Comparison of tree, nut, and kernel characteristics in several walnut species and inter-specific hybrids

This study was carried out to investigate the important characteristics in some cultivars/genotypes of Persian walnut (Juglans regia L.), black walnut (J. nigra L.), and walnut inter-specific hybrids: Paradox (J. hindsii × J. regia) and Royal (J. hindsii × J. nigra) in Kamalshahr Research Station in Karaj in 2011-2012. In the study, vegetative vigor was evaluated based on trunk cross-sectional area (TCSA), canopy diameter, nut related traits such as nut weight and size (thickness, width, height), kernel weight, kernel percentage, kernel color, ease of removal, shell thickness, and oil percent. Results showed very high diversity among the species and interspecific hybrids, so that Paradox with 506 cm2 had the highest TCSA, which was 214% more than that of J. nigra. Based on canopy diameter, Paradox showed the most vigorous growth, with an average of 7.95 m, while J. regia with 5.05 had about 37% less vigor relative to Paradox. Regarding nut height, it varied from 3.02 cm to 1.48 cm in J. regia and Paradox, respectively. Nut weight varied from 11.5 g in J. nigra to 3.62 g in Paradox. Principal coordinates (PCO) and cluster analysis were used to classify walnut cultivars/genotypes and interspecific hybrids. According to multivariate statistical analysis, the cultivars/genotypes were divided into three groups: black walnut, interspecific hybrids, and Persian walnut.