Growth and bioactive phytochemicals of Panax ginseng sprouts grown in an aeroponic system using plasma-treated water as the nitrogen source

Ginseng (Panax ginseng Meyer) sprouts are grown to whole plants in 20 to 25 days in a soil-less cultivation system and then used as a medicinal vegetable. As a nitrogen (N) source, plasma-treated water (PTW) has been used to enhance the seed germination and seedling growth of many crops but has not been investigated for its effects on ginseng sprouts. This study established an in-situ system for N-containing water production using plasma technology and evaluated the effects of the PTW on ginseng growth and its bioactive phytochemicals compared with those of an untreated control. The PTW became weakly acidic 30 min after the air discharge at the electrodes because of the formation of nitrate (NO3‒) and nitrite (NO2‒) in the water. The NO3‒ and NO2‒ in the PTW, together with potassium ions (K+), enhanced the shoot biomass of the ginseng sprout by 26.5% compared to the untreated control. The ginseng sprout grown in the PTW had accumulated more free amino acids and ginsenosides in the sprout at 25 days after planting. Therefore, PTW can be used as a liquid N fertilizer for P. ginseng growth and phytochemical accumulation during sprouting under aeroponic conditions.

Ginseng (Panax ginseng Meyer) sprouts have been recently cultivated as a medicinal vegetable in Korea due to their relatively short period of growth in a soil-less cultivation system. Ginseng sprouts can be grown in less than 25 days after planting and still have a high content of bioactive compounds, including ginsenosides and amino acids [1][2][3][4] . Many functional and evolutionary analyses have shown that only Panax plants actively synthesize various ginsenosides [5][6][7] . Ginsenosides, showing numerous pharmacological effects in humans, including antiinflammation 8 , highly accumulate in the ginseng shoot during the early growth stage, particularly within the first two years of planting 9,10 . In the case of other phytochemicals, non-protein amino acids are known to highly accumulate in the shoots of 1-to 3-year-old ginseng plants 1 , and they have beneficial effects against stress 11 and immune disorders 12 . Many efforts have been given to develop a soil-less cultivation system for ginseng plants using a nutrient solution 13,14 . In the nutrient solution, nitrogen (N) is the most important element affecting plant growth. Plants synthesize amino acids through N metabolism (NO 3 -→ NO 2 -→ NH 4 + → Glutamine → Glutamate → Amino acids), and these amino acids are used to synthesize proteins, enzymes, and chlorophyll 15,16 . Because ginseng plants grow slowly during the early growth stage 17 , earlier application of N is essential not only for its growth but better production of bioactive phytochemicals. Previous studies reported that ginseng growth and ginsenoside synthesis were improved after N treatment through enhanced plant architecture and increased nutrient uptake in the soil [18][19][20] . Most commonly, synthetic N products (e.g., synthetic urea or ammonium nitrate) have been used to provide a N source for crops. The use of commercial N products requires appropriate procedures for proper storage, transportation, and disposal of the chemicals. Recent studies have reported that plasma technology is a promising tool for in-situ production of N-containing water in food and agriculture 16,21 . In an apparatus for plasma treatment, a high electric discharge produces N-containing ions in water through the dissolution of nitrogen oxides generated near the electrodes during air discharge [22][23][24][25] .

OPEN
Institute of Plasma Technology, Korea Institute of Fusion Energy, Gunsan 54004, Republic of Korea. * email: jongseoksong@kfe.re.kr Plasma treated water (PTW) and aeroponic conditions. Lab-scale experiments were performed with two aeroponic systems (length 1.1 m × width 1.4 m × height 2.0 m), each equipped with a 98-L plasma treatment chamber ( Supplementary Fig. S1). Deionized water inside the chambers was either untreated as the control or treated for 30 min at a distance of about 15 cm from 8 surface dielectric barrier discharge (SDBD) electrodes connected to four power supply units at the top of the lid. The same SDBD electrodes were used as in Song et al. 25 , and they generated a high electric discharge at an average power of 255 W with a driving frequency of 18 kHz and a peak-to-peak voltage of 6 kVp-p.
Plasma treated water or untreated deionized water was transferred daily into a 40-L water tank from the chamber. The plasma-treated water was adjusted to the same pH range of 6.5 ± 0.11 as the untreated deionized water with an 8.0 M potassium hydroxide (KOH) solution. Before each spraying, the P. ginseng rhizomes were planted in a 196-hole square plate at a distance of 5.0 × 5.0 cm from each other on the top of a 30-cm deep bath. To effectively spray the water, three of six water-distributing pipelines were placed at the bottom of the bath, and the other three were mounted at a height of 50 cm above the rhizomes. The water-distributing pipelines were equipped with spraying nozzles (Pure Water Tech., Korea) that were adjusted to deliver the water at a spray capacity of 28 mL·min −1 and a controlled spraying pressure of approximately 5 kgf·cm 2 . Up to 25 days after planting, the P. ginseng plants were repeatedly sprayed for 2 min at an interval of 58 min with the PTW or untreated deionized water in each aeroponic system maintained at 20 °C with a 24-h light. Immediately after spraying, the used water was drained from the bath.
Ion chromatographic analysis of the PTW. Nitrogen-containing ions of the PTW or untreated deionized water were sampled immediately after air discharge for 30 min and measured using ion chromatography (Dionex ICS-2100, Thermo, USA) equipped with an IonPac AG25 column (4 × 50 mm) and an ASRS-300 (4 mm) suppressor. All the other analytical conditions were the same as those described by Song et al. 25 . Additionally, potassium ions used to adjust the pH of the PTW were measured using ion chromatography (Dionex ICS-1600, Thermo, USA) with an IonPac CG12A column (4 × 50 mm) and a CSRS-300 (4 mm) suppressor. The column temperature was 30 °C, and the suppressor was used at a current of 59 mA. Methanesulfonic acid (20 mM) as an eluent was used at a flow rate of 1.0 mL·min −1 .
Analysis of growth characteristics. The emergence and early growth of P. ginseng were evaluated up to 25 days after planting. Panax ginseng was regarded as emerged if its hooked stem with folded leaves was fully visible 17 . The number of emerged P. ginseng was counted daily for the emergence rate, and the shoot biomass was weighed for the growth rate every 5 days. Shoot emergence was expressed as a percentage of the total number of emerged shoots out of 98 P. ginseng rhizomes. The shoot biomass was averaged from 14 P. ginseng shoots harvested for each sampling date.
A logistic model was used to describe the emergence and early growth of P. ginseng with time after planting as follows (e.g., Shen et al. 29 ; Torra et al. 30 where Y is an estimate of the cumulative shoot emergence (%) or shoot biomass (g·plant −1 ) at days ( x ) after planting; C is the maximum shoot emergence or shoot biomass at which the lag time is infinite; B is the rate of increase of the shoot emergence or shoot biomass, and M is the time lag to reach 50% of the maximum cumulative shoot emergence or shoot biomass. The goodness of fit of the model to the data was assessed with the adjusted R 2 .
Analysis of ginsenosides. The shoots and roots of the P. ginseng were separately harvested every 5 days.
Each part was pooled and lyophilized at a temperature below − 70 °C and then ground to a powder. For each sample, a 0.5 g aliquot was added to 25 mL of 80% aqueous methanol and extracted using an ultrasonicator for 2 h at 40-50 °C. The extract was centrifuged at 13,000 rpm for 10 min, and the supernatant was filtered through a 0.45-μm syringe filter (Whatman) before analysis. Statistical analysis. All the data were initially subjected to analysis of variance, and a mean comparison was made by Tukey's HSD (honestly significant difference) test at P = 0.05. Data from each sampling date were analyzed with the treatment as a fixed factor and the replicate as a random factor. All the statistical analyses were done with Origin Pro 8.0 (Origin Lab Co., Northampton, MA, USA).

Results and discussion
Formation of nitrogen-containing ions in the PTW. The chemical properties of the water were evaluated immediately after air discharge for 30 min. The water became weakly acidic during the plasma treatment; therefore, it was adjusted to a pH of 6.6 ± 0.34 for the P. ginseng growth using a KOH solution. The acidity of the PTW was accompanied by the formation of nitrate (NO 3 -) and nitrite (NO 2 -) in the water. The concentration of the NO 3 and NO 2 in the PTW was 5.2 and 0.1 mg·L −1 , respectively, while the K + concentration was 5.0 mg·L −1 (P < 0.05) ( Table 1). As previously reported, NO 3 and NO 2 can be formed in PTW through the dissolution of nitrogen oxides (NO, NO 2 , and N 2 O 3 ) formed near the electrodes when a high electric discharge is applied to air [22][23][24][25] . The dissolution of the nitrogen oxides in the water contributes to the low pH of the PTW, which can be explained by the reaction between NO 2 and H 2 O (2NO 2 + H 2 O → NO 2 -+ NO 3 -+ 2H + ) 16 . Our results thus indicate that plasma treatment can produce N-containing ions in water. The N-containing ions in the PTW can be used to supply a N source to P. ginseng plants. In particular, the NO 3 concentration of 5 mg·L −1 might be sufficient for P. ginseng growth during sprouting. The KNO 3 at similar concentration has been previously used as a macro nutrient for P. ginseng plants under hydroponic conditions 13,14 . Ginseng growth after spraying K + -containing PTW. The emergence and early growth of P. ginseng were accurately described by the logistic model (Fig. 1). The shoot of P. ginseng sigmoidally emerged and grew vigorously during sprouting; conversely, the root biomass decreased ( Fig. 1; Supplementary Fig. S2). For the shoot emergence, the maximum value and time to reach 50% of the maximum value were estimated to be 99.5% and 3.6 days for the untreated control and 101.8% and 3.1 days for the P. ginseng sprayed with the K + -containing PTW for 14 days ( Table 2). The maximum shoot emergence showed little or no difference between the untreated control and the P. ginseng sprayed with the K + -containing PTW (Fig. 1a). The time required for 50% of the maximum shoot emergence was approximately 0.5 days earlier when sprayed with the K + -containing PTW compared to the untreated control. For the shoot biomass, the maximum value and time to reach 50% of the maximum value were estimated to be 0.64 g·plant −1 and 11.0 days for the untreated control and 0.72 g·plant −1 and 10.3 days for the P. ginseng sprayed with the K + -containing PTW for 25 days (Table 2). At 25 days after planting, the actual shoot biomass showed a significant difference between the P. ginseng sprayed with the K + -containing PTW and the untreated control (P < 0.05) (Fig. 1b). The shoot biomass of the P. ginseng sprayed with the K + -containing PTW was 26.5% higher than that of the untreated control. The time required for 50% of the maximum shoot biomass was approximately 0.7 days earlier for the P. ginseng sprayed with the K + -containing PTW compared to the untreated control (Table 2). These results indicate that P. ginseng can emerge faster and grow rapidly with the K + -containing PTW for 25 days. The N and K in the PTW are major macro elements affecting plant Table 1. Ion concentration of the plasma-treated water (PTW) containing potassium ions (K + ) compared with the untreated deionized water (DW). *An asterisk indicates a significant difference between the untreated deionized water and plasma-treated water (P < 0.05). www.nature.com/scientificreports/ growth; for example, NO 3 is consequently reduced to amino acids which can be used to synthesize proteins, enzymes, and chlorophyll 16 , and K maintains cation-anion balance within the plant 31 . Previous studies reported that K + -containing PTW enhanced the seed germination and seedling growth of crops due to the N and K supply to the crops. The seeds of sweet basil and spinach grew more rapidly into seedlings after being sprayed with K + -containing PTW 31,32 . Thus, N in the PTW, together with K, could be used as main constituents for plant growth.
Amino acid contents after spraying P. ginseng with the K + -containing PTW. Nitrogen-containing ions produced in the PTW were involved in the enhanced free amino acid contents in the P. ginseng sprouts. Spraying with the K + -containing PTW increased the total free amino acid contents by 27.7% in the P. ginseng root compared to the untreated control (P < 0.05) ( Fig. 2; Supplementary Table S1). In particular, 3 essential amino acids (threonine, lysine, and histidine) and 6 non-essential amino acids (phosphoserine, aspartic acid, alanine, ornithine, arginine, and proline) were increased significantly (P < 0.05). Arginine and asparagine (derivatives of aspartic acid) have been reported to be used as N storage and transport compounds in ginseng roots 1,33,34 . Threonine, lysine, and alanine are derived from aspartic acid in amino acid catabolic pathways in plants 34,35 . Phosphoserine is an intermediate in the production of threonine 35 . Arginine can be converted to proline via ornithine 36 . Thus, P. ginseng sprouts might accumulate excess N as arginine and aspartic acid in roots and consecutively, convert them into their related amino acids. Additionally, the P. ginseng plant might be exposed to some stress due to the K + -containing PTW sprayed for 25 days. Of the related amino acids, proline acts as an osmolyte and a chemical chaperone and accumulates in plants under various stress conditions 34 . Moreover, histidine has an important role as an antioxidant in free radical scavenging under stress conditions [37][38][39] .
After spraying P. ginseng with the K + -containing PTW for 25 days, the non-essential amino acid phosphoethanolamine was significantly decreased in the root (P < 0.05) (Fig. 2). In our study, the P. ginseng sprout grew in the absence of added phosphorus (P), although N and K were readily available for rapid growth. For P. ginseng growth, the P-starved root might require some metabolism of P-containing proteins to cope with the P starvation. Under a P-starved condition, a slight but significant reduction of phosphoethanolamine was detected in Arabidopsis roots; this result indicates that phosphoethanolamine can be dephosphorylated in the phospholipids under P starvation 40,41 .
Spraying the P. ginseng with the K + -containing PTW had little effect on the total free amino acid contents of the shoot but significantly affected the composition of the free amino acids compared with the untreated control The shoot biomass was averaged from 14 P. ginseng shoots harvested for each sampling date. An asterisk indicates a significant difference between the DW and PTW + K + at each day (P < 0.05). www.nature.com/scientificreports/ (Fig. 2). After spraying the P. ginseng with the K + -containing PTW for 25 days, 3 non-essential amino acids (phosphoserine, urea, and 2-aminoethanol) were significantly increased in the shoot (P < 0.05). In contrast, another 2 non-essential amino acids (phosphoethanolamine and hydroxyproline) and the essential amino acid methionine were decreased significantly (P < 0.05). Urea acts as N storage and a long-distance transport compound in many plants 34 . 2-aminoethanol functions as a signal for initiating stress tolerance and serves as a membrane stabilizer 42 . Hydroxylation of proline to hydroxyproline normally occurs during its deposition in the cell walls, indicating that a deficiency of hydroxyproline in cell wall proteins can be related to some stress conditions [43][44][45] . The P. ginseng sprouts might respond to the P deficiency in our study even though N and K were readily available for rapid growth. Under the P-starved condition, metabolic conversion of phosphoethanolamine to 2-aminoethanol occurs in the phospholipids to cope with the P starvation 40,41 . Following the decrease of phosphoethanolamine, methionine might be required less in P. ginseng sprouts as a methyl group donor for phosphoethanolamine. The methyl groups originating from methionine are utilized to methylate phosphoethanolamine to various phosphoethanolamine derivatives involved in the synthesis of phospholipids 46 .
Ginsenoside contents after spraying P. ginseng with the K + -containing PTW. Regardless of the PTW treatment, the total content of the 18 ginsenosides increased more rapidly in the shoot than in the root up to 25 days after planting the P. ginseng in the aeroponic system ( Supplementary Fig. S3). Spraying with the K + -containing PTW increased the total ginsenoside contents by 30.0% in the P. ginseng shoot compared to the untreated control at 25 days (P < 0.05). Two types of ginsenosides accounted for more than 90% of the total ginsenosides: 7 PPD-type ginsenosides (Rb1, Rb2, Rb3, Rc, Rd, Rg3, and Rh2) and 5 PPT-type ginsenosides (Re, Rf,   , and histidine (His)) and 10 non-essential amino acids (phosphoserine (P-Ser), phosphoethanolamine (PEA), urea, aspartic acid (Asp), alanine (Ala), 2-aminoethanol (EOHNH 2 ), ornithine (Orn), arginine (Arg), hydroxyproline (Hypro), and proline (Pro)). Each bar represents the mean of four replicates with each replicate containing three shoots (or roots). The error bars represent the standard error of that mean. An asterisk indicates a significant difference between the DW and PTW + K + at 25 days (P < 0.05). www.nature.com/scientificreports/ Rg1, Rg2, and Rh1) ( Fig. 3; Supplementary Fig. S3). The ratio of PPD-type to PPT-type ginsenosides increased from 0.6 to 1.0 and from 1.0 to 1.6 in the P. ginseng shoot and root up to 25 days after planting, respectively (Fig. 3). These results correspond to the previous findings of Kim et al. 47 . The PPD-type and PPT-type ginsenosides are highly produced in the shoot during the early growth stage of 3-year-old P. ginseng. The ratio of PPDtype (Rb1, Rb2, Rc, and Rd) to PPT-type (Re, Rg1, and Rh1) ginsenosides is always lower in the shoot than in the root 47 . The PPD-type/PPT-type ratio is an important factor for the pharmacological efficacy of P. ginseng 48 . The low PPD-type/PPT-type ratio of P. ginseng has been reported to enhance neurocognitive function 48 . Thus, P. ginseng shoots can be pharmacologically beneficial for human health. The composition of individual ginsenosides differed in the different parts of P. ginseng (Fig. 4). Three PPDtype ginsenosides (Rb1, Rb2, and Rc) accounted for approximately 7.3-18.1% of the total ginsenosides in the shoot, while Rd accounted for approximately 11.9-30.9%; in the root, Rb1, Rb2, and Rc accounted for approximately 39.1-50.6% and Rd for approximately 3.8-5.8%. Two PPT-type ginsenosides Re and Rg1 accounted for approximately 41.8-64.5% and 32.0-38.5% of the total ginsenosides in the shoot and the root, respectively. The difference in the composition of each part of P. ginseng might be attributed to the movement of individual ginsenosides from the root to the shoot, or vice versa during the early growth stage. Two PPT-type ginsenosides Re and Rg1 seem to be preferentially synthesized and stored in the shoot, while three PPD-type ginsenosides Rb1, Rb2, and Rc seem to be preferentially stored in the root 47,49 . The PPT-type ginsenosides Re and Rg1 exert anti-inflammatory effects 8 , and the PPD-type ginsenosides Rb1, Rb2, and Rc stimulate immune responses 50 . Thus, the whole plant of P. ginseng can be used as a medicinal vegetable.
When P. ginseng was sprayed with the K + -containing PTW, the total content of PPD-type ginsenosides were increased more in the shoot compared to the untreated control (Fig. 3a). Spraying the P. ginseng with the K + -containing PTW increased the content of the PPD-type ginsenosides by about 1.7-fold compared to the Days after planting (days) www.nature.com/scientificreports/ untreated control at 15 days (P < 0.05). The higher content of PPD-type ginsenosides was extended for 10 days (P < 0.05). In particular, ginsenoside Rd was the most abundant PPD-type ginsenoside in the shoot (Fig. 4a).
The ginsenoside Rd accounted for approximately 24.6% and 20.1% of the total ginsenosides in the treated and untreated shoot, respectively; 3 ginsenosides Rb1, Rb2, and Rc accounted for approximately 13.9% and 12.1%. Conversely, 3 ginsenosides Rb1, Rb2, and Rc were the most abundant PPD-type ginsenosides in the root (Fig. 4b).
Three ginsenosides Rb1, Rb2, and Rc accounted for approximately 46.1% and 45.3% of the total ginsenosides in the treated and untreated root, respectively; ginsenoside Rd accounted for approximately 4.9% and 4.8%. In the case of the PPT-type ginsenosides, there was little or no significant difference between the P. ginseng sprayed with the K + -containing PTW and untreated control in the shoot and root (P > 0.05) (Fig. 3c,d). Among the 5 PPT-type ginsenosides, ginsenoside Re was the most abundant PPT-type ginsenoside regardless of the different parts of P. ginseng (Fig. 4). The ginsenoside Re accounted for approximately 36.6% and 40.7% of the total ginsenosides in the treated and untreated shoot, respectively, and approximately 28.7% and 28.8% in the treated and untreated root. Thus, our results indicate that spraying with K + -containing PTW can affect the production of PPD-type ginsenosides in the shoot, especially ginsenoside Rd. To our knowledge, information about the change in individual ginsenosides versus KNO 3 during the wholeplant growing period has not been reported much. In our study, the enhanced content of the PPD-type ginsenosides by KNO 3 could be related to the increased transcription of ginsenoside biosynthesis-related genes. A similar study with KNO 3 reported an enhanced saponin content in suspension cultures of P. ginseng 51 . Later, transcriptomic analysis revealed that a similar concentration (about 5 mg·L −1 ) of KNO 3 could enhance the transcript levels of genes associated with ginsenoside biosynthesis under cold stress 52,53 . In addition, the concentration of KNO 3 can enhance the expression of genes encoding antioxidant enzymes and the activities of corresponding enzymes 53 . Potassium seems to be a major contributor to oxidative stress tolerance by activating antioxidant enzymes 53 and increasing ginsenoside production 51 . Therefore, the enhanced content of ginsenoside Rd could be reasonably expected under stress conditions (e.g., P deficiency) because it exerts antioxidant activities 54 .
The use of PTW for growth and bioactive phytochemicals. Plasma treated water can supply a N source to P. ginseng plants during the early growth stage. The PTW, which can have a broad concentration of N-containing ions, is effective as a liquid N fertilizer. A low concentration (about 5 mg·L −1 ) of NO 3 can be used for P. ginseng growth during sprouting under aeroponic conditions. In the case of bioactive phytochemicals, the NO 3 in the PTW, together with K + , can significantly help ginseng plants accumulate free amino acids and ginsenosides, although the detailed mechanisms have not been investigated yet.
The changes in bioactive phytochemicals depend on biological (e.g., organ, growth and development stages, and age) 2,9,47,55,56 and environmental factors (e.g., stress, light quality, and cultural practices) 4,14,19,49,57,58 . In our study, NO 3 in the PTW, together with K + , was involved in the enhanced free amino acid and ginsenoside contents in the P. ginseng sprout. Nitrogen significantly affects ginsenoside biosynthesis, although ginsenosides are non-N-containing metabolites 20,59,60 . Later, the variations in different organs have been mainly attributed to the movement of individual phytochemicals from the root to the shoot or vice versa throughout the growing season 1,47 . Further studies are needed to understand better the biosynthesis and accumulation of individual phytochemicals in P. ginseng sprouts treated with K + -containing PTW.   www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.