Hoya wallichii (Wight) C.M.Burton subsp. tenebrosa S.Rahayu & Rodda, 2021

Hoya wallichii (Wight) C.M.Burton subsp. tenebrosa S.Rahayu & Rodda , subsp. nov. — Fig. 3 View Fig

Similar to Hoya wallichii subsp. wallichii in habit (slender plant with ovate to oblong,thinly coriaceous glabrous leaves) and inflorescence and corolla type (slender peduncle with 1 or 2 flowers open at a time, broadly campanulate corolla).It can be separated on corolla colour (white-cream in subsp. wallichii vs dark purple with a paler edge in subsp. tenebrosa ); corona lobe shape and surface (lobes kidney-shaped, erect and with a well-developed acuminate inner process, purple with a shiny surface in subsp. wallichii vs broadly elliptic, spreading and with a minute inner process,almost black with a velvety surface in subsp. tenebrosa ). — Type: S. Rahayu 1251 (holo BO), Indonesia, West Kalimantan, Putussibau,Kapuas Hulu,lowland, 100 m alt. Sept.2018.

Etymology. The specific epithet ‘ tenebrosa ’ refers to the dark colour of the corolla and corona (Latin tenebrosus = dark).

Perennial climber, likely epiphytic, slender, latex white, all vegetative parts glabrous. Stems cylindrical, slender, 1–2 mm diam; internodes 1–4(–10) cm long. Roots mostly basal, adventitious roots developing only when the stem is in contact with the substrate. Leaves: petiole terete, 2–3 by 0.4–0.8 mm; lamina ovate to oblong, thinly coriaceous when dry, 8–16 by 5–12 mm, apex rounded, shortly apiculate, base rounded; venation pinnate, secondary veins 3–5 on each side, barely visible; basal colleters 3–5, conical, 0.5–0.8 mm long. Inflorescence one per node, positively geotropic, with only 1 (rarely 2) flowers open at one time; peduncle extra-axillary, terete, 5–7 cm by 0.8–1 mm, glabrous; older peduncles forming a rachis from previous flowerings, pedicels positively geotropic, filiform, 15–22 by 0.6–0.9 mm, glabrous. Calyx 3.5–5 mm diam, lobes almost round, 1.5–2 by 1.5–2.2 mm, apex obtuse (round), glabrous, ciliate; basal colleters one at each calyx lobe sinus, ovate-oblong, 0.3–0.4 mm long. Bud globose, 5-ridged. Corolla broadly campanulate, 3–3.5 cm diam when fully open, glabrous; tube 1.5–2 cm long, deep purple inside and white or cream towards the base of the lobes, outside glabrous, inside very finely pubescent, more densely so towards the middle; lobes reflexed, broadly deltate, 0.6–0.8 by 1.8–2.2 cm, apex acute. Corona staminal, c. 2.5 mm high, c. 7 mm diam; lobes broadly elliptic and spreading, 2.8–3.1 by 1.6–1.8 mm, inner process acute, slightly raised apically, outer processes spreading, slightly raised apically, apex rounded, with basal revolute margins, almost black, with a velvety surface. Pollinia oblong, c. 500 by 200 µm, with pellucid margin; corpusculum oblong, c. 200 by 100 µm; caudicles attached to the lower half of the corpusculum, c. 200 µm long. Style head 5-angled, c. 2 mm across, slightly depressed, apex apiculate, c. 0.5 mm tall. Ovary broadly conical, c. 1.5 by 0.7 mm, glabrous. Fruit and seed not observed.

Distribution — Only collected in an oil palm plantation in Kapuas Hulu, West Kalimantan at about 100 m above sea level.

Habitat & Ecology — Hoya wallichii subsp. wallichii was growing epiphytically about 3–5 m above ground. The area was formerly heath forest. In cultivation it grows best in shade, with 30–40 % sunlight.

Conservation status — Data Deficient (DD, IUCN 2012). Hoya wallichii subsp. tenebrosa is only from the type locality in Kapuas Hulu, West Kalimantan, and we do not know the popula- tion size, threats and possible decline. Ex situ collections are present in Bogor Botanic Gardens (from the type locality). The taxon is also widely cultivated elsewhere as it is easily propagated by cuttings. Hoya wallichii subsp. wallichii was first described from material collected in Singapore, where it is now extinct in the wild while it is still extant in Peninsular Malaysia ( Johore) and Borneo ( Brunei) and it is considered as critically endangered (CR B2ab(iii)) ( Rodda et al. 2016).

Additional specimen examined. Cultivated in Singapore, Singapore

Botanic Gardens, 20 Sept. 2019, Yap E.H YEH 73 ( SING), paratype .

Notes — Hoya wallichii subsp. tenebrosa is indistinguishable from H. wallichii subsp. wallichii when sterile: both are slender climbers with ovate to oblong laminae, thinly coriaceous when dry. They can be easily distinguished from their corolla colour and corona colour and surface, as mentioned in the diagnosis. Based on these differences, H. wallichii subsp. tenebrosa could have been alternatively classified at species rank. However, we chose to conservatively publish the new taxon at subspecies rank for the following reasons: both taxa co-occur in Borneo, only one collection of H. wallichii subsp. tenebrosa has been made, few collections of H. wallichii subsp. wallichii are available for study ( Rodda et al. 2018), and we are not sure if the corolla colour and corona morphology are stable features.

The corona of H. wallichii subsp. tenebrosa is also similar in shape to that of H. sammannaniana A.Lamb, Gavrus, Emoi & Gokusing. However , the colour is different (cream with red centre in H. sammannaniana , dark purple with a paler edge in H. wallichii subsp. tenebrosa ) and the corolla is a deeper bellshape in H. sammannaniana , while it is broadly campanulate in H. wallichii subsp. tenebrosa .