Hoya acuminata (Wight) Benth. ex Hook.f.

Nguyen, Van Canh, Averyanov, Leonid V., Nguyen, Quoc Bao, Dang, Van Son, Nguyen, Van Khuong, Maisak, Tatiana V. & Truong, Ba Vuong, 2023, A new species, Hoya honglenae and the first report of H. acuminata and H. phuwuaensis (Apocynaceae, Asclepiadoideae) in the flora of Vietnam, Phytotaxa 632 (1), pp. 60-68 : 61-62

Hoya acuminata (Wight) Benth. ex Hook.f. in Hooker (1883: 53).

H. sect. Pterostelma Hooker (1883: 53) .

( Fig. 1 ).

≡ Pterostelma acuminatum Wight (1834: 39) .

Type: — BANGLADESH. Silhet: Wall Asclep 129, Wallich 8170 Pterostelma acuminata (lectotype K000438717 photo!, designated by Rodda et al., 2019, isolectotype E00179581 photo!) .

Description (based on specimens from Vietnam):—Epiphytic, entirely glabrous undershrub to 0.8 m tall without white latex. Stem terete, glossy green, sub-scandent or pendulous, 2–2.5 mm in diameter; internodes (4.5)5–7(8) cm long. Leaves grassy green, shortly petiolate; petiole (3)3.5–5.5(6.5) mm long, (1.4)1.6–2(2.2) mm in diameter; leaf blade rather thin, leathery, oblong narrowly ovate, acuminate, base cuneate to almost round, (5)6–10(11) cm long, 1.5–2.5 cm wide, margins entire to slightly undulate, with prominent median vein and 3–5 rather obsure lateral veins from each side. Inflorescence terminal or extra-axillary, pendulous, (3)4–5(6)-flowered umbelliform raceme; peduncle slender, glossy green, (1.8)2–2.5(2.8) cm long. Pedicels (2.2)2.5–3.2(3.8) cm long, dull pinkish. Calyx persistent, deeply 5-lobed, lobes dull pinkish to pale greenish, oblong lanceolate, obtuse, 4–5 mm long, 0.8–1 mm wide, glabrous. Corolla with 5 white, acute, narrowly triangular, puberulous, reflexed back lobes, (1.6)2.8–2(2.2) cm long, joined near the base into a short tube 2.2–2.4 mm long, 6–7 mm in diameter, puberulous within. Corona glossy, hyaline, white or very light yellowish pink, 5.5–6 mm in diameter; outer angle of corona lobe flattened from sides, obscurely rectangular, about 2 mm wide and 3 mm tall; inner angle of corona lobes in form of erect, subulate, acute, processus, twice exceeding anther appendages. Anther appendages yellow with a white apical part, erect, narrowly oblong, truncate, about 4 mm long, distinctly shorter than inner corona angles. Pollinarium about 1 mm long; pollinia pale yellow, oblong; caudicles short, hyaline, pale yellow; translators pale yellow; corpusculum purple brown, elliptic. Pistil dull pink, about 2.5 mm long, of two separate, glabrous, wine bottle shaped ovaries. Stigma head white, broadly conoid. Fruit and seeds not seen.

Habitat and phenology: —Submontane evergreen broad-leaved forests at elevations about 1000 m a.s.l., very rare. Flowering in February–March.

Distribution: — Vietnam (Lai Chau Province). Bhutan, Bangladesh (Sylhet), NE India (Meghalaya, Sikkim), W China (Tibet), Myanmar ( Gui et al. 2017, Rodda et al. 2019).

Conservation status: —A very rare species that has been known in Vietnam until now from a single location in the extreme northwest part of the country, near the Laotian border. Taking into consideration the very small area of its occurrence in Vietnam, wide regional deforestation, and commercial collecting, the conservation status of this species may be preliminary estimated as Nationally Critically Endangered (Nationally CR) following to formal criteria proposed by IUCN (2022) as following: A2a–c; B1a,b(i–iii,v)+2a,b(i–iii,v); C1+2(i,ii); D1;—expected population reduction for 10 years or 3 generation ≥ 80% are not reversible (A2), based on direct observation (a), observed decline of the area of occupancy, extent of occurrence and habitat quality (b), and actual levels of exploitation (c);—the area of occurrence <100 km ² (B1) and the area of occupancy <10 km ² (B2), with 1 known population (a) and continuing observed decline of extent of occurrence (bi); the area of occupancy (bii); area, quality of habitat (biii); the number of mature individuals (bv);—the number of mature individuals <250, estimated or projected continuing decline in 25% in 3 years or 1 generation (C1), and observed, estimated, projected or inferred continuing decline when the number of mature individuals in each subpopulation (C2(i)) ≤ 50 and % of mature individuals in one subpopulation = 90–100% (C2(ii));—the number of mature individuals <50 (D).

Notes:—The discovery of this species in Vietnam essentially expands its known area to the east by at least 500 km. Specimens from Vietnam as well as plants known from Myanmar and Tibet differ from the type in distinctly longer peduncles 1.8–2.8 cm long (vs. 1.2 cm), longer pedicels 2.2–3.8 cm long (vs. 2.5 cm), glabrous sepals (vs. sepals ciliate along the margin), and puberulous, much narrower corolla lobes (vs. corolla lobes adaxially glabrous, relatively wide) and may represent a separate allopatric “oriental” variety. A newly discovered plant represents a certain interest in ornamental cultivation and breeding. Commercial collection of plants in nature for sale may be a serious factor in their extinction in the wild in the coming future.

Studied specimen: — Vietnam, Lai Chau Province, submontane evergreen forest at elevation about 1000 m a.s.l., wild collected plant from street market, November 2022, Nguyen Van Canh s.n., photo and herbarium prepared from cultivated plant, 18 February 2023, Nguyen Van Canh, L. Averyanov, AL 2040 (LE 01170321 https://en.herbariumle. ru/?t=occ&id=165964, photos—LE 01123815 https://en.herbariumle.ru/?t=occ&id=165104).

Figures

FIGURE 1. Hoya acuminata (Wight) Hooker. A. Flattened flowering plant, all leaves are shown from abaxial side. B. Intact flowering plant. All photos made from plant used for the preparation of the herbarium specimen AL 2040 by L. Averyanov.