Gailiangqingza No. 3 is a variety of Jiazhou cabbage, the most popular Chinese cabbage variety and is of a high quality, disease resistance, with a high yield, and outstanding storage and transportation resistance in the Jiaozhou area, which was used in the study. We planted Gailiangqingza No.3 on August 15th, 2020, in the field of Qingdao Jiaozhou Chinese Cabbage Research Institute (Jiaozhou, Shandong, China) and the field of the Vegetable Research Institute, Shandong Academy of Agricultural Sciences (Jinan, Shandong, China) in terms of climatology and soil composition. Jiaozhou Chinese cabbage (JA) area was located in Anjiagou Village, Jiaozhou City, Qingdao (119°83${}^{\prime }$97.55${}^{\prime }$${}^{\prime }$ N, 36°13${}^{\prime }$92.41${}^{\prime }$${}^{\prime }$ E). It belongs to the continental climate in the warm temperate semi-humid monsoon region. Its characteristics include sufficient light, rich heat, rain and heat in the same season, four distinct seasons, and a long frost-free period. The daytime temperature in autumn is between 15–24 °C, and the temperature difference between day and night is about 10 °C. The soil is divided into brown, fluvo-aquic, Shajiang black, saline, and paddy soil [10].

Jinan Chinese cabbage (CQ) was located in Pangzhuang Village, Changqing District, Jinan City, Shandong Province (116°73${}^{\prime }$38.89${}^{\prime }$${}^{\prime }$N, 36°38${}^{\prime }$92.29${}^{\prime }$${}^{\prime }$E). Warm temperate semi-humid continental monsoon climate with four distinct seasons. In autumn, the daytime temperature is 18–28 °C, and the temperature difference between day and night is about 10 °C. The soil is mainly brown loam, brown soil, and sandy terroir. Standard field management was carried out during the plant growth using conventional management in open fields [11], and the tight leafy heads were harvested in November 2020.

Fresh Chinese cabbages were randomly collected from Jiaozhou and Jinan, respectively. After removing the roots and outer leaves, the leafy heads were cut up and mixed evenly. The mixed samples were collected using the quartering method to detect quality-related substances and metabolites. Each sample had three biological replicates, and each repeat included three individual plants. The physical and chemical characteristics of the tested soil are shown in Table 1.

The contents of soluble protein, crude fiber, dry matter, vitamin C, and total soluble sugar are usually used to evaluate the nutritional quality of Chinese cabbage. The study determined soluble protein content using the Kjeldahl nitrogen method (GB/T 5009. 5. 2016) [12]. Protein was decomposed to produce ammonia reacted with sulfuric acid, producing ammonium sulfate under catalytic and heating conditions. Boric acid absorbs ammonia during alkaline distillation, titrating with sulfuric acid or hydrochloric acid in a typical volumetric solution. The nitrogen content was calculated based on the acid consumption and then multiplied by the conversion coefficient to get the protein content. The crude fiber was determined according to GB/T 5009.10-2003 [13]. The sample’s sugar, starch, fructose, and hemicellulose contents were hydrolyzed and eliminated by sulfuric acid. Crude fiber, as the remaining substance, was subjected to an alkali treatment to remove the protein and fatty acids. The contaminants were removed after ashing if they were insoluble in acid and base. The dry matter was determined using direct drying experiments according to GB 5009.3-2016 [14]. The dry weight loss in the sample, including absorbent water, partially crystallized water, and substances that are volatile under these conditions, was calculated using the volatile method using the physical properties of water in food at 101.3 kPa (one atmospheric pressure) and the temperature of 101–105 °C. Then the moisture content was calculated using the weight values before and after. Vitamin C content was determined according to GB/T 5009. 86. 2016 [15]. After dissolving ascorbic acid in the sample with metaphosphoric acid and sonicating it, an ion pair reagent was used as the mobile phase. It was separated using a reverse phase column. L(+)-ascorbic acid and D(+)-ascorbic acid were measured directly using a liquid chromatograph with a UV detector (wavelength 245 nm). Following the reduction in L(+)-dehydroascorbate in the samples by L-cysteine solution, the content of L(+)-dehydroascorbic acid was determined by measuring the total amount of L(+)-ascorbic acid with a UV detector (wavelength of 245 nm) or subtracting the L(+) ascorbic acid content measured in the original sample. Total soluble sugar was determined using the Shaffer-Somogyi method [16].

Hierarchical clustering analysis (HCA), principal component analysis (PCA), and orthogonal partial least squares discriminant analysis (OPLS-DA) of the identified metabolites were performed using the statistics function within R (https://www.r-project.org/), and an independent T-test was conducted using SPSS (Version 13.0., SPSS Inc., Chicago, IL, USA).

The leafy head appearance of Gailiangqingza No.3 had no significant differences between the Chinese cabbages planted in Jinan and Jiaozhou (Fig. 1a). However, the flavor was significantly different between JA and CQ according to the taste assessments of 10 people following previous research [22]. Jiaozhou Chinese cabbage was much crunchier and sweeter based on the testing taste of 10 respondents (Supplementary Table 1). We chose 10 people as a representative from 24–56 years old males and females, of which 4 of 10 respondents were male living in Jinan and Qingdao. The study determined the contents of soluble protein, crude fiber, dry matter, vitamin C, and total soluble sugar to assess the quality differences between Chinese JA and CQ. The results showed no significant differences in soluble protein, crude fiber, dry matter, or vitamin C. In contrast, the total soluble sugar content of JA was higher than that of CQ (Fig. 1b). The N and P supply in JA was higher than CQ, illustrating that N and P affected soluble sugar content (Table 1). Our result is in accordance with a previous study which found that the enhancement of total soluble sugar content has a positive correlation to the N and P supply [23]. Similarly, another previous study stated that higher K content in soil significantly reduces the total soluble sugar content [24]. Moreover, the organic matter positively correlated with K absorption, where organic matter considerably promotes the initial fast rate of K adsorption and has more easily accessible adsorption sites for K [25]. We assumed that the lower total soluble sugar content in CQ was related to the soil’s N, P, and K content. Furthermore, this may be the reason why JA tasted sweeter than CQ [26].

Fig. 1.

Quality detection of Chinese cabbage in Jiaozhou (JA) and Jinan (CQ). (a) Leafy head of Chinese cabbage Gailiangqingza No.3 planted in JA and CQ, bar = 5 cm. (b) The contents of soluble protein, crude fiber, dry matter, vitamin C, and total soluble sugars of JA and CQ. *: Difference significance at p 0.05 level according to independent T-test between JA and CQ.

Carbohydrates, organic acids, amino acids, and phenolics play important roles in taste determination [28]. In the study, many kinds of carbohydrates, organic acids, amino acids, and phenolics were detected in Chinese cabbage (Fig. 3a). These metabolites may play important roles in the specific taste of Chinese cabbage.

Chinese cabbage has been reported to have good medicinal value for many diseases, such as cancer, tumors, inflammation, pathogen infection, diabetes, and so on [29, 30, 31]. However, there is no comprehensive identification of the metabolites related to human disease in Chinese cabbage [32, 33, 34]. Therefore, all the metabolites detected in the study were used to perform a query in the TCMSP database to obtain those related to human disease resistance [35]. A total of 108 out of the 531 metabolites were found to be the chemical compositions of Traditional Chinese Medicine, and 79 were found to correspond to at least one disease (Supplementary Table 3). The 79 metabolites contained 17 phenolic acids, 13 amino acids and derivatives, nine lipids, eight alkaloids, seven flavonoids, five nucleotides and derivatives, five organic acids, five vitamins, four lignans and coumarins, two glucosinolates, one saccharide and alcohol, one terpenoid, and one other metabolite (Supplementary Table 3). It was validated that Chinese cabbage is a beneficial vegetable for daily health promotion.

It was reported that the production area had obvious effects on the quality of Chinese cabbage because of the different soil and climatic conditions [36]. However, it is still unclear whether and how production areas affect the metabolite profiling of Chinese cabbage. Jiaozhou is one of the most famous main producing areas for Chinese cabbage. Jiaozhou Chinese cabbage has unique taste and flavor qualities [9]. To study the Jiaozhou-specific metabolite profiling of Chinese cabbage, we analyzed the metabolite differences between JA and CQ, and a total of 89 differential metabolites were identified, which consisted of 38 lipids, 16 phenolic acids, seven flavonoids, six nucleotides and derivatives, four amino acids and derivatives, four saccharides and alcohols, four alkaloids, three glucosinolates, three lignans and coumarins, one organic acid, one vitamin, one terpenoid, and one other metabolite (Fig. 4a, Supplementary Table 4). Of the 89 differential metabolites, 11 were significantly higher in JA, whereas 78 were significantly lower compared with CQ (Fig. 4, Supplementary Table 4). Nicotinic acid adenine dinucleotide, S-(methyl)glutathione, and p-coumaroyltyramine were the top three differentially altered metabolites with the biggest Log2(fold change) value between contents in JA and CQ (the Log2(fold change) values of JA/CQ were 17.89, 14.34, and 2.51, respectively).

Fig. 4.

HCA and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of differentially altered metabolites (DAMs) between JA and CQ. (a) Heatmap of the DAMs detected in all samples. Red indicates high abundance; green indicates low abundance. (b) KEGG enrichment analyses of DAMs between JA and CQ.

Furthermore, lysoPE 16:1 (2n isomer), caffeoyl(p-hydroxybenzoyl) tartaric acid, and lysoPE 16:1 (2n isomer) were the top three with the lowest Log2(fold change) value (the Log2(fold change) of JA/CQ was –3.86, –3.52, and –2.94, respectively) (Table 3). The results indicate that the producing areas had obvious effects on the metabolite profiling of Chinese cabbage. Nicotinic acid adenine dinucleotide and S-(methyl)glutathione were not detected in CQ, whereas their relative levels were high in JA (Supplementary Table 3). These metabolites may be Jiaozhou-specific metabolites of Chinese cabbage. KEGG pathway enrichment analysis was further performed, and the results showed that the DAMs were enriched into 23 pathways, of which tryptophan metabolism (ko00380) and thiamine metabolism (ko00730) were the most significant enrichment pathways (Fig. 4b).

Fourteen out of the 89 DAMs were found to be chemicals related to Chinese traditional medicine, and they corresponded to at least one human disease. The relative contents of 12 DAMs corresponding to human disease were higher in CQ than in JA, whereas only two had lower contents in CQ than in JA (Supplementary Table 4). This indicates that Chinese cabbage planted in CQ may have better health effects for some human diseases.