## 3 Results and Discussion
### Bactericidal Effect of L-Phe Treated With AAGD  
It was reported that in the case of water and other liquids treated by plasma, a low pH of liquid (below 4.7) after treatment is important for a strong bactericidal effect [20]. However, in cases of hydroponic plant growth and other applications in agriculture, a low pH could be harmful for plants, and a bactericidal effect in the neutral pH region (between 5 and 6.5) is desired [18; 24]. The bactericidal effect of L-Phe treated with oxygen radicals in a neutral pH region was reported elsewhere [18]. High RONS concentration in liquids treated with AAGD looks promising for a strong bactericidal effect. To investigate the bactericidal effect of L-Phe treated with AAGD, 10 mL/min flow of L-Phe was treated with AAGD under the standard condition. Then, _E. coli_ bacteria at a concentration of 10\({}^{7}\) mL-1 were suspended in the treated liquid and incubated for various durations. After incubation, the number of survived bacteria was estimated by a CFU assay. Figure 7 demonstrates the effects of incubation time on the number of survived bacteria in pristine L-Phe, L-Phe treated with oxygen radicals, and L-Phe treated with AAGD. In the case of L-Phe treated with AAGD, the number of bacteria gradually decreased with an increase of incubation time and then reached the detection limit of colony count (10 CFU/mL) after 48 h, indicating a 6 log reduction. Additionally, the pH of 80 mL L-Phe did not change after treatment with AAGD and remained at a value of 6.3 owing to the use of phosphate buffer. In the case of L-Phe treated with NEAPRS at conditions used for the plant-promotion experiments reported elsewhere, it required 96 h to achieve the same bactericidal effect [18]. L-Phe treated with AAGD had a higher bactericidal effect in a neutral pH region than that treated with NEAPRS owing to a higher RONS concentration and a presence of different plasma-treatment byproducts. Moreover, bacterial concentration did not  
Figure 6: Plant-growth enhancement effect on radish sprout cultured in varying concentrations of L-Phe treated with AAGD. significantly change in pristine L-Phe after 48 h of incubation, indicating that L-Phe was not responsible for the observed bactericidal effect, and it should have originated from byproducts of plasma treatment and RONS. Undiluted L-Phe treated with AAGD did not have a hydroponic-plant-growth promotion effect; therefore, the bactericidal effect of diluted L-Phe treated with AAGD is important for practical use in hydroponics to suppress various pathogenic bacteria. Even after four-fold dilution, L-Phe treated with AAGD had a bactericidal effect, and a 3 log reduction in the number of survived bacteria could be observed after 48 h of incubation, which was similar to the bactericidal effect of undiluted L-Phe treated with NEAPRS [18]. The observed results showed that the flowing L-Phe treated with glow discharge at ambient air could have a simultaneous hydroponic plant promotion effect and bactericidal properties. Additionally, it was reported that L-Phe treated by plasma could preserve the bactericidal effect for more than one week, which allows to store and transport treated L-Phe before use [18]. The proposed method uses ambient air (which is the cheapest possible gas), utilizing an L-Phe solution (which is a typical fertilizer component) as a base material, and could be scaled considering flowing-liquid treatment. Moreover, the amount of produced plasma-treated L-Phe could be increased by tuning power supplied to the plasma and flow rate, and increasing generator number. If the PAM produced by AAGD is not immediately supplied to the target after treatment, a larger amount of PAM could be produced by high-power discharge (which results in heating PAM to temperatures above 37 \({}^{\circ}\)C) and cooled down before use. The possibility of scaling, inexpensive devices, and low operational cost is promising for the practical use of plasma treatment in hydroponics and other agricultural or biomedical applications.