Composition and content of phenolic acids and flavonoids among the different varieties, development stages, and tissues of Chinese Jujube (Ziziphus jujuba Mill.)

The composition and content of phenolic acids and flavonoids among the different varieties, development stages, and tissues of Chinese jujube (Ziziphus jujuba Mill.) were systematically examined using ultra-high-performance liquid chromatography to provide a reference for the evaluation and selection of high-value resources. Five key results were identified: (1) Overall, 13 different phenolic acids and flavonoids were detected from among the 20 excellent jujube varieties tested, of which 12 were from the fruits, 11 from the leaves, and 10 from the stems. Seven phenolic acids and flavonoids, including (+)-catechin, rutin, quercetin, luteolin, spinosin, gallic acid, and chlorogenic acid, were detected in all tissues. (2) The total and individual phenolic acids and flavonoids contents significantly decreased during fruit development in Ziziphus jujuba cv.Hupingzao. (3) The total phenolic acids and flavonoids content was the highest in the leaves of Ziziphus jujuba cv.Hupingzao, followed by the stems and fruits with significant differences among the content of these tissues. The main composition of the tissues also differed, with quercetin and rutin present in the leaves; (+)-catechin and rutin in the stems; and (+)-catechin, epicatechin, and rutin in the fruits. (4) The total content of phenolic acid and flavonoid ranged from 359.38 to 1041.33 μg/g FW across all examined varieties, with Ziziphus jujuba cv.Jishanbanzao having the highest content, and (+)-catechin as the main composition in all 20 varieties, followed by epicatechin, rutin, and quercetin. (5) Principal component analysis showed that (+)-catechin, epicatechin, gallic acid, and rutin contributed to the first two principal components for each variety. Together, these findings will assist with varietal selection when developing phenolic acids and f lavonoids functional products.


Introduction
Jujube (Ziziphus jujuba Mill.), originated in China [1], has been cultivated for more than 7000 years and has so rich germplasm resources are available. As the main type of dried fruit a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 produced in China, jujube is an economically important forest tree and was known as one of the "five fruits" in ancient times, along with peach [Prunus persica (L.) Batsch], plum (P. domestica L.), apricot (P. armeniacaL.), and chestnut (Castanea spp.) [2,3]. The fruit of jujube contains general nutrients and a variety of functional components, including sugars [4,5], organic acids [6], vitamins C [7], flavonoids [8], cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP) [9], alkaloid [10], etc., making it become a medicine and food homologous product. Phenolic acids and flavonoids are important secondary metabolites with many biological uses, including antioxidants, antiaging, sedation and hypnosis, in lowering blood fat and pressure, as well as protective functions in the liver and cardiovascular, and cerebrovascular systems [11,12]. Phenolic acids and flavonoids are among the most important functional nutrients in jujube, and are found in the fruit [13][14][15], buds [16], leaf [17,18], and kernels [19]. Therefore, analyzing the phenolic acids and flavonoids composition and content among different jujube varieties and tissues is important for the efficient utilization of jujube germplasm resources.
Plant tissues are the sources of phenolics and flavonoids [20][21][22][23]. Phenolic acids in plant include different hydroxybenzoic acids [24] and hydroxycinnamic acids [25] and flavonoids includes flavonols, flavones, flavanols, flavanones, etc. [26,27]. Several recent studies have investigated jujube flavonoids. Zhou et al. [28] showed that the average total flavonoid content of the fruits of 10 jujube varieties ranged from 1.36 to 5.63 mg/g, whereas Kou et al. [29] found significant differences in the total flavonoid content and antioxidant activity among 15 jujube varieties. In addition, using high-performance liquid chromatography (HPLC), Geng et al. [30] showed that Ziziphus jujuba cv. Shanbeiyuanhongzao had the highest rutin content (288.21 μg/g) among eight jujube varieties. Li et al. [31] discovered that the leaves flavonoid content and composition significantly differs among different varieties. Nonetheless, only few studies have been conducted on jujube phenolic acids.
Previous studies on jujube flavonoids have mainly focused on extraction methods [32], determination of the total content, and the content of a few monomer compositions using a narrow range of materials and tissues. Consequently, there is a lack of systematic research on phenolic acids and flavonoids compositions and contents in different varieties, development stages, and tissues. Therefore, in the present study, this variation among 20 representative jujube varieties from the main jujube producing areas in China were analyzed to provide a reference point for further research and for the utilization of jujube phenolic acids and flavonoids.

Plant materials
This experiment was performed at the Shanxi Key Laboratory of Germplasm Improvement and Utilization in Pomology of the Research Institute of Pomology, Shanxi Academy of Agricultural Sciences, China. In all, 20 excellent jujube varieties from the main jujube producing areas in China were selected as test materials (Fig 1, S1 Table), all of which were collected from the National Jujube Germplasm Repository. The sample trees were 30-year-old fruit-bearing trees grown in 2.5 m × 3.0 m spaces and subjected to the same cultivation and management practices. Nine sample trees of each variety, with three biological replicates of three trees each were selected.
Fruit samples were collected from 2~5-year-old sections of the trees at the full red mature stage. All sample fruits were hanged outside the tree canopies, the same size and maturity, free from diseases and insect pests. Collection was according to the standard method for jujube [33]. In addition, fruit samples of Ziziphus jujuba cv.Hupingzaowere collected at five development stages: young fruit (S1; 40 days after the full-blossom period), expanding fruit (S2; 65 days after the full-blossom period), white maturity (S3; when the surface of the fruit had faded

Sample extraction
Sample pretreatment was performed as per the methods of Li et al. [34] and Li et al. [35] with some modification. First, 1.0000 ± 0.0005 g of each prepared sample was weighed into a 50 mL centrifuge tube. Then, 10 mL extract (methanol:water:formic acid = 70:29:1) was added and the mixture was treated ultrasonically for 30 min at 50˚C and then centrifuged for 10 min at 10,000 rpm, following which the residue was extracted once again. The two supernatants were combined and poured into a 25 mL brown volumetric flask, and the volume was brought to 25 mL by adding the extract. This sample was then passed through an SPE column and filtered through a 0.22 μm membrane before use.

Chromatographic conditions
UPLC was performed using Acquity UPLC HSS T3 column (2.1 mm × 100 mm, 1.8 μm) with methanol as mobile phase A, and 0.2% formic acid aqueous solution as mobile phase B, and using the gradient method for elution ( Table 1). The column temperature was maintained at 35˚C, the injected volume was 3 μL, and the flow rate was 0.25 mL/min. UPLC ultraviolet detector was used for detection using wavelengths of 283 nm and 367 nm and an automatic sampler.

Standard solution preparation
A 10.0 mg aliquot of each standard was weighed using an electronic analytical balance, dissolved in methanol, stabilized in a 10mL volumetric flask, and then stored at 4˚C for later use. In addition, the standard mother liquors were diluted to different concentrations to produce standard curves. Detection was performed using the same chromatographic conditions as described above.

Statistical analysis
All statistical analyses were conducted using Excel 2007; all assays were performed in triplicates, and all the data are expressed as mean values ± standard deviation. Statistically

PLOS ONE
Phenolic acids and flavonoids of Chinese Jujube (Ziziphus jujuba Mill.) significant differences were determined using one-way repeated measures analysis of variance performed using SPSS 18.0. Correlation and principal components analyses among the mean values were conducted using Statistical Analysis System v. 9.2 software.

Phenolic acids and flavonoids composition of the samples
Chromatograms of the 14 phenolic acids and flavonoids standards under optimized analytical conditions are presented in Fig 2A. In total, 13 of these standards, including (+)-catechin, epicatechin, rutin, quercetin, kaempferol, isorhamnetin, spinosin, phloridzin, luteolin, gallic acid, chlorogenic acid, ferulic acid, and caffeic acid, were detected from different varieties, development stages, and tissues, with different combinations in each variety. Of the 14 phenolic acids and flavonoids, 12 were detected in the fruits (Fig 2B), 11 in the leaves, and 10 in the stems. Seven phenolic acids and flavonoids, (+)-catechin, rutin, quercetin, spinosin, luteolin, gallic acid, and chlorogenic acid, were common for all tissues.

Changes in phenolic acids and flavonoids composition and content among fruit at different development stages
The total phenolic acids and flavonoids content of Ziziphus jujuba cv. Hupingzao fruit significantly differed depending on the stage of fruit development and decreased as development progressed (Fig 3). There was only a slight decrease from S1 to S2, but this was followed by a rapid decrease from S2 onward. Thus, the phenolic acid and flavonoid content was the highest in S1 (2856.57 μg/g FW) and lowest in S5 (675.34 μg/g FW), showing a difference of 2181.23 μg/g FW. Ten phenolic acids and flavonoids, (+)-catechin, epicatechin, rutin, quercetin, kaempferol, spinosin, phloridzin, luteolin, gallic acid, and chlorogenic acid, were identified in stages S1-S4, whereas isorhamnetin (1.901 μg/g FW) was also detected in S5; The contents of (+)-catechin, rutin, quercetin, spinosin, and luteolin decreased with fruit development ( Table 2). In contrast, the contents of epicatechin, kaempferol, gallic acid, and chlorogenic acid initially increased and then decreased, with the highest content observed in S2, whereas the content of phloridzin peaked in S3.

Phenolic acids and flavonoids composition and content in different tissues
There were significant differences in the total phenolic acids and flavonoids contents of the fruit, leaves, and stems of Ziziphus jujuba cv. Hupingzao at the S3 stage, with the leaves and stems having 5.39 and 3.63 times the phenolic acids and flavonoids content of the fruit, respectively ( Table 3). The types of phenolic acids and flavonoids present also differed among the tissues, with isorhamnetin, ferulic acid, and caffeic acid not being detected in the fruit; epicatechin and caffeic acid not being detected in the leaves; and isorhamnetin, kaempferol, and phloridzin not being detected in the stems. In addition, there were significant differences in the main phenolic acids and flavonoids compositions among three tissues. In the fruit, the content of epicatechin was the highest (57.76% of the total content), followed by (+)-catechin (20.89%), gallic acid (15.08%), and rutin (3.34%). In the leaves, the content of quercetin and rutin was the highest (41.43% and 39.94%, respectively), followed by chlorogenic acid (6.43%) and gallic acid (5.74%). In the stems, the content of (+)-catechin was the highest (54.78%), followed by rutin (30.51%).

Phenolic acids and flavonoids compositions and contents among the fruits of different varieties
The phenolic acids and flavonoids composition and content of the fruits of 20 representative jujube varieties at the full red stage (S5) are presented in  There were also significant differences in the phenolic acids and flavonoids compositions among the different varieties examined. In total, 12 phenolic acids and flavonoids, including (+)-catechin (average of the total content = 55.95%), epicatechin (18.71%), rutin (9.76%), gallic acid (7.30%), chlorogenic acid (3.58%), phloridzin (2.00%), quercetin (1.65%), spinosin (0.89%), luteolin (0.53%), isorhamnetin (0.29%), ferulic acid (0.23%), and kaempferol (0.22%) were identified in the full red fruits of the 20 jujube varieties. Among these, isorhamnetin and ferulic acid were only detected in some varieties, whereas quercetin and spinosin were present in all varieties except Ziziphus jujuba cv.Yunchengxiangzao. In all varieties, the content of (+)-catechin was the highest (31.91%-81.03%), followed by epicatechin, rutin, gallic acid, and chlorogenic acid; the content of other flavonoids, such as quercetin, spinosin, phloridzin, luteolin, and kaempferol were low. The varieties with high total phenolic acids and flavonoids content did not necessarily have a high content of every phenolic acid and flavonoid.

Correlations among the different phenolic acids and flavonoids compositions
Correlation analysis of the different phenolic acids and flavonoids compositions of the 20 varieties (Fig 4) revealed that (+)-catechin and epicatechin contents were significantly positively correlated with the total content (P < 0.01); gallic acid content was significantly positively correlated with the total phenolic acids and flavonoids content (correlation coefficient = 0.45; P < 0.05). Epicatechin content was significantly positively correlated with gallic acid

Principal component analysis
Principal component analysis (PCA) identified two components that explained 97.75% of the total variation in the phenolic acids and flavonoids composition of the 20 varieties (Table 5). The first principal component (PC1) contributed 74.60%. Large, and positive values were associated with (+)-catechin, suggesting that it greatly contributed to PC1. The second principal component (PC2) contributed 23.15% of the total variation. Large and positive values were associated with epicatechin, gallic acid, and rutin, suggesting that these greatly contributed to PC2.   Ziziphus jujuba cv.Yunchengxiangzao, and Ziziphus jujuba cv.Yuanlingzao belonged to the third group characterized by high (+)-catechin and low epicatechin, rutin, and gallic acid levels. The remaining varieties belonged to the fourth group characterized by medium composition levels of each composition. These results provide a reference for the selection of high phenolic acids and flavonoids composition and content varieties.

Discussion
Phenolic acids and flavonoids are some of the most important functional nutrients in jujube. We therefore systematically evaluated the phenolic acids and flavonoids composition and content among different varieties, development stages, and tissues of Chinese jujube. First, we selected Ziziphus jujuba cv. Hupingzao as our experimental material to analyze the phenolic acids and flavonoids composition among the different development stages and tissues. Ziziphus jujuba cv. Hupingzao, from the main production province of Shanxi, China, is a Chinese jujube variety that has been awarded the "National Geographical Indicated Products", validating its selection as a representative sample. Second, we selected 20 excellent varieties of Chinese jujube from the main production areas of Shanxi, Shandong, Henan, Hebei, and Shannxi provinces to determine the phenolic acids and flavonoids composition and content of the fruits during their full maturity stage. Fully matured fruits-namely, full red jujube fruits-are extremely nutritious and can replenish qi; while nourishing and soothing the nerves [36]. Consequently, they are often used to enhance the efficacy of medicines and are the main part of the jujube plant that is the most utilized. From a practical application point of view, it is reasonable that the phenolic acids and flavonoids content of the 20 representative Chinese jujube varieties included in this study should be analyzed for fruits at the full red stage rather than at the young fruit period, during which the content is high. We undertook a comprehensive analysis of the possible phenolic acids and flavonoids in jujube by selecting 14 standards on the basis of previously published research. A total of 13 phenolic acids and flavonoids were detected in the jujube samples. An unknown peak was also observed in the test samples, but because that its peak area was small, it was not counted among the main phenolic acids and flavonoids. However, the identity of this unknown substance should be determined in the future using mass spectrometry and other techniques. We found that the total phenolic acids and flavonoids content of the jujube fruit decreased, as the fruit developed; this is consistent with the results obtained by Zhao et al. [37] and Shen et al. [38] for jujube and Huang et al. [39] for kiwifruit (Actinidia deliciosa Planch) determined using spectrophotometry. In addition to phloridzin, the content of other phenolic acids and flavonoids also decreased with fruit development, supporting the results of Xia et al. [40] who studied the dynamic changes in the flavonoids composition in apricot during fruit development. The dynamic change in the content of each phenolic acid and flavonoid with fruit development is due to the variation in the balance between synthesis, transportation, and decomposition. Our results indicated that the main stage of phenolic acids and flavonoids synthesis in the Chinese jujube fruit occurred in the young fruit, and the compositions in the fruit at this stage were actively metabolized. Young naturally shed fruits can be used to extract phenolic acids and flavonoids and other active ingredients to maximize the utilization of resources.
The detected phenolic acids and flavonoids composition varied among the different varieties, development stages, and tissues, with 12 phenolic acids and flavonoids being detected in the fruits, 11 in the leaves, and 10 in the stems. However, seven phenolic acids and flavonoids were common to all samples. Three flavonoids were reported in the leaves of Amaranthus.tricolor [27,41]; nine in the drought-tolerant vegetable amaranth leaves [24,26], salt-tolerant vegetable amaranth leaves [42], and A.gangeticus leaves [25]; and eight in the leaves of red and green amaranth [43]. Isorhamnetin and ferulic acid were detected only in the fruits of some varieties; therefore, it was speculated that they had varietal specificity. Moreover, epicatechin was not detected in the leaves, isorhamnetin, kaempferol, and phloridzin were not detected in the stems, and caffeic acid was only detected in the stem. It can be inferred that there were differences in the detected phenolic acids and flavonoids among different parts of the plant. In contrast, the leaves and stems had higher total contents than the fruit, and the main phenolic acids and flavonoids also differed among these tissues, with the contents of quercetin, (+)-catechin, and epicatechin being the highest in the leaves, stems, and fruit at the white maturity stage (S3), respectively. The results indicated that the phenolic acids and flavonoids contents varied greatly among different parts. The reasons for these differences in the monomer compositions of the different tissues need to be further studied. Flavonoids in the leaves show a variety of biological properties, such as antioxidant, neuroprotective, and bioprotective properties [44,45], indicating that it would be beneficial to increase the utilization of the flavonoidrich leaves and stems to increase the economic benefit of jujube as a forest tree. Zhang et al. [46] previously showed that the sprouting stage of walnut (Juglans sigillata Dode) leaves had the highest content of flavonoids. Therefore, it would also be useful to study the dynamic changes in jujube leaves at different development stages to determine when the flavonoid content is the highest.
Many studies have confirmed that the composition and content of flavonoids vary among fruit species. For instance, flavanones such as naringin and hesperidin are the main flavonoids in citrus [47,48], whereas the flavonoid dihydrochalcone is found in apples [49]. Furthermore, the flavonoid composition and content also vary among different types and varieties of the same fruit species. For example, different types of peaches have been shown to have different flavonoid compositions, with epicatechin being the main composition in juicy peach and peach and catechin being the main composition in nectarine [50]. In the present study, we found that the main phenolic acids and flavonoids in the fully mature Chinese jujube fruit were the flavonoids (+)-catechin, epicatechin, and rutin; and the phenolic acids gallic acid; of these, the content of(+)-catechin was the highest. The bioactive functions of specific phenolic acids and flavonoids depend on their composition. For example, catechins can inhibit breast, liver, colorectal, and other types of cancers [51]; rutin functions as an antibacterial and antiinflammatory [52,53] substance; gallic acid shows antioxidant, anti-inflammatory, and nephroprotective properties [54]. In this study, we identified the differences between the flavonoids of Chinese jujube fruit and other fruit trees, and analyzed the main phenolic acids and flavonoids in Chinese jujube fruits. These results will provide a reference for the efficient utilization of jujube germplasm resources and deeper research and development of relevant functional products.
Through the multivariate statistical method of PCA, we selected a small number of important variables through linear transformation, that could reflect the information of the original variables as thoroughly as possible; this method is used for dimension reduction in mathematics. In this study, we identified two components using PCA, including four indices-of (+)-catechin, epicatechin, gallic acid, and rutin. Based on these indices, we grouped the 20 different varieties into four categories with distinct characteristics. Of them, Ziziphus jujuba cv. Jishanbanzao, Ziziphus jujuba cv. Jiaochengjunzao, Ziziphus jujuba cv. Yunchengxiangzao, and Ziziphus jujuba cv. Yuanlingzao are rich in (+)-catechin; Ziziphus jujuba cv. Zaoqiangpozao and Ziziphus jujuba cv. Yongjihamazao are rich in epicatechin, gallic acid, and rutin. These varieties could therefore be used as raw materials to develop of functional products or as breeding materials for new varieties with correspondingly high compositions. In the future, it would be useful to investigate the biological functions as well as the metabolic mechanism of the different phenolic acids and flavonoids using molecular biological methods. Moreover, our study found no correlation between the origin of the varieties and PCA results. Although the varieties included in this study originated from different ecological conditions, the samples were collected from the same ecological conditions, and each variety had been introduced to the conservation place since several decades, which may be the reason for this result. This provides us with new avenues of research, to explore the differences in the phenolic acids and flavonoids composition and content of these important varieties under different ecological conditions, to further clarify the impact of ecological conditions on the contents of phenolic acids and flavonoids and other bioactive substances.

Conclusions
We explored the composition and content of phenolic acids and flavonoids among different varieties, development stages, and tissues of Chinese jujube. A total of 13 phenolic acids and flavonoids were detected among the different samples, including (+)-catechin, epicatechin, rutin, quercetin, kaempferol, isorhamnetin, spinosin, phloridzin, luteolin, gallic acid, chlorogenic acid, ferulic acid, and caffeic acid. The total content and composition of phenolic acids and flavonoids decreased with development of the fruit. In addition, the total content varied among different tissues, with the leaves having the highest content and the fruit having the lowest. The main phenolic acids and flavonoids in each tissue also differed, with the highest content of(+)-catechin, epicatechin, and rutin in the fruit; quercetin and rurin in the leaves; and (+)-catechin and rutin in the stem. Finally, (+)-catechin was identified as the main composition in the 20 varieties examined, but there were significant differences in the total contents. Using PCA, we grouped the 20 varieties and screened them with higher contents of important compositions. In future studies, we should purposefully develop and utilize different varieties and tissues according to their individual composition and content.
Supporting information S1