# Plasma-Activated Water Affects the Antioxidant Contents in Water Spinach
## 1 Introduction  
Plasma is generated by feeding different kinds of gases to an electric or electromagnetic field, thereby, providing adequate energy for ionizing or dissociating these gases by exciting collisions [1]. It can be categorized into high-temperature plasma, thermal plasma, and nonthermal or cold plasma [2]. The latter is an emerging nonchemical pretreatment technology that has been recently applied in agricultural applications and food safety to induce changes in plant characteristics [3; 4; 5], eliminate pathogens or inhibit microorganisms [6; 7; 8], promote seed germination [9], increase crop quantity [10; 11], improve crop preservation, and reduce pesticides [12; 13]. Cold atmospheric pressure plasma is applied to water to generate reactive oxygen species (ROS), reactive nitrogen species (RNS), ultraviolet (UV) radiation, and other chemical species in liquids. The newly formed solution is termed as plasma-activated water (PAW), which is acidic in nature resulting from the changes in oxidation-reduction potential (ORP), electrical conductivity (EC), and formation of ROS and RNS, such as the long-lived hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), nitrite (NO\({}_{2}\)\({}^{-}\)), and nitrate (NO\({}_{3}\)\({}^{-}\)) species in PAW [4; 14]. [PERSON] et al. [15] investigated the potential of utilizing cold atmospheric pressure dielectric barrier discharge plasma treatment of tap water (PATW) for agricultural purposes. The reaction between cold plasma's gas-phase substances and tap water leads to the formation of ROS and RNS in water, such as H\({}_{2}\)O\({}_{2}\), NO\({}_{2}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), or ammonia (NH\({}_{4}\)\({}^{+}\)), which can induce and enhance crop growth as well as bicarbonate (HCO\({}_{3}\)\({}^{-}\)) ions, which can promote seedling growth, as a result of PAW-based nitrogen fixation [16; 17; 18]. The species generated depends on the plasma source used, treatment time and gases used, the distance between the liquids, and the nature of the electrode [4]. In addition, the species and reactions generated in the PAW might be different between PAW generated above the water surface and PAWgenerated directly in the liquids [4]. An advantage of generating PAW is that it does not require any additional chemicals to be added to the water, making it eco-friendly [14; 19]. Overall, PAW can disinfect foods, promote the growth of seedlings, increase crop yield, and confront drought [4; 14; 19; 20]. Oxidative stress plays a major role in aging and age-related diseases, such as cancer, cardiovascular disease, chronic inflammation, and neurodegenerative diseases [21]. Epidemiological studies have shown that a high intake of foods of plant origin is strongly associated with a reduced risk of the above-mentioned pathological conditions. These beneficial effects have been partly attributed to the compounds present in plants that possess antioxidant activity. Additionally, antioxidant compounds possess antibacterial, antimutagenic, or antiviral activities to a greater or lesser extent [22; 23]. Antioxidants are substances that prevent damage to cells from free radicals, which are highly reactive, unstable molecules, that lead to cell damage linked to various chronic diseases [24; 25]. Green leafy vegetables contribute to a balanced diet and are rich in bioactive compounds, such as polyphenols, carotenoids, flavonoids, flavones, catechins, vitamins, minerals, and so on [26]. Phytochemicals, which are natural bioactive components sufficient in plants, have been reported to possess antioxidant properties. They are small nonessential nutrients with putative health-promoting actions. Polyphenols and flavonoids represent the largest two categories of phytochemicals and serve as powerful antioxidants; they have received much attention because of their ability to prevent excessive free radicals in oxidative stress that cause several degenerative diseases in humans [27; 28; 29]. Phytochemicals usually increase in abundance during stressful events [28].