The genus Octarrhena (Orchidaceae, Thelasiinae) in Vietnam with description of two new species 

LEONID V. AVERYANOV1,10, АLEN K. ЕSKOV2,11, BA VUONG TRUONG4,5,12, VAN CANH NGUYEN3,13, 

NIKOLAY G. PRILEPSKY6,14, MAXIM S. NURALIEV7,8,15, TATIANA V. MAISAK1,16 & VU MANH9,17 

1 Komarov Botanical Institute of the Russian Academy of Sciences, 2, Prof. Popov str., 197376, St. Petersburg, Russia.

2 Tzitzin Main Botanical Garden of the Russian Academy of Sciences, 4, Botany str., 127276, Moscow, Russia.

3 Institute of Applied Technology, Thu Dau Mot University, No. 6, Tran Van On Street, Phu Hoa Ward, Thu Dau Mot City, Binh Duong Province, Vietnam.

4 Institute of Tropical Biology, Vietnam Academy of Science and Technology, 85 Tran Quoc Toan, District 3, Ho Chi Minh City, Vietnam. 5 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Hanoi, Vietnam.

6 Department of Plant Ecology and Geography, Biological Faculty, M.V. Lomonosov Moscow State University, 1, 12, Leninskie Gory, 119234, Moscow, Russia.

7 Department of Higher Plants, Biological Faculty, M.V. Lomonosov Moscow State University, 1, 12, Leninskie Gory, 119234, Moscow, Russia.

8 Joint Russian-Vietnamese Tropical Scientific and Technological Center, Cau Giay District, Hanoi, Vietnam.

9 Joint Russian-Vietnamese Tropical Scientific and Technological Center, South Department, 3, Str. 3/2, District 10, Ho Chi Minh City, Vietnam.

10 av_leonid@mail.ru; https://orcid.org/0000-0001-8031-2925

11 a.k.eskov@yandex.ru; https://orcid.org/0000-0002-2026-1844

12 bavuong2019@yahoo.com; https://orcid.org/0000-0003-3452-8455

13 nguyenvancanh@gmail.com; https://orcid.org/0000-0001-9578-0342

14 nprilepsky@mail.ru; https://orcid.org/0000-0003-1015-2465

15 max.nuraliev@gmail.com; https://orcid.org/0000-0001-8291-2633

16 tmaisak@mail.ru; https://orcid.org/0000-0001-5919-6755

17 manhvus@yahoo.com; https://orcid.org/0000-0002-2944-3089 

Abstract 

The genus Octarrhena (Orchidaceae) is distributed predominantly in Malesia and Papuasia. Its presence in eastern Indochina was uncovered as recently as in 2015 simultaneously with a description of O. minuscula from the Central Highlands of Vietnam. Here, we describe two more species of the genus, O. emarginata and O. perpusilla, also endemic to this mountain system. All three Vietnamese species of Octarrhena are local endemics with very limited allopatric distribution. We summarize the current knowledge on the genus in Vietnam and present a key for their identification, and a map with location of all known populations. Finally, we briefly discuss modern views on the worldwide species diversity of Octarrhena. 

Keywords: Central Highlands, eastern Indochina, endemism, orchids, plant diversity, plant taxonomy, plant geography 

Introduction 

Octarrhena Thwaites (1861: 305) belongs to a group of morphologically close genera, which also includes the Asian and Australasian genera Phreatia Lindley (1830: 63) and Thelasis Blume (1825: 385). They are sympodial plants with very small flowers, which are difficult for morphological analysis in herbarium material. These three genera belong to the subtribe Thelasiinae of the tribe Podochileae (Dressler 1993). Almost all representatives of this tribe are small epiphytes with short stems bearing flattened or subterete distichous leaves and axillary inflorescences with simultaneously opening resupinate flowers. 

The genus Octarrhena comprises about 50 currently accepted species (Seidenfaden & Wood 1992, Schuiteman 1999, Comber 2001, Schuiteman & de Vogel 2003, Govaerts et al. 2020). The number of species in this genus has increased recently as a result of inclusion of the New Guinean genus Chitonanthera Schltr. in Schumann & Lauterbach (1905: 193) into Octarrhena (Schuiteman & de Vogel 2003). Earlier, a transfer of ten species of Phreatia to Octarrhena has been proposed by Hunt (1970), but these transfers have never been adopted (see also Kores 1989). Octarrhena is widely distributed in tropical regions of Asia, Australia and Oceania from Sri Lanka to Fiji with the highest species diversity in New Guinea (Fig. 1). One species of Octarrhena was reported from each of Sri Lanka (Jayaweera 1981, Fernando & Ormerod 2008), Australia (Clements & Jones 1992, Backhouse et al. 2019), Fiji (Kores 1989, Smith 1991), two species from each of the following regions: Peninsular Malaysia and Singapore (Seidenfaden & Wood 1992), Sumatra (Schuiteman 1999, Comber 2001), Java (Comber 1990, Schuiteman 1999), Borneo (Wood & Cribb 1994, Schuiteman 1999), New Caledonia (Morat et al. 1986), three species from Sulawesi (Schuiteman 1999), four species from the Philippines (Agoo 2007), and 39 species from New Guinea (Millar 1978, van Royen 1979, Schuiteman 1999, Schuiteman & de Vogel 2003, Cámara-Leret et al. 2020). Additionally, Govaerts et al. (2020) indicate Bismarck Archipelago, Solomon Islands, and Vanuatu to be inhabited by some species of Octarrhena. 

Not a single species of Octarrhena has been reported from Thailand, Laos, Cambodia and Vietnam (Schuiteman & de Vogel 2000, Averyanov & Averyanova 2003, Newman et al. 2007, Schuiteman et al., 2008, Pedersen et al. 2011, 2014, Cho et al. 2016) until the recent discovery of O. minuscula Aver. & Duy (2015: 174) endemic to southern Vietnam. Here, we describe and illustrate two more species of this genus found in the same area and provide the key for their identification. Notably, the Vietnamese species of Octarrhena are known from quite a narrow geographical area, all of them found within the southern part of the Central Highlands (Tay Nguyen Plateau). 

Taxonomic treatment 

Octarrhena Thwaites (1861: 305)

Comber (1990: 204–205, 2001: 563–565), Seidenfaden & Wood (1992: 337–338), Wood & Cribb (1994: 235). 

Type:—O. parvula Thwaites (1861: 305). 

About 50 species with the highest number in New Guinea. In Vietnam, the genus is represented by 3 species endemic to the southern part of the Central Highlands (Tay Nguyen Plateau) having very limited, albeit allopatric, distribution areas (Fig. 2). Two of these species are described and illustrated as new to science below. 

Distribution:—Sri Lanka, Vietnam, Peninsular Malaysia, throughout the Malay Archipelago to Papua New 

Guinea, Australia and the Pacific Islands (including New Caledonia and Fiji). 

Key to species of Octarrhena in Vietnam 

1. Leaves terete or obscurely triangular in cross section, less than 1 cm long, blunt or roundish at apex; inflorescence at least twice longer than leaves; peduncle terete with 6–10 sterile bracts, longer than rachis; lip epichile reniform, much broader than hypochile; lip disk with two low callosities ..................................................................................................................................1. O. perpusilla 

- Leaves laterally flattened, narrowly elliptic in cross section, more than 1 cm long, obtuse or acute at apex; inflorescence shorter or slightly longer than leaves; peduncle somewhat flattened, naked or with 1–2 sterile bracts, much shorter than rachis; lip epichile semicircular, slightly broader than hypochile; lip disk without callosities or with a single low callosity ........................................ 2 

2. Petals deltoid, fleshy, blunt or obtuse at apex; lip fleshy, obscurely half-obconoid; lip epichile fleshy, almost straight, truncate or round at apex, entire along margin ............................................................................................................................. 2. O. minuscula - Petals narrowly obovate, thin, roundish at apex; lip thin, flat, obovate; lip epichile thin, down curved, emarginate at apex, finely erose along margin.................................................................................................................................................... 3. O. emarginata 

1. O. perpusilla Aver. & Eskov, sp. nov. (Fig. 3, 4) 

Type:—VIETNAM. Lam Dong Province: Lac Duong District, Bidoup Nui Ba National Park, near Giang Ly forest station, around point 12°11′54.3′′N 108°40′35.0′′E, 1544 m a.s.l., Pinus kesiya woodlands on hills with ericaceous scrub and Brainea insignis in lower strata, epiphyte, 15 November 2018, A.K. Eskov, N.G. Prilepsky, AL 491 (holotype LE: LE01055436 – http://en.herbariumle.ru/ ?t=occ&id=5859). 

Diagnosis:—The new species is morphologically similar to O. minuscula, from which it differs in more tiny habit, twice smaller leaves and floral parts, long inflorescence, reniform lip concave at the base and lip disk with two characteristic callosities at the base of epichile. 

Etymology:—The species name refers to the particularly tiny habit of the described plant. 

Description:—Glabrous miniature perennial sympodial epiphytic herb (0.5)0.8–1(1.2) cm tall. Stems abbreviate, very short, (2)3–4(5) mm long, completely covered by overlapping equitant imbricate leaf sheaths, with several flexuose whitish or pale yellowish roots at the base. Leaves (3)4(5), distichous, articulate at base, fleshy, subterete or triangular in cross section, laterally slightly compressed, shallowly grooved adaxially, oblong, commonly broadest near the middle, narrowing to roundish or blunt apex, straight or slightly recurved, (5)6–7(8) mm long, (2)2.5–3(3.5) mm wide. Inflorescence a sublax raceme, arising from leaf axil, almost straight or slightly curved, somewhat secund, (1.2)1.4–1.8(2.2) cm long, 1–1.2 mm in diameter; peduncle light green, terete, stout, erect, straight or slightly curved, (6)8–10(12) mm long, with 6–10 distant small triangular acuminate sterile bracts; rachis straight or slightly curved, (3)4–10(12) mm long, 0.4–0.5 mm in diameter, with many flowers. Floral bracts light pale yellowish, scarious, triangular, obscurely conduplicate, acuminate at apex, more or less straight or spreading and slightly incurved, 1- veined, (0.8)1–1.4(1.6) mm long, (0.2)0.3–0.4(0.5) mm wide. Flowers subsessile (on very short indistinct pedicel), uniformly light pale greenish, resupinate, campanulate, not widely opening, 0.8–1.0 mm in diameter. Ovary ovoid, 0.5–0.6 mm long. Sepals forward directed or hardly spreading, broadly ovate, concave, somewhat fleshy, obtuse to acute at apex, entire along margin, 0.5–0.6 mm long, 0.5–0.55 mm wide. Petals forward directed, ovate, obtuse at apex, entire along margin, as long as sepals, twice narrower than sepals. Lip sessile, solidly fused with column base, simple, concave in basal half, somewhat fleshy, entire along margin, orbicular at apex, 0.7–0.8 mm long and wide, with transversally recurved elliptic to reniform epichile, much broader than hypochile; disk with two obscure callosities at base of epichile. Column shortly cylindric, 0.55–0.60 mm long, 0.25–0.28 mm in diameter, at front with large orbicular stigma. Anther cap light greenish to dull yellowish-white, hemispheric, 0.2 mm in diameter, 2-celled; pollinia 4, bright yellow. Fruits unknown. 

Habitat, phenology and conservation status:—Miniature branch and canopy epiphyte. Primary and old secondary coniferous open submontane forests and woodlands with Pinus kesiya at elevations about 1550 m a.s.l. Flowering in March–April (under cultivation). Very rare. Estimated IUCN Red List status: DD (IUCN 2019) due to a lack of sufficient data on species distribution. 

Distribution:—Vietnam: Lam Dong Province (Lak Duong District: Bidoup Nui Ba National Park). Endemic. 

Notes:—The single species of Octarrhena earlier reported from eastern Indochina, O. minuscula, differs from all its hitherto known Asian congeners in remarkably dwarf habit, short abbreviated stem and very small, not widely opening campanulate flowers (Averyanov at al. 2015). Octarrhena perpusilla shares with the former species such characters as tiny habit, abbreviated stem and subsessile, very small flowers, but differs in smaller leaves 5–8 mm long (vs. 1–2 cm long), long sublax inflorescence 1.2–2.2 cm long, much longer than leaves (vs. short subdense inflorescence 0.8–1.8 cm long, distinctly shorter than leaves), peduncle with 6–10 distant sterile bracts (vs. 0–2 sterile bracts), smaller flowers 0.8–1 mm in diameter (vs. larger flowers 1.8–2.2 mm in diameter), sepals and petals 0.5–0.6 mm long (vs. sepals and petals 1.2–1.5 mm long), lip 0.7–0.8 mm long and wide, concave in basal half (vs. lip 1.2–1.4 mm long and 1.0–1.1 mm wide, not concave at base), lip epichile reniform, much broader than hypochile, thin and orbicular at apex (vs. epichile ovate, not much broadening, at apex fleshy and truncate) and lip disk with two obscure callosities at base of epichile (vs. disk without callosities). 

Additional material studied:—VIETNAM. Lam Dong Province: Lac Duong District, Bidoup Nui Ba National Park, near Giang Ly forest station, around point 12°11′54.3′′N 108°40′35.0′′E, at elevation 1544 m a.s.l., Pinus kesiya woodlands on hills with ericaceous scrub and Brainea insignis in lower strata, epiphyte, photos made from living cultivated plants on 20 March 2019 by L.V. Averyanov, T.V. Maisak, AL 491.1 (LE: photos LE01073043 – http:// en.herbariumle.ru/?t=occ&id=15141). 

2. O. minuscula Aver. & Duy (2015: 22, fig. 12F, 16) 

Type:—VIETNAM. Dak Nong Province: Dak Song District, 5 December 2014, N.V. Canh, Q.V. Hoi, L.V. Averyanov, N.V. Duy, N.T. Hiep, CPC 7694 (holotype LE: LE01069069 – http://en.herbariumle.ru/?t=occ&id=15266, photos: LE01073129 – http://en.herbariumle. ru/?t=occ&id=15412). 

Habitat, phenology and conservation status:—Miniature branch and canopy epiphyte. Primary and old secondary broad-leaved, evergreen, humid, submontane forests at elevations 800–1000 m a.s.l. Flowering in April–May, September–November (under cultivation). Very rare. Estimated IUCN Red List status: DD (IUCN 2017) due to a lack of sufficient data on species distribution. 

Distribution:—Vietnam: Dak Nong Province (Dak Song, Dak R’Lap and Krong No Districts). Endemic. 

Notes:—Octarrhena minuscula was the first species of the genus discovered in eastern Indochina, found in 2014 (Averyanov et al. 2015). The nearest locations of representatives of the genus were reported earlier from Peninsular Malaysia (Seidenfaden & Wood 1992), Sumatra (Schuiteman 1999, Comber 2001), Java (Comber 1990, Schuiteman 1999), and Borneo (Wood & Cribb 1994, Schuiteman 1999). 

Additional material studied:—VIETNAM. Dak Nong Province: Dak R’Lap District, Nhan Dao Municipality, 950–1000 m, around point 11°53′32′′N 107°36′33′′E, 17 September 2011, N.V. Duy, L.V. Averyanov, CPC 7693 (paratype Herbarium of Tay Nguyen Institute of Scientific Researches; LE: photos LE01073128 – http://en.herbariumle. ru/?t=occ&id=15411); Dak Nong Province: Krong No District, Nam Nung Municipality, evergreen forest at elevation about 800 m a.s.l., 2014, Nguyen Van Canh s.n., photos made on 26 April 2015 by L.V. Averyanov, T.V. Maisak, AL 27 (LE: photos LE01073130 – http://en.herbariumle.ru/?t=occ&id=15413). 

3. O. emarginata Aver., Vuong & V.C. Nguyen, sp. nov. (Fig. 5, 6) 

Type:—VIETNAM. Lam Dong Province: Lam Ha District, evergreen broad-leaved and coniferous submontane forest at elevation about 1000 m a.s.l., epiphyte on tall trees, flowers light greenish, Ngo Quang Dang s.n., herbarium specimen collected from cultivated plant on 28 October 2019, L.V. Averyanov, N.V. Canh, T.V. Maisak, AL 1216 (holotype LE: LE01068469 – http://en.herbariumle. ru/?t=occ&id=15264, photos: LE01061348 – http://en.herbariumle.ru/?t=occ&id=12462, LE01073082 – http://en.herbariumle.ru/ ?t=occ&id=15218). 

Diagnosis:—The new species is morphologically close to O. minuscula, from which it differs in thin, narrowly obovate petals, thin emarginate lip with finely erose margin, and down recurved epichile. 

Etymology:—The species name refers to the emarginate apex of the lip. 

Description:—Glabrous miniature perennial sympodial epiphytic herb (1.8)2–2.8(3.2) cm tall. Stems abbreviate, short, (5)6–9(10) mm long, completely covered by overlapping equitant imbricate leaf sheaths, with many flexuose whitish roots at the base. Leaves (3)4–5(6), distichous, articulate at base, somewhat fleshy, strongly laterally flattened, narrowly elliptic in cross section, narrowly grooved adaxially near the base, commonly broadest near the middle, narrowing to obtuse or acute apex, usually slightly recurved, (1.6)2–3(3.4) mm long, (3.5)4–5(5.5) mm wide. Inflorescence subdense to dense raceme, arising from leaf axil, almost straight or slightly curved, somewhat secund, (1.6)1.8–2.2(2.4) cm long, 3–4 mm in diameter; peduncle light green, somewhat flattened, stout, erect, straight or slightly curved, (5)7–8(10) mm long, with (0–)1(–2) small narrowly triangular acute sterile bracts; rachis straight or slightly curved, (1.4)1.6–1.8(2) cm long, 0.1–1.2 mm in diameter (when fresh), with many flowers. Floral bracts light greenish, later yellowish, scarious, erect, commonly straight, narrowly triangular, cymbiform, obscurely conduplicate, tapering to obtuse or acute apex, 1-veined, (1.4)1.5–2(2.2) mm long, (0.4)0.6–1(1.2) mm wide. Flowers shortly pedicellate, uniformly light pale greenish, resupinate, campanulate, not widely opening, 1.5–1.6 mm in diameter. Pedicel terete, 0.3–0.4 mm long. Ovary obovoid, 0.8–1 mm long, 0.6–0.7 mm in diameter, with 6 shallow longitudinal grooves. Sepals forward directed or hardly spreading, triangular ovate, thin, concave, obtuse to acute at apex, entire along margin, 0.6–0.7 mm long, 0.5–0.55 mm wide. Petals forward directed, narrowly obovate, thin, roundish at apex, finely erose along margin, slightly shorter than sepals, 3–4 mm wide. Lip sessile, solidly fused with column base, simple, broadly obovate, concave in basal half, thin, finely erose along margin, orbicular and emarginate at apex, 0.6–0.7 mm long, 0.55–0.65 mm wide, with recurved semicircular epichile; disk with one obscure callosity in the center. Column shortly cylindric, 0.3–0.4 mm long, 0.3–0.35 mm in diameter, at front with large orbicular stigma and 2 triangular acute stelidia. Anther cap yellow, hemispheric, 0.2 mm in diameter, 2-celled; pollinia 4, bright yellow. Fruits light green, ellipsoid or obovoid, 2.2–2.8 mm long, 1.4–1.6 mm in diameter. 

Habitat, phenology and conservation status:—Miniature branch and canopy epiphyte. Primary and old secondary coniferous evergreen submontane forests at elevations about 1000 m a.s.l. Flowering in September–October, December–January (under cultivation). Very rare. Estimated IUCN Red List status: DD (IUCN 2019) due to a lack of sufficient data on species distribution. 

Distribution:—Vietnam: Lam Dong Province (Da Lat City area and Lam Ha District). Endemic. 

Notes:—Octarrhena emarginata is very close to P. minuscula, sharing with the latter characters such as, laterally flattened, acute leaves, short peduncle, naked or with 1–2 sterile bracts, dense inflorescence, ovate to broadly obovate lip and roundish (not reniform) epichile. The new species differs from P. minuscula mainly in thin, narrowly obovate petals roundish at apex (vs. petals fleshy, deltoid, blunt or obtuse at apex), thin, flat lip (vs. lip fleshy, obscurely half obconoid), and thin, down curved epichile, emarginate at apex and finely erose along margin (vs. epichile fleshy, almost straight, truncate or round at apex, entire along margin). 

Paratype:—VIETNAM. Lam Dong Province: Da Lat City area, 3 December 2018, B. V. Truong, Q. D. Ngo, BV 358 (LE: LE01068470 – http://en.herbariumle.ru/?t=occ&id=15265, photos LE: LE01073080 – http://en.herbariumle. ru/?t=occ&id=15216, LE01073081 – http://en.herbariumle.ru/?t=occ&id=15217). 

Acknowledgements 

We thank Eugene S. Popov for providing the map of Southeast Asia. AKE and NGP are grateful to the Joint Russian- Vietnamese Tropical Scientific and Technological Center for organization of fieldwork. Research fieldwork was supported by the State Assignment of the Tzitzin Main Botanical Garden of the Russian Academy of Sciences (project 118021490111-5), the Russian Foundation for Basic Research (project 18-04-00677) and the governmental contract of the Lomonosov Moscow State University (project АААА-А16-116021660037-7). LVA carried out this research in the framework of the institutional research project of the Komarov Botanical Institute of the Russian Academy of Sciences “Study of the flora of Indochina” (project АААА-А18-118031290070-6) and Russian Foundation for Basic Research (project 20-04-00339 А). The work of MSN on morphological diversity of Octarrhena was supported by the Russian Scientific Fund (project 19-14-00055). 

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