Agro-morphological characterization of summer squash (Cucurbita pepo L.) genotypes based on quantitative morphological descriptors

Introduction

Summer squash (Cucurbita pepo L.; 2n = 2x = 40), a member of the family Cucurbitaceae, is one of the most economically important vegetable crops cultivated worldwide. It is valued for its tender immature fruits, rapid growth, high productivity, and adaptability to diverse agro-climatic conditions. The crop is extensively cultivated in temperate, subtropical, and tropical regions for both fresh consumption and processing. Owing to its short crop duration and continuous fruit production, summer squash has become an attractive vegetable for commercial cultivation, contributing significantly to nutritional security and farmers' income. The fruits of summer squash are rich in dietary fiber, vitamins A and C, potassium, calcium, magnesium, and several bioactive compounds with antioxidant properties. Regular consumption has been associated with improved digestive health and reduced risk of chronic diseases. Growing consumer demand for nutritious vegetables, coupled with increasing adoption of protected cultivation, has further enhanced the commercial importance of this crop. Despite its economic significance, the genetic base of cultivated summer squash remains relatively narrow because modern breeding programs have relied on a limited number of parental lines. Continuous selection for yield, fruit quality, and market preferences has reduced genetic variability in many commercial cultivars. This limited diversity restricts breeding progress and increases vulnerability to biotic and abiotic stresses. Therefore, systematic characterization of available germplasm is essential for broadening the genetic base and identifying promising parental lines for crop improvement. Agro-morphological characterization is the first and most fundamental step in germplasm evaluation. Morphological descriptors are inexpensive, reliable, and easily measurable, providing valuable information on genetic variability, distinctiveness, uniformity, and stability of genotypes. Such characterization facilitates germplasm conservation, variety identification, parental selection, and breeding for improved horticultural traits. Morphological evaluation also serves as the basis for registration and protection of new plant varieties under Distinctness, Uniformity and Stability (DUS) testing. The International Union for the Protection of New Varieties of Plants (UPOV) and Bioversity International have developed standardized descriptor lists for Cucurbita species, enabling uniform characterization across breeding programs. Quantitative descriptors such as cotyledon dimensions, leaf size, petiole length, floral traits, fruit dimensions, seed characteristics, and seed weight provide important information on plant growth, reproductive biology, fruit development, and seed quality. These traits are particularly useful for identifying superior genotypes possessing desirable horticultural characteristics. Evaluation of floral traits including style length, filament length, and anther length is important because these attributes influence pollination efficiency, fruit set, and hybrid seed production. Similarly, fruit length, diameter, ribbing, and average fruit weight determine market preference and consumer acceptance, while seed size and hundred-seed weight are important indicators of seed vigor and propagation efficiency. Although several studies have reported morphological diversity in pumpkin and winter squash, comprehensive agro-morphological characterization of summer squash germplasm under Indian agro-climatic conditions remains limited. Evaluation of diverse genotypes using standardized quantitative descriptors can provide valuable information for identifying elite breeding materials and improving selection efficiency. Therefore, the present investigation was undertaken to characterize eighteen summer squash genotypes using quantitative agro-morphological descriptors and to assess the extent of phenotypic variability among them. The information generated from this study will contribute to germplasm characterization, facilitate the identification of superior genotypes, and provide a foundation for future breeding programmes aimed at improving yield, fruit quality, and adaptation.

Materials and Methods

Plant Material: The present investigation was conducted using eighteen genetically diverse summer squash (Cucurbita pepo L.) genotypes collected from different breeding programmes and germplasm sources. The experimental material comprised advanced breeding lines and cultivated genotypes exhibiting variation in vegetative, floral, fruit, and seed characteristics. These genotypes were selected to represent a broad range of morphological diversity for characterization and evaluation.

Experimental Site

The experiment was conducted at the Research Farm of the Division of Vegetable Science, ICAR-Indian Agricultural Research Institute (IARI), New Delhi, India, during the Rabi season of 2024-2026S The experimental site is located at approximately 28.63° N latitude and 77.15° E longitude, at an altitude of about 228.6 m above mean sea level. The region experiences a semi-arid subtropical climate with cool winters and hot summers. The soil of the experimental field was sandy loam, well-drained, and suitable for cucurbit cultivation. Recommended agronomic practices were followed uniformly throughout the crop growth period to ensure healthy plant establishment.

Experimental Design and Crop Management

The experiment was laid out in a Randomized Complete Block Design (RCBD) with three replications. Each genotype was grown in a single plot consisting of an equal number of plants (specify the number, e.g., 10 plants per replication) at the recommended spacing of 2.0 m × 0.6 m (adjust if different). Standard crop management practices, including irrigation, nutrient management, weed control, and plant protection measures, were adopted uniformly for all experimental plots to minimize environmental variation. Observations were recorded on five randomly selected healthy plants from each replication, and the mean values were used for statistical analysis.

Agro-Morphological Characterization

Morphological characterization was carried out following the descriptors proposed by UPOV (International Union for the Protection of New Varieties of Plants) and Biodiversity International for Cucurbita pepo. Quantitative observations were recorded at appropriate growth stages using standard measuring instruments.

Table 1:

The following quantitative descriptors were evaluated

Character

Cotyledon length (cm) Measured from the base to the tip of fully expanded cotyledons using a digital ruler.

2.

Cotyledon width (cm) Measured at the widest portion of the cotyledon.

3.

Leaf blade length (cm) Distance from the petiole attachment to the leaf apex. 4.

Leaf blade width (cm) Maximum width of the leaf measured across the lamina.

5.

Leaf blade silver patches Presence or absence of silver mottling was visually recorded according to descriptor standards. 6.

Petiole length (cm) Distance from stem attachment to the base of the leaf blade. 7.

Style length (cm) Length from the ovary to the stigma measured in freshly opened female flowers. 8.

Filament length (cm) Length of the filament in freshly opened male flowers. 9.

Anther length (cm) Length of mature anthers measured using a digital Vernier caliper. 10.

Peduncle length (cm) Measured from the fruit attachment point to the stem node. 11.

Fruit length (cm) Measured from blossom end to stem end at marketable maturity. 12.

Fruit diameter (cm) Maximum transverse diameter measured using a Vernier caliper. 13.

Fruit ribs Number or prominence of ribs recorded visually according to descriptor guidelines. 14.

Average fruit weight (g) Average weight of marketable fruits measured using an electronic balance. 15.

Seed length (cm) Mean length of ten randomly selected mature seeds. 16.

Seed width (cm) Maximum width of mature seeds measured using a Vernier caliper. 17.

Hundred-seed weight (g) Weight of 100 well-filled, dried seeds recorded using a precision electronic balance.

Statistical Analysis

The recorded data were subjected to analysis of variance (ANOVA) appropriate for a randomized complete block design to determine the significance of differences among genotypes. Mean values were compared using the least significant difference (LSD) or critical difference (CD) test at the 5% probability level. Descriptive statistics, including mean, range, standard deviation, coefficient of variation (CV), and standard error, were computed for all quantitative traits. Where appropriate, principal component analysis (PCA) and hierarchical cluster analysis (HCA) may be performed to assess genetic diversity and classify genotypes based on agromorphological characteristics. All statistical analyses were performed using R software (version 4.x), SPSS, or SAS (specify the software used).

Based on the four multivariate analyses (correlation matrix, PCA biplot, scree plot, and hierarchical cluster analysis), the following Results and Discussion are suitable for inclusion in a research paper on summer squash (Cucurbita pepo L.) morphological characterization. The interpretation is based on the uploaded figures.

Results

Correlation analysis of morphological traits (Fig-1)

Correlation analysis among the fifteen quantitative morphological characters revealed considerable variation in the degree of association among traits. Strong positive correlations were observed among several fruit-related traits, particularly fruit length, fruit diameter, and fruit weight, indicating that larger fruits tended to possess greater diameter and consequently higher fruit weight. Likewise, seed length, seed width, and 100-seed weight exhibited positive associations, suggesting coordinated seed development.

Vegetative traits such as leaf blade length, leaf blade width, and petiole length also showed moderate to strong positive correlations, indicating that vigorous vegetative growth contributes to larger leaf architecture. Cotyledon length and cotyledon width were positively associated with early seedling vigor. [7,9,6] .

Cluster analysis (Fig-4)

Hierarchical clustering using Ward's method grouped the eighteen summer squash genotypes into three major clusters, demonstrating considerable genetic divergence based on morphological traits. [13,16,12] .

The clustering pattern suggests the existence of appreciable morphological diversity within the evaluated germplasm and identifies genetically divergent parental lines suitable for hybridization programmes.

Discussion

The present study demonstrated substantial morphological diversity among the evaluated summer squash genotypes. [1,8,17] . The presence of weak or negative correlations between certain floral and fruit characters suggests that these traits may be independently inherited or differentially influenced by environmental conditions. Consequently, simultaneous improvement of reproductive and fruit traits may require careful selection strategies rather than indirect selection alone.

Overall, multivariate analysis identified fruit size, seed traits, and vegetative growth characters as the principal determinants of morphological variability in summer squash. These traits can therefore serve as reliable descriptors for germplasm characterization and as useful selection criteria in breeding programmes targeting higher yield and improved horticultural performance.

This interpretation is consistent with  Correlation matrix (trait associations).  PCA biplot (trait contributions and genotype grouping).  Scree plot (26.4% and 19.0% variance explained by PC1 and PC2).  Ward's dendrogram (three major genotype clusters). (2018) [11,6,18] . Principal Component Analysis showed that the first two principal components accounted for 45.4% of the total phenotypic variation, with major contributions from vegetative, fruit, and seed characters, confirming that these traits are the primary sources of variability among the evaluated genotypes. The PCA biplot clearly differentiated the genotypes into three groups, reflecting broad phenotypic divergence.

Hierarchical cluster analysis further classified the genotypes into three distinct clusters, supporting the PCA results and demonstrating considerable genetic divergence within the germplasm. The existence of well-separated clusters indicates the availability of diverse parental lines that can be exploited in hybridization programmes to maximize heterosis and generate superior recombinants. Overall, the combined use of correlation analysis, PCA, and cluster analysis proved effective in identifying key morphological traits contributing to diversity and in discriminating genetically diverse genotypes. Fruit size attributes, seed characteristics, and vegetative growth traits emerged as reliable descriptors for germplasm characterization and should receive priority in selection programmes. The identified divergent genotypes provide valuable genetic resources for developing high-yielding, superior-quality summer squash cultivars with broad adaptability.

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