Seed morphology and seed coat anatomy of Fraxinus, Ligustrum, and Syringa (Oleeae: Oleaceae) and its systematic implications

29 June 2018

Ghimire, Balkrishna; Jeong, MiJIn; Suh, Gang ; Heo, Kweon; Lee, Cheul

A morphological and anatomical study of the seeds of 19 taxa representing two subtribes and three genera of tribe Oleeae (Fraxinus, Ligustrum, and Syringa) and two species of Forsythia (Forsythieae) was carried out using scanning electron and light microscopy to evaluate the usefulness of seed characteristics in the determination of systematic relationships. Considerable differences were found in seed coat morphology both among and within genera. Three types of seed coat pattern – reticulate, colliculate, and tuberculate – were recognized. The shape of testa epidermal cells was either elongated rectangular or polygonal with smooth, striated, or infolded secondary ornamentations. The seed coat was thickest in Ligustrum spp. and thinnest in Syringa spp. The exotesta of Fraxinus species was discontinuous due to the presence of longitudinal grooves on the surface. Based on 22 seed morphological characters, cladistic analysis resolved three well-supported clades that matched the three genera well, with Forsythia species considered be an outgroup. Ligustrum foliosum, with reticulate ornamentation, concave periclinal wall, and raised anticlinal walls, was the most distinct species within the genus, whereas the flat periclinal and sunken anticlinal walls in Syringa fauriei were unique to Syringa. Similarly, infolded secondary ornamentation was only observed in Fraxinus rhynchophylla var. densata. Although the investigated specimens represent a limited sample, the results of this study to some extent provide useful support for the characterization of several taxa, particularly at the specific level. Moreover, surface ornamentation, epidermal cell shape, periclinal and anticlinal wall structure, and nature and structure of the exotesta were shown to be potentially useful in cladistic analysis and thus are applicable for identification of species in this alliance.