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

We have carried out a morphological and anatomical study of the seeds of 19 taxa representing two subtribes and three genera of the tribe Oleeae (Fraxinus, Ligustrum, and Syringa) and two species of Forsythia (Forsythieae) using scanning electron and light microscopy to evaluate the usefulness of seed characteristics for the identification 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 the 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 its surface. Based on 22 seed morphological characters, cladistic analysis resolved three well-supported clades that matched the three genera well, with Forsythia species considered as an outgroup. Ligustrum foliosum, with reticulate ornamentation, concave periclinal walls, 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 in Syringa. Similarly, infolded secondary ornamentation was only observed in Fraxinus rhynchophylla var. densata. Although the investigated specimens represent a limited sample, the results provide valuable support for the characterization of several taxa, particularly at the species level. Moreover, surface ornamentation, epidermal cell shape, periclinal and anticlinal wall structure, and the nature and structure of the exotesta are shown to be potentially useful for cladistic analysis and for identification of species in this alliance.