Hoya thuathienhuensis T.B.Tran, Rodda & Simonsson, 2013

Rodda, M., Trân, Thê Bách, Juhonewe, N. Simonsson & Sâm, Lý Ng ọc, 2013, Hoya thuathienhuensis and Hoya graveolens (Apocynaceae, Asclepiadoideae), a new species and a new record for the Flora of Vietnam, Blumea 57 (3), pp. 243-247 : 243-246

Hoya thuathienhuensis T.B.Tran, Rodda & Simonsson , sp. nov. — Fig. 1

Species insignis, a specibus nobis notae bene distincta. Ad Hoyam lockii similis ob corollam reflexam et folia lanceolata coriacea; differt ob coronae lobos rotundatos non compressos. — Typus: M. Rodda LT10-056 (holo HN; iso K, SING),cultivated in Turin , Italy, 9 March 2011,from specimen originally collected in Vietnam, Th ừa Thiên-Huế, A Roang, 800 m asl, strong twiner in open secondary forest c. 20 m high .

Etymology. Hoya thuathienhuensis is named after the only Vietnamese Province where it has been so far collected, Thừa Thiên-Huế Province in central Vietnam.

Vigorous terrestrial climber with white latex. Leafy stems cylindrical, green, up to 5 mm diam, hirsute; older stems leafless, lignified, c. 10 m long, bark peeling, dark brown or grey, glabrous. Internodes (4.5 –) 15 –25 cm, adventitious roots present only if stems in contact with substrate. Leaves with petiole adaxially channelled, green, 9 –13 by 1.5 – 2.3 mm, sparsely pilose; lamina coriaceous, oval to elliptic, adaxially dark green with sparse whitish spots, lighter abaxially, 3– 6 by 10–15 cm, glabrous, apex cuspidate, base shortly attenuate, penninerved, main vein depressed on adaxial surface, evident on abaxial surface, secondary veins 6–10 pairs evident when dry, held at 60– 80°, anastomosing near leaf margins. Inflorescence positively geotropic, pseudo-umbelliform, convex, 5– 8 cm diam, up to 40-flowered; peduncle extra-axillary, dark green, perennial, presenting scars of previous flowerings, 8 –15 cm long, c. 2 mm diam, hirsute when young; pedicels terete, reddish green, 2 – 3 cm long, c. 0.8 mm diam, glabrous. Calyx reddish green, 3 – 4 mm diam, lobes broadly ovate c. 1.5 by 1 mm with ciliate margins, apex acute, with 1–2 basal colleters at the lobe sinus. Buds globose, slightly 5-angled, ivory white. Corolla reflexed, white, 14–16 mm diam ( 20– 25 mm when flattened), lobes ovate-lanceolate, 9–11 mm long, free lobe c. 8 by 4 – 4.5 mm, margins reflexed, apex apiculate, corolla lobes overall glabrous but densely hirsute underneath the corona only. Corona stami-

© 2013 Nationaal Herbarium Nederland nal, dull yellow, 8–10 mm diam, 4.5–5.5 mm high, filament tube pubescent; lobes from above ovate-round, laterally oblong, c. 4 by 3.5 – 4 mm; inner process acute, converging, outer process apex round, inner process exceeding anthers by 0.5 –1 mm. Pollinaria (all measurements approximate) 750 by 500 μm with two oblong pollinia, 550 by 200 µm, apex and base obliquely truncate, retinaculum 90 by 120 μm, caudicles broad, basally connected to the retinaculum c. 200 μm long. Ovary bicarpellate, each carpel bottle-shaped, light green with pink tips, c. 2 by 0.7 mm, glabrous, apex acute. Fruits and seeds not seen.

Distribution — Thừa Thiên-Huế province in Vietnam.

Habitat & Ecology — Hoya thuathienhuensis has been observed in secondary evergreen lower montane forest up to 20 m high, in proximity to primary forests. It is a strong climbing liana, rooted in the humus-rich forest floor.

Conservation status — The only locality where H. thuathienhuensis has been recorded is within the proposed conservation area of the Hue Green Corridor ( Averyanov et al. 2006) thus supporting the long term preservation of the species in situ. Nonetheless, H. thuathienhuensis is to be considered Data Deficient (DD) according to IUCN Red List criteria (IUCN Standards and Petitions Subcommittee 2011) because it is known from only one collection and thus remains in need of further investigation with respect to future conservation efforts.

Notes — When H. thuathienhuensis was first observed in the field it was not in flower. From its vegetative structures, a strong terrestrial climber with coriaceous glabrous leaves, we first thought it may not belong to the genus. What made us initially think that H. thuathienhuensis may indeed belong to the genus are its extra-axillary, positively geotropic peduncles, bearing scars of previous flowerings thus diagnostic of the genus ( Omlor 1996). After flowering in cultivation it became clear that the taxon belongs to Hoya and bears superficial similarities with co-occurring H. lockii , the only other Hoya species observed in the Hue Green Corridor apart from Hoya carnosa R.Br. so far. It can be easily separated from H. lockii because of the growth habit: H. thuathienhuensis is a strong climbing liana while H. lockii is an epiphytic shrub. Further, the corolla lobes are similarly reflexed and densely hirsute underneath the corona in both species but in H. lockii they are also slightly pubescent elsewhere and ciliate while in H. thuathienhuensis they are glabrous and brilliant; the corona lobes are ovate-round with a short acute inner process depressed among the prominent outer processes ( Fig. 1 ) while in H. lockii the corona lobes are laterally compressed and present a prominent inner process bearing a caudate, erect or slightly converging or diverging appendage with a linear, slightly recurved apex, extending 2 –4 mm above the apex of the outer process.

The general habit of the plant, a strong terrestrial climbing liana, as said, is rare in Hoya and usually typical of species belonging to Hoya sect. Eriostemma . Species in this section are characterised by mainly terrestrial, non-epiphytic habit, inflorescences on short-lived or deciduous peduncles and large and fleshy corollas with proportionally small coronas ( Wanntorp et al. 2011). No species of Hoya sect. Eriostemma are known to occur in Vietnam. Pollinaria in species belonging to sect. Eriostemma consist of club-shaped pollinia lacking pellucid margins, attached to the retinaculum by twisted and winged caudicles ( Wanntorp et al. 2011). With its perennial peduncles, smaller flowers and oblong pollinia with a clearly distinguished pellucid margin attached to the retinaculum by short broad caudicles ( Fig. 1h ), H. thuathienhuensis does not belong to sect. Eriostemma. Its sectional placement is uncertain and on the basis of pollinaria shape, it falls in a phylogenetically unsupported group of species bearing narrow elongate pollinia longer on one side, pellucid margin along the entire dorsal edge, a narrowly rhomboid retinaculum apically pointed and unwinged caudicles ( Wanntorp 2007) whose sectional placement will need to be re-evaluated following recent molecular evidence ( Wanntorp et al. 2011).

Figures

Fig. 1 Hoya thuathienhuensis T.B.Tran, Rodda & Simonsson. a. Sterile branch; b. inflorescence; c. flower, lateral; d. flower, longitudinal median section; e. flower, from above; f. corona, underside; g. pedicel, side and frontal view; h. pollinarium (all: holotype). — Scale bars: a = 5 cm; c–g = 5 mm; b = 1 cm; h = 0.5 mm. — Photos by M. Rodda.