Comparison and Genetic Variation Analysis of Important Fruit Traits in Jujube F1 Hybrids by Different Male Parents

Abstract

The research on the genetic predisposition of key quality traits of fruit in jujube hybrid populations is a fundamental and crucial aspect in the field of jujube genetic breeding. In this study, the jujube hybridization experiments were conducted with the same female parent and different male parents since 2015, and a total of 238 strains were created in two F1 hybrid populations. The genetic variation of fruit external traits and intrinsic quality was analyzed and compared after the hybrid offspring results were stable in 2022. This study can provide a theoretical basis for selecting hybrid-breeding parents, predicting offspring traits, and innovating excellent germplasm resources by investigating the impact of different male parental configurations on the genetic variation of fruit traits in jujube hybrid offspring and identifying superior germplasm resources. The results showed that the inheritance of fruit shape, fruit size, and fruit-intrinsic nutrition-quality traits were quantitative traits controlled by multiple genes, and the offspring populations conformed to normal distribution or partial normal distribution. The six traits of fruit shape in offspring populations exhibited a segregation from their parents. The coefficients of variation of fruit size and intrinsic nutrition quality values ranged from 6.94% to 35.34%. Five intrinsic nutrition-quality traits exhibited significant separation at the super-parental level. This result indicated a rich genetic diversity in offspring traits in two hybrids offsprings. The fruit shape, fruit size, and titratable acidity of jujube hybrid offspring were mainly influenced by the male parent. It was found that the correlation between the single-fruit weight, length, and diameter, as well as the correlation between the fruit length and fruit transverse diameter, was very stable and significantly positive through the correlation analysis. More attention should be paid to fruit length when studying fruit shape, as a significant correlation was found between fruit length and fruit shape index. Finally, 10 superior hybrids were founded and selected as breeding materials for further breeding. Therefore, this study provides a theoretical basis for the early selection of hybrid breeding and the development and application of jujube germplasm resources.

1. Introduction

The jujube (Ziziphus jujuba Mill.) is a deciduous tree belonging to the Rhamnus family. It is native to China and has a long history of cultivation. In ancient times, it was called “five fruits” together with peach, apricot, plum, and chestnut [1]. Jujube fruit is rich in Vitamin C, sugar, and a variety of trace elements, which provide it unique nutritional value and flavor. It can be made dry or eaten fresh and can also be used as medicine, and it is appreciated for its high quality and wide array of economic applications. In addition, it is an important and excellent fruit tree in the northwest of China, highly respected for its advantages of stress resistance, such as drought and salt tolerance [2,3]. The climatic environment of the Tarim Basin in Xinjiang is arid with little rain, and it has large temperature differences between day and night, as well as long sunshine hours, which is very suitable for the production of the jujube. At present, the jujube cultivation area of the Tarim Basin reaches 473,000 hm², which is the dominant area of jujube production in Xinjiang and the largest cultivation in China and has a broad prospect for development [4].

There are problems in jujube production, such as a single-variety structure and a lack of excellent new varieties, which seriously restricts the development of the jujube industry. Therefore, it is the focus of jujube researchers to conduct research on quality improvement and the breeding of jujube and cultivate new varieties with high yield, high quality, and good comprehensive resistance. Hybrid breeding is the most widely used and effective breeding technology in fruit breeding. However, due to the problems of small flowers, difficult emasculation, a low fruit setting rate, and high embryo abortion, the success rate is low, and the development process of hybrid breeding is slow [5]. So far, no new hybrid varieties have been published.

The comprehension and mastery of genetic variation in important traits lays the fundamental basis for hybrid breeding [6]. In China, fruit shape serves as a pivotal characteristic employed by consumers for variety identification and selection [7], while also serving as a significant indicator for determining the economic benefits of jujube and evaluating the visual quality of fruits [8]. Bai et al. [9] analyzed the genetic distribution of the hybrid offspring population of the “Yuluxiangli” and “Huangguan” pear, revealing that the fruit shape adhered to the genetic characteristics of quantitative traits. Furthermore, the fruit shape index exhibited a decreasing regression trend, with the mean value of the offspring being lower than the median value of their parents. The fruit shape of the F1 generation in pear hybrids, as investigated by Lu et al. [10], exhibited a wide range of variation with 10 distinct shapes. Furthermore, the fruit shape of offspring closely resembled that of their parent, indicating a high degree of heritability. Tang et al. [11] discovered that the paternal parent predominantly influenced the inheritance of peach fruit shape in their research. Fruit size is a crucial trait that significantly influences fruit appearance, market value, and economic value [12]. In the early stages of fruit tree domestication, fruit size selection was a primary focus [13], thus emphasizing the increasing importance of fruit size in breeding new cultivars. In the investigation of genetic regulations governing blueberry fruit size, it was observed that maternal genes exerted an influence on the genetics of blueberry fruit size, exhibiting maternal genetic characteristics and representing a polygenic quantitative trait [14]. The study conducted by Han et al. demonstrated that the fruit quality of the interspecific hybrids between Actinidia rufa (Siebold and Zuccarini) Planchon ex Miquel and Actinidia chinensis var. chinensis C.F. Liang exhibited a stronger dependence on the maternal parent [15]. The intrinsic nutritional characteristics of fruits serve as crucial indicators of fruit quality. The genetic analysis of soluble solids content in apple hybrids revealed that the F1 offspring exhibited a predominantly intermediate and high sugar content compared to their parents, suggesting that hybrid breeding can effectively select for high sugar content [16]. In the study of blueberry hybridization [17], it was observed that the acid content was primarily influenced by the male parent [18]. However, contrasting results were obtained when analyzing the titratable acid content of plums, revealing a greater impact from the female parent [19].

It is of great significance to study the genetic variation of jujube hybrid offspring traits for screening excellent hybrid offspring. The genetics research of the quality traits of jujube fruit primarily focuses on jujube variety selection and comparative analysis of phenotypic characteristics in progeny. Jia [20] and Wu [21] et al. comprehensively evaluated and analyzed fruit traits in multiple jujube varieties, aiming to elucidate the richness of phenotypic variation and genetic diversity among different cultivars. Cao [22] conducted a comprehensive analysis of variations, correlations, and principal component evaluation on the yield, biological characteristics, and nutritional content of “Fucuimi” jujube offspring seedlings. As a result, Fu21 was identified as an outstanding plant. Early studies have reported on the analysis of genetic variation in a single population [23,24] to elucidate its underlying genetic principles. However, there has been no report on the influence of paternal traits with different characteristics on the genetic variation of fruit traits in hybrid offspring until now.

Based on stable results obtained from two hybrid offspring populations created in 2015, 10 traits, including fruit shape, size, and intrinsic quality, were analyzed in detail in this study. Meanwhile, superior germplasm resources were identified via an evaluation method utilizing the grey correlation degree. This research lays a foundation for clarifying the genetic rules of jujube hybrid offspring and provides references for parental selection in hybrid breeding and development and the application of jujube germplasm resources.

2. Materials and Methods

2.1. Plant Materials

Female: Yuhong is a male-sterile jujube germplasm resource that exhibits the advantages of mid–late maturity, early fruiting, rapid harvesting, stable yield, high resistance to cracking and fruit shrinkage, excellent quality, and easy management. It represents a novel variety of fresh jujube with mid–late maturation selected and bred from local resources in Mancheng District, Hebei Province [25].

Male: Jiaocheng 5 (referred to as “Jiao 5”) is a superior line of Junzao. Xingtai 16 (referred to as “Xing 16”) is a superior line of sour jujube. The male parent samples were obtained from a seedling site in Xian County, Hebei Province [26].

The hybridization experiment was conducted consecutively for two years from 2015 to 2016 in Alar City, Xinjiang. Previous researchers utilized a partial cover net to isolate the parental plants prior to flowering and subsequently introduced bees into the net for pollination [27]. The bees were manually fed to ensure their survival, and the resulting hybrids were identified using SSR molecular markers. In 2018, the offspring of both populations (“Yuhong” × “Jiao 5” with 98 individuals and “Yuhong” × “Xing 16” with 140 individuals) were cultivated as live seedlings at the Lanpo Bay, Alaer, Xinjiang jujube hybrid-breeding seedling base. The plantation area had irrigated meadow soil and experienced an extreme continental arid desert climate within the warm temperate zone characterized by long hours of sunshine, making it suitable for jujube cultivation. The plant row spacing was set at 0.4 m × 0.6 m with a density of 10–15 plants per row. We applied organic fertilizer to the land during both spring and winter seasons while supplementing K fertilizer during the later stages of fruit development. In case of drought before budding, flowering, fruit development, and soil freezing, we implemented watering measures. Additionally, ring-cutting and canopy-spraying techniques were employed to enhance the fruit-setting rate during flowering. The remaining field management practices followed conventional methods consistently.

The field collection was conducted from 2022 to 2023 in the jujube seedling base of Lanpo Bay during the fruiting period of jujube. At the full red stage of fruit ripening, a random sample of 30 disease-free fruits was collected from each plant’s middle section. Fruit shape traits and size were measured on the day of sampling for these selected fruits. Subsequently, the fruits were broken, mixed, and frozen at −20 °C in a refrigerator. Soluble solids, Vitamin C content, soluble sugars, titratable acids, and flavonoids were quantified through analytical methods. The experiment was replicated three times with averaged final results.

2.2. Determination Method

2.2.1. Fruit Shape Traits

The fruit shape trait was assessed in accordance with the guidelines outlined in “Chinese jujube germplasm resources” [28]. To ensure scientific rigor and adherence to academic standards, we established a Date Germplasm Resources and Genetic-Breeding Team specifically dedicated to evaluating the fruit shape trait based on the criteria specified in this reference. The specific details of our assessment methodology are as follows.

Fruit Shape: At the fully ripe stage, visual inspection was employed as a means of measuring the overall shape of the fruit.

2.2.2. Traits of Fruit Size

The single-fruit weight was measured using electronic balance (precision to 0.01 mg); the fruit length and diameter were measured with digital vernier calipers (precision to 0.01mm). The formula of fruit shape index was as follows: Fruit shape index = fruit length/fruit diameter. [23]

2.2.3. Nutritional Traits of Fruit

Soluble solids content was determined using a handheld digital refractometer. The pulp was homogenized and extracted with a juicer, and a small portion of the resulting juice was placed onto the refractometer for measurement. Three replicates were performed to obtain an average value.

The determination of Vitamin C content was conducted using the molybdenum blue colorimetric method [29]. Soluble sugar content was quantified through anthrone colorimetry [30]. Titratable acid content was determined via an acid-base neutralization method [31]. The total flavonoid content was assessed using aluminum nitrate–sodium nitrite colorimetry [32].

2.3. Data Processing

We tabulated the measured traits and calculated descriptive statistical parameters, including mean, mid-parentage value (V_MP), range of variation, standard deviation (SD), coefficient of variation (CV%), genetic transmission ability (Ta%), mid-parent heterosis rate (R_Hm%), skewness, kurtosis, and over-parent heterosis (including the ratio of higher than high parent [HH%] and the ratio of lower than low parent [LL%]). These calculations were performed using Excel 2010 software to investigate patterns in genetic variation. The formula is stated as follows:

V_MP = (P1 + P2) × 100%;

CV% = (SD/F) × 100%;

Ta% = (F/V_MP) × 100%;

R_Hm% = [(F − V_MP)/V_MP] × 100%;

HH% = (N_H/N_F1) × 100%;

LL% = (N_L/N_F1) × 100%.

(Note: P1 and P2 are the phenotypic value of parents; F is the average of hybrid offspring; NH is the number of individual plants higher than the high parental phenotype value; NL is the number of individual plants lower than the low parental phenotype value; and NF1 is the total number of hybrid offspring plants.)

Furthermore, we conducted correlation analysis and grey correlation analysis on the measured traits using SPSS 26.0. Spearman’s correlation coefficient was employed to quantify the degree of association between two traits while determining both test results and strength. The grey correlation degree considers the traits of the variety as a grey system, and we assigned weights to nine factors closely associated with traits of fruit quality. The specific weightings were as follows: single-fruit weight (0.15), fruit length (0.08), fruit diameter (0.08), fruit shape index (0.05), soluble solids (0.15), Vitamin C (0.12), soluble sugar (0.15), titratable acid (0.12), and flavonoids (0.1). These weighting coefficients are based on Yuan Ye et al.’s study on the “Fengmiguan” jujube [33], as well as expert opinions from our research group members. The correlation degree for each strain was calculated to reflect their comprehensive traits; the higher the correlation, the better the performance [34].

3. Results

3.1. Genetic Analysis and Comparison of Fruit Shape

The analysis results of fruit shape for the two hybrid offspring populations are presented in

Table 1. The segregation results of fruit shape.

Traits	Population	Parents		The Proportion of Segregation Traits in F1 Generation
		Female	Male
Fruit shape	Yuhong × Jiao 5	Coniform	Obovate	Obovate 60.19%	Ovoid 18.45%	Flat cylinder 2.91%	Oblong globose 9.71%	Cylinder 2.91%
				Globose 1.94%	Coniform 1.94%	Millstone 0.97%	Oblate 0.97%
	Yuhong × Xing 16		Oblate	Cylinder 26.76%	Oblate 25.35%	Obovate 19.72%	Flat cylinder 17.61%	Oblong globose 7.04%
				Globose 3.52%	Ovoid 0.70%

. “Yuhong” fruit is coniform, while “Jiao 5” fruit is obovate. The hybrid offspring of “Yuhong” and “Jiao 5” display a variety of distinct phenotypes from their parents, including seven types: ovoid, flat cylinder, oblong globose, cylinder, globose, millstone, and oblate. Among these types, obovate accounts for 60.19% of the F1 generation, while ovoid represents 18.45%, with an overall long fruit shape (Figure 1A). It is noteworthy that 0.97% overflow marks were observed in the F1 generation. The analysis of another hybrid offspring revealed that the majority of fruit shapes in the F1 generation from “Yuhong” × “Xing 16” were predominantly round, although the fruit shape of “Xing 16” was oblate (Figure 1B). Furthermore, the frequency distribution of this progeny included 26.76% cylinder and 25.35% oblate. These findings indicate paternal parent has a greater influence on the fruit shape of hybrid offspring, resulting in a closer resemblance to the paternal character.

The histograms of frequency distribution depicting the fruit shape indexes of two hybrid offspring populations were generated (Figure 1C,D). The fruit shape index changed continuously, conformed to a normal distribution, and was a quantitative trait under polygenic control as evidenced by the values of skewness and kurtosis. The distinction is that the distribution of “Yuhong” × “Xing 16” exhibits a clear right skewness. The study revealed that the coefficients of variation in fruit shape index for both groups of hybrid offspring ranged from 6.94% to 10.79% (

Table 2. The analysis of genetic variation in fruit shape index.

Traits	Population	Parents			F1
		Female	Male	VMP	Mean	Range of Variation	SD	CV%	Ta%	RHm%	HH%	LL%
Fruit shape index	Yuhong × Jiao 5	1.44	1.67	1.55	1.43	0.93–1.81	0.15	10.79	91.87	−8.13	5.43	52.17
	Yuhong × Xing 16	1.44	1.04	1.24	1.09	0.93–1.44	0.08	6.94	87.53	−12.47	0	27.34

), indicating a relatively narrow range of variability. The genetic transmission capacity ranges from 87.53% to 91.87%. The negative mid-parent heterosis rates indicate the heterosis of the fruit shape index within the F1 generation population showed a declining trend. The over-parent heterosis rate was 5.43–52.17%, indicating that the hypersegregation phenomenon widely existed in the F1 generation, and that the overall genetic tendency was lower than that of the parents. The F1 generation of “Yuhong” × “Jiao 5” showed a significant occurrence of extremely low paternity, while the ratio of the lower-than-low parent was 52.17%. It was observed that the fruit shape index of “Jiao 5” was 1.67, exhibiting a significant increase of 0.63 in comparison to “Xing 16”. The mean, standard deviation, coefficient of variation, genetic transmissibility, and mid-parent heterosis rate of the F1 generation coming from the hybrids of “Yuhong” and “Jiao5” were all found to be higher than those of the F1 generation coming from the hybrids of “Yuhong” and “Xing16”. This result suggests that the fruit shape index is significantly influenced by the paternal parent.

3.2. Genetic Analysis and Comparison of Fruit Size

According to the frequency distribution histogram, skewness, and kurtosis values (Figure 2), the traits of the fruit size of the F1 generation of the two hybrid offspring populations showed an obvious single peak, and the results conformed to the normal distribution law with continuous variation, which was a quantitative trait genetic characteristic with minimal polygenic control.

There were certain phenotypic differences in fruit size between the two hybrid populations with different paternal parents (Figure 3). The single-fruit weight, fruit length, and fruit diameter of “Jiao 5” were 15.79 g, 44.72 mm, and 26.82 mm, respectively (

Table 3. The analysis of genetic variation in fruit size.

Traits	Population	Parents			F1
		Female	Male	VMP	Mean	Range of Variation	SD	CV%	Ta%	RHm%	HH%	LL%
Single-fruit mass/g	Yuhong × Jiao 5	11.40	15.79	13.60	14.78	4.70–28.56	5.22	35.34	108.72	8.72	40.22	26.09
	Yuhong × Xing 16	12.40	3.72	8.06	7.12	2.14–15.22	1.84	25.84	88.29	−11.71	0.72	2.88
Fruit length /mm	Yuhong × Jiao 5	35.78	44.72	40.25	39.00	20.13–49.61	5.61	14.38	96.89	−3.11	15.22	26.09
	Yuhong × Xing 16	37.29	19.70	28.50	25.25	15.46–37.29	2.72	10.78	88.60	−11.40	0	2.16
Fruit diameter /mm	Yuhong × Jiao 5	24.83	26.82	25.83	27.43	17.12–35.47	3.72	13.56	106.22	6.22	55.43	22.83
	Yuhong × Xing 16	25.90	18.94	22.42	23.30	15.60–29.96	2.32	9.96	103.91	3.91	10.79	3.60

). The traits of the fruit size of “Xing 16” were 3.72 g, 19.70 mm, and 18.94 mm, respectively. Compared with “Xing 16”, “J5” is a large-fruited jujube variety with a maximum difference in fruit length of 25.02 mm. The traits of the fruit size of the F1 generation were also consistent with that of the male parent. The mean values of single-fruit weight, fruit length, and diameter of the F1 generation of “Yuhong” × “Jiao 5” were larger than those of “Yuhong” × “Xing 16”, and the maximum difference of fruit length was 13.75 mm. The results showed that the genetics of the fruit size of jujube hybrids was affected by the male parent and had the characteristics of male inheritance.

The genetic variation of the three main quantitative traits of fruit size in the two hybrid populations was analyzed (Table 3). The coefficient of variation of quantitative traits reflects the discrete degree of the phenotypic trait. Therefore, the larger the coefficient of variation, the greater the discrete value of the measured trait. According to the variation coefficient of traits of fruit sizes, the variation coefficient of the three traits in the two hybrid F1 populations ranged from 9.96% to 35.34%. The coefficients of variation of single-fruit weight were 35.34% and 25.84%, respectively, and were widely separated. The coefficients of variation of all traits in the hybrid population of “Yuhong” × “Jiao 5” were higher than those in the F1 population of “Yuhong” × “Xing 16”, with the single-fruit weight exceeding about 10% and other traits exceeding about 4%. It can be observed that the coefficients of variation of traits of fruit size in the F1 population of “Yuhong” × “Xing 16” were relatively backward. The over-parent heterosis of the hybrids of “Yuhong” × “Jiao 5” was 15.22–55.43%, and the low parentage rate was 22.83–26.09%, indicating a wide range of over-parent separation. However, the traits of the fruit size of the hybrids of “Yuhong” × “Xing 16” were almost located between the parents, and the over-parent heterosis was low, ranging from 0–10.79% to 2.16–3.60%. In the F1 generation of the two hybrid populations, the mid-parent heterosis rate of fruit length was negative, ranging from −3.11% to 11.40%, indicating that the heterosis of fruit diameter showed a downward trend, while the advantage rate of fruit diameter was positive, ranging from 6.22% to 3.91%.

Spearman’s correlation coefficient analysis for single-fruit weight, fruit length, and diameter was conducted by SPSS26.0 software. The results showed that the correlation of the fruit size of the offspring of the two hybrid groups was very consistent (

Table 4. The correlation analysis of fruit size. Notes: ** indicates that at the 0.01 level (two-tailed), the correlation is significant.

Population	Trait	Single Fruit Weight	Fruit Length	Fruit Diameter	Fruit Shape Index
Yuhong × Jiao 5	Single-fruit weight	1	0.858 **	0.940 **	−0.065
	Fruit length		1	0.718 **	0.384 **
	Fruit diameter			1	−0.304 **
	Fruit shape index				1
Yuhong × Xing 16	Single-fruit weight	1	0.867 **	0.954 **	−0.050
	Fruit length		1	0.758 **	0.370 **
	Fruit diameter			1	−0.256 **
	Fruit shape index				1

). The single-fruit weight was positively correlated with fruit length and fruit diameter significantly (p < 0.01), and both fruit length and fruit width were also significantly correlated with fruit shape index (p < 0.01). Among them, fruit length was positively correlated with fruit shape index; that is, fruit shape index increased with the increase of length. However, there was a negative correlation between fruit diameter and fruit shape index; that is, with the diameter increase, the fruit shape index decreased, indicating that fruit length should have priority in the study of fruit shape. Single-fruit weight was negatively correlated with fruit shape index, but the correlation was not significant.

3.3. Genetic Analysis and Comparison of Fruit Nutritional Traits

The contents of soluble solids, soluble sugars, titratable acids, Vitamin C, and flavonoids were measured to evaluate the nutritional value and flavor of the jujube fruits. Figure 4 shows the frequency distribution histograms of five intrinsic nutritional traits of fruits, including soluble solids, soluble sugar, titratable acid, Vitamin C, and total flavonoids, of the offspring of the two hybrid populations, all showing normal distribution characteristics, suggesting that they are quantitative traits controlled by small-effect polygenes. The titratable acids of the F1 generation of “Yuhong” × “Xing 16” showed an obvious right skewness distribution, indicating that the offspring had extreme values, which was quite different from the frequency distribution of titratable acids of “Yuhong” × “Jiao 5”.

The genetic analysis results of intrinsic nutritional traits of fruits in the two hybrid populations are shown in

Table 5. The analysis of genetic variation in fruit nutritional traits.

Traits	Population	Parents			F1
		Female	Male	VMP	Mean	Range of Variation	SD	CV%	Ta%	RHm%	HH %	LL%
Soluble solid substance content %	Yuhong × Jiao 5	36.67	36.40	36.53	32.15	24.00–43.07	4.23	13.16	88.01	−11.99	16.28	82.56
	Yuhong × Xing 16	37.73	37.13	37.43	33.21	23.80–41.80	2.75	8.27	88.71	−11.29	5.47	93.75
Soluble sugar content %	Yuhong × Jiao 5	24.14	31.60	27.87	21.63	13.13–31.40	3.94	18.21	77.61	−22.39	0	73.17
	Yuhong × Xing 16	24.09	27.74	25.92	27.64	16.33–39.27	3.84	13.91	106.65	6.65	45.05	17.12
Titratable acid content %	Yuhong × Jiao 5	0.48	0.44	0.46	0.52	0.34–0.78	0.10	19.73	112.90	12.90	62.65	21.69
	Yuhong × Xing 16	0.47	1.02	0.75	0.67	0.41–1.51	0.19	27.80	90.12	−9.88	4.50	2.70
Vitamin C mg/g	Yuhong × Jiao 5	2.39	3.01	2.70	2.86	1.28–4.49	0.67	23.26	105.92	5.92	36.59	21.95
	Yuhong × Xing 16	3.02	3.53	3.27	2.84	1.66–4.33	0.49	17.35	86.69	−13.31	7.26	66.13
Total Flavonoid content mg/g	Yuhong × Jiao 5	0.62	0.62	0.62	0.64	0.51–0.84	0.08	12.56	104.56	4.56	54.88	45.12
	Yuhong × Xing 16	0.62	0.90	0.76	0.74	0.54–1.10	0.11	15.36	97.48	−2.52	10.62	13.27

. The coefficients of variation of five traits ranged from 8.27% to 27.80%. The coefficient of variation of titrated acid was the highest among all traits, which is above 19%, indicating a large variation with rich variation information in the titrated acid of hybrid offspring. The comparison of the two combinations revealed significant differences in the content of titratable acids. The content of “Jiao 5” was 0.44%, and that of “Xing 16” was 1.02%. The coefficient of variation of the titratable acid content of the hybrid population of “Yuhong” × “Xing 16” reached 27.80%, with a large variation range of 0.41–1.51%, which was conducive to the screening of individual plants with high acidity. The average titratable acid content of the F1 generation of “Yuhong” × “Jiao 5” was 0.52%, with a variation range of 0.34–0.78%, which was lower than that of “Yuhong” × “Xing 16”, indicating that the titratable acid content of hybrid progeny was greatly affected by the male parent.

The soluble solids content of the two hybrids was not significantly different. The content of the two hybrids in the F1 generation was less than the median value, and the genetic transmissibility was above 88%. The negative heterosis rate of the mid-parent indicates that the heterosis of the offspring had a tendency toward the median genetic trend. In terms of soluble sugar content, there is no significant difference between the two combinations, but in terms of heterosis, they showed opposite heterosis, and the ultra-low parentage rate of the hybrid offspring of “Yuhong” × “Jiao 5” was 73.17%, while the ultra-high parentage rate of “Yuhong” × “Xing 16” was 45.05%, showing that there was a transgressive separation in F1: one was a super-high parent, and the other was a super-low parent. The coefficient of variation, genetic transmission ability, and heterosis rate of Vitamin C in the offspring of “Yuhong” × “Jiao 5” were 23.26%, 105.92%, and 5.92%, respectively, higher than those of “Yuhong” × “Xing 16”, indicating that the trait of “Yuhong” × “Jiao 5” was more stable and advantageous. There is no significant difference in flavonoid content, which has the characteristic of intermediate inheritance.

The correlation analysis of intrinsic nutritional fruit traits (

Table 6. The correlation analysis of nutritional fruit traits. Notes: * indicates that at the 0.05 level (two-tailed), the correlation is significant.

Population	Trait	Vitamin C	Soluble Sugar	Titratable Acid	Soluble Solids Substance	Total Flavonoid
Yuhong × Jiao 5	Vitamin C	1.000	0.072	0.150	−0.098	0.090
	Soluble sugar		1.000	−0.077	0.201	0.166
	Titratable acid			1.000	0.220 *	0.188
	Soluble solids substance				1.000	0.166
	Total flavonoid					1.000
Yuhong × Xing 16	Vitamin C	1.000	0.052	−0.037	−0.044	−0.132
	Soluble sugar		1.000	0.111	−0.212 *	0.160
	Titratable acid			1.000	−0.012	0.000
	Soluble solids substance				1.000	0.056
	Total flavonoid					1.000

) showed that the correlation between various traits was not strong. In the hybrid offspring of “Yuhong” × “Jiao 5”, the soluble solids and titratable acid traits showed a significant positive correlation (p < 0.05), while in the hybrid offspring of “Yuhong” × “Xing 16”, they showed a significant negative correlation.

3.4. Comprehensive Evaluation of Fruit Quality of Hybrid Offspring and Screening of Superior Plants

In this study, the grey correlation of nine representative fruit quality indexes (single-fruit weight, fruit length, fruit diameter, fruit shape index, soluble solids, soluble sugar, titratable acid, Vitamin C, and flavonoids) of the two hybrid offspring populations was analyzed using the grey correlation analysis method and assigned different weight coefficients to different traits (

Table 7. The weight coefficient of fruit quality.

	Single-Fruit Weight	Fruit Length	Fruit Diameter	Fruit Shape Index	Soluble Solids Substance	Soluble Sugar	Titratable Acid	Vitamin C	Total Flavonoid
Weight coefficient	0.15	0.08	0.08	0.05	0.15	0.15	0.12	0.12	0.10

). The top 10 superior lines were screened out (J291 > J284 > JX37 > JX72 > J195 > J204 > JX125 > J243 > J266 > J255) by sorting the two hybrid F1 generations using the grey-weighted correlation degree (

Table 8. Grey-weighted degree correlation sorting of fruit quality of the offspring of two hybrid populations.

Population	Variety	Gray-Weighted Degree	Sorting
Yuhong × Jiao 5	J291	0.694	1
	J284	0.689	2
Yuhong × Xing 16	JX37	0.674	3
	JX72	0.668	4
Yuhong × Jiao 5	J195	0.665	5
	J204	0.663	6
Yuhong × Xing 16	JX125	0.656	7
Yuhong × Jiao 5	J243	0.645	8
	J266	0.633	9
	J255	0.628	10

) based on the weight coefficients of different indexes. The traits of fruit quality were shown in

Table 9. Fruit characters of F1 generation of superior lines.

Variety	Single-Fruit Weight/g	Fruit Length/mm	Fruit Diameter/mm	Fruit Shape Index	Soluble Solids Substance/%	Soluble Sugar/%	Titratable Acid/%	Vitamin C mg/g	Total Flavonoids mg/g
J291	13.29	37.97	25.38	1.50	43.07	31.40	0.60	3.99	0.68
J284	27.19	48.34	33.39	1.45	35.07	25.08	0.49	3.09	0.69
JX37	7.36	33.83	23.97	1.41	37.33	22.84	0.41	2.68	0.72
JX72	9.96	31.01	25.80	1.20	36.07	25.40	0.58	4.33	0.68
J195	27.23	45.47	35.22	1.29	34.60	19.38	0.39	2.09	0.69
J204	28.56	47.34	35.47	1.33	33.53	18.86	0.40	2.01	0.53
JX125	10.29	29.35	27.23	1.08	36.13	30.33	0.48	2.63	1.10
J243	20.94	48.86	29.37	1.66	31.27	19.87	0.35	2.91	0.69
J266	13.17	41.78	23.99	1.74	39.67	29.44	0.50	3.08	0.57
J255	16.58	42.07	29.78	1.41	38.13	25.70	0.67	3.99	0.75

, and the overall degree of the tree and the fruit profile was shown in Figure 5. Among them, seven superior lines were selected from the offspring of “Yuhong” × “Jiao 5”, indicating that the hybrid offspring of this combination had higher advantages in various traits and higher comprehensive evaluation.

4. Discussion

Fruit shape is an important indicator for evaluating the quality of a fruit’s appearance, which can be judged by the fruit shape index [8,35]. In the early days, fruit shape was considered a quality trait. Lesley [36] studied the fruit shape of peaches and believed that round and flat were a pair of relative traits. Later, Ma [37] conducted genetic research on the fruit shapes of the F1 generation of round nectarine and oblate oil pear and found that the fruit shape of the offspring basically conformed to the separation ratio of 1:1. However, in recent years, with the increasing construction of hybrid combinations, it has been found that fruit shape is a quantitative trait controlled by multiple genes, which has been verified in the hybrid offspring of pears [9,10], watermelons [38], grapes [39], citruses [40], and other fruit trees. In addition, a variety of fruit shapes inconsistent with the parents was found in the F1 generation in this study, indicating that fruit shape is a quantitative trait. Liu et al. [41] found that the fruit shape of peach was greatly affected by the male parent, which was consistent with the results of this experiment. “Jiao 5” was obovate, “Xing 16” was oblate, and most F1 generations had the same shape as the male parent, indicating paternal inheritance of fruit shape. However, this is contrary to the study on the hybrid offspring of apricots, which divided the F1 generation strains into four types, with the oval type accounting for 75.22% of the offspring and being consistent with the shape of the female parent, indicating that the fruit shape of the female parent is more easily inherited to the offspring strains [42]. The formation of fruit shape depends not only on the additive and non-additive genetic effects of genes but also on certain parental biases [43]. In this study, the heterosis of the fruit shape index in the two hybrid populations was decreased significantly. The fruit length and fruit shape index showed a significant positive correlation, indicating that there was a relationship between fruit shape and fruit length.

Fruit size is one of the important factors affecting fruit quality and also an important economic trait; it is of great concern for consumers [44]. Fruit size is a quantitative trait controlled by multiple genes [45]. In this study, it was found that the coefficients of variation of the three traits of fruit size ranged from 9.96% to 35.34%, indicating that the population had rich genetic diversity. Among them, the coefficient of variation of single-fruit weight was the largest, which was consistent with previous studies on the jujube [46]. It is suggested that single-fruit weight is greatly affected by genetic regulation and environment and has an enormous potential for genetic improvement. According to correlation analysis, it was found that the correlation of traits of fruit size of the two groups was basically consistent, among which the single fruit weight was significantly positively correlated with the length and diameter of fruits, and the fruit length was significantly positively correlated with fruit diameter, and the result was consistent with the correlation results of fruit size of the hybrid offspring of “Dongzao” × “Jinsi 4” by Xie Huan [23]. It is concluded that these characteristics are improved simultaneously in breeding, which is beneficial to the selection of jujube breeding targets. Meanwhile, the fruit size was greatly affected by the parent in this study, but Han et al. [15] showed that the single-fruit weight traits of the interspecific hybrid offspring of Actinidia rufa (Siebold and Zuccarini) Planchon ex Miquel × Actinidia chinensis var. chinensis C.F. Liang were more biased to the parent. It is possible that the genetic bias of the offspring is also different in different types of fruit-tree hybrids. Therefore, when targeting “large fruit” in jujube breeding, we should choose the parent variety with large fruit as the parent and select suitable hybrid combinations to obtain more offspring with large fruit.

Soluble solids content is an important trait of fruit quality, which directly affects the taste of fruit. Therefore, studying the genetic rules of soluble solids is an important basis for breeding [47,48]. The result of Zhang [49] et al.’s study on soluble solids of apricots showed that the inheritance of soluble solids content of apricots was a quantitative trait controlled by multiple genes and conforms to normal distribution, which is consistent with the results of our study. In the two hybrid populations, the soluble solids traits have the smallest coefficient of variation, and transgressive separation is consistent, showing a stable downward genetic trend. The ratio of the lower-than-low parent is higher than 80%, indicating that soluble solids indicate an inheritance decline trend, which is the same as the study results of apricots by Liu et al. [50], but the opposite to the research results on pears [51].

Soluble sugars and titratable acids are the most important determinants of fruit flavor and the most important nutritional indicators of fruit quality [52]. Both Soluble sugars and titratable acid traits are quantitative traits controlled by small-effect polygenes and show continuous variation with normal distribution in the F1 generation. This study found that the variation coefficient of titratable acid content was the highest in fruit quality, and the variation information was rich. The F1 generation of “Yuhong” × “Xing 16” had a large range of variation and great genetic potential, which was conducive to the screening of individual plants with high acid. Similar results have also been reported in both apples [53] and apricots [54]. At the same time, the content of titratable acid of the two hybrids was greatly affected by the male, which was consistent with the results of Liu et al. [17] in the blueberry hybridization research. However, the content of titratable acid analyzed by Jiao et al. [18] was the opposite, which was greatly affected by the female. The coefficients of variation of Vitamin C content, soluble sugar content, and total flavonoid content were all above 10%, indicating a high degree of dispersion, rich diversity, and high variation level. The correlation of intrinsic nutritional fruit traits between the two hybrid offspring groups was inconsistent, and there was no extremely significant correlation. It may be caused by environmental factors, and unified field management and strict experimental investigation can ensure the accuracy of data.

The variation of hybrids is widespread and many important characters are seriously degenerated, making it difficult to breed new varieties. Fruit quality is a complex trait controlled by both genes and the environment. To screen high-quality varieties of individual plants, nine quality traits of fruit were analyzed and evaluated comprehensively via the grey correlation degree. The method has the advantages of integrality, asymmetry, orderliness, and dynamic calculation and also achieves the goal of comprehensive evaluation of the fresh jujube [55]. According to the evaluation results, a total of 10 strains with excellent comprehensive traits were selected, including J291, J284, JX37, JX72, J195, J204, JX125, J243, J266, and J255, which can be used as further breeding materials. The fruit quality of J291 was significantly better than that of other progeny, which could be used for fresh jujube breeding. The cultivation area of the superior lines should be expanded in the future. Once the characteristics of fruit quality have been stabilised, a comprehensive evaluation of the superior plants should be conducted over a period of 2–3 years to measure their traits. Following this, the variety can be approved and promoted, thereby expediting the process of new variety breeding.

The scientific selection of suitable parents is of great significance for jujube hybrid breeding. When selecting and matching hybrid combinations, excellent fruit quality should be the breeding target, and parents with good resistance, large fruit, and medium or high quality should be selected as outstanding germplasm. This way is not only conducive to the cultivation of excellent varieties, but it can accelerate the breeding process. Meanwhile, it can also select the single plant with the best fruit quality from the F1 generation population as the male parent and continue to hybridize with the female parent of the sterile male. It is expected to obtain the excellent single plant with better comprehensive traits, which provides a basis for the innovative utilization of germplasm resources.

5. Conclusions

In conclusion: The inheritance of fruit shape, fruit size, and intrinsic nutrition quality traits of fruit were quantitative traits controlled by multiple genes, and the offspring populations conformed to normal distribution and partial normal distribution. The fruit shape, fruit size, and titrated acidity were mainly influenced by the male parent in the hybrid jujube and belong to partial male inheritance. The correlations between single-fruit weight and fruit length, single-fruit weight and fruit diameter, and fruit length and fruit diameter were stable and extremely positive. There is a significant correlation between fruit length and fruit shape index, so more attention should be paid to fruit length when studying fruit shape. Ten superior lines were selected as breeding materials for further selection and breeding after comprehensive evaluation.