Hoya buntokensis S.Rahayu & Rodda, 2021

Rahayu, S. & Rodda, M., 2021, Hoya buntokensis (Apocynaceae, Asclepiadoideae), a new myrmecophytic species and Hoya wallichii subsp. tenebrosa, a new subspecies from Borneo (Kalimantan, Indonesia), Blumea 66 (3), pp. 236-241 : 237-239

Hoya buntokensis S.Rahayu & Rodda , sp. nov. — Fig. 1 , 2

Similar to Hoya undulata S.Rahayu & Rodda in corolla shape (rotate) and in the presence of lateral appendages of the corona lobe.The two species can be separated on the lamina margin (undulate in H. undulata , flat in H. buntokensis ) and in the shape of the corona lobes (outer processes elliptic in H. undulata , fan shaped in H. buntokensis ), and in the pollinarium morphology (corpusculum smaller than pollinia in H. undulata , corpusculum larger than pollinia in H. buntokensis ). — Type: S. Rahayu 873 (holo BO), Indonesia, Central Kalimantan, Buntok , lowland heath forest, 100–300 m, July 2017. Etymology. The specific epithet ‘ buntokensis ’ refers to the type locality:

Buntok, Central Kalimantan, Indonesia.

Perennialclimber,epiphytic,latexwhite. Stems cylindrical,slender, 2.5–5 mm diam, pubescent, glabrescent when mature, internodes 1–4(–10) cm long. Roots adventitious, produced along the stem, especially when in contact with substrate as well as growing within megadomatia. Leaves: petiole terete, 0.5–1.5 by 0.25–0.5 cm, pubescent to glabrescent; lamina of two types: the ones produced along internodes generally> 5 cm long are flat, ovate, oblong to lanceolate (5–)9–17 by (2–) 3.5–6 cm, apex (rounded) acute to acuminate, base cuneate, minutely cordate; the ones produced along internodes <5 cm long, forming megadomatia, ovate to obovate 5–10 by 3–4 cm, apex rounded to acuminate, base rounded, minutely cordate, edges inrolled; both types fleshy and glossy, pale to dark green, turning reddish when exposed to bright sunlight above, pale to dark green underneath, often with blackish markings, glabrous; venation pinnate, secondary veins 3–7 on each side. Megadomatia globose, 4–7 cm diam, formed by 2–6 leaves; basal colleters 2–5, aligned perpendicularly to leaf axis, broadly conical, 0.4–0.6 mm long. Inflorescence one per node, pseudo-umbelliform, consisting of 2–5 or up to 12 flowers in cultivation; peduncle extra-axillary, positively geotropic or laterally held, terete, 0.5–1(–7) cm by 3–5 mm, pubescent, older peduncles forming a rachis from previous flowerings, pedicels stout, 30–35 by 2–3 mm, pale pink with purple spots to purple, glabrous. Calyx 5–6 mm diam, calyx lobes semi-circular, 1.5–2.2 by 2.5–3 mm, apex rounded, pink to purple, glabrous, margin irregular, ciliate; basal colleters one at each calyx lobe sinus, narrowly conical, c. 0.4 mm long. Bud globose. Corolla rotate, 2.5–3.5 cm diam when flattened; tube c. 5 mm long, inside white-cream to pale pink, outside white, white with purple spots to purple, inside very finely pubescent, outside glabrous; lobes elliptic-ovate, 10–12 by 8–11 mm, margins recurved, apex acuminate, inside white-cream to pale pink, outside white, white with purple spots to purple, inside very finely pubescent, outside glabrous. Corona staminal, c. 5 mm high, 10–12 mm diam, white-cream; lobes spreading, 5–6 by 4–5 mm, inner process oblong, incumbent on style head, c. 1.5 mm long, outer processes fan shaped, 3.5–4 by 4–5 mm, carinate above, below sulcate with revolute margins, apex truncate, two lateral straight appendages at the junction between outer and inner process. Pollinia kidney-shaped, 1.3–1.5 by c. 0.8 mm, with a very reduced apical pellucid margin; corpusculum oblong, 1.8–2.2 by 0.9–1.1 mm; caudicles attached to the base of the corpusculum, c. 1.5 mm long. Style head 5-angled, c. 3.5 mm across, flat with a slightly raised style head apex. Ovary conical, c. 3 by 1.5 mm, apex truncate, glabrous. Fruit and seed not observed.

Distribution — Hoya buntokensis has only been once collected in Central Kalimantan near Buntok.

Habitat & Ecology — Lowland heath forest habitat above a stream and wetland, at 100–300 m above sea level. It was growing epiphytically on small tree trunks about 5 m above ground, growing in 50–70 % sunlight. Based on observations in situ as well as in cultivation H. buntokensis can grow in low as well as high light levels, but the leaves develop a red or purple colour when exposed to intense sunlight. At the type locality in Buntok, the plants were observed to form megadomatia , with ants nesting within. Roots were also observed forming inside the megadomatia. The formation of megadomatia has not yet been observed in cultivation. In the heath forests in the vicinity of Buntok numerous other Hoya species could be found including H. mitrata , H. elmeri Merr. , H. scortechinii King & Gamble and H. waymaniae Kloppenb.

Conservation status — Data Deficient ( DD; IUCN 2012 ). Hoya buntokensis is known only from the type locality in Buntok, Central Kalimantan, and we do not have information on popula- tion size, threats and possible decline. Ex situ collections are present at the Bogor Botanic Gardens (from the type locality). The species is also widely cultivated elsewhere as it is easily propagated by cuttings .

Additional specimen examined. Cultivated in Thailand, Nakhon Si Tham- marat, Nov. 2019, S . Somadee in M. Rodda MR 1961 ( SING), paratype .

Notes — 1. Hoya buntokensis belongs to a small group of species that can develop megadomatia that includes H. mitrata from Thailand, Peninsular Malaysia, Sumatra and Borneo, H. darwinii Loher , endemic to the Philippines, and H. undulata , endemic to Borneo. The four species develop two types of leaves, the first with a flat or slightly convex lamina, occurring at widely spaced nodes on climbing stems, the other convex, occurring on stems with shorter internodes. The latter form megadomatia with the plant roots growing within and harbouring ant colonies. The first type of leaf is generally oblong to lanceolate in H. buntokensis , H. darwinii and H. mitrata , with an entire edge, while in H. undulata leaves can be ovate, obovate or oblanceolate, with a minutely undulate margin. In H. darwinii and H. buntokensis the megadomatia are globose, 4–7 cm diam and formed by 2–6 convex leaves with an inrolled edge. The megadomatia of H. mitrata are instead cabbage-shaped, usually larger, formed by broadly obovate convex leaves without inrolled edges ( Weissflog et al. 1999, Kleijn & Van Donkelaar 2001) and these of H. undulata are generally looser than those of H. mitrata , H. darwinii and H. buntokensis and formed by 4–10, round to elliptic convex leaves, 3–7(–10) cm long.

2. The inflorescences of H. darwinii and H. mitrata are flat to slightly convex, negatively geotropic, bearing 10–15 flowers, the corolla has a tube longer than the lobes, and the lobes are reflexed. The inflorescences of H. undulata and H. buntokensis are instead positively geotropic or laterally held, and bear 2–5 flowers (up to 12 in cultivation); the corolla is rotate, concave, with almost free lobes.

3. Other common features of the four species are pollinia without pellucid margin (present but much reduced in H. undulata and H. buntokensis ). For these reasons H. buntokensis appears to be more similar to H. mitrata in vegetative morphology, and more similar to H. undulata in inflorescence and flower morphology.

4. Hoya buntokensis differs from H. undulata by the size of the corolla (2.5–3.5 cm diam when flattened in H. buntokensis vs 4–5 cm in H. undulata ), but they have a similar rotate shape. The corona of H. buntokensis is smaller (c. 5 mm high, 10–12 mm diam vs 5–7 mm high, 12–13.5 mm diam in H. undulata ); the outer process of the corona lobes is fan-shaped, with two lateral straight appendages at the junction between outer and inner process vs elliptic, with two lateral hooked appendages at the junction between outer and inner process.

Figures

Fig. 1 Hoya buntokensis S.Rahayu & Rodda. a. Flower, side view; b. corolla and corona, top view; c. corona, top view; d. pedicel, calyx and ovary, side view; e. pollinarium (Rodda M. MR1961; SING). — Drawn by Puay Koon Cheng.

Fig. 2 Hoya buntokensis S.Rahayu & Rodda. Morphological variation of leaves. a. Two convex leaves forming a megadomatium, and one unmodified leaf; b, d, f, h. flat leaves photographed from above; c, e, g, i. same leaves photographed from underneath. The larger leaves are produced on internodes> 5 cm long, while the smaller ones are produced on shorter internodes and are similar to the ones forming megadomatia, showing transition between the two leaf types. Living plant in cultivation in Bogor. — Photos by: a. Abdul Lalang; b–i. Sri Rahayu.