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The research was carried out during the 2015–2017 in an organic certified hazelnut (cv. ‘Çakıldak’) orchard established at 6 × 5 m spacing with ‘ocak’ system (multi-stemmed bush) in Samsun, Türkiye. The aim was to determine effects of plant density on nut traits, nut yield, cluster distribution, kernel oil content, fatty acid composition, total phenolics content (TPC), total flavonoids content (TFC) and total antioxidant activity (TAA) in ‘Çakıldak’ hazelnut cultivar. Plant density (D) in orchard was arranged as 5 (D5), 6 (D6), 7 (D7), 8 (D8), 10 (D10), 12 (D12) and 14 (D14) plants per ocak, respectively. The highest nut yield per hectare was achieved with 2344 ± 129.1 kg in D14, and the highest nut yield per plant was determined with 534 ± 2.0 g in D8. Depending on the plant density, nut weight, kernel weight and kernel ratio ranged from 2.06 ± 0.01 g (D10) to 2.26 ± 0.02 g (D8), 1.18 ± 0.02 g (D10) to 1.27 ± 0.01 g (D8) and 55.10 ± 0.55% (D14) to 57.47 ± 0.40% (D7), respectively. The number of nuts per cluster also varied between 2.24 ± 0.03 and 2.41 ± 0.08. ‘Çakıldak’ kernels contained oil between 58.0 ± 0.02% (D14) and 63.5 ± 0.40% (D8). TPC was determined between 9.10 ± 0.64 and 13.09 ± 0.76 mg GAE g−1. TFC varied from 8.62 ± 0.06 to 13.17 ± 0.32 mg QE 100 g−1. TAA was measured between 39.05 ± 1.24 and 70.27 ± 2.13 mmol TE kg−1by FRAP test and between 25.54 ± 1.49 and 37.37 ± 0.12 mmol TE kg−1by DPPH assay. Results indicated that nut traits, nut yield, oil content, fatty acids, TPC, TFC and TAA were affected by plant density in Çakıldak cultivar.

Hazelnuts, which have high nutritional value, are an important source of fat, fatty acids, phenolics, and antioxidants. In this study, variations in the oil and protein contents, fatty acid compositions, and bioactive compounds of organic hazelnuts (‘Tombul’ and ‘Palaz’ cvs.) grown in seven different organic certified hazelnut orchards established with the ocak (multi-stemmed bush) planting system in Samsun (Türkiye) were evaluated. Except for protein, oleic acid, and linolenic acid, orchard-based variation in other traits investigated was significant. The effect of orchards on these traits varied depending on the cultivar. According to orchards, the oil content in the ‘Tombul’ cultivar varied between 61.1 and 64.4%, protein content between 14.3 and 15.3%, oleic acid content between 82.09 and 82.98%, linoleic acid between 7.49 and 8.44%, total phenolics between 5.51 and 15.92 mg g−1, total flavonoids between 4.86 and 23.20 mg 100 g−1, and antioxidant activity between 10.33–36.98 mmol kg−1and 4.74–28.62 mmol kg−1(according to FRAP and DPPH assays, respectively). ‘Palaz’ cultivar had a range of 59.41–62.92% for oil content, 13.22–15.51% for protein content, 82.05–83.15% for oleic acid content, 6.66–7.89% for linoleic acid, 4.65–9.15 mg g−1for total phenolics, 4.45–7.56 mg 100 g−1for the total flavonoids, and 9.17–23.58 mmol kg−1and 3.54–15.50 mmol kg−1for antioxidant activity (according to FRAP and DPPH assays, respectively). Principal component analysis (PCA) revealed that orchards in both cultivars were clustered at different points on the PCA plane and were associated with different traits. The findings showed that orchard-based variations in protein, oil, fatty acid compositions, and bioactive compounds in hazelnuts are significant.