Taxonomic revision of orchids in the genus Dendrobium SW. section Formosae (Benth. & Hook. F.) Hook. F. (Orchidaceae) in Thailand and adjacent areas

Dendrobium section Formosae is diagnosed by the presence of blackish hairs on leaf blades and leaf sheaths. However, several Dendrobium species were included or excluded from this section regardless of the presence of blackish hairs on plant parts. Therefore, a taxonomic revision of section Formosae in Thailand and adjacent areas was carried out. In total, 39 species were described. In addition, eight taxa in 6 species and 2 varieties, formerly included in section Formosae, were removed from the section. A new species, D. roseiodorum A. Sathapattayanon, T. Yukawa and T. Seelanan is described, and two varieties of D. sanderae, var. luzonicum and var. major, were validated by giving the Latin descriptions and assigning the type specimens. D. hirsutum Griff. was first discovered in Thailand. Moreover, D. ignoniveum J.J. Sm. was also lectotypified. The leaf surfaces of 63 taxa were investigated, representing 42 species of Dendrobium section Formosae, Conostalix, Dendrobium and Distichophyllum. It was found that leaf surface are taxonomic useful for classifying taxa at sectional level. The most important and informative characters are stomatal shape, stomatal type and epidermal hairs, while the cuticular sculpture can be partially used to segregate some species. Molecular phylogenetic analyses of Dendrobium section Formosae sensu lato based on the ITS region of 18S-26S nuclear DNA gene and the maturase (matK) gene of plastid DNA were performed separately and in combined datasets. The results did not support the monophyly of Dendrobium section Formosae. The section Formosae as currently circumscribed fell into 2 separated, well-supported clades and anomalous placements of two taxa, namely D. jerdonianum and D. trigonopus. CladeA represented the taxa with diagnostic character. In contrast, CladeB included taxa with the absence of blackish hairs on any part of plants. Accordingly, section Formosae sensu stricto should be included only taxa in CladeA and more characters, such as stem with far distichous leaves and labellum with more or less callus (excepting for D. kontumense) should be add to the diagnose. The new section established for CladesB is suggested. D. jerdonianm and D. trigonopus should be removed from section Formosae and further investigation of their affinity to the other sections is needed.