A new species of Dischidia (Apocynaceae, Asclepiadoideae) from North-eastern Thailand

Abstract Dischidia phuphanensis Chatan & Promprom, a new species from north-eastern Thailand, is described and illustrated. The new species is similar to D. tonkinensis Costantin, but is distinguished by the shape of its leaves, the apices of the staminal corona lobes, the colour of the corolla and the absence of a corolline corona. The distinguishing characters of similar species are discussed. A key for the identification of those species in Thailand without pitcher-like leaves is provided.


Introduction
Dischidia Brown (1810: 461) belongs to the tribe Marsdenieae of the Apocynaceae and comprises approximately eighty species, distributed in India, Indochina, Malesia, Melanesia and the eastern Pacific (Rintz 1980;Livshultz et al. 2005). Some members of Dischidia are epiphytes living in association with ants (Livshultz et al. 2005;Kidyoo and Suddee 2017). Ants may inhabit the pitcher-shaped leaves of certain species or they may live in the shelter of those with circular leaves which are convex above and concave below. Dischidia exhibits umbel-like inflorescences with small, more or less urceolate flowers that usually have a ring of hairs inside the mouth of the corolla tube and an anchor-shaped staminal corona (Rintz 1980, Forster 1996, Hoffmann et al. 2002. In Thailand, nineteen species of Dischidia were reported by Thaithong et al. (2018). During a floristic survey of the forests in north-eastern Thailand conducted between 2009 and 2018, specimens of Dischidia were collected in Sakon Nakhon Province. After the living plants and herbarium specimens were carefully investigated, the authors concluded that these could not be referred to any previously named species. Consequently, a new species, D. phuphanensis, is described here.

Material and methods
Specimens were collected from Phu Pha Yol National Park, Sakon Nakhon Province, Thailand in 2017. Morphological observations of the new species were carried out on living plants from the field, as well as on herbarium specimens in BK and BKF. Measurements were made with a Vernier caliper or with an ocular micrometer in a dissecting microscope. We consulted the relevant taxonomic literature (e.g. Kerr (1951), Kidyoo and Suddee (2017), Thaithong et al. (2018) etc). We assessed the preliminary conservation status of the new species using our field knowledge and by applying the criteria given by IUCN (2017). Diagnosis. Dischidia phuphanensis is most similar to D. tonkinensis, but the new species differs from the latter in its elliptic or narrowly elliptic or slightly oblanceolate leaves (leaves in D. tonkinensis are ovate to ovate elliptic, rarely obovate), apex of corona lobes obtuse (with tips pointing downward in D. tonkinensis), yellow base of the corolla tube and light yellow or white apices of the lobes (white or orange-yellow corolla tube and lobes in D. tonkinensis) and the absence of a corolline corona (corolline corona present in D. tonkinensis). (Figures 1, 2) Type. Thailand: Sakon Nakhon Province, Phu Pha Yol National Park, 300-400 m, 16°56.126667'N, 104°2.336667'E, 7 August 2017, W. Chatan 2489 (holotype: BKF!; isotype: BK!).
Ecology. This new species grows in both slightly open and in shaded areas in mixed deciduous forest at an elevation of 300-400 m.
Vernacular name. Thao Rag Noi. Etymology. The specific epithet of Dischidia phuphanensis refers to its type locality, the Phuphan mountain range.
Preliminary conservation status. One population of Dischidia phuphanensis was found at the type locality in Phu Pha Yol National Park, Sakon Nakhon Province, north-east Thailand. It is estimated to number fewer than 250 mature individuals. Therefore, it should be considered as "Endangered (EN)" according to the IUCN criteria D (IUCN 2017).
Discussion. Dischidia phuphanensis is similar to D. tonkinensis, from China, Indochina and Thailand (Thaithong et al. 2018). Similarities include their stems (thick, succulent, 2-3 mm in diameter, glabrous), glabrous petiole and lamina, succulent and coriaceous leaves and their glabrous corolla lobes. However, the new species differs from D. tonkinensis in its elliptic or narrowly elliptic or slightly oblanceolate leaves, the obtuse apex of the staminal corona lobes, the yellow base of the corolla tube, the light yellow or white apices of the lobes and absence of a corolline corona. Dischidia tonkinensis has ovate to ovate-elliptic, or rarely obovate lamina, the apices of the staminal corona lobes are retuse; it has a white or orange-yellow corolla tube and lobes and possesses a corolline corona. The new species is similar to D. acuminata Costantin, from Vietnam, in that they share the 1-5 branches to the inflorescence, the short peduncle 0-3 mm long and the absence of a corolline corona. It differs from D. acuminata by the triangular to deltate corolla lobes and with acute apices (narrowly shape, thick and abaxial side nose-like in D. acuminata (Constantin 1912)) The genus Dischidia may be divided into two main groups based on the leaf types, i.e. those with pitcher-like leaves and species with non-pitcher-like leaves. Dischidia phuphanensis has non-pitcher-like leaves. The most recent revision of Dischidia in Thailand was by Thaithong et al. (2018) and nineteen species were recognised. This was made up of one species with pitcher-like leaves and 18 species with nonpitcher-like leaves. After this new species is added to this group, the number of species with non-pitcher-like leaves is 19. A key to the species with non-pitcher-like leaves in Thailand is provided below and is modified from Thaithong et al. (2018). Details of the morphological differences between D. phuphanensis and D. tonkinensis are presented in Table1.