Begonia thuanchauensis C.H.Nguyen, V.H.Bui & C.W.Lin, 2025

Nguyen, Cuong Huu, Bui, Huong Van, Le, Anh Tuan, Pham, Dien Van & Lin, Che Wei, 2025, Five new species of Begonia (Begoniaceae) from Vietnam, Phytotaxa 698 (2), pp. 82-100 : 89-92

3. Begonia thuanchauensis C.H.Nguyen, V.H.Bui & C.W.Lin , sp. nov. § Diploclinium ( Figs. 5 & 6 )

Type:— VIETNAM. Northern Vietnam, Son La province, Thuan Chau distinct, Chieng Pac village , secondary evergreen broadleaved forest, occasionally in shady places on middle part of mountain slopes, around point 21°24'37"N, 103°47'40"E, elevation of about 750 m a.s.l., 24 October 2024, B. V.Huong BVHSL241024 (holotype VNMN; isotype VNF) .

Monoecious tuberous herb. Stem tuberous at base, tubers globose or sub-globose with numerous fibrous roots, erect stem absent. Stipules persistent, very shallowly triangular, 1.8–3 × 4–5 mm, herbaceous, slightly keeled, margin sparsely denticulate, sometimes ciliate. Leaves 2–4, alternate, held horizontally; petiole terete, dusky purplish-red to brownish-red, 1.8–13 cm long, 1.5–5 mm diameter, sparsely tomentose-puberulous; leaf blades symmetric or subsymmetric, slightly oblique, ovate to widely ovate-triangular, 2.8–19 × 2.5–14.5 cm, broad side 1.6–8 cm wide, basal lobes cordate, 0.8–5.2 cm long, apex acute to obtuse or attenuate, margin subentire to sparsely denticulate and ciliate, recurved; leaf chartaceous, succulent, adaxially olive green to malachite green with tinged dark blueish-green to dark brown between tertiary veins, subvelutinous, sparsely to moderately densely puberulous or tomentose-puberulous; abaxially dark magenta, sometimes yellowish-green along veins, sparsely to densely tomentose-puberulous on all veins; venation basally 6–8 palmate, 2 or 3 major lateral veins on either side of midrib, other primary veins branching dichotomously, tertiary veins reticulate. Inflorescences protandrous, 1–3 on one plant, terminal, bisexual, up to 4 orders of branching; peduncle 2.5–8 cm long, purplish-red to brownish-red tinged olive green, erect, glabrous or subglabrous; bracts at lowmost one or two nodes (absent flower) persistent, narrowly triangular, 2.5–3.5 × 1–1.5 mm, margin minutely denticulate and ciliate; bracts at upper nodes (with flowers) deciduous, whitish-green, widely elliptic to obovate-elliptic or oblong, sometimes lanceolate-elliptic, 3.5–7 × 2–5 mm, margin subentire to sparsely denticulate, sometimes with sparsely gland-tipped hairs, decreasing in size towards the more distal inflorescence nodes. Staminate flower: pedicel 5–10 mm long, sparsely to densely gland-tipped puberulous, tepals 2, rosy pink, widely ovate to suborbicular, 7–12 × 8.5–10 mm, glabrous; androecium zygomorphic, ca. 4 mm across; stamens golden yellow, 10–15; filaments fused at base; anthers obovate, ca. 2 mm long, 2-locular, apex acute to mucronate, longer than filaments. Pistillate flower: pedicel ca. 6 mm long, gland-tipped puberulous, tepals 3, rosy pink, glabrous, outer 2 widely ovate to suborbicular, 6–8 × 6–10 mm, inner 1 elliptic-lanceolate, 5–6 × 1–2 mm; ovary trigonous-ellipsoid, ca. 4.5 mm long, 3–4 mm thick (wings excluded), bright pink, very sparsely gland-tipped puberulous, apically beaked, the beak ca. 2 mm long; 3-winged, wings unequal, pink, 5–7 mm long, very sparsely gland-tipped puberulous; lateral wings very narrowly crescent-shaped, ca. 0.5 mm wide, abaxial wing very narrowly trapezoid, ca. 1 mm wide, slightly pointed distally; ovary 3-locular, placenta unilamellate; styles 3, fused at base, yellow, ca. 2 mm long, stigmatic band C-shaped and spirally twisted. Capsule not seen.

Distribution and ecology:— Begonia thuanchauensis is currently only known from a single forest area in Chieng Pac commune, Thuan Chau district, Son La province, northern Vietnam ( Fig. 11 ). The new species usually grows in shady places on middle part of mountain slopes covered by evergreen broad-leaved forests at elevations of 600–700 m a.s.l. Flowers in September–November.

Etymology:— The specific epithet is derived from Thuan Chau district, the type locality of this new species.

Conservation status:— Since no special surveys were carried out for delimiting its distribution, and the information about threat is unclear, so this species was considered as meeting the criteria of Data Deficient (DD) in terms of IUCN Red list categories and criteria ( IUCN 2024).

Notes:— The new species resembles Begonia qingchengshanensis Li et al. (2018: 198) in its tuberous habit, symmetric or subsymmetric leaf blades, 3-tepaled pistillate flowers and unilamellate placenta. However, B. thuanchauensis differs in several aspects. It is acaulescent (vs. caulescent), and has leaf lamina puberulous (vs. glabrous), margins subentire to sparsely denticulate and recurved (vs. biserrate, flat), staminate flowers with 2 tepals (vs. 4), and pedicels gland-tipped puberulous (vs. glabrous or very sparsely tomentose).

Figures

FIGURE 5. Begonia thuanchauensis C.H.Nguyen, V.H.Bui & C.W.Lin. A. Habit; B, B'. Portion of leaf, upper and lower surfaces; C. Stipule; D. Bract at lowmost node; E–E'''. Bracts at upper nodes; F, F'. Staminate flower, face and side views; G, G'. Stamens, dorsal and ventral views; H, H'. Pistillate flower, face and side views; I, I'. Style and stigmatic band, dorsal and ventral views; J. Cross section of an ovary. Drawn from the holotype, BVHSL241024 by Che-Wei Lin.

FIGURE 6. Begonia thuanchauensis C.H.Nguyen, V.H.Bui & C.W.Lin. A. Habits; B. Portion of leaf, upper surface, C, D. same, lower surface; E. Inflorescence; F. Inflorescence, closely view; G. Bract at lowmost node (the two on the left), and upper node (right one); H. Cross section of an ovary.All photos by Huong Van Bui of plants used for the preparation of type herbarium specimen BVHSL241024.

FIGURE 11. Distribution map of Begonia quangnamensis (yellow star), B. zonata (pink circle), B. thuanchauensis (orange square), B. lii (blue triangular) and B. ducii (green hexagon) in Indochina (The map is modified from https://geocat.iucnredlist.org/editor).