The genus Dendrobium, one of the largest genera in Orchidaceae, exhibits enormous vegetative diversification. Such a situation has hindered the establishment of consistent classification systems. To clarify phylogenetic relationships in Dendrobium section Dendrobium and allied groups, we performed molecular phylogenetic analyses of 210 taxa. The results showed that many sections are not monophyletic. Most of the morphological characters that have been believed to reflect phylogenetic relationships are, in fact, the results of convergence. Consequently, recircumscription of the infrageneric classification is required.

Genus Dendrobium (Orchidaceae) contains numerous species. Phylogenetic analyses based on morphological characteristics and DNA sequences indicated that this genus is divided into two major groups: Asian and Australasian clades. On the other hand, little is known about the phytochemical differences and similarities among the species in each clade. In this study, we selected 18 Dendrobium species (11 from the Asian clade and 7 from the Australasian clade) and constructed HPLC profiles, arrays composed of relative intensity of the chromatographic peaks. Next, orthogonal partial least square discriminant analysis (OPLS-DA) was applied to the profile matrix to classify Dendrobium species into the Asian and Australasian clades in order to identify the peaks that significantly contribute to the class separation. In the end, two phenanthrenes, 4,9-dimethoxyphenanthrene-2,5-diol 1 and 1,5-dimethoxyphenanthrene-2,7-diol 2, which contributed to the class separation, were isolated from the HPLC peaks. The existence of 2 was limited to the genetically related Australasian species.