Morphological Diversity among Walnut Genotypes of in Azadshar, Iran

Persian walnut (Juglans regia L.), a monoecious tree with a long history of cultivation in Middle East and Europe, is currently one of the major nut crops in Iran. Since Iran is known to be one of walnut origins, a high genetic diversity can be found in this area. Almost all traditional commercial walnut orchards in Iran propagated by seed, therefore these orchards are rich in genetic resources. Morphological studies on the basis of pomological traits can be used for assessment of genetic variability in fruit trees, as well as for selection superior genotypes. Some researchers studied walnut genetic diversity in several areas of Iran but not in Azadshahr region, an area located in north and north-eastern of Iran with high walnut production. As walnut is highly diverse due to open-pollination system and seed propagation, we intend to study genetic diversity of Azadshar walnut genotypes by using morphological and pomological traits.
This research was conducted in Azadshare, Golestan province, Iran in 2012. Considering walnut orchards distribution, distance and height above sea level, four areas, namely Vamenan, Kashidar, Roodbar and Sidabad, with the distance of 15 Km apart were selected. Based on tree density and distribution, 15 to 40 trees in each area were labeled, and 102 walnut genotypes were generally evaluated. In each genotype, 30 morphological characteristics related to fruit, leaf and tree-growing habit were studied by using IPGRI walnut descriptor with a few modifications. Data analysis was performed by SPSS 16 software. Correlation coefficient of quantitative and qualitative characteristics was performed by using Pearson and Spearman methods, respectively. Cluster analysis was also performed by Ward method.
The results of anatomical characteristics analysis showed that genotypes of this area have high diversity in some pomological traits such as kernel percentage, nut weight, kernel color, easy separation of kernel. Based on the results, genotypes Ka17 and Va31 had the highest average of nut weight (19.79 gr). Va31 genotype had the heaviest kernel (9.4 gr). SID1 genotype had the highest kernel percentage (60.34%). Moreover, e genotypes ROOD4 and Va34 were typified by easy removal of kernel halves (very easy) and fruit flavor desirability (desirable). 26.47% of the genotypes showed very easy separation of kernel from shell. Correlation analysis showed that there was significant correlation among some traits. Fruit weight had high positive correlation with fruit length and diameter, and kernel and shell weight. There was no significant correlation among kernel shape, easy separation of kernel and shell tissue. As the results of cluster analysis of walnut genotypes indicated, clustering of genotypes is mostly similar to collected region and genotypes separated to four main groups in 12.5 distances of 25 in cluster figure. Most walnut genotypes of Vamenan and Kashidat regions, with the exception of ROOD 11 and ROOD 4, were placed in the first and second clusters similar to their geographical distribution. These areas have been the closest to the geographical distance and the height above sea level is not much different from each other. Almost all the genotypes collected from Sidabad region were placed in the third cluster. Sidabad village has high geographical distance from other studied regions, with its elevation is being significantly different from other areas., The majority of Roodbar genotypes, some genotypes of Vamenan and Kashidar, and one genotype of Sidabad (SID4) formed the fourth cluster. Roodbar region was located geographically between Sidabad and Vamenan regions. It has the same elevation as Vamenan and Kashidar have. In general, nut, kernel and leaf characteristics had the major role in clustering of genotypes.
Large genetic diversity in Azadshar walnut genotypes as inferred from morphological markers is advantageous to crop improvement through breeding and selection. Clustering analysis by morphological markers could clearly separate Sidabad walnut genotypes from other populations which have a good similarity with their geographical distribution. According to the results of this study, walnut genotypes collected from Azadshahr region showed a high genetic variation that can be used in breeding programs. Clustering based on morphological characteristics can be an effective method to determine the relationship between genotypes, as well as their relative distance.