Effect of Plasma Activated Water, Hydrogen Peroxide, and Nitrates on Lettuce Growth and Its Physiological Parameters

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 (H2O2, NO2−, NO3−) 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 H2O2 and/or NO3− 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 H2O2 and/or NO3−. 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 H2O2 and NO3− had similar dry weight; however, the PAW induced higher photosynthetic pigment content, higher photosynthetic rate, and lower activity of SOD. The NO3− mainly contributed to the increase of dry weight, photosynthetic pigment content, photosynthetic rate, and overall better appearance of plants. The H2O2 contributed to an increase of dry weight and induced SOD activity. In general, H2O2 and NO3− in proper concentrations can stimulate plant growth and affect their physiological properties.