Chemical Diversity of Essential Oils from Korean Native Populations of Agastache rugosa (Korean Mint)

Agastache rugosa (baechohyang) is one of the most important aromatic plants native to the Republic of Korea. A. rugosa fragrance has been used to prepare incense since the Goryeo Dynasty in Korea. The present study aimed to explore the variation in the composition of essential oils from A. rugosa among native populations in Korea. The seeds of A. rugosa were collected from 90 different sites in Korea and seedlings were raised in the nursery. Essential oils were extracted from these populations by the steam distillation extraction method and their chemical compositions were analyzed by GC-MS. The yield of essential oils of A. rugosa ranged between 0.11% and 0.86%. A total of 204 components were identified from 90 populations of A. rugosa. Out of 204 components, 32 components were common in more than 40 individuals of A. rugosa and these 32 components were selected for principal component analysis (PCA). On the basis of the essential oil compositions, six chemotypes—estragole, pulegone, methyl eugenol, menthone, isopulegone, and nepetalactone—were distinguished according to their major components. As a result of the cluster analysis, 90 individuals of A. rugosa could be classified into three groups: estragole, methyl eugenol, and pulegone. A. rugosa exhibited significant chemical diversity among the individuals. The distribution of chemotypes is associated with the collection of seeds, suggesting that genetic diversity may influence the variations in the chemical compositions and concentrations within the species. This chemical diversity serves as the background to select cultivars for the cultivation and industrial applications of A. rugosa cultivars with high essential oil yield and concentration of its chemical components.


Introduction
Agastache rugosa (Fisch. & C. A. Mey.) Kuntze is an important aromatic edible plant in Korea that belongs to the Lamiaceae family. The A. rugosa plant is mainly found in East Asia, China, Japan, and Taiwan [1]. The common name of A. rugosa is Korean mint (baechohyang) and its aerial parts contain essential oils with a unique fragrance. Therefore, the leaves of A. rugosa are used to enhance the aroma and taste of Korean dishes, such as salads and soups [2,3]. In traditional Korean medicine, A. rugosa is used for the treatment of numerous diseases, especially anxiety, cholera, diarrhea, fever, and nausea [1,4]. Previously, several studies reported that A. rugosa contains a variety of bioactive metabolites that prevent inflammation, cardiovascular diseases, cancer, photoaging, adipogenesis, etc. [4][5][6][7][8].
Previously, few studies reported on the chemical composition of essential oils obtained from A. rugosa in the Republic of Korea [3,9,10]. This plant has a high developmental value as a flavoring crop for various industrial products. Major biologically active constituents of A. rugosa are characterized into two main metabolic classes: terpenoids and phenylpropanoids. The essential oil of A. rugosa is mainly composed of estragole, followed by D-limonene, menthone, and pulegone. However, a huge variation in the chemical composition of essential oils of A. rugosa has been reported, according to different geographical origins [3,6,11]. Knowledge of the chemical composition of essential oils from aromatic plants is an important quality measure for their commercial utilization. Environmental factors play a critical role in determining the chemical polymorphism of plant essential oils [12]. In addition, the quantity and composition of essential oils from aromatic plants are mainly influenced by various factors such as genotype, season, stage of maturity, extraction technique, and drying [13,14]. Principal component analysis (PCA) is an important multivariate analysis method for grouping or clustering quantitative data based on the significance of the sample [11,15,16].
In this context, the present study aimed to provide information on the variation of essential oil compositions of Korean A. rugosa. For this purpose, seeds of A. rugosa were collected from 90 different regions in Korea, the plants were cultivated, and their essential oil compositions were analyzed using gas chromatography and mass spectrometry (GC-MS) analysis. Furthermore, PCA was performed to classify chemotypes in order to identify differences in regional species of A. rugosa in Korea.

Color and Yield of Essential Oils from A. rugosa Populations
A. rugosa is one of the important medicinal and aromatic plants in Korea. The essential oil obtained from this plant has been used for enhancing the aroma of Korean dishes. This oil is mainly composed of monoterpenoids, phenylpropanoids, and other aromatic components [4]. In this study, 90 Korean native A. rugosa populations were selected for essential oil analysis. Seeds collected from 90 individuals were cultivated in the same field under similar environmental conditions and then used for essential oil extraction ( Figure 1). Essential oils were extracted from 90 individuals of A. rugosa by the steam distillation method. For this purpose, seeds of A. rugosa were collected from different cultivated regions in Korea. The yield percent (v/w) and the color of essential oils markedly varied according to the collection sites of the seeds ( Table 1). The extraction yield of A. rugosa essential oils was a minimum of 0.11% and a maximum of 0.86%. The essential oil varied depending on the seed collection area. Furthermore, the color of the essential oils was classified into colorless, pale lemon, and lemon ( Figure 2). Similar to our report, the yield of essential oil obtained from the aerial parts of A. rugosa ranged between 0.15 and 0.49% [3,17].
Previously, few studies reported on the chemical composition of essential oils obtained from A. rugosa in the Republic of Korea [3,9,10]. This plant has a high developmental value as a flavoring crop for various industrial products. Major biologically active constituents of A. rugosa are characterized into two main metabolic classes: terpenoids and phenylpropanoids. The essential oil of A. rugosa is mainly composed of estragole, followed by D-limonene, menthone, and pulegone. However, a huge variation in the chemical composition of essential oils of A. rugosa has been reported, according to different geographical origins [3,6,11]. Knowledge of the chemical composition of essential oils from aromatic plants is an important quality measure for their commercial utilization.
Environmental factors play a critical role in determining the chemical polymorphism of plant essential oils [12]. In addition, the quantity and composition of essential oils from aromatic plants are mainly influenced by various factors such as genotype, season, stage of maturity, extraction technique, and drying [13,14]. Principal component analysis (PCA) is an important multivariate analysis method for grouping or clustering quantitative data based on the significance of the sample [11,15,16].
In this context, the present study aimed to provide information on the variation of essential oil compositions of Korean A. rugosa. For this purpose, seeds of A. rugosa were collected from 90 different regions in Korea, the plants were cultivated, and their essential oil compositions were analyzed using gas chromatography and mass spectrometry (GC-MS) analysis. Furthermore, PCA was performed to classify chemotypes in order to identify differences in regional species of A. rugosa in Korea.

Color and Yield of Essential Oils from A. rugosa Populations
A. rugosa is one of the important medicinal and aromatic plants in Korea. The essential oil obtained from this plant has been used for enhancing the aroma of Korean dishes. This oil is mainly composed of monoterpenoids, phenylpropanoids, and other aromatic components [4]. In this study, 90 Korean native A. rugosa populations were selected for essential oil analysis. Seeds collected from 90 individuals were cultivated in the same field under similar environmental conditions and then used for essential oil extraction ( Figure  1). Essential oils were extracted from 90 individuals of A. rugosa by the steam distillation method. For this purpose, seeds of A. rugosa were collected from different cultivated regions in Korea. The yield percent (v/w) and the color of essential oils markedly varied according to the collection sites of the seeds ( Table 1). The extraction yield of A. rugosa essential oils was a minimum of 0.11% and a maximum of 0.86%. The essential oil varied depending on the seed collection area. Furthermore, the color of the essential oils was classified into colorless, pale lemon, and lemon ( Figure 2). Similar to our report, the yield of essential oil obtained from the aerial parts of A. rugosa ranged between 0.15 and 0.49% [3,17].

Chemical Composition of Essential Oils of A. rugosa Populations
In total, 204 chemical components were identified in the essential oils of 90 individuals of Korean A. rugosa based on the retention indices and mass spectral data (Supplementary Table S1). Among them, methyl chavicol (estragole), D-limonene, isopulegone,

Chemical Composition of Essential Oils of A. rugosa Populations
In total, 204 chemical components were identified in the essential oils of 90 individuals of Korean A. rugosa based on the retention indices and mass spectral data (Supplementary  Table S1). Among them, methyl chavicol (estragole), D-limonene, isopulegone, menthone, pulegone, β-caryophyllene, and β-cubebene were registered as major components in the essential oils of A. rugosa populations. The composition of essential oils of A. rugosa populations differed not only by their major components, but also by the number of different classes of components. Li et al. [18] identified 37 chemical components from the essential oil of A. rugosa, and the major components were methyl eugenol (50.1%), estragole (8.5%), eugenol (7.5%), thymol (3.6%), and pulegone (2.5%). A recent study reported that the most abundant component in the essential oil of A. rugosa was estragole (89.49%), followed by D-limonene (3.40%), menthone (1.80%), and pulegone (1.86%) [3]. Estragole was also the main component in the essential oil of A. ruogsa cultivated in Australia [19]. In other studies, menthone (48.8%) was reported as the most abundant component in the essential oil obtained from the leaves of A. rugosa cultivated in China [6,20]. Based on previous reports, A. rugosa is classified into three chemotypes: pulegone, estragole, and methyl eugenol [18]. These studies suggested that the variation in the essential oil composition may be influenced by various ecological and physiological factors. In addition, ontogenetic cues play a partial role in the varied proportions of essential oil components [11].
In the present study, 32 components were common in more than 40 individuals of A. rugosa. A. rugosa individuals collected from various sites were classified into six chemotypes according to their chemical components ( Table 2). Among them, the estragole chemotype showed the highest number of A. rugosa (53 individuals), followed by the pulegone type (17 individuals), methyl eugenol type (11 individuals), menthone type (6 individuals), isopulegone type (2 individuals), and nepetalactone type (1 individual). Although the collected seeds were grown in the same environmental conditions, significant variations were observed in the essential oil compositions of 90 individuals. Hence, the data may reflect the genetic diversity within the species. It was reported that genetic diversity within the species plays a major role in essential oil compositions and concentrations [21][22][23]. Kang et al. [24] studied the genetic diversity of 65 accessions of A. rugosa germplasms using inter simple sequence repeat (ISSR) markers. The authors reported these accessions were grouped into two major clusters (A and B), and cluster A is subdivided into two subclusters. Furthermore, Dang et al. [25] compared morphological features and essential oil compositions in the pulegone and estragole chemotypes of A. rugosa. Previous studies also found that the morphological variations among different chemotypes were mainly in leaf type, trichome density, plant height, and internode length [26,27]. These studies clearly indicate that morphological and genetic variability among A. rugosa populations may influence the chemical diversity within species.
Zielińska et al. [11] studied the essential oil profiles of the progeny derived from different ages of mother plants of A. rugosa in cultivated plants and in vitro shoot cultures. A phenylpropanoid, estragole, was the most abundant component in A. rugosa essential oil. The quantitative variation among individuals was mainly in the concentration of estragole, menthone/isomenthone ratio in leaves, and pulegone in inflorescences. The authors suggested that the essential oil compositions of A. rugosa were highly dependent on the age of the mother plants. Zielińska and Matkowski [29] reviewed that the essential oil of A. rugosa is mainly dominated by estragole, but other monoterpene components such as limonene, menthone, isomenthone, and pulegone may also be present in high concentrations. Similar to our study, Li et al. [18] isolated the essential oils of A. rugosa aerial parts collected from Zhejiang, Hubei, and Henan provinces of China and found three different chemotypes, pulegone, estragole, and methyl eugenol, were major components, respectively. The morphological variations, essential oil components, and transcriptomic data between the two chemotypes of A. rugosa, pulegone and estragole, were also investigated [25].
As a result of the cluster analysis, 90 individuals of A. rugosa could be classified into three groups according to their chemical composition, as shown in the dendrogram ( Figure 5). Group I included most A. rugosa individuals with the highest estragole content in their essential oils. Group II contains individuals of A. rugosa with the highest methyl eugenol content and group III contains individuals of A. rugosa with the highest pulegone content. Furthermore, group III could be divided into three sub-groups according to the ratio of essential oil components. The chemical composition characteristics of each cluster are shown in Table 5. Dang et al. [25] reported that 46 genes were identified with the biosynthesis of estragole and pulegone. In these, the authors identified chavicol methyl transferase and limonene-3-hydroxylase in A. rugosa. Jang et al. [30] investigated the phenotypic expression and floral dimorphism in five Korean populations of A. rugosa and reported that three phenotypes were found according to their reproductive characteristics. The findings of this study suggest that different chemotypes among A. rugosa populations are not associated with environmental conditions. The changes in the composition of essential oils and the concentration of components may be associated with phenotypic and genetic characteristics of A. rugosa populations. In particular, genetic and environmental variations play a key role in phenotypic variation within the species [31]. are not associated with environmental conditions. The changes in the composition of essential oils and the concentration of components may be associated with phenotypic and genetic characteristics of A. rugosa populations. In particular, genetic and environmental variations play a key role in phenotypic variation within the species [31].

Group Major Chemical Characterization
Ⅰ Estragole Content ratio of estragole in essential oil is the highest Ⅱ

Methyl eugenol
The highest ratio of methyl eugenol content Ⅲ Pulegone isopulegone The highest ratio of pulegone and isopulegone contents Pulegone menthone The highest ratio of pulegone and menthone contents (1:1)

Pulegone
The highest ratio of pulegone content Previous studies demonstrated that various edaphic and ecological factors influenced the essential oil yield and their chemical components in aromatic plants [23,32,33]. In the present study, the statistical analysis results suggest that the essential oil components are predominantly determined by the plant's genotype, because all collected seeds were cultivated under similar environmental conditions. Furthermore, planting season may slightly influence the overall yield of essential oils and their compositions of A. rugosa populations.

Collection of A. rugosa Seeds and Their Cultivation
Korean domestic A. rugosa seeds were collected from 90 different regions of the Republic of Korea in 2019 ( Table 1). The seeds were sown in 128-hole seedling trays (17 cm 3 , Seoul Bio Co., Ltd., Chungbuk, Korea) filled with horticultural topsoil in April 2019. The seedlings were grown for 36 days in a glass greenhouse at the Agricultural Research Institute, Gangwon-do Agricultural Research and Development Institute, where the proper temperature (23-25 °C day) and humidity were maintained. In early May, 3.5-leaf-bearing  The highest ratio of pulegone and isopulegone contents Pulegone menthone The highest ratio of pulegone and menthone contents (1:1)

Pulegone
The highest ratio of pulegone content Previous studies demonstrated that various edaphic and ecological factors influenced the essential oil yield and their chemical components in aromatic plants [23,32,33]. In the present study, the statistical analysis results suggest that the essential oil components are predominantly determined by the plant's genotype, because all collected seeds were cultivated under similar environmental conditions. Furthermore, planting season may slightly influence the overall yield of essential oils and their compositions of A. rugosa populations.

Collection of A. rugosa Seeds and Their Cultivation
Korean domestic A. rugosa seeds were collected from 90 different regions of the Republic of Korea in 2019 ( Table 1). The seeds were sown in 128-hole seedling trays (17 cm 3 , Seoul Bio Co., Ltd., Chungbuk, Korea) filled with horticultural topsoil in April 2019. The seedlings were grown for 36 days in a glass greenhouse at the Agricultural Research Institute, Gangwon-do Agricultural Research and Development Institute, where the proper temperature (23-25 • C day) and humidity were maintained. In early May, 3.5-leaf-bearing herbaceous seedlings were planted in two rows at 25 cm intervals with an area of 1650 m 2 ( Figure 1). The plants were grown until the flowering stage, which is known to contain the highest essential oil content. After that, aerial parts of all samples of A. rugosa were harvested at the blooming stage and immediately cut into 15 cm units used for the essential oil extraction.

Essential Oil Extraction
The essential oils were separately extracted from the aerial parts of 90 different A. rugosa individuals by steam distillation. The essential oil extraction was carried out with a 1 kg sample of A. rugosa for 1 h and 30 min using a steam distillation apparatus (EssenLab Plus, Hanil Lab Tech Co, Ltd., Yangju, Korea). The essential oil yield (%) was calculated as volume (mL) of extracted oil per 1 kg of fresh plant sample. After extraction, water and impurities in the extracted essential oil were removed using anhydrous sodium sulfate and stored at 4 • C for further analysis.

GC-MS Analysis
The GC-MS analysis of the essential oil of A. rugosa was performed by a Varian CP-3800 (GC)/Varian 1200 L (MS) (Varian, Palo Alto, CA, USA) equipped with a capillary column VF-5MS (Agilent, Santa Clara, CA, USA) polydimethylsiloxane (30 m × 0.25 mm × 0.25 µm). The GC oven temperature was programmed to increase from 50 • C (for 5 min) to 250 • C (for 3 min) at a rate of 5 • C/min, then increased to 300 • C at a rate of 20 • C/min, and the final temperature was kept for 5 min. The injector temperature was set at 250 • C and the ion source temperature was set at 280 • C. One µL of the sample was injected with a split ratio of 20:1, and helium was used as a carrier gas at a rate of 1 mL/min. For mass spectra analysis, the ionization voltage was set to 70 eV, and the mass range was set to 30-500 m/z. The components in the essential oil of A. rugosa were identified by comparing the mass spectrum data of the National Institute of Standards and Technology (NIST, 3.0) library and the retention indices (RI) relative to a homologous series of n-alkanes (C 8 -C 20 ) with those reported in the literature [34].

Statistical Analysis
Prior to the statistical analysis, the data of essential oil components obtained from 90 individuals of A. rugosa were integrated and sorted in ascending order based on the RI value. Then, the common chemical components that appeared in 40 or more A. rugosa populations were extracted separately from the raw data. Statistical analysis was done based on the completely extracted data and then cluster analysis was performed. Principal component analysis (PCA) was performed to analyze multiple data on the content of essential oil components and to understand the relationship between the essential oil composition and the collection site of the seeds of A. rugosa. All statistical analyzes were performed using IBM SPSS ver. 24 (IBM Corp. Released 2016, Chicago, IL, USA).

Conclusions
The results reveal a significant chemical polymorphism within Korean native A. rugosa populations according to seed collection sites. Based on the essential oil compositions, 90 individuals of A. rugosa were classified into six chemotypes: estragole, pulegone, methyl eugenol, menthone, isopulegone, and nepetalactone. Furthermore, these 90 individuals of A. rugosa could be classified into three groups-estragole, methyl eugenol, and pulegoneaccording to PCA and cluster analysis. These results can extend our knowledge of the chemical diversity of A. rugosa populations, suggesting the view that essential components are excellent markers at the intraspecific level. Further genetic diversity studies of A. rugosa populations are warranted to understand the variation within the species.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27196341/s1, Table S1. The area percent of 32 components in the essential oils of A. rugosa populations. Table S2. Correlation coefficients between 32 chemicals of essential oils from A. rugosa populations.