## 1 Introduction  
The fast development of low-temperature atmospheric-pressure plasma sources, which happened in last two decades, has led to the appearance of a huge number of new applications [1; 2; 3; 4; 5; 6]. Developments of atmospheric pressure plasma sources allowed treatments of various targets by reactive species not employing vacuum systems; moreover, use of nonequilibrium or so-called \"cold\" plasmas made possible target treatment without thermal damage, which opened a door for use of plasmas in biomedical and agricultural applications [4; 5; 7; 8; 9; 10]. A promising topic is liquid treatment by plasma and following the use of the treated liquid in medicine or agriculture [4; 7; 11; 12; 13]. During plasma treatment, reactive oxygen and nitrogen species (RONS) are delivered from the gas phase to the surface of the liquid with following dissolving and triggering various chemical reactions [14; 15; 16]. Additionally, in the case of organic liquids or other solutions, liquid components could be decomposed through an interaction with plasma or chemical reactions triggered by the plasma treatment [17]. The combination of RONS and other components present in the liquid treated with plasma could have a suppressing effect, such as bactericidal or insecticidal effect and the selective killing of cancer cells, or a promotion effect, such as enhancement of plant-seed germination, plant growth, and even chicken-growth promotion [14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. A promising field for the application of plasma technologies is hydroponic plant growth, where a plant grows in a nutrient medium without soil [24]. Considering numerous reports on plant-growth promotion and seed-germination enhancement by plasma treatment, the promotion effect could be expected from the plasma treatment of a nutrient medium or fertilizers used for hydroponics. There are many reports on the use of organic fertilizers and amino acid fertilizers in particular [25; 26]. L-Phenylalanine (L-Phe, C9H11 NO2) is a commonly used component of amino acid fertilizers; it was reported that use of L-Phe allows the production of tropane alkaloids to increase (up to industrial scale) as a secondary plant metabolite, and achieve folate (which is an important vitamin) biofortification in vegetables [27; 28; 29; 30]. On the other hand, pathogenic bacterial presence in a hydroponic nutrient solution could interfere with plant growth and contaminate the collected crops; therefore, a bactericidal effect of the nutrient solution is desired [31]. A low pH level (below 4.7) is important for the bactericidal effect of plasma-treated liquids; however, a low pH level could be harmful to plants, and the pH of the nutrient solution is commonly kept in range from 5 to 6.5 [20; 24]. It was reported that a bactericidal effect could be achieved by the treatment of solutions of organic compounds with a benzene ring (such as L-Phe) with plasma-generated neutral oxygen reactive species (O3, [O(3P)]), OH, and HOO; hereafter oxygen radicals); additionally, L-Phe treated with oxygen radicals allowed a simultaneous bactericidal effect achievement and hydroponic-plant-growth promotion [18]. Reported results look promising for use in agriculture; however, for practical applications, a more effective and scalable way of plasma treatment is desired. In most cases, low-temperature nonequilibrium atmospheric-pressure plasma jets (NEAPPJs) or dielectric barrier discharges (DBDs) are used for liquid treatments. However, in the case of NEAPPJ, the production rate of reactive oxygen and nitrogen species (RONS) is low, which plays a key role in the bactericidal effect and plant-growth promotion, and liquid irradiation takes a long time (from several minutes to several hours) due to the low density of the plasma [32; 33; 34; 35]. A high-density nonequilibrium atmospheric-pressure radical source (NEAPRS), reported elsewhere, allowed the problem of low RONS concentration appearing in the case of APPJ to be solved [14; 18; 36].