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European hazelnut (Corylus avellana L.) is an economically and nutritionally important nut crop with wild and cultivated populations found throughout Europe and in parts of Asia. This study examined the molecular genetic diversity and population structure of 402 genotypes including 143 wild individuals, 239 landraces, and 20 cultivars from the Turkish national hazelnut collection using simple sequence repeat (SSR) markers. A total of 30 SSR markers yielded 407 polymorphic fragments. Diversity analysis of the Turkish hazelnut genotypes indicated that they fell into three subpopulations according to ad hoc statistics and neighbor-joining algorithm. Although all cultivars clustered together, they overlapped with the wild accessions and landraces. Thus, the dendrogram, principal coordinate, and population structure analyses suggest that they share the same gene pool. A total of 78 accessions were selected as a core set to encompass the molecular genetic and morphological diversity present in the national collection. This core set should have priority in preservation efforts and in trait characterization.

This study has been conducted between 2015 and 2016 at Hazelnut Research Institute in Giresun. The objective of this study was to determine the effects of the pollinizer cultivars on the nut and kernel characteristics. Tombul, Palaz, Çakıldak, Foşa and Allahverdi were used as main cultivars while Tombul, Palaz, Çakıldak, Foşa, Allahverdi, Sivri, Kalınkara and Yassı Badem were used as pollinizer cultivars. Self-pollination was considered as a control and open-pollination was taken as an application. In the study, it was determined that the pollinizer cultivars caused changes in some nut and kernel characteristics with the xenia and metaxenia effects. The effects of pollinizers on nut set and number of nut in clusters were quite obvious. Selfpollination resulted in low nut set and number of nut in clusters, indicating it is self-incompatible. Open pollination and different cross-pollination treatments increased nut set and number of nut in clusters. It is estimated that the changes in the pomological characteristics may be related to the nut set as well as the pollen sources. The kernel percentage and good kernel were generally low in self-pollination. Open-pollination and crosspollination treatments increased kernel percentage and good kernel. It has also been improved that the pollinizer cultivars may cause changes in nut shape. Palaz pollination treatment caused decrease of nut shape index in main cultivars.

Turkey is a rich source of European hazelnut (Corylus avellana) germplasm with nearly 400 accessions in the national collection. This genetic material encompasses cultivars, landraces and wild genotypes which were characterized for 12 nut and 13 kernel traits over 2 years in the 1990s. Analysis of these attributes revealed both the positive and negative impacts that human selection and breeding have had on hazelnut. Thus, while selection has resulted in larger nuts and kernels, cultivars have fewer nuts per cluster and kernels with larger internal cavities. Breeding has also resulted in a propensity for cultivars to have higher proportions of double kernels and empty nuts, two traits which reduce quality and yield. In addition, it is clear that while selection has successfully increased hazelnut fat content it has not impacted overall flavor, a much more complex trait. The nut and kernel phenotypic data were combined with genotypic data from 406 simple sequence repeat marker alleles for association mapping of the quantitative trait loci (QTL) for the traits. A total of 78 loci were detected in the population with the highest proportions for nut (24%) and kernel (26%) appearance parameters followed by quality (19%), shell thickness (16%) and yield-related (15%) traits. It is hoped that some of the identified QTL will be useful for future breeding of hazelnut for improved nut and kernel yield and quality.