Didymoplexiella laosensis S.W.Gale & Kumar, 2020

Kumar, Pankaj, Bouamanivong, Somsanith, Fischer, Gunter A. & Gale, Stephan W., 2020, Didymoplexiella laosensis (Orchidaceae: Epidendroideae: Gastrodieae), a new holomycotrophic orchid from Lao PDR, Phytotaxa 430 (1), pp. 1-8 : 2-5

Didymoplexiella laosensis S.W.Gale & Kumar sp. nov. ( Figs. 1 & 2 )

Type:― LAOS. Vientiane Province: Vang Vieng District, Phone Ngueng Area, 280 m elevation, 26 April 2012, Gale, Kumar, Santainsy and Phunthavong HNL-KFBG 0007 (holotype HNL!, isotype KFBG!)

Diagnosis:— Didymoplexiella laosensis is similar in morphology and colouration to D. ornata , D. siamensis and D. trichechus . However, the lateral sepals of D. laosensis have a rounded apex; the fleshy, longitudinal callus on the labellum terminates more than 0.5 mm before the apex of the midlobe; and the projecting labellum side lobes exceed the midlobe and are truncate-rounded and irregularly denticulate at the apex.

Leafless,holomycotrophic terrestrial herb, 33.1–45.2cm tall.Rhizome horizontal or ascending, tuberous,irregularly fusiform, 5.8–7.5 long, 0.4–0.8 cm in diameter, slightly fleshy, several-noded, beige. Roots 4–7, arising from base of subterranean stem immediately above rhizome apex, slender, wiry, up to about 6 cm long, sometimes bearing 1 or more globose, orange root nodules. Peduncle erect, slender, 4- or 5-noded, glabrous, 28.7–35.2 cm long, 2.0–3.0 mm in diameter, beige to dark brown. Floral rachis 6.5–10.0 cm long, elongating during anthesis, laxly to sub-densely 16–20- flowered; floral bracts ovate-triangular, 1.0–2.0 mm long, acute, dark brown. Flowers resupinate, opening acropetally one or two at a time, white flushed pale lilac, the inner surface of the lateral sepals streaked orange-rose at base, the lip lilac, the column with a blue-violet tinge; pedicel and ovary 6.0– 9.5 mm long, brown; dorsal sepal connate with the petals for 2/3 of its length, forming a hood over the column, elliptic-obovate, 7.2–8.8 mm long, 3.0– 4.2 mm wide, 3-veined, free apical portion ovate, obtuse, outer surface slightly verrucose towards the apex; lateral sepals connate for 2/3 of their length, longitudinally deflexed from their common margin, broadly elliptic-obovate, 6.6–8.4 mm long, 2.8–3.1 mm wide, 3-veined, free apical portion broadly ovate, rounded, outer surface slightly verrucose towards the apex; petals broadly oblong-ovate, slightly oblique, 8.0–9.0 mm long, 4.0– 4.4 mm wide, 2-veined, free apical portions broadly ovate, apical margins erose; labellum projecting forwards, obtriangular when flattened, 5.3–6.6 mm long, 4.5–5.2 mm wide, distinctly 3-lobed above the middle, bearing a long, narrow, raised, fleshy callus extending from a pair of small gland-like domes at the base to midway along the mid-lobe; mid-lobe quadrate, minutely erose along the apical margin; side lobes oblong, 2.0– 2.7 mm long, 1.2–1.4 mm wide, distinctly longer than the mid-lobe, deflexed, truncate-rounded, apical margin irregularly denticulate. Column erect, slightly arched, ca. 5 mm long, with a pair of prominent, decurved, fang-like stelidia ca. 2.2 mm long at the apex, foot absent; anther hemi-spherical, ca. 1.1 mm across; pollinia 4, in 2 pairs; stigma ventral, slightly raised, with a prominent rostellum along apex. Capsule fusiform, ca. 1.2 cm long.

Other specimens examined:― LAOS. Vientiane Province: Vang Vieng District, Phou Din Deng, 353 m elevation, 30 April 2012, Fischer, Gale, Kumar, Bouamanivong, Santainsy and Phunthavong HNL-KFBG 0282 (HNL!)

Flowering:— April–May.

Habitat:— Plants growing in organic debris between rocky crevices under broadleaf forest on limestone karst.

Etymology:— The species is named in honour of the country in which it was discovered.

Taxonomic notes:— Recognition of D. laosensis brings the total number of species in the genus to eight. All share a similar column and column-foot structure, on the basis of which they can be readily distinguished from members of both Didymoplexis and Didymoplexiopsis . Flower colour is variable and so cannot be used for species delimitation. The structure and outline of the labellum are therefore the most important features for identification ( Fig. 3 ).

Conservation assessment:— Three scattered populations of Didymoplexiella laosensis were found in Vang Vieng District of Vientiane Province, central Laos. The extensive limestone karst landscape of the region offers hope that additional populations may yet be found but further surveys are needed to confirm this. The Extent of Occurrence and Area of Occupancy (AOO) are estimated to be <1 km 2 and 8 km 2, respectively ( Moat 2007). On the strength of current knowledge, the total population size is estimated to amount to fewer than 500 mature individuals. Being a holomycotroph, species of Didymoplexiella are expected to be highly specific in their mycorrhizal symbiosis and therefore also their habitat ( Jacquemyn et al. 2016). Clearance of broadleaf forest across the seasonal Asian tropics continues to endanger the unique biota of the region ( Clements et al. 2006), with natural regeneration and restoration of limestone floras being regarded as especially difficult ( Urich & Reeder 1999). However, the immediate threat to this species is habitat destruction due to mining ( Day & Urich 2000). Part of the limestone mountain range where D. laosensis was discovered was leased to a cement production company by the Lao government in 2015. Presently, only 10% of the limestone karst area of central and northern Laos is under statutory protection ( Delang et al. 2013). Applying IUCN (2019) guidelines, the species is assessed as Endangered (EN B1+B2ab(iii)).

Figures

FIGURE 1. Didymoplexiella laosensis. A. Plants in habitat. B. Tuber and roots with orange nodules. C. Inflorescence. D. Side view of flower. E. Front view of flower. F. Fused dorsal sepal and petals. G. Fused lateral sepals. H. Labellum. I. Side view of column. J. Front view of column. K. Operculum. L. Pollinarium (one pollinium missing on either side, cf. Fig. 2-K). M. Gynostemium with ovary, pedicel and pollinia.

FIGURE 2. Didymoplexiella laosensis. A. Inflorescence. B. Tuber and roots. C. Fused dorsal sepal and petals. D. Fused lateral sepals. E. Labellum. F. Gynostemium with ovary and pedicel. G. Front view of column. H. Side view of column. I. Oblique view of column. J. Operculum. K. Pollinarium. (Drawn by Pankaj Kumar from specimen HNL-KFBG 0007.)

FIGURE 3. Comparison of flower and labellum of Didymoplexiella laosensis with those of morphologically similar members of the genus.A–B. Didymoplexiella laosensis. C–D.Didymoplexiella siamensis. E–F.Didymoplexiella ornata. G–H.Didymoplexiella trichechus. Photo credits:A. Stephan Gale; C. Pankaj Kumar; E. Ong Poh Teck; G. Tsukaya Hirokazu.All line drawings drawn by P. Kumar (D drawn from fresh specimen collected in Hong Kong; F drawn from colour photographs of plants in Malaysia provided by Ong Poh Teck and from original description (Ridley 1891); H redrawn from Tsukaya et al. (2014) from Borneo).