Didymoplexis gibbosa Aver. & Nuraliev, 2022

Averyanov, Leonid V., Maisak, Tatiana V., Lyskov, Dmitry F., Kuznetsov, Andrey N., Kuznetsova, Svetlana P. & Nuraliev, Maxim S., 2022, Didymoplexis gibbosa (Orchidaceae), a new mycoheterotrophic species from southern Vietnam, Phytotaxa 556 (2), pp. 207-212 : 208-211

Didymoplexis gibbosa Aver. & Nuraliev , sp. nov. ( Fig. 1 , 2 )

Diagnosis:—The new species is most close to Didymoplexis pallens , differing mainly in circular epichile 3.5–4 mm long and wide, entirely white and round at apex, in clawed lip bearing at base large erect bilobed callus covered at front with long papillae, and in disc fleshy, gibbously recurved, with 2 low broad ridges, each with longitudinal row of fleshy erect papillae.

Type:— VIETNAM. Quang Nam Province: Tay Giang District, A Xan Municipality, Lang Po Mu homestay area, forest on local ridge with Fokienia hodginsii , 15°48’06’’N 107°19’53’’E, elev. 1360 m, 17 March 2022, Nuraliev M. S., Lyskov D. F. NUR 3450 (holotype LE: LE01168149 , https://en.herbariumle.ru/?t=occ&id=134432, photos LE: LE01123093 https://en.herbariumle.ru/?t=occ&id=134264) .

Paratype:— VIETNAM. Quang Nam Province: Tay Giang District, A Xan Municipality, Lang Po Mu homestay area, forest on slope, 15°49’02’’N 107°19’10’’E, elev. 1430 m, 20 March 2022, Nuraliev M. S., Lyskov D. F. NUR 3482 ( LE: LE01168148 https:// en.herbariumle.ru/?t=occ&id=134431, photos LE: LE01123094 https://en.herbariumle.ru/?t=occ&id=134265) .

Etymology:—The specific epithet refers to the fleshy gibbously recurved disc of the flower lip.

Description:— Herb achlorophyllous, perennial, rhizomatous, terrestrial, glabrous. Rhizome hypogeous, horizontal to ascending, light greyish-brown to yellowish or almost white, terete, to 8 cm long, with 1–3(4) irregular tuber-like swellings and constrictions between them; individual swellings fusiform, villose, (0.6) 1–2.5 cm long, (3)4–4.5(5) mm in diameter; constrictions nearly glabrous, sometimes bearing adventitious roots. Roots 1–4, arising from basal neck of rhizome, yellowish white, filiform, flexuose, 0.2–0.3 mm in diameter. Floriferous stem white, slender, erect, straight or slightly flexuose in basal part, (5)6–10(12) cm long, 1.2–1.4 mm in diameter, with 3–4 distant sterile bracts in lower half and several flowers at apex. Sterile bracts inconspicuous, scarious, broadly triangular, obtuse at apex, tubular at base, 2–3 mm long and wide (when flattened). Inflorescence a lax terminal raceme; rachis (0.8)1–1.2(1.4) cm long, sometimes slightly zig-zag, with (2)3–4(5) flowers. Flower-subtending bracts light brownish, triangular ovate, acute at apex, (1.2–)1.4(–1.6) mm long and wide, persistent. Pedicel and ovary suberect to down curved, resupinate, white, terete, (0.9)1–1.2(1.3) cm long, (1.1)1.2–1.4(1.5) mm in diameter; ovary shallowly longitudinally grooved. Flowers short lasting, opening by 1 in succession, cupular, not widely opening, (7)8–9(10) mm long, 6–8 mm in diameter; sepals and petals outside sparsely finely tuberculate; flower mostly white, sepals and petals with pale brownish stripes along median and lateral veins, lip pure white with pale pinkish callus at the base, column and anther cap pure white and somewhat translucent. Dorsal sepal united with petals for 3/4 of its length, forming a hood; free lobes of dorsal sepal and petals broadly triangular, blunt to obtuse and slightly cucullate at apex, (2.8–)3(–3.2) mm long and wide. Lateral sepals similarly united with each other and joined for half way to petals; free lobes of lateral sepals down (abaxially) recurved, almost semicircular, shortly cucullate at apex, (2.4–)2.5(–2.6) mm long. Lip entire, at base with short claw (hypochile) 1–1.2 mm long, 0.8–1 mm wide proximally (at junction with the apex of the column foot), distally abruptly widening into almost circular epichile 3.5–4 mm long and wide (when flattened); claw adaxially bearing large erect bilobed callus covered at front with long fleshy glandular forward directed papillae; epichile strongly abaxially recurved, with erect finely erose margins, almost round at apex; disc fleshy, gibbously recurved, with 2 low broad ridges extending from the epichile base to its apex, each with longitudinal row of fleshy erect papillae. Column erect, slightly curved forward (abaxially), (4)4.2–4.4(4.6) mm long, 1.4–1.6 mm wide, with slender base, in apical half with broad semicircular lateral wings directed abaxially, each wing terminated at apex by erect conoid stelidium; column foot rather stout, at a straight angle to column base, slightly curved upright, 2–2.2 mm long, as wide as column base. Anther cap hemispheric, 0.5–0.6 mm in diameter, finely papillose, dorsally shortly stalked at base; pollinia in form of 4 amorphous masses. Fruits unknown.

Ecology and phenology:—Primary evergreen mixed submontane forests (with Fokienia hodginsii ) on granite at elevations of 1300–1500 m a.s.l. Flowers in March.

Didymoplexis gibbosa is rather common in the area between the locations of the type and the paratype (with an air distance of 2.2 km between them). The area is remarkable for the dense population of Fokienia hodginsii and presence of a considerable number of large trees of this conifer species. Didymoplexis gibbosa seems to be associated with it, frequently occurring on local ridges and slopes, sometimes near bases of huge Fokienia trunks.

The flowers of the new species apparently open at night and remain anthetic till early morning. Around 10 a.m. (and till the sunset) inflorescences were observed bearing only the flower buds, whereas one or two recently abscised flowers were frequently found on the ground under each plant.

Distribution:— Vietnam (Quang Nam Province, Tay Giang District). Known only from the type location.

Conservation status:—Preliminarily estimated as Critically Endangered, CR B2ab(iii,v). Available observations indicate that the species meets the following IUCN (2019) Red List criteria: only one population is known with the extent of occurrence that cannot be calculated, the area of occupancy is less than 10 km 2 (B2a), and decline is expected for the area, extent and quality of habitat (iii) and number of mature individuals (v).

Notes:— Didymoplexis gibbosa is readily distinguished from other species of the genus (e.g. King & Pantling 1898, Hayata 1912, Smith 1914, Tuyama 1941, Summerhayes 1951, Seidenfaden & Wood 1992, Chen et al. 2009, Averyanov 2011, Cribb et al. 2011, 2013, Pedersen et al. 2014, Inoue 2016, Averyanov et al. 2019, Hermans et al. 2021) by the characters of the lip structure, i.e. the fleshy disc and the presence of papillae on callus and along two disc ridges (although the papillae are sometimes hardly noticeable without special observations). The new species is morphologically most close to the widely distributed and highly variable D. pallens Griffith s.l. (1844: 383 tab. 17), but differs from the latter species in densely villose (vs. glabrous or sparsely pubescent) swollen parts of rhizome, 2–5- flowered (vs. 4–20-flowered) inflorescence, epichile almost circular (vs. deltoid to broadly obtriangular), 3.5–4 mm long and wide (vs. 4.5–5 mm long, 6–7 mm wide), entirely white (vs. white with bright yellow spot at apex), round (vs. truncate) at apex, lip with a distinct claw (vs. cuneate at base, without a distinct claw), lip claw adaxially bearing large erect bilobed callus covered at front with long fleshy glandular forward directed papillae (vs. lip adaxially warty at base), and disc fleshy (vs. not much fleshy), gibbously recurved (vs. recurved but without distinct gibbosity), with 2 low broad ridges extending from the epichile base to its apex, each with longitudinal row of fleshy erect papillae (vs. disc warty along median vein). The new species has flowers opening at night and drooping in the morning, whereas flowers of most its congeners last during a day (but see Hermans et al. 2021). This character makes D. gibbosa especially easily overlooked during botanical surveys.

Figures

FIGURE 1. Didymoplexis gibbosa. A. Plants with flowers. B. Rhizomes. C. Inflorescences with flower bud (left) and with withering flower in the evening (right), side views. D. Recently abscised flower, side views. E, F. Recently abscised flowers, front views. G. Recently abscised flower with lateral sepals removed, abaxial view. H. Pedicel, ovary, column and lip, side view. I. Lip, views from different sides. J. Column, column foot and lip, side views. K. Apical part of ovary and column, views from different sides. L. Column apex, front view. Photos by M. Nuraliev from NUR 3450 and NUR 3482; photo correction and plate design by L. Averyanov and T. Maisak.

FIGURE 2. Didymoplexis gibbosa. A. Plant with flowers. B. Swollen part of rhizome. C. Inflorescence. D. Intact flowers, front and side views. E, F. Flattened flower, front and half-side views (lip removed). G. Flattened lip, adaxial and abaxial side. H–J. Lip, abaxial, halfside and adaxial views. K. Column and lip, side view. L, M. Intact lip and sagittal section of lip, side views. N–P. Column, front, half-side views and view half from back. Drawn from specimens NUR 3450 and NUR 3482 by L. Averyanov and T. Maisak.