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. 2025 Sep 12;25:1202. doi: 10.1186/s12870-025-07234-x

Pre-selection-based evaluation of fruit characteristics in Turkish hazelnut (Corylus colurna L.) in Bolu

Tuba Bak 1,✉, Mustafa Arslan 2, Turan Karadeniz 1
PMCID: PMC12427108  PMID: 40936083

Abstract

This study was carried out to determine the fruit characteristics of Turkish hazelnut (Corylus colurna L.) by pre-selection. The aim was to determine the fruit characteristics and the relationship between them. This research was carried out in four different locations in Bolu province. Nut weight was 0.81–2.38 g; kernel weight was 0.41–0.85 g; shell thickness was 1.00-2.86, and kernel percentage was 27.88–55.85%. Correlation analyses showed a high correlation, especially between kernel percentage and shell thickness, kernel weight, and nut weight. According to the heat map clustering analysis results, although the kernel percentage was in a different subgroup, it was clustered in the same group: nut length, nut shape index, kernel length, and kernel shape index. Among the prominent genotypes, MR31, KR8, and MR15 stand out with their nut weights, MR31, MR25, and MR14 with their kernel weight, MR7 with a thin shell, and MR4, MR6, and MR14 with their kernel percentage. As a result, in this study, genotypes with different characteristics of Turkish hazelnuts can be used as material for various purposes in breeding programs. These results may also contribute to conserving hazelnut genetic resources and future hazelnut breeding studies.

Keywords: Hazelnut, Pomological characteristics, Genetic diversity, PCA

Introduction

Hazelnut is a plant species belonging to the genus Corylus, in the subfamily Corylaceae of the family Betulaceae, affiliated with the order Fagales. It is suggested that there are more than 25 different species in this genus [1]. The genus Corylus is widely populated in temperate regions of the Northern Hemisphere. These species spread widely from Japan, Korea, and China to Tibet, India, Northern Iran, Türkiye, the Caucasus, Europe, and North America [2]. Corylus colurna L. is a species that spreads in limited geographies such as the Balkans, Romania, and Northern Türkiye. Corylus is an impressive plant with a wide morphological diversity, from small and multi-stemmed shrubs to tall and showy trees [3]. It is a tree with a robust and well-developed root system that can live up to 700 years [4].

Within the genus Corylus, Corylus colurna L. (Turkish hazelnut) and Corylus avellana L. (Common hazelnut) stand out as the two most common species in Europe. Corylus avellana, known as the European hazelnut, is the most researched and best-known hazelnut species in the genus Corylus. In addition to fresh consumption, hazelnuts are used as edible hazelnut oil, roasted, sliced, chopped, quartered, crushed, or crushed; hazelnut flour is used as feed for poultry and in the soap and cosmetics industry [5]. Corylus colurna L. is often called "Kaya hazelnut", "Ağaç hazelnut", "Ayı hazelnut" [6], "Turkish hazelnut" or "Bolu hazelnut" [2]. Turkish hazelnuts are used in local desserts, products such as "Nut Candy" and "Bolu Chocolate," or consumed as nuts. In addition, Turkish hazelnuts provide economic contributions to the livelihoods of local people by providing additional income [6]. Its fruits, which have many benefits for humans, are also an important food source for wildlife living in forests and rural areas [7, 8]. In addition, flavonoids found in fruits and leaves, compounds such as rutin, kaempferol, luteolin, quercetin, and caffeic acid constitute an essential source in terms of biological activity [9–11]. Corylus colurna is widely used in urban landscape designs in Europe and America [6]. It is especially preferred in boulevards, parks, avenues, streets, and cemeteries [12]. In addition, the waste of Corylus colurna can be used to dye leather in testicular products such as shoes and bags due to the tannin in their shells [13].

Although Corylus colurna can be grown from seed, the germination barrier seen in its seeds must be broken [14]. Long-term research and selection processes of its genotypes provide strong evidence that this species is a suitable rootstock for hazelnut varieties [15–20]. However, other Corylus species have not been studied sufficiently in genetic resource collections, breeding programs, and research studies as much as Corylus avellena [2].

Temel et al. [21] stated that the natural distribution area of ​​Turkish hazelnut is the Western Karadeniz Region (Bolu, Düzce, Kastamonu, Karabük, and Sinop). Turkish hazelnuts with preserved genetic characteristics for many years have been identified in different regions of Türkiye. Researchers reported that the elevation values ​​of the species in 18 natural Turkish hazelnut population areas varied between 700 and 1700 m. Ünalan [22] presented reports on new Turkish hazelnut populations in the Kastamonu region, and Kabak et al. [23] on the northern slope of Şaphane Mountain in the Kütahya provincial borders.

Although studies on Corylus colurna are limited, it is known that Turkish hazelnut has a more expansive natural distribution. Determining and using hazelnut resources through selection effectively encourages hazelnut breeding [24]. Determination of existing populations and examination of morphological and especially pomological characteristics can be carried out through selection studies. The most important examples are obtained by selection from a large population in naturally growing areas [25], and cultural varieties may arise from these studies [26].

It has excellent potential because Turkish hazelnut grows spontaneously in natural environments and is not subject to any special care, cultivation, fertilization, etc., cultural processes [27]. Therefore, selection studies are critical to evaluate the affluent population [28]. Based on these objectives, this study aimed to determine (1) fruit characteristics and (2) the relationship between the examined characteristics in Turkish hazelnut (Corylus colurna L.) genotypes.

Material and method

Plant material

Corlus colurna L. is a large tree characterized by its broad crown. Their trunks are rough and dark grey. The nut skin is thick and generally small in weight. These characteristics distinguish it from other Corylus species. For this reason, two main criteria were established for selection in this study: a large sample, nut weight, and thin shell. In line with the selection objectives, genotypes with thin shells and nut weight were identified in 2018. Therefore, genotypes with thick shells and small nuts were eliminated in line with the breeding objectives. And 50 Turkish hazelnut (Corlus colurna L.) genotypes with high nut weight and thin shells were selected (Fig. 1).

Fig. 1.

Fig. 1

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Photo of tree (a), fruits (b), and nuts (c) from C. colurna. (original photo by M. Arslan)

This study was conducted on 50 Turkish hazelnut genotypes in the districts of Bolu Central (Afşar, Güneyfelakettin, Merkeşler) and Seben (Korucuk) in 2019–2020, which are natural distribution areas of Turkish hazelnut (Corylus colurna L.). The research material consisted of Turkish hazelnut genotypes that grow naturally in the wild and range in age from 50 to 300 years. In the study, seven sample groups were selected in Güneyfelakettin, two in Afşar, nine in Korucuk, and thirty-two in Merkeşler. The sampled populations are located at altitudes ranging from 980 to 1280 m in Güneyfelakettin, 1050 m to 1150 m in Afşar, 660 m to 1040 m in Merkeşler, and 1020 m to 1290 m in Korucuk (Fig. 2).

Fig. 2.

Fig. 2

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Locations examined where Turkish hazelnut

Climate characteristics of the experimental area

The monthly average temperature and precipitation values ​​of the research area are presented in Fig. 3. According to the long-term climate data (2005–2020) recorded in the research area, the annual average temperature in Bolu Central district is 11.3 °C. The hottest is August (21.2 °C), while the coldest is January (1.3 °C). In Seben district, the long-term average temperature is 11.7 °C, the hottest is August (22.7 °C), and the coldest month is January (0.6 °C). The lowest and highest precipitation for the central district were recorded in August (16.5 mm) and June (71.0 mm), respectively, while in Seben district, it was recorded in August (11.8 mm) and January (47.9 mm). The monthly average precipitation is 41.2 mm and 32.0 mm in Bolu Central and Seben districts, respectively (Fig. 4) [29].

Fig. 3.

Fig. 3

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Long-term temperature (°C) data of Bolu center (a) and Seben (b) districts (2005–2021)

Fig. 4.

Fig. 4

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Long-term precipitation (mm) data of Bolu center (a) and Seben (b) districts (2005–2021)

Pomological characteristics

Turkish hazelnut nut weight, nut length, nut width, nut thickness, kernel weight, kernel length, kernel thickness, shell thickness, kernel percentage, nut and kernel shape index, and nut and kernel size characteristics were determined. Nut and kernel characteristics were weighed on a precision scale, and their weights (g) were determined (FLY 3000). Nut and kernel fruit length, width, thickness, and shell thickness were measured with a digital caliper with 0.01 mm precision (mm) (Loyka 5110). The kernel percentage was determined by the kernel to nut. It was calculated with kernel percentage (%) = (Kernel weight/nut weight)*100. Nut and kernel shape index was obtained by dividing the average width, length, and thickness values by length (NSI = NL/(NWT + NT/2). Size measurements were calculated by taking the arithmetic average of the fruits' width, length, and thickness measurements [30, 31]. Analyses were performed on 40 healthy fruits of each genotype.

Statistical analysis

The general characteristics of Turkish hazelnuts were determined using descriptive statistics, using 14 attributes with 50 genotypes. The coefficient of variation (CV) was used to determine the diversity between genotypes (Table 1). In addition, principal component analysis (PCA), hierarchical cluster analysis, and correlation analysis were performed from multivariate analyses, and prominent genotypes were identified. JMP 17 Pro software (trial) was used in statistical analyses.

Table 1.

Descriptive statistics of the investigated characteristics of the examined Turkish hazelnuts

Traits Unit Minimum Maximum Mean  ± SD %CV
Nut weight g 0,81 2.38 1.59 0.30 18.70
Nut length mm 14.42 22.24 17.51 1.54 8.77
Nut width mm 11.99 17.69 15.46 1.26 8.15
Nut thickness mm 9.83 14.47 12.20 1.03 8.44
Kernel weight g 0.41 0.85 0.57 0.09 16.57
Kernel length mm 11.77 15.82 13.84 1.01 7.32
Kernel width mm 9.92 13.26 11.23 0.83 7.40
Kernel thickness mm 6.33 10.03 7.70 0.73 9.49
Shell thickness mm 1.00 2.86 2.14 0.36 17.07
Kernel percentage % 27.88 55.85 36.31 4.81 13.25
Nut shape index - 1.04 1.63 1.28 0.11 8.85
Kernel shape index – 1.19 1.82 1.48 0.14 9.36
Nut size mm 12.32 16.96 14.82 1.03 6.95
Kernel size mm 9.58 12.05 10.56 0.60 5.70
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Results and discussion

Pomological analysis of genotypes and descriptive analysis

This research examined the physical properties of 50 hazelnut genotypes, and their descriptive statistics are given in Table 1. This study showed that the examined Turkish hazelnuts were remarkable in nut and kernel caliber. While large hazelnuts are preferred, especially for marketing, small and medium-sized hazelnuts are selected in the confectionery industry [32]. There were significant differences in the examined nut and kernel size properties. While 90% of the nut sizes were over 12.00 mm, the kernel size was over 9.00 mm (Table 1.). Nut weight (18.70%), kernel weight (16.57%), shell thickness (17.07%), and kernel percentage (13.25%) had high CV values. Popović et al. [33] reported CV values ​​for nut weight, length, width, thickness, and shape index as 23.15%, 8.11%, 9.50%, 10.22%, and 10.77%, respectively. Nut weight, nut length, and nut width CV values ​​are similar to the findings of [33]. High CV values ​​clearly show that pomological markers can separate genotypes.

Nut weight, kernel weight, and kernel percentage are the essential quality traits that directly affect yield in hazelnuts [34], and their heritability levels are 0.63, 0.67, and 0.87, respectively [35]. A high kernel percentage is desired for hazelnut breeding and industry [28]. The kernel percentage is affected by nut weight, kernel weight, and shell thickness [36]. While nut weight was between 0.81 g and 2.38 g, kernel weight was between 0.41 g and 0.85 g (Table 1, Fig. 5). The kernel percentage is a highly heritable trait [35]. The kernel percentage in the examined Turkish hazelnut genotypes was 36.31%. Genotypes with a kernel percentage of over 36% can be considered promising for Turkish hazelnuts. In our study, the eleven genotypes' kernel percentage is over 40% (MR2, MR3, MR4, MR5, MR6, MR7, MR12, MR14, MR17, MR18, and MR19). According to the findings of [37–40], we have remarkable results. In particular, MR7 has a high kernel percentage (55.85%) and can be used to develop varieties with a high kernel percentage.

Fig. 5.

Fig. 5

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Graphical distribution according to NW, KW, KP traits

A thin shell is an essential criterion in hazelnut breeding studies. Because shell thickness is a feature that affects fruit quality and kernel percentage. In addition, a thin shell is preferred in industrial processing [41]. Shell thickness in Turkish hazelnut genotypes varied between 1.00–2.86 mm (Table 1, Fig. 6). The examined shell thickness is similar to [37], thinner than Ayan et al. [38], and thicker than the genotypes of [39]. While the cultivated varieties have thinner shells, one of the characteristic features of Turkish hazelnuts is that they have thick shells. Shell thickness has a relatively high heritability [35]. The MR7 genotype stands out in our study with its 1.00 mm shell thinness. This genotype can be considered as a suitable parent for breeding programs aiming to obtain thin shells. According to market demands and breeding goals, it is possible to develop more hazelnut varieties with superior characteristics and rich genetic diversity in the future [42].

Fig. 6.

Fig. 6

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Graphical distribution of genotypes according to ST, NSI and KSI features

The most important examples of hazelnut varieties are obtained by selection from a large population in natural growing areas. Selection is one of the most widely used methods in hazelnut breeding [25]. Recently, it has focused on developing varieties with higher kernel sphericity and smaller size for hazelnut processing and confectionery applications [43]. While the hazelnut shape index in Turkish hazelnuts varies between 1.04 and 1.63, the kernel shape index was found between 1.19 and 1.82 (Table 1, Fig. 5). Köksal [44], classified hazelnuts as 0.81–1.19 round, 1.20–1.40 pointed, > 1.41 long and < 0.8 short. Our genotypes are longer than the findings of other researchers [33, 39], and partially similar results in terms of shape index [33, 38]. Miletić et al. [45] reported that Turkish hazelnuts with cultivars are long-shaped, similar to our research results. This selection is close to the demands of the confectionery industry and high-quality fruits in terms of its characteristics. However, environmental factors and cultural practices are essential in fruit sizes. The vast differences between hazelnut sizes may be due to environmental factors and cultural practices. Balta et al. [46], reported this situation.

Principle component analysis

Fourteen features were used for principal component analysis; three PC eigenvalues ​​explained 87.37% of the total variation. The first two components explained 69.34% of the variability. PC1 (47.23%) was mainly related to nut weight, nut length, nut width, nut thickness, kernel weight, kernel width, kernel thickness, shell thickness, hazelnut, and kernel size. PC2 was related to the nut and kernel shape indexes, explaining 22.11% of the total variability. PC3 also explains that 18.03% of the variability is related to kernel percentage. The most significant features affecting PC1 were nut weight, nut width, and nut size (0.92, 0.91, and 0.90, respectively), while the most critical feature in PC2 was the nut shape index (0.88). Kernel percentage had the highest effect on PC3 (0.85) (Fig. 7). Traits in PC1, PC2, and PC3 were highly correlated. These findings play a critical role in understanding the complex relationships and variations among traits of hazelnut genotypes. They indicate that nut and kernel traits are essential for distinguishing and analyzing breeding material for hazelnut characterization.

Fig. 7.

Fig. 7

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Biplot graph of two main components of examined Turkish hazelnuts (a), vector loadings, eigenvalues, and variances in the studied principal component analysis (b)

The examined fruit traits of 50 Turkish hazelnuts were determined by PCA analysis (Fig. 7). PCA graph shows how Turkish hazelnuts are distributed and grouped over two main components (PC1, PC2). The confidence ellipse explains 95% of PC, and MR10 remains outside the ellipse. It is distinguished from other genotypes in terms of shape index traits. MR10, MR22, MR8, and GF4 are located in the same position on the PC axis. MR7 and MR27, MR29, MR17, MR5, MR12, M6 and MR4 show similar traits among themselves. KR8, MR25, MR24, and MR31 are positively related to PC1. Similarly, AF2 and KR6, KR2, and MR30 are positively related to PC1 and PC2. GF2 and GF5 were in different planes and positively related to PC1. The distribution of genotypes reveals that those with similar characteristics are located close to each other, while those with different characteristics are located further away. Examining the genetic relationships and population structure between wild forms, genotypes, and varieties in a region, can provide information about gene flow between individuals [47].

The fact that genotypes show similar characteristics indicates that they are almost close regarding fruit characteristics. While PC1 explained differences in fruit characteristics such as size, shape, or shell thickness, PC2 was closely related to nut and shape length and shape index. PC1 was related to weight, size, and width characteristics. Fruit and kernel weight were inversely related to kernel percentage. Similarly, the kernel percentage has inverse vectors with shell thickness. These results are similar to the reports of other researchers [34, 48].

Correlation analysis

This comprehensive correlation analysis reveals complex relationships between 50 Turkish hazelnut genotypes and 14 fruit traits (Fig. 8). It is seen that there is a high positive correlation between nut weight, nut width, nut thickness, kernel weight, nut size, and kernel size. However, there is a negative (− 0.50) correlation between kernel percentage and nut weight, a positive (0.10) correlation with kernel weight, and a strong negative (− 0.79) correlation between shell thickness. The correlation analysis shows strong positive correlations between nut width and nut size (0.93) and kernel weight and kernel size (0.94). Again, a strong negative (− 0.66) correlation exists between kernel thickness and kernel shape index. There are positive, strong relationships between nut length and kernel length, nut size, and kernel size, as well as negative relationships between shell thickness, kernel thickness, and kernel percentage. Correlation analyses evaluate the relationships between different traits. Positive correlations indicate that improving one trait can improve another trait simultaneously according to the examined traits. Karakaya et al. [41], reported high positive correlations between nut weight, kernel weight, kernel, and kernel size characteristics in nut varieties. There are strong positive correlations between nut weight and nut thickness, and this is similar to the research findings of Milošević and Milošević [49]. Similarly, there is a negative relationship between weight and kernel percentage and between kernel percentage and shell thickness. Indeed, other studies support this [34, 41]. Again, there is a positive correlation between nut length and kernel weight. Indeed, hazelnut length is one of the critical factors affecting kernel weight and percentage [50].

Fig. 8.

Fig. 8

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Correlation analysis of the investigated Turkish hazelnuts

Hierarchical cluster analysis_heatmap

The results of the heatmap hierarchical clustering analysis showing how Turkish hazelnut genotypes are grouped according to their similarities are given in Fig. 9. When the dendrogram is examined, it is seen that the genotypes are clustered in two main groups, A and B. These two main clusters are divided into two sub-clusters (A1 and A2 and B1 and B2), and genotypes showing differences are divided into many sub-clusters in this way. Similarly, the 14 variables examined are divided into two main groups, X and Y. 9 of the examined traits are included in the X central cluster. While nut weight, width, size thickness, and shell thickness are included in the X1 sub-cluster, Kernel weight, size, width, and thickness are included in the X2 sub-cluster. The Y1 cluster is clustered with nut and kernel length and shape index traits. Only kernel percentage is included in the Y2 cluster. The internal ratio is one of the main factors affecting yield, thin shell, and other characteristics in hazelnut breeding.

Fig. 9.

Fig. 9

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Hierarchical clustering analysis results show the relationships between Turkish hazelnut genotypes and traits. The heatmap shows the similarities and differences between the samples and the traits examined with color coding. Red tones represent high values, and blue tones represent low values

Although cluster A1 is in two different groups within itself, it has also merged into a single cluster according to its characteristics. In cluster A2, 16 genotypes showed similar characteristics among themselves. In cluster B1, four genotypes and in cluster B2, 18, a total of 22 genotypes were grouped within themselves in terms of the examined characteristics. Regarding the examined characteristics, cluster B1 constitutes the cluster with the largest hazelnut and kernel weight. Turkish hazelnuts in clusters B2 and A1 show similar characteristics in terms of kernel percentages. Cluster A1 draws attention with its thin shell characteristic (Table 2). In the analysis, similar characteristics of genotypes starting with GF1 and ending with MR31 were grouped within group B. In group A, similar genotypes started with GF2 and were completed with MR7. These results show that the examined characteristics have strong relationships in Turkish hazelnuts. The formation of the clusters provides valuable information that can be used in Turkish hazelnut breeding studies on the examined genotypes and characteristics. In fruit breeding, Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) have been used in many studies as a critical tool from selecting parental lines representing wide genetic variation to simplifying complex multi-trait data and identifying new varieties with desired phenotypes [51–56].

Table 2.

Average values of the clusters formed in the cluster analysis in terms of the studied features

Cluster Number of ındividuals Group NW NL NWT NT KW KL KWT KT ST KP NSI KSI NS KS
1 8 B2 1.67 18.14 16.00 12.47 0.65 14.13 11.89 8.32 1.98 39.58 1.28 1.40 15.22 11.16
2 4 B1 2.21 18.06 17.31 14.06 0.74 13.95 12.56 9.13 2.32 33.57 1.17 1.31 16.26 11.53
3 10 B2 1.80 18.91 16.52 12.80 0.61 14.93 11.53 7.50 2.43 34.06 1.30 1.57 15.83 10.86
4 16 A2 1.46 16.20 14.99 12.15 0.49 12.82 10.77 7.52 2.27 33.62 1.20 1.41 14.28 10.08
5 4 A1 1.41 19.02 14.79 11.45 0.51 14.68 10.24 6.71 1.87 36.25 1.46 1.74 14.75 9.98
6 8 A1 1.30 16.72 13.95 10.73 0.54 13.77 10.94 7.48 1.72 42.61 1.36 1.50 13.56 10.37
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Conclusion

This study analyzed 14 different fruit traits of 50 Turkish hazelnut (Corylus colurna L.) genotypes in detail. The findings show that there is a wide variation among genotypes, and this variation reflects genetic diversity. In particular, traits such as fruit size, shape, and kernel percentage are essential in characterizing genotypes. Correlation analysis shows a strong positive correlation between kernel weight, size, and width. Strong positive relationships exist between nut weight and kernel weight, nut thickness, nut width, and nut size. Strong negative relationships were determined between kernel percentage and shell thickness, positive relationships with kernel weight, and strong negative relationships with nut weight. Negative relationships were determined between shell thickness, nut size, and nut shape index. Principal Component Analysis (PCA) emphasized the effect of fruit traits on genetic variation for Turkish hazelnuts and showed that these traits create differences among genotypes. Hierarchical cluster analysis grouped the genotypes into two main clusters and revealed the unique profiles of each cluster. The kernel percentage is mainly found in the different subgroups in hierarchical clustering. Among the prominent genotypes, MR31, KR8, and MR 15 stand out with their nut weights, MR31, MR25, and MR14 with their kernel weights, MR7, MR4, MR6, and M14 with their kernel percentages, and MR7 with their thin shell trait. The variation among traits can be evaluated as an indicator of genetic diversity, and this can be used for various purposes in breeding programs. It is known that Turkish hazelnuts grow in their habitats, under competition with other plant species, and without any cultural practices. Therefore, this study has provided a basis for future research by determining the fruit traits of Turkish hazelnuts in their natural distribution areas through pre-selection. Increasing and protecting these studies is vital in evaluating important traits for hazelnut cultivation in our country.

Acknowledgements

We thank Assoc. Prof. Dr. Emrah GÜLER for his suggestions regarding the evaluation of our statistical data.

Abbreviations

NW

Nut weight

NL

Nut length

NWT

Nut width

NT

Nut thickness

KW

Kernel weight

KL

Kernel length

KWT

Kernel width

KT

Kernel thickness

ST

Shell thickness

KP

Kernel percentage

NSI

Nut shape index

KSI

Kernel shape index

NS

Nut size

KS

Kernel size

Author contributions

TB: Investigation, methodology, conceptualization, formal analysis, writing—original draft, validation, visualization. MA: Investigation, identification and collection of materials, methodology, validation. TK: Supervision, methodology, conceptualization, writing—review editing, validation. All authors contributed equally to the manuscript and read and approved the final version of the manuscript.

Funding

This research did not receive any funding.

Data availability

The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Plant material was collected from the natural distribution areas of Turkish hazelnut (Corylus colurna L.) in the Central and Seben districts of Bolu province, Güneyfelakettin, Afşar, Merkeşler and Korucuk locations. Fruit samples from 50 different genotypes located outside the public area were collected with the permission of the tree owners in the villages. No fee was paid for the fruit samples. This study does not involve any human or animal testing.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

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