In general, H<sub>2</sub>O<sub>2</sub> and NO<sub>3</sub><sup>−</sup> in proper concentrations can stimulate plant growth and affect their physiological properties.
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