Taxonomic revision of Dendrobium Sw. Section Stachyobium Lindl. (ORCHIDACEAE) in Thailand

Abstract

Taxonomic revision of Dendrobium Sw. section Stachyobium Lindl. in Thailand has been conducted from 2015–2018 based on natural habitats observation in Thailand as well as various herbaria visits, i.e. BCU, BK, BKF, BM, C, CMUB, E, K, KKU and QBG. Key to species, taxonomic descriptions, ecological data, illustrations and geographical distribution of all taxa are provided. Twenty four taxa with including 21 species and 3 varieties were recognized. Among these, two species namely Dendrobium chiangdaoense and D. obchantiae are new to Thailand and two species viz. Dendrobium sp.2 and Dendrobium sp.3 are being proposed as new species. Besides, Dendrobium sp.1 is also being proposed as new variety. Plant status was investigated, 7 species, i.e. D. confinale, D. dixonianum, D. elliottianum, D. garrettii, D. gregulus, D. mucronatum and D. proteranthum are endemic to Thailand. In addition, 6 species viz. D. cuspidatum, D. denudans, D. kanburiense, D. peguanum, D. perpaulum and D. strongylanthum are also rare species. One taxon namely D. delacourii is reduced from species to the rank of variety under the name D. venustum var. delacourii Promm., Kidyoo & Suddee, stat. nov. Furthermore, five names are recognized as synonyms. D. eserre is reduced to synonym of D. dixonianum, D. kratense and D. erostelle are reduced to be synonyms of D. incurvum. The other two names, D. nanocompactum and D. wilmsianum are reduced to be synonyms of D. compactum. Two species namely D. eriaeflorum and D. monticola were record from Thailand. In fact, they were misidentified and are not found in Thailand.

The leaf anatomical characters were examined by using epidermal peeling, paraffin embedding and scanning electron microscope (SEM). The results indicated that 5 significant anatomical characteristics for species grouping and identification of some species are cuticular ornamentations, type of stomata, distribution of stomata, type of trichomes and type of inclusions.

The palynological study by unacetolyzed pollens were investigated under light microscopy (LM) and scanning electron microscopy (SEM). The pollinia shape of the Stachyobium can separate into 2 types, i.e. obliquely ellipsoid and globose. Pollen of most species are tetrad, inaperture and the exine sculpturing are psilate. Furthermore, the degree of surface sculpturing (pitting) can be categorized into 2 types, i.e. 1. flattened and shallowly grooved and 2. convex and deeply grooved. These characters are useful for species grouping.