New species, Hoya sungwookii (Apocynaceae, Asclepiadoideae ), from southern Vietnam

A new species, Hoya sungwookii, discovered in southern Vietnam near Nha Trang Town (Khanh Hoa Province) is described and illustrated. It belongs to the type section of the genus and is distinguished from the morphologically closest H. ablongacutifolia (= H. graveolens) in glabrous pedicels, smaller flower 1.1–1.4 cm across, greenish ciliate calyx, corolla abaxially light green, small green to dark green corona 4.5–5 mm in diameter, 1.6–1.8 mm tall, small, 2–2.2 mm long, corona segments, and short corona segment inner angle, not exceeding anthers. Additionally, data on ecology, phenology, distribution of newly described species are presented, as well as its evaluated conservation status and color illustrations. Taking into consideration our new data, the genus in Vietnam currently includes 45 species.


Summary.
A new species, Hoya sungwookii, discovered in southern Vietnam near Nha Trang Town (Khanh Hoa Province) is described and illustrated.It belongs to the type section of the genus and is distinguished from the morphologically closest H. ablongacutifolia (= H. graveolens) in glabrous pedicels, smaller flower 1.1-1.4cm across, greenish ciliate calyx, corolla abaxially light green, small green to dark green corona 4.5-5 mm in diameter, 1.6-1.8mm tall, small, 2-2.2 mm long, corona segments, and short corona segment inner angle, not exceeding anthers.Additionally, data on ecology, phenology, distribution of newly described species are presented, as well as its evaluated conservation status and color illustrations.Taking into consideration our new data, the genus in Vietnam currently includes 45 species.

Introduction
Hoya R. Br. (Brown, 1810) is one of the largest genera in the family Apocynaceae (subfamily Asclepiadoideae, tribe Marsdenieae), which includes 400-450 currently accepted species found mostly in tropics of Asia, Pacific islands and NE Australia (Li et al., 1995;Forster, Liddle, 1996;Forster, 2006;Liddle, 2009;Rodda, 2015;Nguyen et al., 2023).Species of this genus are mostly small herbaceous epiphytic vines or subshrubs with main centers of their diversity in Indochina, Borneo, Philippines and New Guinea (Wanntorp et al., 2014).Many species are widely cultivated as significant ornamental plants known commonly by the name wax plants.The Indochinese Peninsula represents the largest center of genus speciation and diversity in mainland Asia, with about 60 species reported, mainly from Thailand and Vietnam (Pham, 2003;Tran, 2005;Averyanov et al., 2017;Rodda, Rahayu, 2018).The last preliminary assessment of the genus in Cambodia, Laos, and Vietnam documented 40 Hoya species in the flora of Vietnam (Averyanov et al., 2017).However, the true genus diversity in Vietnam still remains insufficiently studied.Further investigation of the genus in Vietnam led to the discovery of additional species, Hoya lamthanhiae Pham et Kloppenberg (Pham et al., 2020), as well as three novelties, H. acuminata (Wight) Benth.ex Hook.f., H. honglenae Aver., Vuong, Bao et V. C. Nguyen, and H. phuwuaensis Kidyoo (Nguyen, 2023).During a special field investigation of Hoya diversity in southern Vietnam, we collected an unidentified species near Nha Trang Town (Khanh Hoa Province) and introduced it into cultivation.After studies of relevant literature and available Hoya specimens in P, K, SING, VNM, BM, L, LE, and PKF herbaria, we assume that it is a new species that is here described as H. sungwookii.In addition, data on ecology, phenology, distribution of newly described species are presented, as well as its color illustrations and evaluated conservation status.Taking into consideration our new data, the genus in Vietnam currently includes 45 species.

Materials and Methods
The measurements and species description are based on living plants and herbarium specimens.Type herbarium and additional material are stored at the Herbarium of the Institute of Tropical Biology, Vietnam Academy of Science and Technology (VNM).All photographs were taken with a Canon 700D body and an EF-S 60mm f/2.8 Macro USM lens.The terminology for the morphological description follows Harris J., Harris M. (2006), Randal (2008) , Hickey, King (2013), Beentje (2016),  and Simpson (2019).Scanned images and associated supplementary analytical photos are kept in the database "Herbarium LE" (https://en.herbariumle.ru).In the citation of the taxa distribution in Vietnam, including the provinces, we follow the modern official administrative division of the country (Vietnam Administrative Atlas, 2015).The online version of the IUCN Red List of Threatened Species (IUCN 2024) was used for the assessment of the species conservation status.Institutes where studied specimens are kept are indicated by their internationally accepted herbarium acronyms (Thiers, 2024).
H. sect.Hoya 80-100 species spread across the whole general area.In Vietnam about 16 species.
In terms of the number of species, this section is probably the largest in the genus.Species of this group share such diagnostic features as an epiphytic vine living form, many-flowered inflorescence, relatively large flowers, small calyx dissected from the base into 5 lobes, mostly flat, adaxially hirsute corolla dissected on more than half of its length into 5 spreading lobes, and rotate corona with horizontally spreading segments having ovoid outer angle and subulate inner processus.Although the morphological borders of the section remain unclear (Kloppenburg, 1990;Randal, 2008), it forms a monophyletic clade based on molecular data (Wanntorp et al., 2014).The evolution of this group was connected with speciation in Sundaland and the Philippines, from ancestral group inhabited mainland Southeast Asia (Wanntorp et al., 2014) (Costantin, 1912;Rodda et al., 2013).Diagnosis.The new species is morphologically closest to Hoya oblongacutifolia, from which it differs in glabrous pedicels, smaller flower 1.1-1.4cm across, greenish ciliate calyx, corolla abaxially light green, small green to dark green corona 4.5-5 mm in diameter, 1.6-1.8mm tall, small corona segments, 2-2.2 mm long, and short corona segment inner angle, not exceeding anthers.
Etymology.The species is named after Mr. Um Sung Wook, who first found this plant in nature and kindly shared his discovery for our study.

Fig. 1 .
Fig. 1.Hoya sungwookii: A -plant in natural habitat in locus classicus; B -fresh flattened flowering plant; C -flattened shoot of plant in cultivation; D -intact inflorescences; E -spotted and uniform green leaves, adaxial and abaxial side; F -separated inflorescence; G -separated flowers, side view; H -pedicels with calyx and ovary; I -calyx and ovary; J -flower, frontal view; K -corolla, view from below; L-N -corona, views from above, from side, and from below; O -sagittal section of corona and ovary.Photos by Van Canh Nguyen and Ba Vuong Truong from plant used for the preparation of the holotype specimen VNM 00070356, photo correction and design by L. Averyanov and T. Maisak.

Fig. 2 .
Fig. 2. Hoya sungwookii; A -flower (black asterisks on figures A, E, and F indicate areas of corolla lobes magnified on figures B-D); B-D -different kinds of indumentum on adaxial surface of corolla; E -magnified corolla lobe and corona; F -flattened and magnified basal portion of the corolla; G -apex of pedicel and calyx, side view; Hcorona, view from below; I-K -sagittal section of corona, gynostegium and apex of pedicel; L -pollinaria, view from different sides; M -pollinia with corpusculum, caudicles, and translators.All photos by Ba Vuong Truong from alcohol preserved material used for the preparation of the holotype specimen VNM 00070356, photo correction and design by L. Averyanov and T. Maisak.

Table
The comparable of morphological characters of Hoya sungwookii and the morphologically most similar species, H. oblongacutifolia.Secondary, rather dry, short, sclerophyllous, wind-formed coastal forest and scrub on steep, sea-faced rocky slopes on granite at an elevation of about 50 m a. s. l.Very rare.Flowers in September -December.