Hoya phuwuaensis Kidyoo, 2016

Kidyoo, Manit, 2016, Hoya phuwuaensis (Apocynaceae: Asclepiadoideae), a new species from Northeastern Thailand, Phytotaxa 282 (3), pp. 218-224 : 218-223

Hoya phuwuaensis Kidyoo , spec. nov. ( Figs. 1 & 2 )

Hoya phuwuaensis is similar to H.caudata in having prominent silver blotches on the adaxial leaf surface, positively geotropic inflorescences with slender peduncles, white corollas with pubescent adaxial surface and long hairs along the margins. The new species is however clearly distinguished by having elliptic leaves with cuneate base, obovate-oblong corona lobes and triangular anther appendages, while H. caudata has ovate leaves with rounded to cordate base, ovate corona lobes, and lanceolate anther appendages.

Type:— THAILAND. Phu Wua Wildlife Sanctuary, Bueng Kan Province, 200 m, 12 September 2008, M. Kidyoo 1035 (holotype BKF, isotype BCU).

Climbing epiphyte of about 2 m long. Latex yellowish white in all vegetative parts. Stem and branches cylindrical, 2.5–3 mm in diam., greenish brown to greyish brown, scabrous-strigose, internodes 4–10 cm long. Leaves: petiole stout, dark brown, strigose, 3–6 mm long, 2–3 mm in diam.; blade thick, coriaceous, elliptic, 5.5–11 × 2.5–3.7 cm, margins entire to slightly undulate; adaxial surface dark green with irregular white blotches, scabrous-strigose, abaxial surface pale green with scattered strigose hairs; apex acute to acuminate, base cuneate to acute with a small ovate to oblong colleter, 0.3–0.5 × 0.7–0.9 mm; midrib and nerves inconspicuous on both sides, lateral veins 5–6 pairs, held at an acute angles to the midrib. Inflorescences extra-axillary, positively geotropic, slightly concave in top view, 18–20-flowered; flower buds pentagonal shaped; peduncle slender, perennial, 4.5–7 cm long, 1–1.2 mm in diam., scabrous-strigose. Pedicels of the peripheral flowers 1.5–2 cm long, ca. 0.8 mm in diam. and those of flowers towards the centre of the inflorescence 0.5–1 cm long, ca. 0.8 mm in diam., pinkish or greenish white with scattered reddish purple spots, scabrous with scattered hairs. Sepals greenish, ovate-lanceolate, 1.5–1.7 × 0.7–0.9 mm, apex acute, abaxial surface scabrous with scattered hairs, adaxial surface with an ovate basal colleter, ca. 0.2 × 0.14 mm, between lobes. Corolla rotate, creamy to yellowish white, 1–1.1 cm in diam.; adaxial surface densely pubescent with long straight hairs along the margins of the lobes, abaxial surface glabrous, connate part 3–3.2 mm long; lobes reflexed, revolute, ovate, 3.2–3.5 × 4–4.3 mm, apex acuminate. Corona purple red, 5–5.2 mm in diam., coronal scales obovate-oblong, 2.2–2.4 × 1.2–1.3 mm, upper surface convex, lower surface sulcate; outer process slightly upcurved with obtuse apex, inner process raised up higher than outer process, apex of inner process dark purple red, acute; anther appendages yellow, apices white, triangular, 0.8– 1 mm long, erect, longer than apex of inner process, covering the stigma head. Pollinarium: pollinia obliquely oblong, yellow, 0.3–0.31 × 0.09–0.1 mm, apex truncate, margins pellucid extending over the dorsal margins of pollinia; caudicles stout, hyaline, with broad wing, yellow, 0.12–0.13 × 0.04–0.05 mm; Corpusculum reddish brown, ovate-oblong, 0.12–0.13 × 0.05–0.06 mm. Pistil: two distinct ovaries glabrous, ca. 2 mm long; stigma head conical, subquadrangular. Follicles linear-lanceolate in outline, pale green with scattered reddish brown spots, ca. 12.0 cm long, 0.45 cm in diam. Seeds narrowly oblong, 8–8.5 × 1.1–1.3 mm, with a single coma of silky white hairs 3–3.5 cm long.

Distribution:— Thailand (Bueng Kan Province)

Habitat & Ecology: — Hoya phuwuaensis occurs in sun-exposed areas in mixed deciduous forest at about 200 m elevation. The plant climbs on tree trunks, its roots originating and developing on the gnarled surfaces of the trunks ( Fig. 3 a-b). Adventitious roots, produced at both nodes and internodes, usually only on the side appressed to the tree trunk, anchor this climbing epiphyte to crevices in the tree bark. Flowering in July–September.

Conservation status: —The area where the new species was found is easily accessible and frequently visited. The plant from which the type specimen was collected was located near the office of Phu Wua Wildlife Sanctuary, growing on a small tree trunk where it was intermingled with the common species Hoya micrantha Hooker (1883: 55) . During a follow-up visit to the type locality several months after my collection of the type specimen, I discovered that this individual of H. phuwuaensis had been cut near the base of the stem and had died. Hoya spp. are frequent targets of illegal collection for commercial propagation and since H. phuwuaensis is now commercially available it is possible that the original stock was obtained from the plant that I observed. In repeated careful surveys in the surrounding forests and rock fields from the type locality to about 500 m elevation, conducted during several subsequent visits to the site, I found no other individual of H. phuwuaensis . The failure to locate additional individuals during the follow-up studies leads to the frightening conclusion that the plant may have already vanished from its natural habitat, at least in Phu Wua Wildlife Sanctuary. Until other wild populations are found, H. phuwuaensis can be considered to be facing high risk of imminent extinction in the wild. It is possible that the individual found near the office of the Sanctuary originated from a seed dispersed from a parent plant growing far out of reach in the canopy of nearby mixed deciduous forest.As H. phuwuaensis is presently known only from the type locality, its preliminary conservation status is thus Data Deficient (DD; IUCN 2016). Further studies to search for individuals of this species growing in forest are warranted. However, as mentioned above, the plant has been commercially propagated and thus it is currently widely available in cultivation. This cultivated stock can serve as an alternative source for reintroduction back into the native habitat.

Observations:— Hoya phuwuaensis is a new representative of the Hoya section Peltostemma . All species of this section produce positively geotropic, extra-axillary indeterminate corymbs with slender peduncles. They have a rotate corolla and ovate lobes. The corolla lobes are densely pubescent on the adaxial surface and villous with long hyaline hairs along the margins. Anther appendages are longer than the inner angles of the corona lobes. The pollinaria have winged caudicles. Four of the five species, i.e. H. caudata , H. flagellata , H. phuwuaensis and H. soidaoensis have ovate, ovate-lanceolate or elliptic leaves which are flat to slightly sulcate, whereas H. imbricata mostly produces dome-shaped or sometimes flat circular leaves that are appressed against the tree-trunk on which the plant grows and offer accommodation to ant colonies. Within the group of plants that do not bear circular leaves appressed against the tree-trunk, H. phuwuaensis appears most closely related to H. caudata . Both have large leaves with prominent silver blotches on the adaxial surface and corrugated margins, whereas H. soidaoensis and H. flagellata clearly differ from H. caudata and H. phuwuaensis in their leaf and flower characters ( Kidyoo 2013).

Hoya phuwuaensis can be clearly distinguished from H. caudata by differences in leaf shape and size and in flower characters ( Fig. 2 , Table 1). The former has elliptic leaves with cuneate base and slightly corrugated margins. Its flower is smaller and bears an obovate-oblong corona with rounded apex and triangular anther appendages. Hoya caudata , on the other hand, has ovate leaves, each with a rounded to cordate base, acuminate apex, and undulate and sometimes revolute margins. The flower of H. caudata bears ovate corona lobes with obtuse apex and lanceolate anther appendages. Moreover, flowers of H. phuwuaensis bloom at night, then re-close in the next morning ( Fig. 3 c-d). The cycle is repeated for at least a second day. In this respect, H. phuwuaensis is similar to H. flagellata that is also night blooming. In contrast, the flowers of H. caudata remain open during both the day and the night for one to two days after opening. These differences in flowering phenology suggest the need of comparative studies in the pollination biology of these species.

The highly restricted geographical distribution of H. phuwuaensis contrasts with that of the three other flat-leafed members of the section. Hoya caudata commonly occurs in the lowland forest and limestone hills in Peninsular Malaysia, Singapore, Sumatra, Borneo ( Hooker 1883, Rodda & Ang 2012) and has never been reported from natural habitats in Thailand ( Kidyoo, 2013). Hoya flagellata , H. soidaoensis and H. phuwuaensis are endemic to Thailand. They are respectively found in lowland evergreen forest, hill evergreen forest, ( Hooker 1883, Ridley 1923, Rintz 1978, Kidyoo 2013) and exposed areas in mixed deciduous forest.

Figures

FIGURE 1. Hoya phuwuaensis: a. Flowering branch; b. Open flower in top view; c. Open flower in side view; d. Calyx; e. Corona from underneath; f. Pollinarium; g. Fruit; h. Seed. Drawn by M. Kidyoo based on M. Kidyoo 1035.

FIGURE 2. Comparison between Hoya phuwuaensis, H. caudata, H. soidaoensis and H. flagellata. H. phuwuaensis: a. Leaf; e. Flower in top view; i. Pollinarium (reference specimen M. Kidyoo 1035). Hoya caudata: b. Leaf; f. Flower in top view; j. Pollinarium (reference specimen M. Kidyoo 1393). Hoya soidaoensis: c. Leaf; g. Flower in top view; k. Pollinarium (reference specimen M. Kidyoo 1526). Hoya flagellata: d. Leaf; h. Flower in top view; l. Pollinarium (reference specimen M. Kidyoo 1425). All photographs by M. Kidyoo.

FIGURE 3. Hoya phuwuaensis: a. Habitat; b. Habit; c. Blooming flowers photographed at night; d. Closing flowers in the morning; e. Latex.All photographs by M. Kidyoo.