Dendrobium obchantiae Promm., Suddee & Kidyoo, 2018

Prommanut, Phattaravee, Suddee, Somran & Kidyoo, Manit, 2018, A narrow endemic new species of Dendrobium sect. Stachyobium from Thailand (Orchidaceae: Malaxideae), Phytotaxa 348 (2), pp. 90-98 : 93-97

Dendrobium obchantiae Promm., Suddee & Kidyoo , sp. nov. ( Figs. 2 , 3 )

Type:— THAILAND. Tak Province: Umphang District, near the Check Point to Thi Lo Su Waterfall, c. 600 m, 10 Oct 2014, cultivated and flowered in Chiang Mai, 25 Oct. 2015, Prommanut & Buddhawong 703 (holotype: BKF).

Dendrobium obchantiae is similar to D. incurvum in having a slender stem, bracteoles shorter than pedicel plus ovary and labellum with three keels, but the former is distinguished by its pale green or greenish-yellow flowers with 3–4 longitudinal purple veins on sepals and petals, elliptic-obovate dorsal sepal, widely triangular or falcate-triangular lateral sepals, ovate or suborbicular labellum with three distinct lobes, obliquely triangular-obovate, raised sidelobes and a short, stout mentum.

Epiphytic herbs with smooth roots. Pseudobulbs erect, cylindric or narrowly fusiform, base slightly contracted, becoming narrower above middle, green or greenish purple, 9.5–26.0 cm tall, internodes 1.6–3.7 cm long, 2–4 mm in diameter, with 2–8 nodes, developing young pseudobulbs enclosed by leaf sheaths; leaf sheath membranous, glabrous on both surfaces, light creamy brown, mature old pseudobulbs naked, dull purple with pinkish purple longitudinal lines. Leaves 2–4, distichous, spreading, slightly recurved, ovate, lanceolate, narrowly lanceolate or oblong-lanceolate, 4.5–8.0 × 0.5–1.3 cm, apex unequally and shortly bilobed, each lobe acute, base expanded into sheaths, glabrous on both surfaces, the lowermost smaller than the uppermost, green or dull green above, pale green or purplish green beneath, mid-vein grooved above and ribbed below. Inflorescences with 1–4 subterminal and axillary racemes, suberect or nodding, 2.5–5.5 cm long, not exceeding leaves, lax, 2–3-flowered; peduncle and rachis glabrous, dull purple or purplish green, peduncle 1–2 cm long, rachis 1–3 cm long, covered with bracts; bracts slightly concave, triangular to lanceolate, 3.0–4.0 × 1.5–2.0 mm, apex acuminate, base truncate, margin entire, 1-veined, glabrous on both surfaces, green or purplish green, abaxial surface distinctly keeled below. Flowers resupinate, waxy, strongly fragrant, 1.4–1.6 cm in diameter. Sepals greenish yellow with 3 longitudinal purple veins, spreading, often recurved distally, margin entire, abaxial surface distinctly keeled; dorsal sepal elliptic-obovate, 6.5–7.0 × 3.8–4.0 mm, apex acute, base truncate, 3–4-veined, mid-vein grooved above; lateral sepals widely triangular or falcate-triangular, 6.5– 7.0 × 5.0– 5.5 mm wide at base, apex acute, base obliquely truncate. Petals pale green or greenish yellow, with 3 longitudinal purple veins, spreading, oblanceolate, 6.5–6.9 × 1.9–2.0 mm at middle, apex acuminate, base truncate, margin entire, sometime slightly undulate in upper half, 3-veined, hardly visible. Labellum bright greenish yellow or dull green, recurved at apex, ovate or suborbicular and distinctly trilobed, 7.0–7.5 × 5.0–8.0 mm across side lobes, shortly clawed at base; side lobes greenish yellow with dull green lines, obliquely triangular-obovate, forwards and upwards pointed, 4.0–5.0 × 1.9–2.0 mm, apex acute, margin entire; disc narrowly oblong, vesicular on both surfaces, waxy with 3 greenish yellow thickened keels, raised at base, continuing to base of midlobe, all of same length; midlobe green, suborbicular, recurved, apex acuminate, margin cristate-corrugate. Mentum green, broadly conical, 3.0–4.0 × 2.5–3.0 mm in diameter, margins entire, straight or incurved distally. Column green, 1.5–1.9 × 2.5–2.8 mm at base, margin entire, side wings strongly developed; foot concave, waxy, 4.0–5.0 × 2.0– 2.5 mm at base; stigmatic cavity green, circular or ovate shaped, stelidia obtuse, anther cap pale green with pale brown patches, subglobose or widely obovate, 0.5–0.8 × 0.8–1.0 mm, apex emarginate, basal margin minutely ciliate, adaxial surfaces sulcate; pollinia 4, in 2 pairs, yellow, each pollinium narrowly elliptic, 0.4–0.6 × ca. 0.2 mm, without caudicles and visidium. Ovary (including pedicel) narrowly clavate, curved, 5–8 mm long, glabrous, waxy, 6-grooved. Capsule green, ellipsoidal, 1.8–2.0 × 0.8–1.0 cm, with 3 obtuse keels, apex with scarious, persistent sepals and petals.

Etymology:— The specific epithet is in honour of Obchant Thaithong, Department of Botany, Chulalongkorn University, Thailand, who has long contributed to the study of Thai orchids.

Distribution and ecology:— Dendrobium obchantiae is known only from near the Check Point to Thi Lo Su Waterfall, Umphang District, Tak Province and Jokkrading Waterfall, Thong Pha Phum National Park, Thong Pha Phum District, Kanchanaburi Province. It is an epiphytic orchid, growing on Gigantochloa ridleyi , Wendlandia tinctoria and Syzygium cumini in the mixed deciduous forest at 600–800 m elevation.

Phenology:— Flowering October–November, fruiting January–March.

Conservation status:— Dendrobium obchantiae is currently known only from two populations as mentioned above. The number of mature plants found in each population during the expeditions in 2015 and 2016 is less than ten. In addition, these habitats are being destroyed by animal farming and human activities. Based on this evidence D. obchantiae faces an extremely high risk of extinction. According to the IUCN Red List Categories and Criteria (2016), it is strongly suggested that D. obchantiae should be treated as a critically endangered species (CR).

Taxonomic notes:— Dendrobium obchantiae is closely related to D. incurvum . They are similar in having fusiform stems, narrowly lanceolate or oblong-lanceolate leaves of chartaceous texture, inflorescences that do not exceed the leaves and three rounded keels on the inner surface of labellum. However, the new species can be clearly distinguished from D. incurvum by several characters as follow: D. obchantiae has pale green or greenish yellow flowers, lax inflorescence (2–3-flowered per inflorescence), elliptic-obovate dorsal sepal, widely triangular or obliquely triangular lateral sepals, oblanceolate petals that are narrower than sepals and recurved, an ovate or suborbicular labellum with side lobes, short and stout mentum that is broadly conical. Dendrobium incurvum has dense inflorescence (5–12- flowered per inflorescence), white flowers, oblong-linear dorsal sepal, narrowly falcate or obliquely falcate-lanceolate lateral sepals, linear-oblanceolate and not recurved petals, narrow lanceolate labellum without side lobes and narrowly conical mentum forming a spur ( Table2, Fig. 2 ). Additionally, D. incurvum appears to be restricted to mangrove forest or evergreen forest in southeastern and Peninsular Thailand, eastern Cambodia, peninsular Myanmar, Peninsular Malaysia and eastern Java. In contrast, D. obchantiae grows in mixed deciduous forest in northern and southwestern Thailand. These anatomical, morphological and ecological differences support that hypothesis that D. obchantiae is a new species.

Additional specimens examined:— Dendrobium incurvum . MYANMAR. Myeik Murgui, 12° 26’ 27’’ N, 98° 36’ 42’’ E, 1835, Griffith 808 (type K–L). NORTH ANDAMAN ISLAND, 1 November 1991, Rao s.n. (C). CAMBODIA. November 1920, Tixier 7 I/63 (C). MALAYSIA. Bukit Wang: North Kadah, Haniff 646 (C). Thailand. Chanthaburi: Makham District, 30 m, 1 November 2015, Rakthai s.n. (BK); Khao Soi Dao: 50 m, 25 October 1956, Smitinand 3604 (BKF); ibid, 100 m, 22 November 1956, Smitinand 3639 (BKF); ibid, 400 m, 12 November 1969, Beusekom & Smitinand 2162 (BKF). Chumphon: 21 January 1927, Kerr 0353 (C). Prachinburi: 21 October 1968, Chermsirivathana 1005 (BK), 3 November 1978, Sukhakul s.n. (K). Ranong: Kraburi, 10 m, 24 December 1928, Kerr 0666 (BK); Klong Naka Wildlife Sanctuary, 200 m, 28 November 2015, Chanla s.n. (BKF). Trat: in mangrove forest, near sea level, 1 August 2004, Phengiao s. n. (BKF); Khao Saming District, near sea level, 16 October 2016, Prommanut 850 (BCU!).

Dendrobium obchantiae . THAILAND. Kanchanaburi: Thong Pha Phum District, Thong Pha Phum National Park, trail up to Jokkrading Waterfall , 760 m, 24 October 2015, Prommanut & Suddee 584 ( BK) ; ibid, 5 January 2016, Prommanut & Suddee 674 ( BCU). Tak: Umphang District , near the Check Point to Thi Lo Su Waterfall, c. 600 m, cultivated and flowered at Chiang Mai, 25 October. 2015, Prommanut & Buddhawong 703 (holotype: BKF) .

Figures

FIGURE 2. Dendrobium obchantiae. A. Habitat. B. Inflorescence. C. Flower. D. Fruit. E. Labellum with three keels and side lobes. F. Host in natural habitat. Dendrobium incurvum. G. Habit and inflorescence. H. Flower, side view. I. Labellum with three keels and no side lobes. J. Host in natural habitat. A–C, photos by W. Buddhawong; D–J by P. Prommanut.

FIGURE 3. Dendrobium obchantiae. A. Habit. B. Inflorescence. C. Flower front view. D. Flower side view. E. Lateral sepal. F. Dorsal sepal. G. Petals. H. Labellum with three keels and side lobes. I. Column. J. Operculum. K. Pollinia. L. Fruit. Drawn by P. Prommanut from Prommanut & Buddhawong 703.