Wheat (Triticum sp.) is one of the most important food crops of the world, which is known to provide nearly 44% of the total edible dry matter and 40% of food crop energy consumed by the most developing countries of the world [1]. Of the seventeen different wheat species, only three species, namely allohexaploid Triticum aestivum, known as bread wheat, whereas other two allotetraploid species, Triticum durum (Triticum turgidum ssp. durum) and Triticum dicoccum, known internationally for pasta making and locally in specific food preparations, are cultivated in the world. Of the world wheat cultivated area, Triticum aestivum occupies 90%, whereas of the remaining two species, Triticum durum contributes 9-10% and Triticum dicoccum contributes to a negligible extent [2, 3]. As for the production at a global level, the production of wheat was estimated to be 735 million tons in 2018 to 2019, of which bread wheat accounted for about 94%. In 2017-2018, the production of durum wheat Triticum turgudum ssp. durum was recorded 37.5 million tons with more than a 5% share of the global wheat production [4-6]. Countries in the Mediterranean and Southern Europe, the Balkans, North Africa, and southwest Asia, where initial wheat domestication and cultivation are observed, the durum wheat cultivars are still predominant [7-11]. These indigenous durum wheat cultivars form an important source of genetic materials due to their ease in adapting to adverse biotic and abiotic stresses and grain quality. These could be the products of natural selection through domestication performed by the farmers through ages [4].

The Sultanate of Oman is rich in diversity of the most important crops like wheat, which not only has a global significance but is also regarded as one of the strategic crops in the country [12]. It is grown in diversified environments, as evidenced by the number of indigenous accessions collected during several collecting missions undertaken in Oman involving international organizations like the Food & Agriculture Organization (FAO) of the United Nations (UN) [13-16]. This is because of its position in the Middle East and its ancient trade relations tracing back to 3000 BCE with Iraq in the fertile crescent region that covers Mesopotamia, the home to the earliest known human civilizations and origin of several wheat species, including allohexaploid bread wheat (6x=2n=42) around 4500 BCE [17-19]. Durum wheat (Triticum turgidum sub sp. durum (4x=2n=28) having two genomes, A and B twice in its somatic cells (AABB), was domesticated at the same time [20, 21].

There exists enormous variation among different indigenous accessions of durum wheat in terms of quantitative characters that are directly related to yield and yield-related traits or qualitative traits like pigmentation and other descriptors of wheat, which are very important for crop improvement programs through breeding and selection [22-26]. Among the yield-related traits, number of tillers, number of grains/ spike, and test weight are known to influence directly and positively to the grain yield [25, 27-29]. Several approaches and analyses like those of principal components, clusters, and factors have been suggested by many researchers for estimation of genetic diversity or distances among and between groups of cultivars studied based on not only morphological and growth attributes [29-34] but also molecular markers [4, 35, 36]. Most of these authors have used Hierarchical Cluster Analysis (HCA) to estimate genetic dissimilarity and similarity, and Factor Analysis (FA)/ Principal Component Analysis (PCA) to determine the factors that contribute to the variation of quantitative characters in durum wheat. There have been no studies conducted so far on understanding the genetic diversity among the available indigenous durum landraces of Oman. Hence, in the present study, indigenous durum wheat landrace accessions repatriated from the USDA gene bank were investigated to estimate the potentiality of genetic diversity using both HCA and FA/PCA for the selection of appropriate parents for hybridization programs to improve the productivity of durum wheat.

The Sultanate of Oman has an ancient cultivation history of both bread wheat (Triticum aestivum L.s.l.) and durum wheat (Triticum turgidum ssp. durum) because of its characteristic location on the eastern edge of the Arabian Peninsula [13, 43-45]. Wheat landraces constitute the prime genetic resources of cultivated wheat not only in Oman but also in several MENA (the Middle East and North Africa) [42, 46-52]. Indigenous landraces have paramount significance for their potential utilization in crop improvement and conservation programs. A large number of wheat landraces have been collected from different parts of Oman during collecting missions during the 1980s and 1990s after Oman’s inclusion in worldwide activities for the collection of plant genetic resources under the FAO program [13]. The T. turgidum USDA accessions used in the present investigation were part of the collected genetic material during the 1990s [13]. Although the wealth of the Omani landraces has been emphasized [53], the morphological variation, tolerance to biotic and abiotic stresses, and their quality characters have not been significantly exploited in crops like wheat [52, 54]. Besides, information on the extent and pattern of genetic diversity in these durum wheat accessions is not available.

Fig. (3). Dendrogram of cluster analysis of 17 indigenous durum wheat accessions using Wards’ method.

The results of the present investigations clearly revealed that 17 indigenous accessions differed significantly from one another with respect to all morpho-agronomic and pigmentation characters and showed a considerable level of phenotypic variation in terms of diversity (Tables 2-6; Figs. 1-3). In view of the requirement to use new traits in breeding procedures and statistical methods for integration of disciplinary investigation [49], in the present study, the pigmentation on different parts of the plant as presence (score 1) or absence (score 0) has been considered in the analyses of diversity [55] for their potential use as genetic identification markers in DUS tests of varieties [56-58]. Our results are in agreement with those of previous studies underlining the existence of genetic variability or diversity among durum wheat accessions studied not only in terms of agro-morphological characters like tiller number, spikelets/spike, grains/spike, 1000 grain weight [3, 25, 29, 59-62] but also for days to flowering and maturity [3, 25].

Correlation coefficients for the agro-morphological and qualitative characters were examined for potential correlations to determine whether the selection for stability in one character might influence the stability in other characters of growth of the crop. In this study, only two of the six significant correlations were found positive among the different agro-morphological and quality traits in durum wheat indigenous accessions investigated (Table 4). Accordingly, a positive association among the characters indicated that improvement of one character might simultaneously improve the other desired trait [29, 63]. For example, a positive correlation among the traits related to inflorescence, such as spikelets/spike and grains/spike showed that an improvement in physical properties of the inflorescence also results in an improvement in the number of grains that contribute to yield. Conversely, negative associations between the growth and yield attributes would help in maintaining the magnitude of the characters in balance for achieving optimum yield. For instance, spikelets/ spike or grains/ spike could be balanced to achieve the required grain length as correlations between spikelets/grains and grain length are negative and significant. Similarly, the tiller number should be optimum to get maximum spikelets and grains (Table 4). Similar observations were done by other researchers in their durum wheat germplasm [64].

The structure of the genetic diversity among the set of 17 indigenous durum wheat accessions representing six governorates of Oman extending from north of the country to its south, was assessed by multivariate analyses such as PCA and CA based on aggregate phenotypic means over two cropping seasons. Using a PCA based on the correlation matrix, it appeared that the first eight principal components accounted for over 90% of the total variance of which the first two components contributed 46.705%, which was considered to be substantial to scatter the positions of accessions over the four quadrants of the biplot graph (Fig. 2). Such a magnitude of the contribution of each of the first three PCs rightly agreed with the results of research conducted in 2014 [65], which indicated the contribution of the PC1, PC2, and PC3 as 25.9%, 17.1%, and 13.3%, respectively and that in 2018 [42] which found PC1, PC2 and PC3 contributing 45.13%, 17.85%, and 14.71%, respectively in wheat germplasm studied. Similar results were also obtained by other researchers [66-71]. In PC1, tiller number, grain length, spikelets/spike, grains/spike, and leaf sheath color were effective main sources of variation, whereas, in PC2, accessions were discriminated mainly through days to flowering, days to maturity and internode pigmentation. This is because, in PCA, differentiation of accessions into different clusters was ensured with a relatively high contribution of few characters rather than a small contribution from each character [72, 73].

In the present investigation, the results of two multivariate analyses viz. PCA and CA based on Euclidian distances showed different patterns of clustering, and they did not appear to support each other. PCA analysis formed four clusters viz. cluster I of 4 accessions, cluster II of 5 accessions, cluster III of 6 accessions, and cluster IV of 2 accessions (Fig. 2) while a dendrogram of cluster analysis (Fig. 3) revealed the formation of four clusters viz. cluster A of 2 accessions, cluster B of 9 accessions, cluster C of only one accession and cluster D of 5 accessions based on the principle of discrimination each one adapted. Further, these results revealed that accessions within each cluster might belong to the same or different governorates (Figs. 2 and 3), thus suggesting the fact that there existed no clear relationship between accessions and geographical diversity. For instance, in the composition of clusters from PCA, the accessions of each of all the four clusters were found to be from different governorates. Cluster IV consisted of two accessions, namely PI 532303 and PI 532304, which belonged to Al Batinah South (Rustaq) and Dhofar (Salalah) governorates, which are about 1000 km distant. Similarly, all the four accessions of cluster I were the ones collected from the four different governorates viz. PI 532 302 (Dhofar), PI 532305 (Al-Dhahirah), PI 532306 (Al-Dakhiliya), and PI 532307 (Alsharqiya South). A similar trend of clustering was observed in cluster II and cluster-III. This might be due to the exchange of landraces between the farmers of different geographic regions either voluntarily among relatives or through trading in the markets. Conversely, the dendrogram of CA, tended to include accessions of the same geographical area or governorate in one cluster. For instance, Cluster A consisted of two accessions (PI 532279 and 532281) that belonged to Al-Dakhiliya governorate, and all the five accessions collected from Khasab such as PI 532287, PI 532288, PI 532289, PI 532291, and PI 532292 were included in Cluster-B, although these emerged from three different branches or nodes of the dendrogram. This further showed that the diversity in indigenous durum wheat accessions collected from various geographical parts of Oman might be similar or different in genetic constitution. Therefore, more emphasis on the selection has to be directed to landraces per se rather than considering geographical location, alone, as a source of diversity in this germplasm.

The results of multivariate cluster analyses revealed the existence of substantial variation among the durum wheat landrace accessions in the present study, which could be implied in the selection of appropriate accessions based on characters to be improved, as parents for producing transgressive segregants in a minimum period of time in the crop improvement program [42, 74, 75]. Accordingly, the accession PI 532306 of Al Dakhiliya can be chosen from the cluster-I as one of the parents for manipulating days to flowering with leaf sheath as identification marker; Similarly, PI 532291 and/ or PI 532 292 of Musandam can be chosen from cluster-II for improving grain weight with leaf color as marker-trait, PI 532239 of North Al- Batinah and/ or PI 532281 of Al Dakhiliya from cluster-III for improving spikelets/spike or grains/spike with coleoptile color as marker-trait, and PI 532303 or PI 532304 of Al-Dhofar governorate from cluster-IV for improving tiller number and grain length along with internode color as marker-trait. The durum wheat accessions of the present study are part of landraces of Oman, which are well known for their desirable features of bread-making quality for the local culinary items besides their resilience nature to survive abiotic stresses like drought, heat, salinity [34, 45, 47, 76]. It is interesting to note that as many as six accessions, PI 532239, PI 532242, PI 532288, PI 532289, PI 532292, and PI 532302, have been included in the core collections of USDA gene bank based on their potential characters of growth and tolerance to several diseases of wheat like stripe rusts [62]. Hence, it is essential to conserve and use these landraces for improving the production of quality wheat used in local preparations in Oman.

Seventeen indigenous durum wheat landraces/ accessions were significantly different (p<0.05) with respect to nine quantitative characters except in respect of a number of tillers and had a higher degree of variation in respect of most of these quantitative characters. Only six correlations involving four characters viz. tiller no., spikelets/ spike, grains/spike, and grain length were significant (p<0.05). Two multivariate analyses were found not supporting each other in discerning diversity, with each forming four diverse clusters with different compositions of accessions. However, their overall results led to the selection of parents for hybridization to improve the characters of growth for higher yield or productivity with one or two identifying markers of pigmentation on plant parts.