Heterostemma cucphuongense (Apocynaceae, Asclepiadoideae), a new species from Vietnam

Abstract Heterostemma cucphuongense (Apocynaceae, Asclepiadoideae), a new species from Vietnam is described, illustrated and compared with the similar species Heterostemma succosum Kerr. Heterostemma cucphuongense differs from H. succosum by the morphology of the rachis of the inflorescence, the margins of the corolla lobes and the colour of the adaxial surface of the corolla.


Introduction
The genus Heterostemma Wight and Arn. comprises approximately 30 to 40 species and is widely distributed from India and China to Australia and the Western Pacific Islands (Li et al. 1995;Swarupanandan et al. 1989;Forster 1992). The number of species has increased in recent years and three new species were published in 2019 alone. These are: Heterostemma barikiana P.Agnihotri et al. from India, Myanmar and Thailand, (Agnihotri et al. 2019), H. ficoides A.Kidyoo (Kidyoo 2019) from Thailand and Heterostemma trilobatum A.Kidyoo & Thaithong also from Thailand (Kidyoo and Thaithong 2019). However, this third species is indistinguishable from H. barikiana. Furthermore, the specimen C. Maknoi & P. Srisanga 2258 (QBG) Costantin 1912;Ho 1993;Li et al. 1995;Tran 2005;Rodda 2016), as well as a recent revision (Tran 2017 [in Vietnamese]), where seven species have been recorded for the country.
Etymology. The species is named after the type locality, Cuc Phuong National Park, in Ninh Binh Province, northern Vietnam.
Distribution and ecology. Heterostemma cucphuongense was only collected once, near one of the main trails in the Cuc Phuong National Park. It was found in primary evergreen forest on soils derived from degraded limestone. It was collected in flower in June.
Conservation status. Heterostemma cucphuongense is endemic to the Cuc Phuong National Park. Since it is known from a single collection, its conservation status is Data Deficient (DD; IUCN 2012).
Notes. Heterostemma cucphuongense is similar to H. succosum Kerr (1939), a species found in Thailand and Laos. Both have shortly pedunculate inflorescences, in which the flowers generally open one at a time and are relatively large and rotate (mostly > 14 mm diam.). The two species can be easily separated because, in H. cucphuongense, the inflorescences form a rachis to 7 mm long with age while the inflorescences of H. succosum do not develop any rachis. Furthermore, the pedicels of H. cucphuongense are 5-10 mm long, while H. succosum has pedicels 15-30 mm long. Further distinguishing characters (that, however, are less obvious in dried material) are the margins of the corolla lobes that are recurved in H. cucphuongense (vs. flat in H. succosum). The two species also differ in that the colour of the adaxial surface of the corolla is red with white-yellow spots in H. cucphuongense (vs. yellow-orange with reddish-brown spots in H. succosum). These and additional diagnostic characters separating the two species are listed in Table 1.