## 4 Discussion
Compared to other studies on both direct and indirect plasma applications, ROS (reactive oxygen species) are the main factor affecting germination and plant growth and defense [7; 40; 41]. The SDBD generator device used in this study mainly produced a higher NO\({}_{3}\)\({}^{-}\) concentration when a longer plasma treatment time was used. In addition, the change of pH and conductivity was also significant at longer plasma treatment. This change was predicted to affect the plant growth and development. In a previous study, tobacco grown on medium containing plasma treated water had increased cotyledon growth but suppressed primary root hair growth at PAW40 [25]. _Arabidopsis_ seedlings showed self-sufficient under minimal conditions for weeks without external supply of nutrients [42]. Therefore, we used the minimal media of only DW and PAW to study the more dramatic effect of the PAW treatment. By using this method, we expected to see more specific pathways that are affected by PAW. Even though it is important to study the effect of all component changes in PAW such as conductivity and osmolality, in this study we focused on the changes in the nitrate concentration which is a main nitrogen source that affects plant growth. Nitrogen is an essential macronutrient for plants and an important constituent of key molecules such as amino acids, nucleic acid and chlorophyll [43]. Nitrogen availability in the soil is a major factor in plant growth, and its availability is a key determinant of plant yield [43; 44; 45; 46]. From this perspective, the increment of the nitrate concentration from plasma treatment potentially can serve as a very important nitrogen source for plant growth and development. Concerning root development, the PAW treatment condition (PAW5, 7 and 12) promoted an increased total root length compared with the DW treatment condition in _Arabidopsis_; however, root development was inhibited by the PAW that was generated with a longer plasma treatment time (PAW19 and 40). In detail, the morphological differences at 15 days after planting in the root showed that the root length for the PAW5 treatment condition increased by 87%; however, it was suppressed by the PAW40 treatment condition by 63% compared to the DW control, indicating that root growth occurred independent of the plasma treatment time used to generate the PAW. Thus, _Arabidopsis_ needs specific growth conditions (Figures 2 and 3). In contrast to the specific pattern of activation and inhibition of root growth, the effect of the PAW treatment in the cotyledon tissue did not show any inhibition. The cotyledon size increased according to the plasma treatment time of the PAW due to an enlarged palisade cell size, not by cell number. Intobacco, the cotyledon size was also regulated by cell size not cell proliferation at 10 days after planting [25]. This result implies that they share the same mechanism in cotyledon development in dicotyledonus plants. In the seedlings, the roots become activated in the PAW5, 7 and 12 treatment conditions and inhibited in the PAW19 and 40 treatment conditions, and the cotyledon expands according to the plasma treatment time of the PAW as a tissue-specific plasma effect. Therefore, we need to understand it as an overall balance of the plant developmental growth and investigating the optimal treatment conditions for _Arabidopsis_ is important to achieve the desired effect. Interestingly, the PAW treatments showed an increased number of root hairs in all the PAW conditions; however, they showed a decreased root hair cell length shown by cell length measurements, in other words, a decreased trichoblast cell length (Table 2). In addition, root hair number in the _WER:GFP/cob19-1_ line increased induced by the PAW treatment while root cell elongation shortened. In the Col-0 background, the root hair number increased by 18-32% dependent on the PAW treatment time (Table 2). Root hair number in the _cob19-1_ mutant background was dramatically increased by at least two times by the PAW treatment at 5 days after planting (Table 4).