Phenotypic and pomological diversity of selected native walnut (Juglans regia) populations: implications for breeding and cultivation programs

There is a significant genetic diversity among Iranian local populations of Persian walnut (Juglans regia L.). Therefore the walnut cultivated for nut production provided a good opportunity for the identification of genotypes with desirable traits. In the present study, the pomological and phenological diversity traits of a total of 167 seed-originated walnut genotypes were investigated based on International Plant Genetic Resources Institute and the International Union for the Protection of New Varieties of Plants descriptors for two consecutive years. Considerable variations were found among the studied genotypes for all the evaluated traits. Nut weights, kernel weights, and kernel percentages among the selected genotypes ranged from 5.78 to 18.04 g, 2.4 to 10.78 g, and 18.33 to 78.20 respectively. Lateral bearing (LaBe) was positively and significantly correlated with yield (Yi) (r = 0.88). In addition, a negative correlation was observed between chilling damage and either LaBe or Yi. According to principal component analysis, the first two components explained 56% of the total variance among the studied genotypes. Phenological traits such as bud break date, first (FPSh), and last pollen shedding date (LPSh) were the most important traits contributing to the first component (PC1). PC2 was mainly contributed by nut weight (NuWe), kernel weight (KeWe), and kernel percentage (KePe). Cluster analysis divided the genotypes under study into 3 main groups. After 2-year evaluation, promising genotypes with lateral bearing habit, light kernel color, high yield, and late-leafing have been identified, including 22Fa-BC167, 22Fa-BC93, 22Fa-BC162, 22Fa-BC105, and 22Fa-BC163. These genotypes showed great potential for direct introduction into commercial cultivation in the orchards or as parental candidates for future walnut breeding programs.