Phenotypic classification of mulberry (Morus) species in Taiwan using numerical taxonomic analysis through the characterization of vegetative traits and chilling requirements

Mulberry (Morus spp.) has been introduced and cultivated in Taiwan for more than 300 years, and therefore a large and diverse population has been formed. The complex and diverse morphology of the genus has resulted in long-standing problems related to nomenclature, classification and identification of species. The purpose of the present study was to establish and identify an unbiased phenotypic classification system for members of the Morus genus, which show various expressions patterns of sexual traits. A total of seventy-seven plants of 27 mulberry accessions from 7 Morus spp., and several hybrids as well as unidentified populations, were studied using a numerical taxonomic analysis based on principal component analysis (PCA) and cluster analysis. This analysis involved the characterization of 20 vegetative traits and chilling requirement. Moreover, an additional investigation of style length for 11 female and 1 bisexual identified accessions was also conducted to determine whether a classification system similar to the conventional classification system using flower traits can be obtained from the present study. A classification system was defined and suggested three clusters: (1) Morus laevigata; (2) Morus atropurpurea, Morus bombycis, Morus australis and Morus formosensis; (3) Morus alba and Morus latifolia. The hybrids and three unidentified accessions were related to cluster with the similarity species based on the phenotypic data. Interestingly, M. atropurpurea, often regarded as a member of M. alba, showed its similarity to M. bombycis, M. australis and M. formosensis in this study. According to the character loading determined by the PCA, nine of the investigated traits were representative, including 4 quantitative traits (leaf width, petiole width, ratio of leaf length to width, chilling requirements) and 5 qualitative traits (bud shapes during dormancy and growing period, color of the mature leaf, shape of the leaf margin and the leaf apex). The cluster based on the style length in the study met the grouping of the 20 vegetative traits plus chilling requirements and also the 9 representative characters. The representative traits can also yield a classification similar to that resulting from the conventional classification system using reproductive traits. It was indicated that a new, simplified and rectified key for identifying different sexual forms of mulberry in Taiwan was built to facilitate species classification and to support further breeding programs.