Leaf anatomy of five epiphytic Bulbophyllum Thouars (Orchidaceae) species from the Cerrado of southeastern Brazil: adaptive strategies and taxonomic value

The epiphytic habit is predominant in Bulbophyllum. Anatomical studies can elucidate the adaptive strategies of epiphytic species under extreme environmental conditions such as high temperatures and water scarcity. We aimed to: (i) characterize the leaf anatomy of five species of Bulbophyllum: Bulbophyllum chloroglossum Rchb.f. & Warm., B. epiphytum Barb.Rodr., B. exaltatum Lindl., B. gladiatum Lindl., and B. plumosum Barb.Rodr., to identify the adaptive responses to the epiphytic habit; (ii) investigate the presence of anatomical characters useful for the taxonomy of the group. Individuals of the studied species were collected at different heights on the phorophyte whenever possible. The leaves were processed for structural characterization by light microscopy, micromorphometry, and calculation of stomatal index. All species have unistratified epidermis with thick external periclinal walls and hypostomatic leaves with tetracytic stomata. The mesophyll contains hydrenchyma with sclereids and calcium oxalate crystals. The vascular bundles are collateral and surrounded by fibers. The individual of B. chloroglossum collected at the highest height on the phorophyte has a thicker leaf blade compared to the individual collected at a lower height. However, the individuals of B. exaltatum and B. gladiatum had a thicker leaf blade, even at a lower height. Significant differences in the stomatal index occur between B. chloroglossum and B. plumosum. All species of Bulbophyllum possess anatomical adaptive strategies, such as the thick outer periclinal wall of epidermal cells, hydrenchyma, hypostomatic leaf, and crystals along the leaf blade. We identified distinctive anatomical characters among the species, allowing the elaboration of an identification key.