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Research Article
Didymocarpus phuquocensis, a new species of Gesneriaceae from Phu Quoc Island, South-western Vietnam
expand article infoThi-Lien Tran‡, Ngoc-Sam Ly§|, Minh-Ngoc Tran‡, Xuan-Truong Nguyen‡, Ngoc-Giang Cao‡, Hong-Dung Pham
‡ National Institute of Medicinal Materials, Ha Noi, Vietnam
§ Graduate University of Science and Technology, Ha Noi, Vietnam
| Institute of Tropical Biology, Ho Chi Minh City, Vietnam
¶ Phu Quoc National Park, Duong Dong Town, Vietnam

Corresponding author: Ngoc-Sam Ly ( lysamitb@gmail.com )

Academic editor: Alan Paton
© 2020 Thi-Lien Tran, Ngoc-Sam Ly, Minh-Ngoc Tran, Xuan-Truong Nguyen, Ngoc-Giang Cao, Hong-Dung Pham.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Tran T-L, Ly N-S, Tran M-N, Nguyen X-T, Cao N-G, Pham H-D (2020) Didymocarpus phuquocensis, a new species of Gesneriaceae from Phu Quoc Island, South-western Vietnam. PhytoKeys 159: 35-44. https://doi.org/10.3897/phytokeys.159.47442
Open Access

Abstract

A new species of Gesneriaceae, Didymocarpus phuquocensis, is described and illustrated from Phu Quoc National Park, Kien Giang Province, South-western Vietnam. It is most similar to D. pulcher, D. hookeri and D. punduanus in having 3-verticillate petiolate leaves, morphologically similar calyx, corolla, stamens, pistil and fruit, but differs from all in the glandular-pubescent stems, petioles and leaf blades, 1(–2)-flowered cymes, longer corolla and fruit and longer and densely glandular-puberulent ovary. Data on distribution, ecology, phenology and provisional conservation assessment of the new species are given along with an illustration and a colour plate.

Keywords

Didymocarpus, Kien Giang, new species, taxonomy, Vietnam

Introduction

The genus Didymocarpus Wallich was established in 1819, based on the species D. primulifolius D.Don from Nepal. The genus has previously been considered to comprise about 180 species distributed in tropical Asia with a few scattered in Africa and Australia (Wallich 1819; Don 1825; Wang et al. 1998; Weber and Burtt 1998; Weber et al. 2000). The taxonomic delimitation of the genus Didymocarpus has varied considerably over time (e.g. Burtt 1998; Weber et al. 2000; Möller et al. 2011; Möller and Clark 2013; Li et al. 2015). By combining molecular phylogenetic data and morphological revision of Didymocarpus, recent studies have remodelled and reduced this number (Vitek et al. 2000; Weber et al. 2000, 2011a, b; Möller et al. 2011; Li et l. 2015; Möller et al. 2017), with some species being placed in other genera, particularly to Henckelia Spreng. (ca. 60 species (Weber et al. 2011b, Middleton et al. 2013)), Petrocodon Hance (more than 20 species (Weber et al. 2011a)) and Tribounia D.J.Middleton (2 species (Middleton and Mӧller 2012)). The genus, as currently recognised, has around 98 species (Möller et al. 2017). Species of Didymocarpus are distinguished from other genera of Gesneriaceae by having: lithophyte perennial habit, ovate to ovate-cordate lamina which is mostly glandular-hairy, tubular corolla with an oblique limb (rarely trumpet-shaped or bell-shaped), two fertile stamens, three staminodes, a capitate stigma, an orthocarpic ovary and a bivalve capsule which dehisces loculicidally (Weber et al. 2000). As currently circumscribed, Didymocarpus ranges from northwest India, eastwards through Nepal, Bhutan, northeast India, Burma (Myanmar), southern China, Vietnam, Laos, Cambodia, Thailand and the Malay Peninsula, with the highest species diversity being found in China and Thailand (Weber and Burtt 1998; Weber et al. 2000; Nangngam and Maxwell 2013; Nangngam and Middleton 2014; Möller et al. 2016). In Vietnam, five species of Didymocarpus have been reported, namely D. bonii Pellegr., D. kerrii Craib, D. poilanei Pellegr., D. pulcher C.B.Clarke and D. purpureobracteatus W.W.Sm. (Pellegrin 1930; Pham 2000; Phuong and Xuyen 2012; Phuong et al. 2014), but D. bonii Pellegr. [= Calcareoboea bonii (Pellegr.) Burtt] has been recognised as Petrocodon bonii (Pellegr.) A.Weber & Mich.Möller (Weber et al. 2011a). Recently, Didymocarpus puhoatensis X.Hong & F.Wen was described from Central Vietnam (Hong et al. 2018).

During medicinal plant investigations in Phu Quoc National Park (NP), Kien Giang Province, south-western Vietnam, several interesting plants of a small species of Gesneriaceae were collected by the authors in 2018–2019. The flowers of these plants have a capitate stigma and other features characterising this plant as Didymocarpus (Wang et al. 1998, Weber et al. 2000). A critical examination of living flowers, herbarium specimens of these plants and comparison with type material and protologues of all closely-related species in Vietnam and neighbouring countries (e.g. Clarke 1874, 1883, 1884; Wallich 1829; Wang et al. 1998; Pham 2000; Nangngam and Maxwell 2013; Phuong et al. 2014; Nangngam and Middleton 2014; Sinha and Datta 2016; Roy 2017), suggested that these specimens were different from the other known Didymocarpus species. These plants with 3-verticillate, petiolate leaves, campanulate calyx, funnelform corolla found in Phu Quoc NP show similarities in these characters with D. pulcher, D. hookeri C.B.Clarke and D. punduanus Wall. ex R.Br. However, it shows significant differences in its vegetative and floral structures (see Table 1) and we describe it here as a species new to science (see also Taxonomic Notes).

Table 1.
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Morphological comparison of D. puhoatensis with its most closely-related taxa (based on Wallich 1829; Clarke 1874, 1883, 1484; Sinha and Datta 2016; Roy 2017).

Characters D. phuquocensis D. pulcher D. hookeri D. punduanus
Plant height (8–)10–19 cm ca. 30.5 cm ca. 40 cm 20.3–25.4 cm
Stem glandular-pubescent puberulous, eglandular villous, eglandular pubescent,
eglandular
Petiole glandular-pubescent puberulous, eglandular villous, eglandular pubescent, eglandular
Leaves 3-verticillate petiolate leaves, terminal whorl of smaller sessile leaves 3–4- verticillate petiolate leaves, terminal whorl of smaller sessile or subsessile leaves usually 3–4- verticillate petiolate leaves, uppermost leaves sessile 3- verticillate petiolate leaves, 2 sessile leaves at the apex
Leaf blade
Margins serrate crenulate to serrate crenulate shallowly crenulate
Base attenuate or cuneate auriculate cordate cordate cordate
Apex attenuate to acute short acute subobtuse subobtuse
Indumentum glandular-pubescent pubescent, eglandular somewhat villous eglandular puberulous, eglandular
Inflorescence axillary or terminal 1(–2)-flowered cyme axial and terminal many-flowered cyme terminal many-flowered cyme terminal many-flowered cyme
Bracts oblong-lanceolate, abaxially densely multicellular glandular-pubescent rounded/suborbic-ular, sparsely viscous pilose rounded, glabrescent ovate, nearly glabrous
Corolla 4.8–5.3 cm long, light purple, glabrous 2.5–3 cm, violet-purple, glabrous or outside sparsely multicellular-villous ca. 1.4 cm long, pale yellow with rose marks or nearly white, outside pilose ca. 2.2 cm long, purple, nearly glabrous
Ovary ca. 3 cm long, densely multicellular glandular-puberulent 1.1–1.8 cm, glabrous 1.1–1.7 cm long, pubescent 0.7–1.3 cm long, glabrous
Style sparsely glandular-puberulent glabrous glabrous glabrous
Capsule 4.4–5.5 cm long ca. 4 cm long 1.3–3.5 cm long ca. 4 cm long

Materials and methods

The descriptions are mainly based on measurements from flowering material of living plants in the field, supplemented by measurements from herbarium specimens. Type specimens of the most closely-related species were examined from the herbaria material from the following herbaria: E, K, HN, IBK, P, VNM, VNMN and W (herbarium codes follow Thiers (2020)), as well as digitised specimen images of Didymocarpus species also being accessed from botanical websites (e.g. https://science.mnhn.fr/, http://www.cvh.org.cn/, https://plants.jstor.org/). All morphological characters were studied under a dissection microscope and are described using the general terminology and standard work of Wang et al. (1998) and Harris and Harris (2001). A distribution map was created using SimpleMappr (http://www.simplemappr.net/) (Shorthouse 2010). Conservation status was assessed using the IUCN Red List Categories and Criteria version 3.1 (IUCN 2019) and inferring from the GeoCAT website (http://geocat.kew.org/editor) (Bachman et al. 2011).

Taxonomic treatment

Didymocarpus phuquocensis N.S.Lý, T.L.Tran & N.G.Cao, sp. nov.

urn:lsid:ipni.org:names:77211380-1
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).

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 Ngọc-Sâm Lý.

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).

Figure 2. 

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

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.

Acknowledgements

This study was financial supporting by the grant of the project TNB.ĐT/14-19/C16 belonging to the Program of Tây Nam Bộ (Vietnam) enabling our expedition in Phu Quoc Island of Kien Giang Province where the new species was discovered. The authors are thankful to Director of the Board of the NIMM and Phu Quoc National Park and their staff for the field trip permit and facilities and to Mr Ha Van Long, Nguyen Minh Hung, Le Duc Thanh and Mrs Ngo Thi Minh Huyen for field assistance. Special thanks to Mr Stephen Maciejewski, Gesneriad Society and Prof Dr Michael LoFurno, Temple University, Philadelphia, the USA for editorial assistance.

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