Hoya phuwuaensis Kidyoo (2016: 218)

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 : 65-67

Hoya phuwuaensis Kidyoo (2016: 218) .

H. sect. Peltostemma Schlechter (1916: 14)

( Fig. 3 ).

Type:— THAILAND. Phu Wua Wildlife Sanctuary , Bueng Kan Province, 200 m, 2008, M. Kidyoo 1035 (holotype BKF, isotype BCU, not seen, fide Kidyoo 2016) .

Description (based on specimens from Vietnam):—Slender creeping epiphytic herbaceous vine to 2 m long with pure white latex in all vegetative parts. Stem terete, 2–2.5 mm in diameter, pale gray brown, scabrous; internodes (2.5)3–6(7) cm long. Leaves shortly petiolate; petiole gray brown, scabrous, (3.5)4–7(8) mm long, (1.8)2–2.2(2.5) mm in diameter; leaf blade thick, coriaceous, elliptic, acute, base cuneate, scabrous, (5.5)6–9(11) cm long, (1.8)2– 3(3.2) cm wide, margins entire to slightly undulate, wrinkled, or revolute; adaxial surface dark grassy dark green with large, irregular white blotches; abaxial surface uniformly light pale green; median and lateral veins almost invisible. Inflorescence extra-axillary, pendulous, 20–28-flowered umbelliform raceme; peduncle slender, dirty purple brown, (4.5)5–5.5(6) cm long, scabrous; rachis perennial, fleshy. Pedicels of the peripheral flowers 1.4–1.5 cm long, light pinkish to almost white. Calyx persistent, deeply 5-lobed, lobes white to pinkish, narrowly ovate, obtuse, 1.4–1.6 mm long, scabrous outside. Corolla discoid, with 5 broadly triangular lobes, white, 0.8–1.2 cm in diameter; adaxial surface densely pubescent with long straight hairs along the margins of the lobes; abaxial surface glabrous; lobes reflexed, distally revolute, 3.2–4 mm long, apex acute. Corona purple red, 4.8–5 mm in diameter; outer angle of corona lobe narrowly ovoid, 2–2.2 mm long, almost straight or distally slightly up-cured, with round apex, upper surface convex; inner angle of corona lobes erect, narrowly conoid, acute, 1.3–1.4 mm tall, much higher than outer angles. Anther appendages yellow with a white apical part, erect, narrowly triangular, acute, 0.8–1 mm long, exceeding the apex of inner corona angles. Pollinarium about 0.3 mm long; pollinia pale yellow, obliquely oblong, 0.3–0.31 mm long, narrowly winged, truncate, margins pellucid; caudicles short, hyaline, with broad wing, light pale yellow; translators, pale yellow, obscurely triangular; corpusculum brown, obtriangular, laterally compressed. Pistil dull pink, about 1 mm long, of two separate, glabrous, wine bottle shaped ovaries. Stigma head white, broadly conoid, 0.7–0.8 mm tall and wide. Fruits not seen.

Habitat and phenology:—Epiphyte on tall trees in broad-leaved evergreen mountain forests, commonly along stream valleys, often on Ficus sp. Flowers in May.

Distribution:— Vietnam, Dak Lak Province (Lak District, Chu Yang Sin Mountains). NE Thailand (Bueng Kan Province, Phu Wua Wildlife Sanctuary).

Conservation status: —In Vietnam, only one small population of this rare species has been discovered until now. It was found in an area affected by occasional deforestation, seriously impacting habitat quality. The species may also be threatened in the future by commercial collecting. Additionally, a very small area of the occurrence and occupation identifies conservation species status as Nationally Critically Endangered (nationally CR), following the formal criteria proposed by UIUCN (2022) as follows:A3c,d; B1a,b(i–iii,v)+2a,b(i–iii,v); C1+2(i,ii); D1;—population reduction suspected to be met in the future (A3), a decline in area of occupancy (AOO), extent of occurrence (EOO), and habitat quality (c), and actual or potential levels of exploitation (d);—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). The discovered population of this species in Vietnam is distant from its alone early-known location in NE Thailand, more than 800 km to the SE. According to available information, the species may already be extinct from nature in Thailand (Kydoo 2016), and the Vietnamese population represents currently the only natural refugium of this remarkable taxon in nature.

Note:—The species is easy to cultivate and remarkable for its beautiful flowers and white-green tessellated leaves. The plant has good prospects for cultivation as an ornamental plant.

Studied specimens:— Vietnam, Dak Lak Province, Lak District, Chu Yan Sin Mountains, epiphyte on Ficus sp. , 16 September 2021, Truong Ba Vuong & Nguyen Van Canh, BV 1667 (VNM 00043091, photos—LE 01124052 https://en.herbariumle.ru/?t=occ&id=165926). Vietnam, Dak Lak Province, Lak District, herbarium prepared from cultivated plants by Nguyen Van Canh s.n. in May 2022, det.: May 2022, Nguyen Van Canh, AL 1397 (LE 01169047 https://en.herbariumle.ru/?t=occ&id=140563).

Figures

FIGURE 3. Hoya phuwuaensis Kidyoo. A, B. Flattened portion of plant with flowers (A) and leaves (B). C. Latex on the stem cut. D. Inflorescence, view from above. E. Inflorescence, frontal view. F. Flowers, side view. G. Pedicel and flowers, frontal view and view from back. H. Corolla, adaxial side. I. Corona, view from above and from below. J. Corona, side view. K. Flower, sagittal section. L. Pollinarium, view from different sides. M. Calyx, with exposed pistil.All photos made from plant used for the preparation of the herbarium specimen BV 1667 by Nguyen Van Canh, photo correction and design by L. Averyanov and T. Maisak.