Development of a core collection in Iranian walnut (Juglans regia L.) germplasm using the phenotypic diversity

Persian walnut (Juglans regia L.) is a very important nut crop with increasingly attention in the world. It was among the fruit tree species that has been propagated sexually, up to some decades ago. So, there were no or limited cultivars and the breeding works are usually new. In order to release new cultivars, a walnut improvement program started in Iran in 1980s. There was established a walnut collection, with selection of some superior genotypes from the orchards together with some introductions. As, the comprehensive characterization of germplasm is necessary in the breeding programs, in this study a collection of 104 Persian walnut accessions has been evaluated especially in order to create a core or breeding collection with reduced number of accessions and keeping maximum variability. To carry out this work, 18 walnut important traits were evaluated during 2014 and 2015 in the walnut collection of Horticultural Science Research Institute, Karaj, Iran. A wide variation was observed for several important characteristics such as leafing date (from March, 5 to April, 27), nut weight (6.60-15.33 g), kernel weight (2.67-8.21 g), and kernel percentage (35.39-71.09%). Significant correlations were observed between traits. Factor analysis showed that the six major factors affecting traits were explaining 79.9% of the total variance. The analysis, using maximization strategy through a heuristic algorithm, determined a core collection consisted of 27 accessions with similar variability respect to primary collection (mean difference percentage = 9.88% and coincidence rate of range = 95.05%). The parameters showed that this core includes 100% coverage of the primary diversity. According to result the F-test revealed no-significant differences for all traits. In this paper, for the first time, a walnut core collection was established from a primary collection. Conservation of this core collection could be more economic and better manageable for breeding purposes.