Didymocarpus phuquocensis N.S. Ly , T.L.Tran & N.G.Cao, 2020

Tran, Thi-Lien, Ly, Ngoc-Sam, Tran, Minh-Ngoc, Nguyen, Xuan-Truong, Cao, Ngoc-Giang & Pham, Hong-Dung, 2020, Didymocarpus phuquocensis, a new species of Gesneriaceae from Phu Quoc Island, South-western Vietnam, PhytoKeys 159, pp. 35-44 : 35

Didymocarpus phuquocensis N.S. Ly, T.L.Tran & N.G.Cao sp. nov. Figures 1 , 2

Diagnosis.

Didymocarpus phuquocensis is most similar morphologically to D. pulcher , D. hookeri and D. punduanus in the 3-verticilate, petiolate leaves, the morphologically-similar calyx, corolla, stamens, pistil and fruit, but differs from all in the glandular-pubescent stems and petioles (vs. puberulous, villous, pubescent and eglandular of the latter three, respectively), glandular-pubescent leaf blades (vs. pubescent, somewhat villous, puberulous and eglandular, respectively), 1(-2)-flowered cymes (vs. many-flowered cymes of the latter three), longer corolla 4.8-5.3 cm long (vs. 2.5-3 cm in D. pulcher , ca. 1.4 cm in D. hookeri and ca. 2.2 cm in D. punduanus ), longer and densely multicellular glandular-puberulent ovary ca. 3 cm long (vs. 1.1-1.8 cm and glabrous in D. pulcher , 1.1-1.7 cm and pubescent in D. hookeri and 0.7-1.3 cm and glabrous in D. punduanus ) and longer fruits 4.4-5.5 cm long (vs. ca. 4 cm long in D. pulcher , 1.3-3.5 cm long of D. hookeri and ca. 4 cm long in D. punduanus ).

Type.

Vietnam. Kien Giang Province: Phu Quoc District, Phu Quoc NP, Suoi Mo, 18 September 2018, 10°14'40.90"N, 104°2'14.15"E, 82 m elev., Lý Ngọc Sâm, Cao Ngọc Giang, Nguyễn Thị Liên, Ngô Minh Huyền, Hùng, Hà Văn Long, TNB-305 (Holotype: VNM, isotype: P, NIMM).

Description.

Deciduous, perennial, epilithic herb, (8-)10-19 cm tall, stems 2.5-3.5 mm in diameter. Dry season: new vegetative buds produced from the rhizome which then develops during the rainy season. Rainy season: stem erect, (3-)4 nodes, pale greenish, densely white multicellular glandular-pubescent; the longest node separated from the base of stem 5.7-12.2 cm long, the medium nodes at the middle stem 1.5-4.5 cm long, the shortest nodes very shortly distanced (0.2-0.5 cm long) at the apex. Leaves 3- verticillate, petiolate in the 2nd and 3rd whorls, other whorls with smaller and sessile or subsessile leaves; blades coriaceous, adaxially light greenish, abaxially whitish-green, asymmetrically narrowly elliptic to elliptic-ovate, the largest ones 7.2-10 × 2.4-3.7, the smaller ones 1-4.6 × 0.5-2.1 cm, adaxially densely white multicellular glandular-pubescent, abaxially sparsely white multicellular glandular-pubescent, apex attenuate to acute, base lightly oblique, attenuate to cuneate, margin serrate; venation pinnate, with 5-6 of ascending secondary veins on each side of midrib, somewhat opposite, adaxially obscure, abaxially prominent, densely covered with indumentum as the stem; petioles terete, unequal in length, 0.5-3.2 cm long [the longest ones 3-3.2 cm, the shortest ones 0.5-1 cm], 2-2.5 mm in diam., whitish-green, sometimes tinted greenish-purple above, with indumentum as the stem. Inflorescences terminal or subterminal, cyme 1(-2) flowered, pendent; peduncle slender, (1.3-)3.1-4.5 cm long, ca. 0.5 mm in diam., tinted reddish-green, covered with white multicellular glandular and glandular-pubescent; pedicels 5-11 mm long, ca. 0.7 mm in diam., pale green, with indumentum as the peduncle, but more sparse; bracts paired; lanceolate to oblong-lanceolate, 4-5 × 1-1.5 mm, apex round to acute, margin entire, green, adaxially sparsely multicellular glandular-pubescent, abaxially densely white multicellular glandular-pubescent. Calyx campanulate, 5-6.5 mm long, dull reddish, outside sparsely multicellular glandular-puberulent; tube 3-4 mm long, 2-2.5 mm in diam.; lobes triangular, (sub)equal, 5-lobed, symmetrical, 2-2.5 mm long, ca. 1.5-1.8 mm wide at base, apices acute. Corolla funnelform, 4.8-5.3 cm long, glabrous, light purple, paler at base; tube 3.8-4.1 cm long, base narrow, 2-3 mm in diameter, widening abruptly at 1.9-2.1 cm from the base, widest at throat, 1-1.1 cm in diam.; lobes (sub)orbicular; anterior (lower or abaxial) lip 3-lobed, unequal, the middle one 5-7 × 7-10 mm, the lateral ones 5-7 × 4.5-5 mm, apices rounded; posterior (upper or adaxial) lip 2-lobed, slightly equal, 5-6 × 7.5-8.5 mm, apices rounded. Stamens 2, inserted at 2.2-2.4 cm above the base of the corolla; filaments slender, white, glabrous, 7-8 mm long, glandular-puberulent on the connective; anthers brownish, oblong, 2-2.2 × ca. 1 mm, tips and bases rounded, white-bearded; staminodes 3, inserted ca. 3 mm below the stamens, reduced to filaments, equal in length, 2.5-3.5 mm long, glabrous, tips with few glandular-puberulent. Disc cupular, ca. 1 mm high, margin irregular sinuate. Pistil ca. 3.2 cm long; ovary cylindrical, greenish with white towards the base, ca. 3 cm long, 1-1.5 mm in diam., densely glandular-puberulent; style continuous with the top of the ovary, ca. 2 × 1 mm, whitish, sparsely glandular-puberulent; stigma irregular capitate, ca. 1 × 1 mm, concave, white, papillose. Capsules straight to slightly curved, linear, glabrous, 2-valved, loculicidal dehiscent, 4.4-55 cm long, 1-1.5 mm in diam., turning brown when ripe, calyx and style persistent. Pollen and Seeds not studied.

Distribution and habitat.

This species grows in moist places and shaded areas in primary tropical evergreen forests, on granite bedrock along streams or on moist and mossy cliffs in Phu Quoc NP, at 80-530 m elev. (Fig. 2 ).

Phenology.

Flowering and fruiting from May to October.

Etymology.

The specific epithet " phuquocensis " was named after the type locality.

Provisional conservation status.

At present, four relatively-large subpopulations consisting of about 2000 mature individuals/mature clumps have been observed in Phu Quoc NP. The extent of occurrence (EOO) and the area of occupancy (AOO) were estimated using the web Geospatial Conservation Assessment Tool or GeoCAT ( Bachman et al. 2011) and the auto-value cells width 2 km were calculated to be 53.7 km2 and 16 km2, respectively. These two values meet the criteria B1 (EOO <100 km2) for Critically Endangered and B2 (AOO <500 km2) for Endangered, following the IUCN Red List Categories and Criteria Version 3.1 ( IUCN 2019). Although the known habitat of the new species is protected as part of the National Park, its habitat is fragmented and still faces some risk due to loss of the habitat within some parts of its range (in particular the clearing of forest land for agricultural fields and building of numerous roads and tourism areas). Based on the IUCN Red List Criteria ( IUCN 2019), we therefore provisionally assess this species as Endangered (EN B2ab(iii), C).

Other specimens examined

(Paratypes). Vietnam. Kien Giang Province: Phu Quoc Island, Phu Quoc NP, K7 peak, 10°21'53.42"N, 104°0'31.22"E, 356 m elev., 21 May 2019, Lý Ngọc Sâm, Hà Văn Long, TNB-430 (VNM); the same locality, Nui Chua peak, 10°22'40.09"N, 104°2'6.24"E, 532 m elev., 22 July 2019, Cao Ngọc Giang, Ngô Minh Huyền, Hà Văn Long, TNB-502 (VNM); the same locality, Ham Rong Mount, 10°24'6.34"N, 103°58'6.47"E, 351 m elev., 10 July 2019, Cao Ngọc Giang, Ngô Minh Huyền, Hà Văn Long, TNB-508 (VNM).

Vernacular name.

Vietnamese language: Song bế phú quốc.

Taxonomic notes.

Morphologically, the 3-verticillate petiolate leaves of D. phuquocensis are shared with several species of Didymocarpus , such as D. insulsus Craib (north-eastern, Thailand), D. tristis Craib (Chanthaburi Province, south-eastern Thailand), D. dongrakensis B.L.Burtt (northeast Thailand) D. newmanii B.L.Burtt (Chanthaburi Province, south-eastern Thailand), D. pulcher (from India, Buhtan, Nepal, China to Vietnam), D. hookeri (Assam, Arunachal Pradesh, Meghalaya and Sikkim, India), D. punduanus (Assam, Meghalaya and Nagaland, India) ( Wang et al. 1998; Nangngam and Maxwell 2013; Phuong et al. 2014; Sinha and Datta 2016; Roy 2017). Of these, the campanulate calyx of the new species is similar to D. pulcher , D. hookeri and D. punduanus , but distinguished from D. insulsus , D. newmanii and D. tristis which have the calyx 5-lobed to the base. Didymocarpus phuquocensis is most similar to D. pulcher , D. hookeri and D. punduanus in the morphological characters of 3-verticillate petiole leaves, the same shape of calyx, corolla, stamens, pistil and fruit. The major differences between the new species and the three latter are outlined above in the diagnosis. Moreover, the shorter plant height ((8-)10-19 cm), the narrowly elliptic to elliptic-ovate leaf blades that have serrate leaf margins, attenuate to cuneate leaf base and attenuate to acute leaf apex, the oblong-lanceolate bracts being abaxially densely multicellular glandular-pubescent, the light purple corolla and sparsely glandular-puberulent styles of D. phuquocensis also distinguish it from D. pulcher , D. hookeri and D. punduanus . A detailed morphological comparison between D. phuquocensis , D. pulcher , D. hookeri and D. punduanus is provided in Table 1 .

Figures

Figure 1. Didymocarpus phuquocensis A flowering plants in natural habit B flowering plants showing 3 - verticillate leaves (red arrow) C mature plant with dried fruit and new stems D flower (top view) E flower (side view) F flower (front view) G inflorescence with peduncle, bract and calyx H calyx (top view) I longitudinal section of calyx showing disc J longitudinal section of corolla showing anterior (lower) lip with fertile stamens K longitudinal section of corolla showing posterior (upper) lips and staminodes (red arrow) L close-up of fertile stamens M ovary and calyx N dried fruits. The coloured plate prepared by Ng ọc-Sam Ly.

Figure 2. Distribution map of Didymocarpus phuquocensis in Phu Quoc National Park, Phu Quoc Island, Kien Giang Province.