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Article

Identification of Key Soil Mineral Elements Affecting Sugars and Organic Acids of Jujube Fruit

by
Panpan Tong
1,2,
Guanglian Liao
3,
Fengzhi Liang
1,2,
Dengyang Lu
1,2,
Cuiyun Wu
1,2 and
Jiangbo Wang
1,2,*
1
College of Horticulture and Forestry, Tarim University, Alar 843300, China
2
National-Local Joint Engineering Laboratory of High Efficiency and Superior Quality Cultivation and Fruit Deep Processing Technology on Characteristic Fruit Trees, Alar 843300, China
3
National Key Laboratory for Germplasm Innovation & Utilization of Horticultural Crops, Huazhong Agricultural University, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(6), 652; https://doi.org/10.3390/horticulturae10060652
Submission received: 25 May 2024 / Revised: 12 June 2024 / Accepted: 17 June 2024 / Published: 19 June 2024
(This article belongs to the Section Plant Nutrition)
Browse Figures
Versions Notes

Abstract

:
Soil mineral elements are the key factors affecting fruit quality, but which soil elements affect the sugars and organic acids of jujube fruit is still not clear. In this study, the fruit and soil of 18 major producing areas of the Tarim Basin were studied. By measuring the sugar and acid content in the fruits and element contents in the soil, the main soil mineral elements affecting the sugar and acid content in jujube fruits were identified. The results showed that the sugar components were mainly sucrose, glucose and fructose, and the organic acid components were mainly succinic acid, citric acid and malic acid. The fruits near the Kunlun Mountains had a higher ratio of sugar-to-acid and sweet-to-acid. Some elements in the soil of the Tarim Basin are abundant, such as the elements Ca, Fe, Mn and B, but the contents of the elements NO3-N, NH4-N, Zn and Mo are low. The contents of Ca, Mg and Mn were positively correlated with the contents of glucose, fructose and galactose. And the Fe, Ca, Mg and Mn were the main factors affecting the sugars and organic acid contents. Our study provides theoretical support for rational fertilization and efficient cultivation management of jujube.

1. Introduction

Ziziphus jujube Mill. is the largest dried fruit tree and an important economic forest tree in China [1,2]. Jujube fruit is not only rich in amino acids, sugars, vitamin C, polyphenols and other active substances [3,4], but also rich in micronutrients and cellulose [5], which together give jujube fruit unique nutritional and medicinal value. In addition, it is important that the jujube tree also has the advantages of being windproof, sand fixation, drought tolerance and salt tolerance, so it is widely cultivated in Xinjiang, China, in which the Tarim Basin is one of the main production areas of jujube.
Fruit flavor quality is the main factor in determining fruit quality and commercial value, which is mainly affected by the contents of sugars and organic acid components [6,7]. The sugars and organic components in the fruit are easily affected by the external environment, especially the soil [8]. A close relationship between fruit quality and soil nutrients has been confirmed in kiwifruit [9], apple [10], citrus [11] and other fruit crops [12,13]. In jujube, there was also a report that a significant correlation between tree nutrients and soil minerals [14], and the soil factors affecting jujube fruit quality were identified by using multivariate statistical methods [15,16]. However, the effect of soil mineral elements on the sugars and organic acid components of jujube fruit is still not well understood.
Therefore, the fruit and soil of the main jujube-producing areas in the Tarim Basin were taken as the research objects, and the sugars and organic acid component contents in the fruits was determined, and principal component analysis was used to rank the producing areas. At the same time, the soil mineral element content was determined, and correlation analysis and the linear regression equation were used to identify the key soil mineral elements that affect the sugars and organic acid quality of fruit. Our research provides a theoretical basis for guiding reasonable fertilization and efficient cultivation of jujube trees.

2. Materials and Methods

2.1. Materials

Fruit and soil samples were collected from 18 main jujube-producing areas (35°–42° N, 75°–90° E) in Tarim Basin, China (Supplementary Table S1, Figure 1). Two jujube plantations with the same management level and tree age in each producing area were selected. Nine jujube trees were randomly selected from each plantation, and three trees from each were used as biological replicates. A total of 60 fruits at full red stage were collected from the east, south, west and north directions of each jujube tree, and soil at a depth of 20–40 cm outside the canopy projection was also collected [17]. The jujube pit was removed in the field and the flesh was treated with liquid nitrogen, then stored at −80 °C for future use. Around 1 kg of soil samples was collected according to the quartering method [18], the plant residues and debris were removed, and then the soil was naturally air-dried indoors.

2.2. Determination of Sugar and Organic Acid Components

Extraction and derivation were performed following the previous description [19]. A 0.5 g sample was weighed into a 10 mL tube, 7 mL of 80% methanol was added into it, and it was incubated at 70 °C for 30 min. Then, it was extracted under ultrasonic conditions for 90 min, centrifuged at 4000 rpm/min for 10 min, and 2 mL of the supernatant was transferred into a new tube, and then centrifuged at 12,000 rpm/min for 15 min. An amount of 0.5 mL of the supernatant was taken and dried at 60 °C with a vacuum rotary evaporator. After drying, 0.8 mL of hydroxylamine hydrochloride solution was added to the sample for a derivatization reaction, and the reaction was carried out at 70 °C for 1 h. After cooling, 0.4 mL of HMDS and 0.2 mL of TMCS were added successively and heated at 70 °C for 2 h. Finally, 0.5 mL of the supernatant was transferred to the sample vial for GC-FID analysis. Finally, the content of sugar and organic acid components was determined by gas chromatography using ISQII (GC-MS, Thermo, Waltham, MA, USA) [20]. Gas chromatography used 5%-Phenyl-methyl polysiloxane (Agilent, Santa Clara, CA, USA) with high-purity nitrogen as the carrier gas at a flow rate of 45 mL/min. Other parameters were default parameters [21]. The total sugar and total acid are the sum of all sugar and organic acid components, respectively [22,23,24].

2.3. Calculation of Sugar–Acid Ratio, Sweetness Value and Sweet–Acid Ratio

The sugar–acid ratio is the ratio of total sugar content to total acid content. Sweetness value = Suc × 1 + Fru × l.750 + Glu × 0.700 + (Rha × 0.330 + Xyl × 0.450 + Gal × 0.300 + Man × 0.300)/4. In the formula, Suc, Fru, Glu, Rha, Xyl, Gal and Man are, respectively, the contents of sucrose, fructose, glucose, rhamnose, xylose, galactose and mannose, and the numbers are the sweetness values assigned to each sugar component [25]. The sweet–acid ratio is the ratio of the sweetness value to the total acid content.

2.4. Determination of Soil Mineral Elements

The air-dried and sieved soil was sent to Hubei QAL Testing Science and Technology Co., Ltd. (Wuhan, China) for testing of mineral elements. The soil samples were ground into powder and mixed with acidic buffer solution. After ultrasonic treatment, the content of nitrate nitrogen and ammonium nitrogen were determined using a UV spectrophotometer [26]. The dissolved soil solution was then injected into an inductively coupled plasma spectrometer (iCAP7200 Radial, Thermo, Waltham, MA, USA). By exciting the plasma, emission spectra were generated. The instrument detected the intensity and wavelength of the emitted spectra to determine the content of various mineral elements in the samples [26]. Additionally, the electrode of a pH meter was immersed in the soil extract to measure the pH and electrical conductivity [26].

3. Data Analysis

Data statistics and visualization were performed by GraphPad Prism [27] and TBtools-II [28]. The difference analysis was conducted using a one-way ANOVA test with Duncan’s method and a significance level set at p < 0.05 [29].

4. Results and Discussion

4.1. Evaluation of Sugar and Acid Components of Fruit from Different Producing Areas

The total sugar, total acid and sweetness values of jujube fruits in different producing areas varied from 101.59 to 328.96 mg/g, 2.71 to 10.33 mg/g and 100.35 to 325.04 (Figure 2A–C, Supplementary Table S2). The sugar–acid ratio and sweet–acid ratio ranged from 23.14 to 56.13 and 22.69 to 50.34 (Figure 2D,E), respectively. Among them, the sugar–acid ratio and sweet–acid ratio of fruits near the Kunlun Mountains were higher. In addition, seven sugar components and six acid components were detected in the fruit. The main sugar components were sucrose, glucose and fructose, among which sucrose had a great difference between different producing areas, ranging from 26.12 to 189.00 mg/g (Figure 2F). The acid components were mainly succinic acid, citric acid and malic acid, among which citric acid had a great difference between producing areas, ranging from 0.17 to 5.35 mg/g (Figure 2G).

4.2. Determination of Soil Mineral Elements in Different Producing Areas

The basic physical and chemical properties and mineral elements of soil in different producing areas were analyzed, and the overall soil was moderately alkaline, with a pH value ranging from 7.66 to 8.32 (Figure 3A). The electrical conductivity of the soil in different producing areas varied widely, ranging from 0.16 to 2.14 mS/cm (Figure 3B), and the coefficient of variation was 60.54%. The macronutrients in the soil were mainly the elements Ca, Mg and S (Figure 3C), and the content of the element Ca was much higher than other elements (5567.51–20,895.95 ppm; coefficient of variation is 49.25%), especially in the soil located in the northwest of the Tarim Basin (Supplementary Table S3). The micronutrients in the soil were mainly Na, Cl, Fe, Al and Mn, which were 54.87–531.41 ppm, 7.76–310.28 ppm, 33.07–159.40 ppm, 8.04–161.44 ppm and 22.82–79.27 ppm, respectively (Figure 3D).

4.3. Principal Component Analysis of Sugar and Organic Acid Components

Three principal components with eigenvalues greater than 1 were selected for analysis. The contribution rates of the three principal components were 53.79%, 17.62% and 15.13%, respectively, and the total contribution rate was 86.54% (Table 1). The first principal component was composed of total acid, the second principal component was composed of sucrose, and the third principal component was composed of sweet–acid ratio. Taking each principal component as weight, the scoring formulas of the three principal components and the total scoring formulas were obtained (Supplementary Table S4). After comprehensive evaluation, the ranking of different producing areas is as follows: ‘50R’ > ‘MF’ > ‘33R’ > ‘PS’ > ‘QM’ > ‘46R’ > ‘SY’ > ‘48R’ > ‘12R’ > ‘ZP’ > ‘RQ’ > ‘YT’ > ‘3R’ > ‘47R’ > ‘29R’ > ‘41R’ > ‘LP’ > ‘42R’ (Table 2). Among them, the top producing areas are distributed around the Tianshan Mountains and Kunlun Mountains, but the surface of these areas is bare and vegetation is sparse, which may have a greater correlation with soil mineral elements. The lower ranking regions are mainly due to the lower total sugar content.

4.4. Correlation Analysis between Soil Elements and Sugar, Organic Acid Components

Among the macronutrient elements, the Ca and Mg element content was significantly positively correlated with the sugar components, including glucose, fructose and galactose. Among the micronutrient elements, only Mn had a significant positive correlation with glucose, fructose and galactose, and Mn also had a significant positive correlation with citric acid. In addition, the Fe element content was negatively correlated with total sugar, sucrose and succinic acid (Figure 4).

4.5. Regression Equation Analysis of Key Soil Elements and Main Sugar and Organic Acid Indexes

Multivariate statistical analysis can better reflect the actual relationship between independent variables and dependent variables. Therefore, the multiple linear regression equation was established with soil mineral elements and sugar and acid quality. The results showed that the total sugar, sucrose, succinic acid content and sweetness value were all affected by the element Fe. Glucose, fructose, galactose and citric acid content were mainly affected by the elements Ca and Mn. Mannose, citric acid and tartaric acid were also affected by the element Mn, and the sugar–acid ratio was mainly affected by the element B (Table 3).

5. Discussion

Soluble sugars and organic acids are the main flavor substances in fruits, which directly affect the flavor of fruits [30,31]. There are differences in sugar and organic acid components in different horticultural crops. The sugar components found in jujube fruits in this study are consistent with reported studies [10], which are mainly glucose, sucrose and fructose. The difference is that pear fruits are mainly fructose [32], grape fruits are mainly glucose and fructose [33] and jujube is mainly sucrose. The organic acid components of jujube fruits found in this study are different from previous studies. It was previously reported that jujube fruits are mainly composed of malic acid, citric acid and succinic acid, among which citric acid is the most important acid component [14], while our study found that jujube fruits from the Tarim Basin are mainly composed of succinic acid. This is also different from the major organic acid components previously reported in apple and peach fruit [34]. This is mainly due to the soil environment in different regions, resulting in the low citric acid content of some sampling sites. In general, citric acid values are high with respect to other acids.
Soil element content is closely related to plant development; lack or excess will seriously affect plant photosynthesis, and further affect fruit quality [35]. These soil elements can act as enzyme activators or enzyme cofactors in plants and participate in many metabolic processes [36]. In our study, the content of the element Mg in the three dominant producing areas obtained by principal component analysis was much higher than that of the three producing areas at the end of the ranking, which indicated that the element Mg played an important role in regulating the fruit quality of jujube. There have been a large number of studies on Mg treatment to improve the sugar content in many fruit crops, including navel orange, apple and so on [37,38]. In addition, it is worth noting that the content of the elements Ca, Fe, Mn and B in the soil of the jujube-producing area of the Tarim Basin is much higher than that of other areas [39], which is most likely because the Tarim Basin is located in an arid area with almost no rainfall and serious wind erosion [40,41]. Therefore, in the process of fertilization, it is necessary to consider supplementing the missing nutrients in the soil to improve the ability of plants to adapt to adverse environmental conditions [42]. Suitable fertilization can help reduce the loss of elements, and improve crop yield and quality.
Some micronutrients are necessary for plant growth, and excessive or too few micronutrients in the soil will cause adverse reactions in plants. It has been reported for many fruit trees that the application of Fe fertilizer can significantly increase the sugar content of fruits [43,44], but there are also reports that an excess of the element Fe will affect the root development and nutrient absorption of plants, thus interfering with the synthesis of sugars in plants, and thus reducing the sucrose content in fruits [45,46]. In addition, an excess of the element Fe may cause oxidative stress in the cells, affecting the normal progress of the tricarboxylic acid cycle, which produces succinic acid [47]. However, this study found a significant negative correlation between the total sugar, sucrose and sweetness value and the element Fe, which was caused by an excess of the element Fe.

6. Conclusions

There were significant differences in the content of sugar and organic acid components among the 18 ‘Huizao’ producing areas in the Tarim Basin. Among them, the producing areas near the Kunlun Mountains had a higher sugar–acid ratio and sweet–acid ratio of fruits. The soil of the Tarim Basin is rich in some large elements, such as Ca, but also lacks some elements, such as NO3-N and NH4-N. Among the micronutrients, the elements Fe, Mn and B were significantly higher in other regions, while the elements Zn and Mo were significantly lower. By correlation and linear regression analysis, it was found that Fe, Ca, Mg and Mn were the main factors affecting the fruit sugar and acid quality in the Tarim Basin, and the fruit sugar and acid quality could be improved by increasing the content of the element N and decreasing the content of the element Fe in the soil. The results of this study enrich the study of the association between soil elements and fruit quality, and facilitate the optimization of soil management and fertilization strategies, reducing the pollution of soil and water resources from agricultural activities.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/horticulturae10060652/s1, Table S1: Information of sampling points in Tarim Basin; Table S2: Content of sugar and organic acid components in different producing areas of Tarim Basin; Table S3: Content of mineral elements in different producing areas of Tarim Basin; Table S4: Comprehensive score calculation formula.

Author Contributions

P.T.: sample collection and writing—original draft. F.L. and D.L.: data measurement. J.W. and G.L.: reviewing and editing. C.W.: supervision and conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (32260729) and the key industry support plan project of XPCC (2017DB006). The funding bodies played no role in the design of the study, collection, analysis, and interpretation of the data, and in writing the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article and Supplementary Material, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Horticulturae 10 00652 g001
Figure 1. Distribution of sampling points in major ‘Huizao’ producing areas in Tarim Basin.
Figure 1. Distribution of sampling points in major ‘Huizao’ producing areas in Tarim Basin.
Horticulturae 10 00652 g001
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Figure 2. Content of sugar and organic acid components in fruits from producing area of Tarim Basin. (A–G) represent the content of total sugar, sweetness value, total acid, sugar–acid ratio, sweet–acid ratio, sugar composition, and acid composition in the fruits from each production areas, respectively. The blue line represents the median, while the upper and lower purple lines represent the 75% and 25% quartile, respectively. Small triangles represent different regions.
Figure 2. Content of sugar and organic acid components in fruits from producing area of Tarim Basin. (A–G) represent the content of total sugar, sweetness value, total acid, sugar–acid ratio, sweet–acid ratio, sugar composition, and acid composition in the fruits from each production areas, respectively. The blue line represents the median, while the upper and lower purple lines represent the 75% and 25% quartile, respectively. Small triangles represent different regions.
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Figure 3. Content of mineral elements of producing area soils. (A–D) represent the content of pH, electrical conductivity, macronutrients, and micronutrients in the soil of each production area, respectively. The blue line represents the median, while the upper and lower purple lines represent the 75% and 25% quartile, respectively. Small triangles represent different regions.
Figure 3. Content of mineral elements of producing area soils. (A–D) represent the content of pH, electrical conductivity, macronutrients, and micronutrients in the soil of each production area, respectively. The blue line represents the median, while the upper and lower purple lines represent the 75% and 25% quartile, respectively. Small triangles represent different regions.
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Figure 4. Correlation analysis between soil mineral elements and sugar and organic acid components. The redder it is, the more positively correlated it is, and the bluer it is, the more negatively correlated it is. * and ** were significant at p < 0.05 and p < 0.01 levels, respectively.
Figure 4. Correlation analysis between soil mineral elements and sugar and organic acid components. The redder it is, the more positively correlated it is, and the bluer it is, the more negatively correlated it is. * and ** were significant at p < 0.05 and p < 0.01 levels, respectively.
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Table 1. Characteristic features and contribution rates in principal component analysis.
Table 1. Characteristic features and contribution rates in principal component analysis.
Quality IndexesFeature 1Feature 2Feature 3
Total sugar 0.2790.2580.022
Sucrose0.1530.367−0.162
Glucose 0.2750.0760.175
Fructose 0.2740.0930.193
Galactose 0.2780.0720.186
Rhamnose 0.248−0.2960.008
Mannose 0.234−0.2380.141
Xylose 0.261−0.1670.024
Sweetness value 0.2770.2710.038
Total acid 0.292−0.015−0.239
Malic acid 0.0970.270−0.396
Citric acid 0.265−0.281−0.067
Quinic acid 0.244−0.2660.041
Tartrate 0.212−0.1270.384
Succinic acid 0.1920.262−0.297
Fumaric acid 0.2510.2680.062
Sugar-acid ratio −0.1530.2720.408
Sweet-acid ratio −0.1030.2820.471
Eigenvalue9.6833.1722.723
Contribution rate/%53.79217.62115.126
Total contribution rate/%53.79271.41386.539
Table 2. Principal component analysis scores and comprehensive evaluation.
Table 2. Principal component analysis scores and comprehensive evaluation.
Producing AreasF1F2F3FtotalRank
50R5.709 −1.950 2.529 3.593 1
MF2.617 1.152 2.165 2.240 2
33R3.095 1.939 −1.664 2.028 3
PS3.489 −2.466 0.611 1.774 4
QM0.989 3.332 1.621 1.576 5
46R1.507 1.784 −1.169 1.095 6
SY0.523 1.474 0.610 0.732 7
48R1.865 −2.506 −1.088 0.459 8
12R0.456 −1.058 −1.496 −0.194 9
ZP0.587 −0.574 −2.764 −0.235 10
RQ−1.008 0.887 0.550 −0.350 11
YT−1.082 0.502 0.689 −0.450 12
3R0.040 −2.458 −0.254 −0.520 13
47R−0.771 1.994 −2.783 −0.559 14
29R−1.553 0.143 −0.386 −1.004 15
41R−5.392 0.129 2.216 −2.938 16
LP−5.120 −0.467 1.483 −3.018 17
42R−5.950 −1.857 −0.870 −4.229 18
Table 3. Multivariate statistical analysis of selection of soil mineral elements and sugars and organic acids. * and ** were significant at p < 0.05 and p < 0.01 levels, respectively.
Table 3. Multivariate statistical analysis of selection of soil mineral elements and sugars and organic acids. * and ** were significant at p < 0.05 and p < 0.01 levels, respectively.
Indexes Soil Nutrient Factors Regression Equation F-Value
Total sugar: y1Fe: x1, Mg: x2y1 = 0.168x1 + 30.601x2 + 183.0013.320 *
Sucrose: y2Fe: x1, Cu: x3y2 = −0.700x1 − 0.392x3 + 192.8984.168 *
Glucose: y3Mg: x2, Ca: x4, Mn: x5y3 = 0.101x2 + 0.001x4 + 1.078x5 + 5.1013.512 *
Fructose: y4Mg: x2, Ca: x4, Mn: x5y4 = 0.040x2 + 0.001x4 + 0.440x5 + 6.3513.202 *
Galactose: y5Mg: x2, Ca: x4, Mn: x5, Cl: x6y5 = 0.007x2 + 0.001x4 + 0.111x5 + 0.007x6 − 0.5114.347 **
Rhamnose: y6Ca: x4y6 = 0.001x4 + 0.3033.758 *
Mannose: y7Mn: x5y7 = 0.009x5 + 0.0064.590 *
Xylose: y8P: x6y8 = −0.031x6 + 5.4313.766 *
Sweetness value: y9Fe: x1y9 = −1.023x1 + 318.2724.170 **
Malic: y11Mo: x7y11 = 11.266x7 + 1.6513.962 *
Citric: y12Ca: x4, Mn: x5y12 = 0.001x4 + 0.029x5 + 0.1743.200 *
Tartrate: y14Mn: x5y14 = 0.002x5 − 0.0124.977 *
Succinic: y15Fe: x1y15 = −0.018x1 + 3.8647.911 **
Sugar-acid ratio: y17B: x8y17 = 4.089x8 + 24.5304.067 **
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Tong, P.; Liao, G.; Liang, F.; Lu, D.; Wu, C.; Wang, J. Identification of Key Soil Mineral Elements Affecting Sugars and Organic Acids of Jujube Fruit. Horticulturae 2024, 10, 652. https://doi.org/10.3390/horticulturae10060652

AMA Style

Tong P, Liao G, Liang F, Lu D, Wu C, Wang J. Identification of Key Soil Mineral Elements Affecting Sugars and Organic Acids of Jujube Fruit. Horticulturae. 2024; 10(6):652. https://doi.org/10.3390/horticulturae10060652

Chicago/Turabian Style

Tong, Panpan, Guanglian Liao, Fengzhi Liang, Dengyang Lu, Cuiyun Wu, and Jiangbo Wang. 2024. "Identification of Key Soil Mineral Elements Affecting Sugars and Organic Acids of Jujube Fruit" Horticulturae 10, no. 6: 652. https://doi.org/10.3390/horticulturae10060652

APA Style

Tong, P., Liao, G., Liang, F., Lu, D., Wu, C., & Wang, J. (2024). Identification of Key Soil Mineral Elements Affecting Sugars and Organic Acids of Jujube Fruit. Horticulturae, 10(6), 652. https://doi.org/10.3390/horticulturae10060652

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