Begonia xuansonensis T.V.Do, Nuraliev & Y.M.Shui, 2021

Do, Truong Van, Shui, Yu-Min, Le, Phuc Van, Huynh, Hue Thu Thi, Kuznetsov, Andrey N., Kuznetsova, Svetlana P. & Nuraliev, Maxim S., 2021, Begonia xuansonensis, a new orange-flowered species of Begoniaceae from northern Vietnam, Phytotaxa 510 (3), pp. 251-262 : 252-257

Begonia xuansonensis T.V.Do, Nuraliev & Y.M.Shui , sp. nov. ( Figs. 1 , 2 , 3A–E )

B. sect. Platycentrum ( Klotzsch 1855: 243) de Candolle (1859: 134).

Diagnosis: —The new species is most similar to B. lowiana King (1902: 67) and differs from the latter species mainly in stipules 4–6 × 1.5–2 mm (vs. 15–20 × 8–10 mm), reddish-orange to orange (vs. white to pink) coloration of tepals and ovary, abaxially densely red setose (vs. glabrous) inner tepals of staminate and pistillate flowers, and 3 (vs. 2 or 3) styles. In addition, it is similar to B. cathayana Hemsley (1908: 8202) , differing in stipules 4–6 mm (vs. 10–30 mm) long, uniformly dark green adaxial side of leaf blade (vs. adaxial side with a red-purple or pale green V-shaped band), reddish-orange to orange (vs. pink or orangish) tepal coloration, abaxially densely red setose (vs. glabrous) inner tepals of a staminate flower and 3 (vs. 2) styles in a pistillate flower.

Type: — VIETNAM. Phu Tho Province: Thanh Son district, Xuan Son commune, Xuan Son National Park , N 21°07.325’, E 104°56.269’, elev. 851 m, 26 October 2020, Do Van Truong ĐVT 388 (holotype: VNMN!; isotypes: HN!, MW: MW0595760!) .

Description:—Herb terrestrial, perennial, caulescent, without tuber. Stem upright, cane-like, reddish-brown, 30–50 cm long, 4–6 mm in diam., fleshy; internodes 3–6 cm long, densely covered with red setae. Stipules pale green, lanceolate to narrowly triangular, 4–6 × 1.5–2 mm, margin entire, densely villous on both sides, caducous. Leaves distichous. Petiole brown, terete, (2)3.5–10(12) cm long, 2–3.5 mm in diam., fleshy, densely covered with red setae. Leaf blade asymmetric, oblique, ovate or broadly ovate, 7–15 × 6–9 cm, papery, base obliquely cordate with basal lobes 1–2.5 cm long, apex acuminate, margin biserrate, adaxially dark green, setulose, abaxially grayish-green and sometimes with reddish veins, sparsely setulose but with age becoming subglabrous and sparsely hairy on veins; venation palmate- pinnate, primary palmate veins 6–8, secondary pinnate veins in 2–3 pairs, tertiary veins obviously reticulate, venation slightly sunken adaxially and prominently raised abaxially. Inflorescence axillary, cymose, bisexual, with 2 pistillate flowers at base and 3–4 distal staminate flowers; all axes red, densely red setose; peduncle erect, 3–3.2 cm long; bracts caducous, unknown. Staminate flower zygomorphic, disymmetric; pedicel straight, 18–22 × 1 mm; tepals 4, unequal, reddish-orange to orange, densely red setose abaxially and glabrous adaxially; outer tepals 2, ovate to triangular, 12–15 × 9–11 mm, apex acuminate, margin with narrow teeth protruding into hairs; inner tepals 2, oblong lanceolate, 9–10 × 4–5 mm, apex acute to obtuse, margin entire; androecium actinomorphic, with a short androphore; stamens ca. 60–80, yellow, glabrous; filaments 0.8–1 mm long; anthers obovate to oblong, 1–1.2 × 0.8–1 mm, apex rounded, extrorse-latrorse, dehiscing by longitudinal slits. Pistillate flower asymmetric; pedicel straight, 11–14 × 1.2–1.5 mm; tepals 5, unequal, reddish-orange to orange, densely red setose abaxially and glabrous adaxially; outer tepals 2, ovate, 11–13 × 7–9 mm, apex acuminate, margin with narrow teeth protruding into hairs; inner tepals 3, unequal, lanceolate, 8–10 × 4–5 mm, apex acute to acuminate, margin of two smaller tepals entire, margin of larger tepal with narrow teeth protruding into hairs; styles 3, greenish-yellow, 2.5–3 mm long, free; stigmatic band 2-lobed (U-shaped), twisted; ovary reddish-orange, obovate or turbinate, prominently winged, 0.9–1 × 0.6–0.7 cm, densely red setose, trilocular; placentation axile, placentas bilamellate. Capsule with thin fruiting pedicel 2.8–3.2 cm long, yellowish-brown, obovate or turbinate, 1–1.2 × 0.8–0.9 cm, with three unequal wings; dorsal wing larger, 1.6–1.8 × 1.5–1.7 cm; two lateral wings smaller, 1.3–1.4 × 0.3–0.4 cm; capsule sparsely covered with reddish trichomes becoming glabrous when mature, with persistent styles. Seeds numerous, brown, ovate, 0.1–0.2 mm long.

Additional specimen examined: — VIETNAM. Phu Tho Province: Thanh Son district, Xuan Son commune, Xuan Son National Park , in the forest, along marked trail from Du village to the peak 1200 m ( Ten Mountain ), N 21°07’18’’, E 104°56’24’’, 15 June 2014, elev. 800 m, Nuraliev M.S. NUR 1070b (photo LE: LE01093236 http:// en.herbariumle.ru/?t=occ&id=74364) .

Etymology: —The specific epithet refers to Xuan Son National Park, where this species was found and collected.

Phenology: —Flowering in June; fruiting from October to November.

Distribution and ecology: — Begonia xuansonensis is currently known only from the slope of Ten mountain in Xuan Son National Park, Phu Tho Province, northern Vietnam ( Fig. 4). It grows under the canopy of broadleaved evergreen forest, in humid areas with exposed limestone, at elevations of 700–850 m a.s.l.

Conservation status: —Five locations comprising more than two hundred individuals of Begonia xuansonensis were found in the core zone of Xuan Son National Park. The core zone is almost entirely covered by well-protected areas, including those with primary forests. Furthermore, the new species is likely to regenerate successfully, as many seedlings were observed surrounding the mature plants and not yet gathered by the local people. Thus, the persistence of the new species might probably not be at risk in the near future. According to the IUCN Categories and Criteria ( IUCN 2019), B. xuansonensis is preliminarily assessed here as NT (Near Threatened) on the basis of the number of locations (≤ 5) and number of mature individuals (<250).

Sectional placement: —We preliminarily assign the new species to the morphologically diverse B. sect. Platycentrum sensu Shui et al. (2019b), whose infrageneric system of Begonia is based on investigation of plastid genomes. The placement of B. xuansonensis within the taxonomic system of the genus is to be verified by employment of molecular phylogenetic data.

Taxonomic relationships: — Begonia xuansonensis is morphologically most close to B. lowiana endemic to Pahang State of Peninsular Malaysia ( Kiew 2005, Hughes et al. 2018). The sectional placement of B. lowiana is currently uncertain ( Shui et al. 2019b). The similarity between B. xuansonensis and B. lowiana is remarkable for the reason of their habitation in different floristic regions, with more than 1850 km between known populations of these two species. The most important differences of B. xuansonensis from B. lowiana are: stipules 4–6 × 1.5–2 mm vs. 15–20 × 8–10 mm, reddish-orange to orange vs. white to pink coloration of tepals (in both staminate and pistillate flowers) and ovary, and abaxially densely red setose vs. glabrous inner tepals of both staminate and pistillate flowers ( Table 1). Although the presence of three styles in a pistillate flower is currently accepted for B. lowiana ( Kiew 2005) , presence of two styles is indicated in the protologue of this species ( King 1902, see also Ridley 1922), which is likely to be one more difference from B. xuansonensis .

Among the known species of Begonia sect. Platycentrum , B. xuansonensis is also similar to B. cathayana in having a cane-like, caulescent stem, prominent red hairiness of vegetative parts and inflorescence axes, orange hues in flower coloration, staminate flowers with four unequal tepals and outer tepals abaxially covered with red setae, pistillate flowers with five tepals abaxially covered with red setae, and a hairy ovary. Begonia xuansonensis differs from B. cathayana mainly in stipules 4–6 mm (vs. 10–30 mm) long, uniformly dark green adaxial side of leaf blade (vs. adaxial side with a red-purple or pale green V-shaped band), abaxially densely red setose (vs. glabrous) inner tepals of a staminate flower and three (vs. two) styles in a pistillate flower. Detailed morphological comparison of these species is provided in Table 1 and Figure 3 .

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Begonia xuansonensis resembles B. baviensis Gagnepain (1919: 195) , another member of B. sect. Platycentrum , in several aspects, including the pronounced reddish hairiness of most parts of the plant ( Table 1, Fig. 3 ). The main differences of the new species from B. baviensis are: stipules 4–6 mm (vs. 15–20 mm) long, unlobed (vs. lobed) leaf blade, oblong lanceolate (vs. obcordate) abaxially densely red setose (vs. glabrous) inner tepals of a staminate flower, inner tepals of a pistillate flower shorter than outer tepals (vs. as long as outer tepals), and three (vs. two) styles in a pistillate flower.

Additionally, B. xuansonensis differs remarkably from most other species of B. sect. Platycentrum in reddishorange to orange tepal coloration. According to Shui et al. (2019b), the tepals are white, pink, rarely red, orange and yellow in this section.

The new species shares the same vegetative habit and groundplans of staminate and pistillate flowers with two Vietnamese representatives of B. sect. Petermannia ( Klotzsch 1854: 124) de Candolle (1859: 128): B. rubrosetosa Aver. in Averyanov & Nguyen (2012: 20) and B. saolaensis Y.M.Shui, T.A.Le & C.T.Vu in Shui et al. (2019: 61) ( Fig. 3 ). Both these species are endemic to Thua Thien Hue Province; they are illustrated in Fig. 3 for the purpose of easy field recognition of species belonging to this morphological group. Finally, B. xuansonensis is somewhat similar to B. alta Aver. in Averyanov & Nguyen (2012: 6), a member of B. sect. Petermannia ( Shui et al. 2019b). These species grow sympatrically on slopes of Ten Mountain in Xuan Son National Park at elevations of 700–1000 m a.s.l. Begonia xuansonensis is readily distinguished from B. alta by hairy (vs. glabrous) leaves, tepals and ovary, five (vs. three) tepals in a pistillate flower and reddish-orange (vs. white to pink) tepal coloration. Anthetic flowers of Begonia xuansonensis were recorded in June, whereas B. alta is known to flower in January–February.

Figures

FIGURE 1. Begonia xuansonensis. A. Habit; B, C. Leaf, adaxial and abaxial surfaces; D. Distal part of shoot; arrow indicates stipule; E. Portion of internode; F. Inflorescence. Photos: A, C, F from Nuraliev NUR 1070b by M.S. Nuraliev; B, D, E from Do Van Truong ĐVT 388 by T.V. Do.

FIGURE 2. Begonia xuansonensis, flowers and fruit. A. Pistillate flower bud, lateral view; B. Pistillate flower, lateral view; C. Pistillate flower, front view; D. Inflorescence branch with two dehisced capsules; note numerous small seeds scattered around; E. Staminate flower bud, lateral view; F. Staminate flower, front view. Photos:A–C, E, F from Nuraliev NUR 1070b by M.S. Nuraliev; D from Do Van Truong ĐVT 388 by T.V. Do.

FIGURE 3. Morphological comparison of Begonia xuansonensis (A–E) with similar Vietnamese species: B. cathayana (F–H), B. baviensis (I–M), B. rubrosetosa (N–R) and B. saolaensis (S–W). A, F, I, N, S. Habit; B, J, O, T. Inflorescence; C, G, K, P, U. Staminate flower; D, H, L, Q, V. Pistillate flower; E, M, R, W. Ovary/capsule. Photos: A–E, I, K by M.S. Nuraliev; F–H by Y.-M. Shui; J, L, M by T.V. Do; N–R by L.V. Averyanov (LE: LE01088955 http://en.herbariumle.ru/?t=occ&id=65474, LE01088957 http://en.herbariumle. ru/?t=occ&id=65476); S–W by T.A. Le (Shui et al. 2019a).