In-situ assessment of the traditional Lebanese walnut germplasm using morphological characteristics and ISSR Markers

The characterization of walnut cultivars is an important step towards conservation and exploitation of the Lebanese walnut diversity. In this study, we aimed to evaluate the genetic and phenotypic diversity within traditional Lebanese walnut cultivars grown in different regions of Lebanon. A total of 35 accessions were submitted to an ampelographic assessment; 26 of them were genetically studied using Inter Simple Sequence Repeat (ISSR) markers. A set of 27 morphological characters, including 13 qualitative traits and 14 quantitative traits related to the tree, leaf, nut, and kernel, was examined. Seven specific characters of the walnut were revealed to be the most discriminating. These characters included: leaflet shape, shell texture, shell color, nut shape, nut weight, kernel weight, and kernel color. Based on these traits, the clustering analysis identified three distinct groups that are clearly differentiated by the leaflet shape and margin. Molecular assessment was conducted using nine ISSR primers already used for walnut trees. These primers generated 72 bands, of which 67 were polymorphic. The average of the observed alleles was equal to eight in each locus. ISSR clustering based on Jaccard distance did not reveal similarity between accessions with the molecular studies. The highest genetic distance was observed between genotypes T34 and T2, differing by 45 bands (62.5%), while the lowest was between genotypes T2 and T6, differing by only four bands (5.5%). Accordingly, the results revealed a high diversity among Lebanese walnut accessions that should be further investigated to determine the major walnut genotypes and to evaluate the effect of agroclimatic conditions on the morphotypes.