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Article

Integrated Analysis of Morphological and Physicochemical Traits in “Liuyuehong” Soft-Seed Pomegranate Fruit

by
Shubin Zhang
1,†,
Shuaishuai Sha
1,†,
Quanlin Cui
1,
Jin Zhang
2,
Fenfen Yang
1,
Wei Lin
1,3,* and
Yuansong Xiao
1,4,*
1
Research Center for Crop Biotechnology Breeding and Smart Cultivation in Southern Xinjiang, School of Advanced Sciences, Kashi University, Kashi 844000, China
2
Kashgar Prefecture Forest and Grassland Bureau, Kashi 844000, China
3
Institute of Urban Agriculture, Chinese Academy of Agricultural Sciences, Chengdu 610000, China
4
College of Horticultural Science and Engineering, Shandong Agricultural University, Tai’an 271000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Horticulturae 2025, 11(11), 1369; https://doi.org/10.3390/horticulturae11111369
Submission received: 6 October 2025 / Revised: 5 November 2025 / Accepted: 11 November 2025 / Published: 13 November 2025
(This article belongs to the Special Issue Bioactivity and Nutritional Quality of Horticultural Crops)
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Abstract

The grain-level heterogeneity of fruit morphological characteristics significantly determines their sensory performance and intrinsic quality, providing a quantitative basis for commercial grading. This study utilized “Liuyuehong” soft-seeded pomegranate (Punica granatum L.) as experimental material. Fruits were classified into three size grades based on individual fresh weight: large (107–125 g), medium (74–92 g), and small (47–67 g). Fresh weights of whole fruits, exocarp, and outer seed coat were measured for each grade, followed by analysis of key quality indicators, including seed count, 100-seed weight, Brix degrees, pH, single-seed dimensions, vitamin C content, and edible fraction. Subsequently, correlation analysis, principal component analysis (PCA), and the entropy weight-TOPSIS method were employed to evaluate the integrated quality of different fruit grades comprehensively. The results indicate that the fruit morphological characteristics of “Liuyuehong” soft-seed pomegranate have a significant impact on its sensory and physicochemical qualities. (1) Large and medium fruits are superior to small fruits in terms of single fruit size, exocarp color uniformity, seed color, and mouthfeel, with large fruits having the highest comprehensive evaluation score (0.7). (2) Mouthfeel is correlated with the number of seeds in the fruit; the number of seeds in large and small fruits shows a significant negative correlation with Brix degrees (p < 0.05). (3) Small fruits exhibit greater individual variation within the group, with outliers and a tendency for late maturation. In conclusion, the fruit morphological characteristics of “Liuyuehong” soft-seed pomegranate significantly affect seed maturity and quantity, thereby determining the fruit’s sensory quality and physicochemical properties. The results indicate that fruits with a single- weight below 70 g commonly exhibit delayed development. It is therefore recommended to raise the lower threshold for commercial grading to above 75 g to enhance overall fruit quality and market consistency.

1. Introduction

Fruit quality grading must align with consumer preferences, as it directly determines market value and competitiveness [1]. Among quality attributes, sensory characteristics (e.g., size, color, texture) serve as primary criteria for grading, as they are the primary indicators influencing consumer selection [2]. For instance, larger fruit dimensions are typically associated with premium pricing in commercial markets [3], while higher color saturation consistently enhances visual appeal and purchase intention [4]. However, the mechanistic associations between external morphological characteristics and intrinsic quality parameters—such as flavor and nutritional attributes—remain a critical limitation in conventional grading systems. Conventional fruit grading systems exhibit an over-reliance on external morphological parameters (e.g., size, color, and surface integrity), while neglecting the physiological trade-offs between intrinsic quality attributes and external appearance. Of particular note is the “nutrient dilution effect”, a phenomenon wherein fruit size augmentation is typically concomitant with a reduction in the concentration of bioactive compounds and flavor-related metabolites [5,6]. This phenomenon directly reveals a potential non-linear relationship between external morphology and intrinsic quality, thereby highlighting the necessity and complexity of elucidating their interplay.
Current research on pomegranate fruit morphology and quality has primarily focused on comparative studies among different cultivars [7,8,9]. These studies have yielded valuable insights for optimizing agronomic traits to align with market requirements [10] and improve ecological resilience [11]. Notably, substantial variations in sensory quality traits (e.g., fruit shape index, pericarp color uniformity) and nutrition/flavor-related characteristics (e.g., sugar-acid ratio, aril firmness) can occur even within individual cultivars grown under identical field conditions, indeed on the same tree, due to microenvironmental heterogeneity (e.g., light gradients, nutrient allocation patterns) and asynchronous fruit development [12,13]. If the relationship between pomegranate fruit morphological traits and physicochemical quality is explored solely from the perspective of cultivar breeding, it remains insufficient to comprehensively encompass quality variations across different cultivars, thereby failing to provide an adequate basis for establishing widely applicable fruit grading criteria. A grading protocol must explicitly embed cultivar-specific covariance structures between visual and internal quality axes to maximize both accuracy and operational utility. This requires systematic quantification of intra-cultivar variance in sensory descriptors (e.g., shape regularity, color homogeneity) and physicochemical metrics (°Brix, titratable acidity) and mandates that cultivar identity be treated as a fixed-effect predictor in any market-grade classification model.
Fruit sensory attributes not only serve as the core decision-making basis for consumer purchasing behavior but also constitute key metrics for developing market grading systems. Existing research has advanced grading methodologies through technological innovation: Kumar et al. developed classification models based on morphological features (e.g., fruit size, weight, and color) using machine learning and digital image processing techniques [14]; Fashi et al. achieved grading assessment by integrating external and aril appearance parameters via artificial intelligence and image analysis algorithms [15]; Hemmati et al. demonstrated high consistency between prediction models for juice percentage and pH, derived from visible-near infrared spectroscopy combined with destructive sampling, and visible spectral results [16]. These technological advancements provide a crucial empirical foundation for rapid and objective grading of pomegranate fruits. However, the associations between morphological traits and intrinsic physicochemical properties in the “Liuyuehong” soft-seeded pomegranate cultivar remain insufficiently elucidated, necessitating further systematic investigation.
“Liuyuehong” is an early-ripening, soft-seeded pomegranate cultivar independently developed in Mengzi City, Yunnan Province, China. It has garnered widespread attention for its prominent commercial traits, including early maturity, vibrant external appearance, deep red seed, superior flavor, and strong stress tolerance. Upon introduction to the characteristic fruit tree production region of Kashgar, Xinjiang Uygur Autonomous Region, China, this cultivar has markedly enriched the diversity of local pomegranate germplasm resources. Field observations indicate that “Liuyuehong” exhibits considerable yield potential in the Kashgar region; however, fruit size uniformity is poor, and no scientific grading criteria based on fruit size (e.g., diameter classification intervals, single- weight thresholds) have yet been established. The absence of such scientific criteria (i.e., standardized grading based on fruit size for “Liuyuehong”) has resulted in ambiguous consumer perceptions of fruit quality, for instance, the lack of empirical data regarding differences in flavor and nutritional quality between “large” and “small” fruits, thereby constraining the enhancement of product brand premium capacity. To this end, the present study systematically measured and analyzed morphological indicators (longitudinal diameter, transverse diameter, single- weight), structural components (seed-to-pericarp ratio, seed number per fruit), intrinsic quality attributes (soluble solids content, titratable acidity, sugar-acid ratio, vitamin C content), and skin/seed color parameters of “Liuyuehong” fruits across different size categories. The objectives were to (i) elucidate correlations between fruit size and multiple quality metrics and (ii) rank and evaluate comprehensive fruit quality across size categories using the entropy weight-TOPSIS integrated assessment method. Taken together, our findings furnish the quantitative framework and empirical data required to establish a science-driven quality index and market-facing grading standard for “Liuyuehong” pomegranates.

2. Materials and Methods

2.1. Experimental Site and Plant Materials

The experimental materials were transplanted in March 2024 to the cold greenhouse test area located in Saqikanmailisi Village, Yingwusitan Township, Kashgar City, Xinjiang Uygur Autonomous Region, China. The detailed planting layout is shown in Figure 1.
The test cultivar was “Liuyuehong”, which was used in the form of 3-year-old fruit-bearing young trees. The rootstock was “Qiancenghua” pomegranate (P. granatum “Qiancenghua”). The graft union showed good healing, with a trunk diameter (thickness) of ≥2.5 cm, a tree height of 1.2–1.4 m, and a crown width of 0.8–1.2 m. All trees exhibited moderate and uniform growth vigor. The cold greenhouse extended in an east-west direction, with a single-span steel structure covered with a 0.12 mm PO-coated film. It had an eave height of 2.3 m and a ridge height of 3.5 m, covering a total area of 408 m2 (51 m × 8 m). The pomegranate trees were planted in double rows in a north-south direction, with a row spacing of 4.0 m and a plant spacing of 2.0 m.
The experiment exhibited a characteristic warm-temperate continental arid climate defined by significant diurnal temperature fluctuations, minimal precipitation, and extensive sunshine duration. The irrigation water source was underground well water. Throughout the entire annual growth cycle of the young trees, systematic field management practices, including flower thinning, pesticide spraying, weeding, and fertilization, were implemented as scheduled. These measures effectively ensured the supply of water and nutrients required for the growth of young trees, as well as pest and disease control.
Fruit samples were collected on 19 October 2024. For this study, five adult “Liuyuehong” pomegranate trees with consistent growth vigor were selected as sample trees. From the outer and middle parts of the crown of each tree, fruits with uniform light exposure and similar bearing positions but significantly different sizes were chosen to ensure that the samples covered the natural variation range of fruit sizes during this period. Exactly three fruits were collected from each tree, resulting in a total of fifteen experimental samples.

2.2. Determination of Fruit Physical Indices

Fruit samples were harvested at commercial maturity and promptly transferred to the laboratory in insulated foam boxes containing ice packs to minimize postharvest metabolic activity. Upon arrival, the vertical diameters (VD) and horizontal diameters (HD) of each fruit were measured using a digital caliper with an accuracy of 0.01 mm, and the fresh weight (FW) was determined using an analytical balance with a precision of 0.0001 g. Each fruit was then carefully dissected along the equatorial axis using a stainless-steel scalpel to separate the seeds from the exocarp. The total fresh weight of the seed (TFWS) and the exocarp (TFWE) were recorded separately, and the number of seeds per fruit was counted. A random subset of 100 seeds was weighed to determine the hundred-seed weight, and individual seed weights were measured from a representative sample. The vertical and horizontal diameters of the seeds were also measured using the digital caliper. The edible rate (ER) was calculated according to Formula (1) as follows:
E R = T F W S F W

2.3. Chemical Indices Measurements of Sarcotesta

The Brix degrees of the seed were measured using a Nohawk digital refractometer (Shenzhen Tiansu Calibration and Testing Co., Ltd., Shenzhen, China). Briefly, 5.0 g of fresh seed was ground into a homogenate, which was then centrifuged at 5000 rpm for 10 min at 4 °C. The supernatant was collected and equilibrated in a constant-temperature water bath at 25 °C for 5 min before Brix degree determination.
The pH value of the seed was determined using a PHS-3C precision pH meter (INASE Scientific Instrument Co., Ltd., Shanghai, China). The electrode was calibrated with three standard buffer solutions (pH 4.00, 6.86, and 9.18) before measurements. Fresh seed (5.0 g) was homogenized, diluted to a final volume of 100 mL, and transferred to a conical flask. The electrode was immersed in the homogenate, and the pH value was recorded after stabilization (defined as a change of less than 0.02 pH units per minute).
The content of vitamin C was quantified via the 2,6-dichlorophenolindophenol titration method. Briefly, 1.0 g of fresh seed was ground into a homogenate, which was then volumetrically adjusted to 10 mL using an oxalic acid solution. After the solution was clarified, 0.5 g of the supernatant was volumetrically made up to 10 mL. The dye titer was calibrated using a standard ascorbic acid solution of known concentration. Each sample was titrated until a pink color persisted for 15 s without fading, which was taken as the endpoint, and the volume of dye consumed was recorded.

2.4. Sensory Profile of Pomegranate Fruit

Five members quantified four key commercial quality attributes of pomegranate (exocarp color uniformity, outer seed coat color intensity, and mouthfeel) using a 10-point scale (Table 1). All samples were randomly coded for a blind test. The evaluation followed a strict “appearance-internal quality-taste” sequence: peel color, then aril color, followed by the sweetness-acidity balance of 30 seeds, and residual astringency after 30 s of chewing. Panelists rinsed with purified water between samples. Final scores were the mean of the five independent ratings. The experiment was repeated three times with different fruit batches to ensure robustness.

2.5. Statistical Analysis

Differences in physicochemical quality indices among fruit size categories (large, medium, and small) were analyzed using one-way analysis of variance (ANOVA), followed by Duncan’s multiple comparison test for post hoc comparisons at a significance level of p < 0.05. All statistical analyses were performed using Origin software (Version 2024, OriginLab Corp., Northampton, MA, USA). To reveal the comprehensive differences in physicochemical quality among different fruit types and the intrinsic correlations between indicators, Principal Component Analysis (PCA) was employed for data dimensionality reduction and ordination, combined with Hierarchical Clustering Analysis to achieve sample classification. The above analyses and visualizations were performed using the Principal Component Analysis App module in Origin software. The Pearson correlation coefficient method was employed to analyze the relationships between fruit morphological traits (e.g., vertical diameter, horizontal diameter) and physicochemical quality parameters (e.g., Brix degrees, pH, vitamin C content). Correlation coefficients were calculated, and heatmaps were generated using the Correlation Plot App module in Origin software.
To objectively evaluate the comprehensive quality of fruits categorized by large, medium, and small sizes, the Entropy Weight Method was utilized to determine the weights of various physicochemical indices (aiming to eliminate subjective weighting biases). A comprehensive evaluation model was then constructed by integrating the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). Finally, the quality ranking of each fruit size category was achieved by calculating its respective Closeness Coefficients.

3. Results

3.1. Sensory Characteristics of “Liuyuehong” Fruits with Different Sizes

Fruit sensory attributes are core evaluation indicators determining the commercial value of “Liuyuehong” pomegranate. Based on a standardized sensory evaluation system (Table 1), this study systematically quantified the exocarp color, outer seed coat, and taste flavor of fruit samples from three size categories (large, medium, and small). The results demonstrated that individual fruit weight had a significant influence on fruit sensory quality (Figure 2).
Compared with small fruits, large and medium-sized fruits demonstrated superior uniformity in exocarp coloration and a more intense red hue in their seeds. Furthermore, the seeds of large and medium fruits were characterized by a sweet taste and absence of astringency, contributing to an overall flavor profile that was significantly more desirable (Figure 3).

3.2. Physicochemical Properties Across Fruit Size Categories of “Liuyuehong”

The fruit physicochemical quality of the “Liuyuehong” soft-seed pomegranate correlates with individual fruit weight (Figure 4). Specifically, compared with medium-sized fruits, large fruits exhibited significantly greater individual fruit weight, fruit diameter, and hundred-seed weight (increases of 32.67 g, 4.64 mm, and 3.27 g, respectively, p < 0.05) (Figure 4A–D).
Similarly, medium fruits significantly surpassed small fruits in these traits (increases of 22.75 g, 11.06 mm, and 6.43 g, respectively; p < 0.05). Although large fruits exhibited numerically higher aril number and outer seed coat fresh weight than medium and small fruits, no significant differences were observed (Figure 4E,F, p > 0.05). Regarding biochemical traits of the seed, medium-sized fruits contained higher vitamin C content than large and small fruits by 0.04 mg·g−1 and 0.03 mg·g−1, respectively (Figure 4G). A similar trend was observed for Brix degrees, while the pH was 0.036 units lower in medium-sized fruits compared with both large and small fruits (Figure 4H). However, no significant differences were detected in the vitamin C content, Brix degrees, or pH among the different fruit size categories (Figure 4G–I, p > 0.05).

3.3. Correlation Analysis of Quality-Related Traits in “Liuyuehong” Fruits Across Size Classes

There are significant differences in the correlation patterns between sensory indices and chemical quality indices of seeds among different fruit size grades (Figure 5). In large-sized fruits (Figure 5A), the correlations are primarily concentrated in three aspects: (1) There is a significant positive correlation between the pH value and vitamin C (Vc) of seeds (p < 0.05), while no significant correlations are observed between these two indices and other indices (p > 0.05); (2) there is a significant negative correlation between the Brix degrees (BD) and number of seeds (NS) (p < 0.05); (3) there is a significant positive correlation between the hundred-seed weight (HSW) and seed vertical diameter (SVD) (p < 0.05).
For the medium-fruited type (Figure 5B), significant correlations were only observed in two pairs: (1) there was a significant negative correlation between the pH value of seeds and the fresh weight of fruit (p < 0.05); (2) there was a significant negative correlation between the number of seeds (NS) and the seed horizontal diameter (SHD) (p < 0.05). For the small-fruited type (Figure 5C), the correlation network was the most complex: (1) A significant positive correlation was identified between the Brix degrees (BD) of seeds and seed horizontal diameter (SHD) (p < 0.05), while significant negative correlations were observed between BD and both fruit horizontal diameter (FHD) and number of seeds (NS) (p < 0.05); (2) there was a significant positive correlation between the seed vertical diameter (SVD) of seeds and hundred-seed weight (HSW) (p < 0.05), as well as a significant negative correlation between SVD and NS (p < 0.05).

3.4. Principal Component Analysis and Comprehensive Evaluation of Fruit Quality in “Liuyuehong” with Different Fruit Sizes

Principal component analysis (PCA) clearly revealed the two-dimensional structure of fruit quality traits in “Liuyuehong” pomegranate (Figure 6). The first two principal components (PC1 and PC2) collectively explained 68.2% of the total variance. Among them, PC1 (with a contribution rate of 44.5%) represented the fruit morphological dimension, reflecting the overall size of sink organs. This component was primarily determined by morphological traits, including fruit longitudinal diameter (loading = 0.38), transverse diameter (loading = 0.41), single fruit weight (loading = 0.41), exocarp weight (loading = 0.41), and hundred-seed weight (loading = 0.38). A higher PC1 score indicated a larger overall fruit size, greater hundred-seed weight, and heavier exocarp.
In contrast, PC2 (with a contribution rate of 23.7%) corresponded to the seed resource allocation dimension, showing strong correlations with the number of seeds (loading = 0.56), Brix degrees (°Brix, loading = −0.46), seed horizontal diameter (loading = −0.38), and seed vertical diameter (loading = −0.32). A higher PC2 score was associated with a greater number of seeds but smaller seed size and lower sugar content—revealing a systematic trade-off between “seed quantity” and “individual seed quality” under conditions of fixed resource supply.
Two-dimensional ordination based on PC1 (morphological indicators) and PC2 (physicochemical quality indicators) showed distinct boundaries in the clustering distribution of large, medium, and small fruit types, indicating a stable association between fruit size and quality traits. Notably, the phenotypic variation within the small-fruit group was significantly greater than that within the medium- and large-fruit groups. Further analysis confirmed that this dispersion primarily originated from the heterogeneity of seed number, seed vertical/horizontal diameters, hundred-seed weight, Brix degrees, and pH value.
This phenomenon suggests that the drivers of fruit miniaturization in “Liuyuehong” pomegranate may not be a single specific factor. Instead, it may involve disturbances in multiple physiological regulatory pathways, which in turn manifest as coordinated variations in multi-dimensional quality traits.
To comprehensively evaluate the quality differences among different fruit types, this study employed the entropy weight-TOPSIS method to perform weighted scoring on 11 indicators, including single fruit weight, fruit vertical diameter and horizontal diameter, exocarp weight, hundred-seed weight, seed number, seed vertical diameter and horizontal diameter, Brix degrees, pH, and vitamin C content (Table 2). The results showed that large fruits had the highest comprehensive score (0.7), with their advantages mainly reflected in morphological indicators such as single fruit weight, fruit length and diameter, exocarp weight, hundred-seed weight, and seed number, as well as sensory indicators including pericarp coloration, exocarp color, and mouthfeel; medium fruits ranked second in comprehensive score (0.3), exhibiting superiority only in the indicator of seed vertical diameter and horizontal diameter; small fruits scored the lowest in both morphological and physicochemical quality indicators, showing the poorest comprehensive quality performance.

4. Discussion

4.1. Fruit Quality in “Liuyuehong” Soft-Seed Pomegranate Is Governed by a Size-Dependent Trade-Off Rather than a Simple Scaling Effect

The prevalent trade-off between fruit size and flavor quality has long been a focus in pomology research, often attributed to carbohydrate dilution [17,18] and source-sink competition [19,20]. Our study reveals the association between fruit size and flavor quality in “Liuyuehong” soft-seeded pomegranate, providing a new perspective to clarify the classic controversy between the “nutrient dilution effect” and “source-sink competition relationship”. Specifically, the nutrient dilution effect in large fruits and insufficient nutrient supply in small fruits of “Liuyuehong” soft-seeded pomegranate essentially reflect a trade-off in photosynthate allocation strategies: the former reduces resource allocation per seed due to excessive seed number, while the latter inhibits seed development due to insufficient total resources. Our data show non-linear relationships between fruit size and seed vitamin C (Vc) content, pH, sugar content, and seed size (Figure 4G–K). Compared with medium-sized fruits, the seed number is significantly negatively correlated with sugar content (Brix°) in both large and small fruits (Figure 5A,C). This finding reveals a non-linear “sink strength-quality” relationship, indicating an optimal sink strength range for achieving the best flavor, which contrasts sharply with previous observations in other species. For instance, in grapes (Vitis vinifera), smaller fruits exhibit higher sugar content and anthocyanin levels due to a higher skin-to-pulp ratio [21,22]; in strawberries (Fragaria × ananassa), however, larger fruits show higher vitamin C content, sugar content, and anthocyanin levels [23].
The seemingly contradictory results mentioned above precisely highlight the species-specific nature of the relationship between fruit size and quality. We hypothesize that such differences may arise from two aspects: on one hand, they stem from the unique resource allocation strategies of different species—grape quality mainly depends on the concentration of metabolites in the peel [24], whereas anthocyanin and Vc synthesis in strawberries may rely more on sufficient carbon source supply [25]; on the other hand, they result from the differentiation of fruit functional types. As a “seed-functional fruit”, pomegranate’s resource allocation is regulated by the “seed number-resource matching degree” (increased seed number triggers a dilution effect), while “pulp-functional fruits” such as grapes and strawberries depend on the metabolic activity of pulp cells (e.g., sugar transport, secondary metabolism) [26,27]. This indicates that the key factor determining fruit quality is not size itself, but a complex network of dynamic trade-offs between “source” capacity, “sink” strength, and metabolic pathway specificity, which is determined by the species’ developmental and genetic background. This finding demonstrates that in “seed-functional” fruits, the fruit size-quality relationship is determined by the dynamic thresholds of three factors—seed number, carbon quota, and metabolic dilution—rather than a single size effect. This threshold is species-specific across different fruit functional types (seed vs. flesh), providing a generalizable framework for cross-crop quality prediction.

4.2. Quality Dilution Effects in “Liuyuehong” Soft-Seed Pomegranate Fruits of Contrasting Sizes

Fruit growth and development constitute a complex biological process involving cell division, expansion, biomass accumulation, and metabolite synthesis and transformation, in which the allocation and transport of photosynthetic assimilates play a decisive role [28]. During fruit development, continuous carbohydrate supply is essential for cell division and expansion [29], thereby promoting fruit enlargement [30]. However, studies have indicated that if cellular expansion proceeds too rapidly relative to the rates of sugar synthesis and transport, intracellular sugar concentration may become diluted [17,31]. It has also been reported that an excessive number of seeds (i.e., an abundance of sink organs within the maternal fruit) may reduce nutrient allocation per seed, leading to decreased nutrient concentration in individual seeds [20].
In pomegranate, it has been observed that seed maturity improves seed quality through optimized cell expansion and sugar accumulation, rather than by increasing seed quantity [32,33]. Our data demonstrate that although large fruits exhibit advantages in seed number and hundred-seed weight, their seed longitudinal and transverse diameters are smaller than those of medium-sized fruits (Figure 4D,J,K), indicating a “weight increase without volume expansion” dilution effect—where the weight gain in large fruits primarily results from water accumulation rather than dry matter accumulation. This finding aligns with reports in citrus and tomato, revealing a potential universal constraint between fruit expansion and intrinsic quality [34,35]. This commonality manifests as a shift in photosynthetic allocation toward “seed number proliferation” rather than “individual seed expansion” when sink strength significantly increases during fruit development [19,20,36]. Furthermore, it suggests an inherent regulatory logic in large fruits where “quantitative compensation” takes precedence over “individual development”, reflecting a strategic trade-off in resource allocation among internal organs and forming a “quantity-volume” trade-off mechanism.

4.3. Fruit Size-Dependent Quality Variation in “Liyuehong” Soft-Seeded Pomegranate Is Driven by Heterogeneous Maturation

Although the regulatory role of the source-sink relationship in fruit development is well-established across multiple crops, the mechanism by which fruit size modulates internal seed development and flavor formation in pomegranate (a unique fruit with a multi-seeded structure) remains poorly understood. Fruit heterogeneity, a common phenomenon, significantly influences fruit composition and quality [22]. Our study reveals that fruit size drives heterogeneous maturation and determines seed flavor quality through a two-factor interaction between “sink strength regulation” and “carbohydrate dilution”. Specifically, in the soft-seeded “Liyuehong” pomegranate, seeds from medium and large fruits not only exhibited a superior sugar-acid balance and lower astringency (Figure 3) but also had higher 100-seed weight, total seed number, and exocarp weight (Figure 4E,F,K). These traits collectively indicate that, compared with small fruits, medium and large fruits leverage their stronger sink competitiveness to secure a more substantial quota of photosynthate allocation for sugar accumulation [20,37]. This, in turn, drives a more coordinated and complete maturation program, ultimately enhancing the overall harmony of flavor-related metrics, such as sugar-acid balance and the efficiency of astringent compound degradation [38]. Conversely, for small fruits, an overall insufficient nutrient supply is likely the primary cause of restricted seed development.
Maturity heterogeneity—the temporal mismatch in ripeness among fruit on the same tree or within a single fruit—has been attributed to flowering date, microclimate, and crop load [39,40,41,42]. Roch et al. proposed that medium-to-large fruit require both a longer developmental window and a greater resource supply [28], enabling them to emit stronger assimilate-demand signals to source leaves and thus secure a disproportionate share of photoassimilates during the rapid-growth phase [43]. Under uniform environmental and management conditions, such heterogeneity is driven primarily by two developmental biology factors: flowering order and fruit number. This interpretation aligns tightly with published evidence. Fattahi et al. demonstrated a strict correspondence between flowering order and fruit size in pomegranate: the first flush produces dominant, large fruit, whereas the last flush yields small, inferior fruit [40]. Cano-Lamadrid et al. further showed that hand-thinning (i.e., artificial reduction of sink competition) markedly improves individual fruit quality [44]. Thus, the immature traits we observed in the small fruit of “Liuyuehong”—pronounced astringency, lighter color, and smaller size (Figure 2)—likely reflect their origin as “late sinks” that are disadvantaged in the resource hierarchy. This conclusion mirrors Fattahi’s finding that fruit derived from the final wave of flowering consistently exhibits lower maturity [40]. Systematically dissecting the spatiotemporal coupling between flowering phenology and fruit development is therefore essential for elucidating quality formation in “Liuyuehong” pomegranate. Such knowledge will underpin precision-thinning strategies: Selectively removing the last flush of flowers or their resultant fruit can re-allocate assimilates to the first and second flushes, thereby elevating overall fruit quality and commercial value.

4.4. Establishing Grading Standards and Optimizing Production Practices for the “Liyuehong” Soft-Seeded Pomegranate

Establishing an objective, indicator-based fruit-quality evaluation system is essential for accurately determining harvest maturity and formulating commercial grading standards. In pomegranate, quality can be assessed using morphological traits combined with physicochemical indices of the seeds [6], both of which are critical for identifying optimal harvest timing and for tracking quality changes during fruit development [45,46]. Here, we systematically characterized the sensory (Table 1, Figure 2 and Figure 3) and physicochemical (Figure 4) attributes of “Liuyuehong” soft-seed pomegranate, providing the insights needed to achieve these goals.
In terms of commercial grading, fruit weight is closely associated with core quality traits: compared with medium and large fruits weighing over 75 g, small fruits (weighing less than 70 g) exhibit poorer performance in exocarp color uniformity, seed color, and mouthfeel, specifically manifested as lighter seed color and prominent astringency (Table 1, Figure 2 and Figure 3). This result provides direct evidence for establishing a minimum weight threshold (70 g) for commercial harvest of this cultivar.
Regarding the assessment of physiological maturity, this study identified a positive correlation between peel chromaticity values and seed sugar content. This finding corroborates the conclusions of Fawole et al., indicating that peel color can serve as an effective external indicator for predicting seed sugar content [47]. Although large fruits exhibit non-linear changes in indices such as pH, sugar content, and Vc with increasing fruit diameter, attributed to the nutrient dilution effect, comprehensively, medium- and large-sized fruits outperform small fruits significantly in terms of seed color, Vc content, and sugar content, whereas the trend in pH shows the opposite pattern. This pattern supports the conclusion proposed by Fashi et al. that pH can be used as an indicator for quality prediction [15].
From the perspective of flavor development, seeds from medium-to-large fruits exhibited desirable characteristics, including deep color, a balanced sugar-acid ratio, and low astringency, a finding highly consistent with Fawole et al.’s description of mature pomegranates [32]. Notably, astringency is generally regarded as a marker of immaturity in plant development; for instance, in persimmon (Diospyros kaki) fruits, astringency degrades gradually during the ripening process until it disappears [48]. This suggests that the smaller fruits in our study, which had pronounced astringency, were likely not fully mature at harvest. An extended developmental period could therefore promote nutrient accumulation and flavor improvement in these fruits.
In summary, this study establishes a systematic framework for the scientific grading and harvest strategy optimization of “Liuyuehong” soft-seeded pomegranates from both morphological and physicochemical dimensions. It not only defines the minimum weight standard for commercial grading but also verifies the effectiveness of exocarp color and seed pH as predictive indicators for maturity and quality.
Building upon the findings of this study, future research should focus on the following key questions: (1) the regulatory mechanisms by which different flowering periods (e.g., initial and late flowering phases) influence fruit development dynamics and final quality formation in “Liuyuehong” pomegranates; (2) construction of quantitative relationship models between fruit load and morphological-quality indicators; (3) development of quality improvement approaches based on flowering regulation and load optimization, along with their production feasibility. Advancing these investigations will not only enhance our physiological understanding of fruit quality differentiation but also translate into precision orchard management strategies (such as targeted water-fertilizer supply during specific flowering stages and differentiated fruit thinning protocols), ultimately achieving synergistic improvement of resource use efficiency with both fruit yield and quality.

5. Conclusions

The size of “Liuyuehong” soft-seeded pomegranate fruit is co-determined by flowering phenology and fruit load, which drives flavor divergence primarily through a “seed number-sugar dilution” effect. Our study establishes that both seed number and size are key negative regulators of sugar accumulation. Specifically, larger fruits experience a nutrient dilution effect due to excessive seed proliferation, whereas smaller fruits suffer from restricted sugar accumulation caused by source-sink imbalance. These variations in fruit morphology and quality are coordinately regulated by flowering timing and crop load, with smaller fruits exhibiting pronounced immaturity characteristics. Based on these findings, we propose a grading threshold of 70 g per fruit to enhance market homogeneity. Future work should decipher the spatiotemporal coupling between flowering regulation and fruit development, thereby informing precision fruit-thinning strategies to achieve synergistic gains in resource-use efficiency, yield, and quality.

Author Contributions

S.Z.: Formal analysis, Investigation, Writing—Original Draft, Visualization, and Writing—Review and Editing. W.L. and Y.X.: Writing—Review and Editing. J.Z.: Conceptualization, S.S. Investigation, Editing, F.Y. and Q.C. Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Kashi University Talent Introduction Project (grant No. GCC2024ZK-001) and the Kashi University Campus Project (grant Nos. [2024]2884 and [2024]2938).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

We sincerely thank everyone who contributed their time and effort to the manuscript. We thank Fang Wang, Yifan Ga, Zulalai Yilajiding, and Naiergezai Aizezi, who were involved in collecting the data.

Conflicts of Interest

The authors declare that they have no conflicts of interest. The funder had no role in the study design, data collection, and analysis; decision to publish; or preparation of the manuscript.

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Horticulturae 11 01369 g001
Figure 1. Cultivation of “Liuyuehong” pomegranate in a cold greenhouse and a schematic of its structure.
Figure 1. Cultivation of “Liuyuehong” pomegranate in a cold greenhouse and a schematic of its structure.
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Figure 2. Representative photographs of peel and arils from large, medium, and small “Liuyuehong” fruits, randomly selected across five biological replicates.
Figure 2. Representative photographs of peel and arils from large, medium, and small “Liuyuehong” fruits, randomly selected across five biological replicates.
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Figure 3. Sensory evaluation results for fruits of different sizes. Data are presented as the mean of five biological replicates (n = 5).
Figure 3. Sensory evaluation results for fruits of different sizes. Data are presented as the mean of five biological replicates (n = 5).
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Figure 4. Physicochemical properties across fruit size categories of “Liuyuehong” pomegranate. (A) fresh weight of fruit; (B) fruit vertical diameter; (C) fruit horizontal diameter; (D) hundred seed weight; (E) number of seeds; (F) fresh weight exocarp; (G) vitamin C content; (H) pH; (I) brix degrees; (J) seed vertical diameter; (K) seed horizontal diameter. Data are presented as the mean ± standard deviation (SD) of five biological replicates (n = 5). * p < 0.05.
Figure 4. Physicochemical properties across fruit size categories of “Liuyuehong” pomegranate. (A) fresh weight of fruit; (B) fruit vertical diameter; (C) fruit horizontal diameter; (D) hundred seed weight; (E) number of seeds; (F) fresh weight exocarp; (G) vitamin C content; (H) pH; (I) brix degrees; (J) seed vertical diameter; (K) seed horizontal diameter. Data are presented as the mean ± standard deviation (SD) of five biological replicates (n = 5). * p < 0.05.
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Figure 5. Correlation analysis of fruit quality attributes in “Liuyuehong” peach with different fruit diameters. Abbreviations: HSW, hundred-seed weight; BD, Brix degrees; SHD, seed horizontal diameter; SVD, seed vertical diameter; FHD, fruit horizontal diameter; FVD, fruit vertical diameter; FWE, fresh weight exocarp; fresh weight fruit, NS, number of seeds; VC, vitamin C; pH, pH value. * p < 0.05. Figures (A–C) show the large, medium, and small fruits, respectively.
Figure 5. Correlation analysis of fruit quality attributes in “Liuyuehong” peach with different fruit diameters. Abbreviations: HSW, hundred-seed weight; BD, Brix degrees; SHD, seed horizontal diameter; SVD, seed vertical diameter; FHD, fruit horizontal diameter; FVD, fruit vertical diameter; FWE, fresh weight exocarp; fresh weight fruit, NS, number of seeds; VC, vitamin C; pH, pH value. * p < 0.05. Figures (A–C) show the large, medium, and small fruits, respectively.
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Figure 6. Principal component analysis of fruit quality in “Liuyuehong” with different fruit diameters. Abbreviations: pH, pH value; NS, number of seeds; VC, vitamin C; FVD, fruit vertical diameter; FHD, fruit horizontal diameter; FWF, fresh weight fruit; FWE, fresh weight exocarp; HSW, hundred-seed weight; SVD, seed vertical diameter; SHD, seed horizontal diameter; BD, Brix degrees. The blue, red, and yellow ellipses represent the 95% confidence intervals for the large, medium, and small fruit groups, respectively, delineating the region in which 95% of the samples from each group are expected to fall.
Figure 6. Principal component analysis of fruit quality in “Liuyuehong” with different fruit diameters. Abbreviations: pH, pH value; NS, number of seeds; VC, vitamin C; FVD, fruit vertical diameter; FHD, fruit horizontal diameter; FWF, fresh weight fruit; FWE, fresh weight exocarp; HSW, hundred-seed weight; SVD, seed vertical diameter; SHD, seed horizontal diameter; BD, Brix degrees. The blue, red, and yellow ellipses represent the 95% confidence intervals for the large, medium, and small fruit groups, respectively, delineating the region in which 95% of the samples from each group are expected to fall.
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Table 1. Standardized scoring system for the sensory evaluation of fruits.
Table 1. Standardized scoring system for the sensory evaluation of fruits.
IndexScoresCriteria
Exocarp
color		10Uniform, natural red hue
7–9Moderately uniform, natural red hue
4–6Non-uniform distribution; natural red hue
1–3Patchy appearance, mottled with dull and natural red areas
Outer seed
coat color		10Uniformly distributed, deep, dark red color
7–9Uniformly distributed, deep red color
4–6Uniformly distributed, light red color
1–3No uniform distribution; pink color
Mouth
feels		10Balanced sweet-sour flavor
7–9Sour-dominated flavor with a sweet note
4–6Pronounced acidity
1–3Pronounced astringency
Table 2. Comprehensive evaluation of fruit quality of “Liuyuehong” with different sizes.
Table 2. Comprehensive evaluation of fruit quality of “Liuyuehong” with different sizes.
Fruit SizeThe Proportion of the Sample IndexComprehensive ScoringComprehensive Ranking
HSWECSCMFSHDSVDFHDFVDFWEFWFNS
Large0.60.50.60.60.40.40.60.70.60.710.71
Medium0.40.50.40.40.60.60.40.30.40.300.32
Small0000000000003
Abbreviations: HSW, hundred-seed weight; EC, exocarp color; SC, seed color; MF, mouthfeel; SHD, seed horizontal diameter; SVD, seed vertical diameter; FHD, fruit horizontal diameter; FVD, fruit vertical diameter; FWE, fresh weight exocarp; FWF, fresh weight fruit; NS, number of seeds.
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Zhang, S.; Sha, S.; Cui, Q.; Zhang, J.; Yang, F.; Lin, W.; Xiao, Y. Integrated Analysis of Morphological and Physicochemical Traits in “Liuyuehong” Soft-Seed Pomegranate Fruit. Horticulturae 2025, 11, 1369. https://doi.org/10.3390/horticulturae11111369

AMA Style

Zhang S, Sha S, Cui Q, Zhang J, Yang F, Lin W, Xiao Y. Integrated Analysis of Morphological and Physicochemical Traits in “Liuyuehong” Soft-Seed Pomegranate Fruit. Horticulturae. 2025; 11(11):1369. https://doi.org/10.3390/horticulturae11111369

Chicago/Turabian Style

Zhang, Shubin, Shuaishuai Sha, Quanlin Cui, Jin Zhang, Fenfen Yang, Wei Lin, and Yuansong Xiao. 2025. "Integrated Analysis of Morphological and Physicochemical Traits in “Liuyuehong” Soft-Seed Pomegranate Fruit" Horticulturae 11, no. 11: 1369. https://doi.org/10.3390/horticulturae11111369

APA Style

Zhang, S., Sha, S., Cui, Q., Zhang, J., Yang, F., Lin, W., & Xiao, Y. (2025). Integrated Analysis of Morphological and Physicochemical Traits in “Liuyuehong” Soft-Seed Pomegranate Fruit. Horticulturae, 11(11), 1369. https://doi.org/10.3390/horticulturae11111369

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