Distribution Pattern of Volatile Components in Different Organs of Chinese Chives (Allium tuberosum)

Abstract

Volatile compounds are important components of the flavor quality of Chinese chives, but the distribution of flavor components in different organs of Chinese chives is still unclear. In this experiment, two Chinese chive varieties, ‘Fu Jiu Bao F1’ and ‘Jiu Xing 22’, were taken as test materials, and the contents of volatile compounds in different stages and organs of Chinese chive were determined by HS-SPME/GC-MS technology. A total of 70 and 85 volatile organic compounds (VOCs) were detected in various organs of two varieties at the commodity harvesting stage and physiological maturity stage, respectively. The total volatile compound content of Fu Jiu Bao F1 in the stage of commodity harvesting was higher than that of the physiological maturity stage, but Jiu Xing 22 showed the opposite trend. The organ distribution pattern of total volatile compounds in Fu Jiu Bao F1 and Jiu Xing 22 at the commodity harvesting stage was consistent, as follows: leaf > pseudostem > root. However, at the physiological maturity stage, the distribution pattern of Fu Jiu Bao F1 was different from that of Jiu Xing 22. Further, sulfur-containing compounds at different stages showed different organic distributions. Comprehensive analysis indicated that organic-common and organic-specific compounds varied from different cultivars and growth stages of Chinese chive, and organ differences in VOC distribution were greater than the varieties’ differences based on PCA analysis. The results of this study clarify the composition and organ distribution of volatile compounds in Chinese chive and provide a direction for the study of Chinese chive flavor quality.

1. Introduction

Chinese chive (Allium tuberosum Rottller ex Sprengel) is a perennial herbal plant in the Allium and Lily family [1,2]. It originated in China and is now widely cultivated in the world. Chinese chive can be eaten or stored as a medicinal material, which has high medicinal value. The leaves and pseudostem of Chinese chive are the main edible organs, and the sauce made from Chinese chive flowers is also a traditional seasoning in northern China. It can promote gastrointestinal motility and prevent constipation, and can also be used to prevent and treat cancer, diabetes, and cardiovascular diseases due to being rich in dietary fiber and a variety of vitamins and minerals [3,4,5,6]. Chinese chive can be divided into four kinds, including leaf use type, floral axis use type, mature flower use type, and root use type [7]. The leaf use type of Chinese chives is generally with fresh leaves and the pseudostem as the edible parts, which are cultivated on the largest scale in China. The floral axis use type of Chinese chives is mainly harvested with strong chives’ floral axis. The mature flower use type of Chinese chives always takes inflorescence and young seeds as edible parts and is mostly used for pickling processing. The root use type of Chinese chives produces fleshy roots as the main edible parts, which are mainly grown in the southwest of China. In the present study, two varieties of Chinese chives belong to the leaf use type and floral axis use type, respectively.

The flavor of a crop depends on its own quality attributes and the human taste and olfactory system. Quality attributes include being sour, sweet, bitter, astringent, and possessing a unique aroma. The main components of this unique aromatic smell mainly result from volatile organic compounds, and the volatile organic compounds are numerous, including aldehydes, ethers, ketones, and esters [8]. When the tissues of Chinese chive are crushed or cut, volatile compounds with a distinctive odor, similar to that of garlic, are quickly generated through enzymatic degradation of S-alk (en) yl cysteine sulfoxides by alliinase (E.C. 4.4.1.4). Further studies have shown that the main components of this odor are sulfur-containing compounds, which are secondary metabolites of Chinese chive [9,10,11]. Therefore, it is necessary to analyze the content of sulfur-containing compounds. The volatile sulfur compounds in Chinese chive are known to include allyl methyl sulfide (AMS), diallyl disulfide (DADS), methyl 2-propenyl trisulfide (MPTS), dimethyl tetrasulfide (DMTeS), di-2-propenyl trisulfide (DPTS), dimethyl disulfide (DMDS), methyl 2-propenyl disulfide (MPDS), and dimethyl trisulfide (DMTS) [10]. According to a study by Yabuki et al., sulfur-containing compounds in Chinese chive include sulfide, disulfide, trisulfide, and tetrasulfide with ethyl, butyl, and amyl groups. Among all, the highest content of the compound is dimethyl disulfide, followed by allyl methyl disulfide and methyl propenyl disulfide [12]. Furthermore, volatile sulfur compounds in steam-dried Allium hookeri root are the main volatile compounds, with detection of allyl methyl trisulfide (20.1%), diallyl trisulfide (13.8%), isobutyl isothiocyanate (9.3%), dimethyl trisulfide (8.7%), methyl propyl trisulfide (6.5%), and dipropyl trisulfide (5.5%) [13]. In dried garlic, sulfur compounds account for 95.1%, including diallyl trisulfide (36.6%), diallyl disulfide (35.8%), allyl methyl trisulfide (6.0%), methyl propyl trisulfide (3.6%), and allyl methyl disulfide (2.6%) [14].

Accurate qualitative and quantitative analysis of volatile organic compounds is a prerequisite for the study of Allium flavor [11]. At present, the separation methods of volatile organic compounds mainly include the headspace analysis method, steam distillation method, solid-phase microextraction (SPME), simultaneous distillation extraction (SDE), solvent-assisted flavor evaporation (SAFE), and supercritical fluid extraction. The identification methods mainly include gas chromatography–mass spectrometry (GC-MS), gas chromatography–ion mobility spectrometry (GC-IMS), gas chromatography–olfactometry (GC-O), comprehensive two-dimensional gas chromatography time-of-flight–mass spectrometry (GC×GC-TOF-MS), and so on [11,15]. SPME is a pretreatment method invented by Pawliszyn in 1990. Compared with solvent extraction, this method has the advantages that it does not require organic solvents, requires fewer analytical samples, is simple and fast to operate, and is cost-effective [12,16,17]. It integrates the sampling, extraction, and concentration processes of volatile organic compounds and can be used in combination with GC-MS to achieve rapid qualitative and quantitative determination of compounds [18,19,20]. In our previous study, we optimized seven parameters and established the best conditions. Therefore, we have adopted this method [11].

Many studies have shown that the type and content of volatile substances produced by different organs of plants are different. Flamini et al. [21] research showed that in lemon leaves, the content of limonene in old leaves was half of that in young leaves, but in the flower buds, as the flower opens, the amount of limonene increases, and the limonene is not produced in pollen. Gao et al. [22] found that 39, 28, and 27 volatile substances were detected in the roots, pseudostem, and leaves of green onion, respectively. Among the different compound classes, three organs had the highest levels of sulfur-containing compounds, especially trans-propenyl-propyl-disulfane, dipropyl disulfide, dipropyl trisulfide, and 3,4-dimethyl thiophene. In Welsh Onions, Liu et al. [23] detected 30, 37, and 28 compounds in the roots, pseudostem, and leaves of “Zhangqiu”, while 21, 27, and 20 compounds were detected in the corresponding organs of “Tenko”. The distribution of sulfur compounds in the roots, pseudostem, and leaves of “Zhangqiu” accounted for 72%, 83%, and 26% of the total content, while those of “Tenko” accounted for 55%, 84%, and 57%, respectively. In addition, aldehydes are the second largest class of volatiles in Welsh onions.

Chinese chive is a very valuable horticultural crop; therefore, it is necessary to study the volatile organic compounds in different organs of Chinese chive. However, the study on the composition of volatile substances in different types and organs of Chinese chive is limited. The current study focused on the distribution of volatile substances content in different organs of leaf use type and floral axis use type of Chinese chives at different periods to clarify the composition and distribution pattern of volatile substances in different organs. This study will provide a reference to evaluate the flavor quality and flavor product development of Chinese chive.

2. Materials and Methods

2.1. Sample Collection and Preparation

This experiment was conducted on the analysis of flavor in different plant organs of two specific Chinese chives varieties. Chinese chive varieties, i.e., Fu Jiu Bao F1 (leaf use type) and Jiu Xing 22 (floral axis use type), were selected to determine and compare volatile organic compounds in different plant organs. Three organs at the leaf, pseudostem, and root at the vegetative harvesting stage and six organs at the leaf, pseudostem, root, floral axis, mature flower, and seed at the fruiting stage were chosen and analyzed (Figure 1). The seed of the Fu Jiu Bao F1 variety was obtained from the Alibaba Platform, while the seed of variety Jiu Xing 22 was obtained from Henan Jiuxing Biotechnology Research Institute Co., Ltd. (Pingdingshan, China). Firstly, the seeds were sown initially for nursery raising, and, after one month age of this nursery, the seedlings were transplanted in the hydroponic container. The samples were taken at the commodity harvesting stage and the physiological maturity stage. At the commodity harvesting stage, the plant was dug out and divided into three parts: leaf, pseudostem, and root. At the physiological maturity stage, six organs of leaf, root, pseudostem, floral axis, mature flower, and seed were obtained. Six organs were selected in one replication for one cultivar. All samples were crushed, frozen in liquid nitrogen, and kept in a −80 °C refrigerator. The samples were carefully used to avoid interference with external impurities, as suggested by Yabuki et al. [12]. The volatile organic compounds of Chinese chives were measured by HS-SPME/GC-MS.

2.2. HS-SPME Procedure

The test method was adopted by referring to Xie et al. [11]. Briefly, the Chinese chive samples were taken out of liquid nitrogen and ground swiftly to homogenize them, and the 1.5 g of homogenate was put in a screw-head headspace vial (15 mL) with a magnetic stirring rotor (the was purchased from ANPEL Laboratory Technologtes Inc. Shanghai, China) and in ultrapure water (2 mL) to stir fully. Later on, 0.75 g of firm Na₂SO₄ (was purchased from Sinopharm Group Chemical Reagent Co., Ltd. Shanghai, China) was taken and 4 μL of difurfuryl sulfide (was purchased from TCI Development Co., Ltd. Shanghai, China) was added as an internal standard. The sample vial was tightly fitted using a PTFE silicon stopper. Later, the headspace bottle was kept in equilibrium at 70 °C for 15 min on a metal heating agitation platform (500 rpm). After that, the removal and adsorption were performed by inserting pretreated 85 μm carboxen-polydimethylsiloxane (CAR/PDMS) fiber into a headspace bottle for 50 min with uninterrupted heating and agitation. After extraction, the fiber was desorbed into a GC injection port for 5 min and subject to GC-MS analysis. Three repetitions were set for each sample.

2.3. GC-MS Analysis

Characterization of volatile compounds was carried out by GC-MS analysis [12] using an Agilent 7890B/7000D GC-MS (Agilent, Santa Clara, CA, USA). The standard mass Spectrometry Library workstation was used for the separation and identification of the volatile organic compounds (VOCs). A DB-WAX elastic quartz capillary column (30 m × 0.25 mm, 0.25 μm) was used as the stationary phase (Agilent, Santa Clara, CA, USA). The VOCs of Chinese chive were analyzed using an Agilent 7890 B/7000 D GC-MS under the following conditions: capillary column of DB-WAX (30 m × 0.25 mm, 0.25 μm) with He (≥99.999% purity) as the carrier gas at a flow rate of 1 mL/min in non-diversion mode; an initial temperature of 40 °C held for 1 min, raised to 80 °C at 8 °C/min, then raised to 130 °C at 2 °C/min, and, finally, raised to 220 °C at 6 °C/min and held for 3 min; a total analysis time of 49 min; MS ionization, EI, 70 eV; MS source temperature to 230 °C; scan area, 30–660 amu.

2.4. Qualitative and Quantitative Analysis of Volatile Compounds

After the program started, the VOCs were separated and identified by the GC-MS with the automatic deconvolution system (AMDIS) and mass spectrometry library (NIST 2014, Standard Spectrum Library of the National Institute of Standards and Technology of the United States, https://www.nist.gov/srd). According to the mass spectrometry library, only those with a matching score of more than 70 were identified. The quantitative concentration of VOCs was analyzed by the internal standard method, using the following formula:

$$\mathrm{V}\mathrm{O}\mathrm{C}\mathrm{s}\left(\mu \mathrm{g}/\mathrm{g}\right)={\displaystyle \frac{\mathrm{A}1}{\mathrm{A}2}}\times {\displaystyle \frac{\mathrm{M}1}{\mathrm{M}2}}$$

In this formula, A1 and A2 define the peak areas of the sample and the internal standard, respectively; M1 and M2 show the weight of the internal standard (μg) and sample (g), respectively.

2.5. Statistical Analysis

The data analysis was conducted using Excel 2019. The column chart and stacking diagram were performed using sigmaplot software (Version 15.0). The upset plot was drawn using online sites (https://www.chiplot.online/, accessed on 6 July 2024); Venn diagrams were made using online sites (http://www.ehbio.com/test/venn/#/, accessed on 8 July 2024).

3. Results

3.1. Total Volatile Organic Compounds

As shown in Figure 2, there were certain differences in the total amount and quantity of volatile organic compounds in the two Chinese chives varieties at different periods. At the commodity harvesting stage (Figure 2A), it was seen that the total amount and number of volatile organic compounds (VOCs) of Fu Jiu Bao F1 were higher than that of Jiu Xing 22. The highest content of VOCs was observed in the leaves of both varieties of Chinese chive. However, the largest number of VOCs in Fu Jiu Bao F1 was obtained in the pseudostem, while in Jiu Xing 22 was obtained in the roots. Accordingly, the total VOC content in the roots, pseudostem, and leaves of Fu Jiu Bao F1 accounted for 7.57%, 17.34%, and 75.09%, and the total VOC content in the roots, pseudostem, and leaves of Jiu Xing 22 accounted for 2.35%, 3.45%, and 94.19%, respectively. The number of VOCs of the two Chinese chives in each organ was different. The distribution of the VOC number of Fu Jiu Bao F1 was as follows: pseudostem > root > leaf, accounting for 81.82%, 54.55%, and 48.48%, respectively. The distribution of the VOC number in Jiu Xing 22 was as follows: root > leaf > pseudostem, accounting for 85.96%, 61.40%, and 31.58%, respectively. At the physiological maturity stage (Figure 2B), the total amount of volatile organic compounds in Fu Jiu Bao F1 was lower than that in Jiu Xing 22. Noticeably, the VOCs in the Fu Jiu Bao F1 were mainly concentrated in the seeds, while that in Jiu Xing 22 was concentrated in the pseudostem. Total VOC content in roots, pseudostem, leaves, floral axes, mature flowers, and seeds of Fu Jiu Bao F1 accounted for 10.12%, 4.69%, 2.95%, 6.2%, 2.17%, and 73.86%, while it accounted for 2.37%, 82.66%, 1.37%, 1.75%, 10.34%, and 1.52% for Jiu Xing 22. A similar distribution of VOC numbers between Fu Jiu Bao F1 and Jiu Xing 22 was observed. The quantity distribution in the flower axis was the highest, while that in the seed was the lowest. The number of VOCs in Fu Jiu Bao F1 and Jiu Xing 22 in the floral axis accounted for 72.22% and 86.84%, respectively, and the number of VOCs in seeds accounted for 20.83% and 13.16%, respectively.

3.2. Different Classes of Volatile Organic Compounds

3.2.1. At Commodity Harvesting Stage

A total of 70 VOCs were detected in various organs of two varieties (Table S1). Particularly, 35, 53, and 31 volatile organic compounds were detected in the root, pseudostem, and leaf of Fu Jiu Bao F1, and 48, 18, and 35 volatile organic compounds were detected in the root, pseudostem, and leaf of Jiu Xing 22, respectively (Figure 3 and Table S1). These two varieties of Chinese chives have two unique compounds: one is a root-specific compound (6,6-Dimethylhepta-2,4-diene), and the other is a pseudostem-specific compound (Trisulfide, di-2-propenyl). The study results showed that the highest content was aldehydes (79.97%) among the roots of Fu Jiu Bao F1 (Figure 4A and Table S1). In the pseudostems, the highest content was acids (21.62%), followed by sulfur compounds (18.95%). In the leaves, the highest content was esters (37.22%), followed by sulfur compounds (27.15%). For Jiu Xing 22, in the roots, the highest content was sulfur compounds (35.24%). The highest content of ethers (43.32%) was in the pseudostems, followed by sulfur compounds (38.96%). In the leaves, the highest content was hydrocarbons (29.20%). These results indicate that there are differences in VOCs in different varieties and organs of Chinese chive.

3.2.2. At Physiological Maturity Stage

A total of 85 VOCs were detected in various organs of two varieties. Among them, 50, 44, 49, 51, 25, and 15 volatile organic compounds were detected in the root, pseudostem, leaf, floral axis, mature flower, and seed of Fu Jiu Bao F1, and 39, 61, 37, 65, 24, and 10 volatile organic compounds were detected in the root, pseudostem, leaf, floral axis, mature flower, and seed of Jiu Xing 22, respectively. Neither tetradecanoic acid nor hexadecanoic acid existed in the root (Figure 5, Tables S2 and S3). As shown in Figure 6, the floral axis (31.87%) and mature flowers (30.05%) of Fu Jiu Bao F1, and mature flowers (48.13%) and seeds (54.65%) of Jiu Xing 22 had the highest content of ethers. In the seeds of Fu Jiu Bao F1, the highest content of VOCs was heterocyclic polymers (57.42%). In the pseudostems of Jiu Xing 22, the highest content of VOCs was hydrocarbons (30.58%). The highest content of sulfur compounds (42.57%) was in the leaves. In the mature flowers, the ether (48.13%) content was highest, followed by sulfur compounds (35.13%).

3.3. Volatile Sulfur Compounds

3.3.1. At Commodity Harvesting Stage

As can be seen from Figure 7, the total amount of volatile sulfur compounds in Fu Jiu Bao F1 was higher than those in Jiu Xing 22. Similarly, the total volatile sulfur compounds of both varieties were concentrated in the leaves, followed by pseudostems and roots. The content of sulfur compounds detected in Fu Jiu Bao F1 accounts for 24.40% of the total VOCs, while the content of sulfur compounds detected in Jiu Xing 22 accounts for 19.64% of the total VOCs (Table S4). A total of eight sulfur compounds were detected in Fu Jiu Bao F1, namely AMS, DADS, MPTS, DMTeS, DPTS, DMDS, MPDS, and DMTS, and seven sulfur compounds were detected in Jiu Xing 22. Only AMS was not detected. In Fu Jiu Bao F1, DADS (98.87%) and MPDS (98.01%) were mostly concentrated in leaves. DPTS were all concentrated in pseudostems, and DMTS (68.63%) was mostly concentrated in roots (Figure 7B). In contrast, in Jiu Xing 22, DADS (95.66%), MPTS (95.94%), and DMDS (95.82%) were mostly present in leaves. The sulfur compound distribution of pseudostems was relatively equal, and DMTS (100.00%) was all concentrated in the roots (Figure 7C).

3.3.2. At Physiological Maturity Stage

The total amount of volatile sulfur compounds in the two varieties was opposite at the physiological maturity stage to the commodity harvesting stage (Figure 8 and Table S5). At the physiological maturity stage, almost three times more volatile sulfur compounds in Jiu Xing 22 were found than in Fu Jiu Bao F1. The distribution pattern of sulfur compounds in different organs varied between Fu Jiu Bao F1 and Jiu Xing 22. The total volatile sulfur compounds of Fu Jiu Bao F1 were mainly concentrated in seeds, while volatile sulfur compounds of Jiu Xing 22 were mainly concentrated in pseudostems and mature flowers. Eight sulfur compounds were detected in both Fu Jiu Bao F1 and Jiu Xing 22, namely AMS, DADS, MPTS, DMTeS, DPTS, DMDS, MPDS, and DMTS. As shown in Figure 8B, the main kinds of volatile sulfur compounds in seeds of Fu Jiu Bao F1 were DADS (57.02%), DMTS (28.16%), and DMDS (13.14%). Moreover, DADS (95.09%), DMDS (67.06%), and DMTS (63.17%) were mostly concentrated in the seeds, while most MPDS (69.96%) and a portion of DMTS (27.81%) were concentrated in the floral axis. However, in Jiu Xing 22, DADS (71.33%), DMTeS (79.10%), DPTS (82.24%), and AMS (92.47%) were mostly concentrated in pseudostems, while DMDS (81.46%) and DMTS (85.47%) were mostly gathered in mature flowers.

3.4. Evaluation of Flavor Characteristics in Different Organs of Chinese Chive

3.4.1. At Commodity Harvesting Stage

The difference of volatile substances among different organs of two Chinese chive varieties during the commodity harvesting stage. A Venn diagram was used for analysis, as shown in Figure 9.

As can be seen from Figure 9A, for Fu Jiu Bao F1, there were 14 common substances in roots, pseudostems, and leaves, among which there were five acids, four esters, three ethers, and two heterocyclic polymers. There were 8 special substances in roots, 14 special substances in pseudostems, and 2 special substances in leaves. The roots contained four hydrocarbons, two ethers, one aldehyde, and 1 ketone. The pseudostems contained three acids, three ethers, two esters, two heterocyclic polymers, one aldehyde, one ketone, one furan, and one phenol, respectively. In the leaves, one belonged to the ethers and the other to heterocyclic polymers. For Jiu Xing 22, there were 9 substances in common among three organs, of which there were five ethers, three acids, and one heterocyclic polymer. Sixteen unique substances were observed in the roots, of which there were four hydrocarbons, three ketones, three ethers, two heterocyclic polymers, two esters, one aldehyde, and one acid, respectively. Furthermore, there were three specific substances that belonged to ethers in the pseudostems, and two specific substances that belonged to acids and hydrocarbons in the leaves, as shown in Figure 9B.

Principal component analysis (PCA) was performed on the volatile compounds of two different varieties of Chinese chives, as shown in Figure 10. In two kinds of Chinese chives, the differentiation contribution rate of the first principal component (PC1) was 46.43%, and the differentiation contribution rate of the second principal component (PC2) was 27.78%. The total contribution rate reached 74.21%. We investigated that the roots and pseudostems of the two kinds of Chinese chives were more clustered, while the leaves were dispersed. The roots and pseudostems of both kinds of Chinese chives showed negative responses to PC1, while the roots of Fu Jiu Bao F1 and leaves of Jiu Xing 22 were positive on PC2. It indicates that the differences in volatile compounds among the organs are greater than those among the varieties. Combined with Table S4, the differences between the leaves of the two varieties may be due to the large differences in the contents of hydrocarbons, furfuryl, furans, and phenols.

3.4.2. At Physiological Maturity Stage

As can be seen in Figure 11A, during the physiological maturity stage of Fu Jiu Bao F1, these six organs had five substances in common. Among them, two belonged to ethers and the remaining three belonged to aldehydes, ketones, and heterocyclic polymers, respectively. There were two specific substances in the roots (all belonging to esters), one special substance in the leaves (furfuryl and furan), one special substance in the floral axis (ether), three special substances in the mature flowers (acids, ethers, and furfuryl and furan), and one special substance in the seeds (terpenoid). However, only one common substance belonging to ethers among the six organs of Jiu Xing 22 was observed. The pseudostems, flower axis, mature flowers, and seeds contained only one specific substance, which belonged to acids, ketones, hydrocarbons, and diterpenes, respectively. There were no specific substances in the roots and leaves (Figure 11B).

Principal component analysis was performed on the volatile compounds of two different varieties of Chinese chives at the physiological maturity stage, and the results of PCA analysis are shown in Figure 12. The differentiation contribution rate of the first principal component (PC1) was 52.36%, and the differentiation contribution rate of the second principal component (PC2) was 26.96%. The total contribution rate reached 79.32%. The response of the seeds of Fu Jiu Bao F1 and the pseudostems of Jiu Xing 22 to PC1 was positive, while the roots, pseudostems, and seeds of Fu Jiu Bao F1 were positive on PC2. Different from the commodity harvesting stage, the distribution of pseudostems and seeds in the physiological maturity stage of the two varieties was relatively dispersed, indicating that there are more obvious differences in the volatile compounds of pseudostems and seeds in the physiological maturity stage. According to Table S5, the significant differences in the contents of aldehydes, hydrocarbons, and phenols may be the reason for the large differences in pseudostems of the two varieties. There were significant differences in the contents of aldehydes, esters, and terpenoids, so the distribution of the seeds of the two varieties was relatively dispersed.

4. Discussion

In this study, it was found that there were certain differences in the volatile substances of different varieties, stages, and organs of Chinese chive. This might be possible due to some differences in different varieties. These findings were consistent with the results of previous studies [23,24]. The differences between breeds may be controlled by certain genes. This study found that furfuryl, furans, phenols, terpenoids, and diterpenes can distinguish the difference between varieties very well. Furfuryl, furans, and phenols are the specific compounds in Fu Jiu Bao F1 at the commodity harvesting stage and at the physiological maturity stage. Further, terpenoids are the specific compounds in Fu Jiu Bao F1, and diterpenes are the specific compounds in Jiu Xing 22. The authors of [25] found that the intensity and quality of volatiles in different parts of Allium were affected by varieties and growth stages. The results of the present study showed that the volatile substances content in Fu Jiu Bao F1 was higher than that in Jiu Xing 22 at the commodity harvesting stage, but at the physiological maturity stage, the volatile substances in Jiu Xing 22 were higher than in Fu Jiu Bao F1. This phenomenon is consistent with previous studies, suggesting that it may have relationships with Chinese chive variety. The authors of [11] detected 57 kinds of VOCs in Chinese chive using HS-SPME/GC-MS, among which the highest content was ethers, which is in agreement with our study’s results. The highest content of Fu Jiu Bao F1 in the commodity harvesting stage and Jiu Xing 22 in the physiological maturity stage were both ethers. The authors of [22] reported 39, 28, and 27 volatile compounds in the roots, pseudostems, and leaves of green onion, respectively. In this experiment, the pseudostems in Fu Jiu Bao F1 had the most volatile substances, with 54, while the roots in Jiu Xing 22 had the most volatile substances, with 49. This suggests that there are not only differences in volatile compounds among Allium plants but that there are also differences among organs. During the growth process, the chemical composition of different parts of the plant may differ due to the different growth environments and nutrient uptake by plants. Many studies have been conducted on the biological and physiological effects of the characteristic components of Allium vegetables, and the relationship between sulfur compounds and human health has been reviewed [11,26,27]. For example, many sulfur compounds found in Allium vegetables may protect against cancer [28,29] as well as cardiovascular [5,30] and inflammatory diseases [31,32]. Some studies reported that compounds with an allyl group in their structure (such as DAS, DADS, or DATS) inhibited tumor initiation more effectively than molecules containing methyl or propyl groups [33,34]. Chinese chive contains a large number of sulfur compounds; however, it is important to extract sulfur compounds from Chinese chive to make functional products. At the commodity harvesting stage, the sulfur compounds of the two kinds of Chinese chives are mainly concentrated in the leaves, while at the physiological maturity stage, Fu Jiu Bao F1 is transferred to the seeds, and Jiu Xing 22 is transferred to the pseudostem and mature flower. Moreover, Fu Jiu Bao F1 had more sulfur compounds during the commodity harvesting stage. According to the characteristics of each stage, it can provide a reference for development and utilization. In the vegetative growth stage, the leaves can be processed into pickled Chinese chive. In the reproductive growth stage, the mature flower can be processed into Chinese chive flower sauce. Further, Allium plants have a distinctive odor that comes mainly from sulfur metabolites known as thioethers [11]. In fact, intact onion cells have no odor, but when the cell is destroyed, S-alk (en) yl cysteine sulfoxide is hydrolyzed, in turn producing volatile sulfides [23,35,36]. The production of S-alk (en) yl cysteine sulfoxide went through three steps: (1) sulfate assimilation; (2) glutathione biosynthesis; and (3) CSO biosynthesis [2]. These sulfur compounds are derived from the same precursor, but different reaction pathways lead to differences in their chemical structures, mainly due to the number and relative positions of S atoms, methyl groups, and carbon-carbon double bonds [11]. The authors of [12] identified 11 sulfur-containing compounds in shredded leaves of Chinese chive, among which methiin was the main one on the basis of their mass spectral and GC-retention data. The authors of [11] used HS-SPME/GC-MS to detect 11 sulfur-containing compounds in the leaves of Chinese chive. A total of eight sulfur compounds were found in this study, with more methyl sulfides than allyl sulfides, among which six methyl sulfides (allyl methyl sulfide; methyl 2-propenyl trisulfide; dimethyl tetrasulfide; diallyl disulfide; methyl 2-propenyl, disulfide; and dimethyl trisulfide) were found to be similar to the results of Yabuki et al. [12] Jun et al. [13] found that the sulfur compounds in onion root were more than those in steamed onion root, and the sulfur compounds decreased with the extension of steam-drying time. This research result is consistent with that of Bi et al. [37] that, under the same conditions, the higher the processing accuracy, the less volatile the matter. Through our study of these two varieties of Chinese chive, we found that the sulfur compounds in the vegetative growth stage are higher than in the reproductive growth stage, which is an interesting phenomenon and worthy of further study.

In the present study, at the commodity harvesting stage, the sulfur compounds in Fu Jiu Bao F1 are mainly methyl allyl disulfide and diallyl disulfide, and the sulfur compounds in Jiu Xing 22 are mainly diallyl disulfide and methylallyl trisulfide. At the physiological maturity stage, the main sulfur compounds in Fu Jiu Bao F1 are diallyl disulfide and methyl trisulfide, while the sulfur compounds in Jiu Xing 22 are mainly methyl trisulfide and dimethyl tetrasulfide. But Iida et al. [38] identified dimethyl disulfide, dimethyl trisulfide, allyl methyltrisulfide, and allyl methyldisulfide as the most important in the steam distillate of Chinese chive. Some studies documented that the content of dimethyl disulfide is the highest among volatile substances, followed by allyl methyl disulfide and methyl propenyl disulfide [12]. These differences in volatile substances may be explained by the results of previous research. [10,39], who found large differences in volatiles at different organs.

PCA mainly uses dimensionality reduction algorithms, extracting some principal components from the original variables to approximate them, thus they retain as much of the original information as possible. In our study, PCA was used to distinguish volatile substances, and the results showed that the leaves of the two varieties were relatively dispersed at the commodity harvesting stage, indicating that the differences between organs were greater than the differences between varieties, which was similar to the previous findings [23]. However, the main reason for the difference between the organs of the two varieties is the difference in the varieties, which can be explained by the difference in the content and composition of the volatile compounds of the two varieties. Previous studies have revealed that there were some differences in the number of volatile mixtures in different organs, which is consistent with the results of this study [40,41,42]. According to previous research [23], three root-specific compounds and two pseudostem-specific compounds were detected in Welsh onion. We also found two unique compounds: one was a root-specific compound (6,6-Dimethylhepta-2,4-diene), and the other was a pseudostem-specific compound (Trisulfide, di-2-propenyl).

5. Conclusions

The organ distribution pattern of volatile substances in Chinese chive depends on different growth periods and varieties. At the commercial maturity stage, the organ distribution rule was consistent between two varieties of Chinese chives, as follows: leaf > pseudostem > root. Nevertheless, at the physiological harvesting stage, volatile substances in two varieties of Chinese chives showed different organ distribution patterns. In general, volatile substances are more concentrated in reproductive organs than vegetative organs. As is characteristic of volatiles, the organ distribution of sulfur compounds was consistent with that of the total VOCs at the commodity harvesting stage. The results clarified the distribution pattern of volatile components, especially sulfur compounds, in different organs, growth stages, and varieties of Chinese chive. This can provide a theoretical foundation to study the regulation of volatile substance synthesis and explore organ-oriented functional products for Chinese chive in the future.