Leaf morpho-anatomy and taxonomic significance in five Dendrobium sect. Stachyobium species from China

Dendrobium Sw. is one of the three largest genera in Orchidaceae, which has been disputed due to controversial taxonomy and difficult species identification. Leaf morpho-anatomy of five sect. Stachyobium species from China were investigated using microscope and paraffin section to provide anatomic evidence for taxonomic significance. (1) Leaf size and apex of blade were varied among species and could be used for species delimitation. (2) Two shapes of ordinary epidermal cell were recognized, consisting of narrow-polygonal cell and broad-polygonal cell. The adaxial ordinary epidermal cells were conspicuously larger than the abaxial epidermal cell. The anticlinal wall of ordinary epidermal cells was straight-arched, with obvious thickenings. Peltate glandular trichomes occurred on both sides of leaf blade, with higher density on the abaxial surface. (3) Tetracytic stomata was observed only in D. compactum, while co-occurrence of tetracytic and anomocytic stomata was found in other four species. (4) Two types of outlines of leaf midrib were recognized, including flat type and carinate type. (5) The bicollateral vascular bundle was only observed in D. strongylanthum, but the amphivasal vascular bundle was commonly observed in the rest four species. (6) Two types of peltate trichome were firstly described anatomically, including a uni-celled stalk type and a bi-celled stalk type. Two types of crystals were present in idioblasts, including the spherical silica body in D. sinominutiflorum, and prismatic crystal in the rest four species. The monophyletic sect. Stachyobium was strongly supported by a comparative morpho-anatomy of leaf. Some morpho-anatomic features were selected and used for species discrimination and taxonomy of Dendrobium, including leaf size, leaf shape, apex of blade, the ordinary epidermal cell shape, stomatal density, leaf midrib outline, type of vascular bundle, anatomic type of peltate trichome, and crystals. Additionally, a key to five sect. Stachyobium species from China was proposed based on leaf morpho-anatomical features.