Sonerila phaluongensis K.S.Nguyen, Aver. & C.W.Lin, 2025

Nguyen, Khang Sinh, Averyanov, Leonid V. & Lin, Che Wei, 2025, New species, Sonerila cornuta and S. phaluongensis (Melastomataceae), from northern Vietnam, Phytotaxa 696 (2), pp. 158-168 : 162-165

Sonerila phaluongensis K.S.Nguyen, Aver. & C.W.Lin , sp. nov. ( Figs. 3 & 4 )

Type: — VIETNAM. Son La Province: Van Ho District, Chieng Xuan Municipality, Co Hong Village, Xuan Nha Nature Reserve, Pha Luong Mountain, primary coniferous and mixed forest with Pinus cernua on very steep slopes along ridge edge composed by brown sandstone at elevation of 1200–1400 m a.s.l., around point 20°41’40.5’’N 104°39’24.7’’E, terrestrial undershrub, flowers pink, anthers yellow, locally common, 13 November 2013, L. Averyanov, N. T. Hiep, N. S. Khang, N.D. Thang, L.D. Qui, CPC 7044 (holotype: LE LE 01253138 http://re.herbariumle.ru/01253138, photos LE LE 01124786 http://re.herbariumle.ru/01124786).

Diagnosis: — Sonerila phaluongensis is a distinctive species among Vietnamese Sonerila , easily distinguishable from others by its dimorphic leaves in each pair. It superficially resembles S. betongensis J.Wai & J.M.Hu (2023: 114) , but differs in having a 4-winged stem (vs. terete), glabrous leaves (vs. densely hispid), a cylindric hypanthium (vs. cupuliform to campanulate) that is sparsely covered with gland-tipped hispid hairs (vs. relatively densely hispid), and the anthers with two prominent basal lobes (vs. basal lobes insignificant).

Etymology:— The specific epithet refers to the name of mountain (Pha Luong), where the new species was discovered.

Description:— Terrestrial perennial herb or small undershrub. Stems suberect, few- to many-branched, quadrangular in cross section, 4-winged, up to 30 cm tall, internodes 0.8–3 cm long, 3–6 mm thick (wings 1–2 mm wide), shorter on the distal branches, glabrous; green, turning yellowish-brown and slightly lignified at age. Leaves glabrous, strongly dimorphic in each pair and held nearly horizontally. Larger leaves: petiole 0.5–1.2 cm long; leaf blade thick, obliquely oblanceolate, 4.5–11 × 1–2.5 cm, at base asymmetric, subcordate, margin denticulate on the upper two-thirds and entire on the lower one-third, apex acuminate to caudate; adaxial surface emerald green; abaxial surface pale green; venation acrodromous, with 3 or 5 veins, median nerve grooved and lateral two or four veins slightly impressed to flat on the adaxial surface, and prominent on the abaxial surface, secondary and tertiary veins inconspicuous. Smaller leaves: petiole 1–2.5 mm long; leaf blade ovate to widely ovate, 0.5–2.5(3.3) × 0.4–1.1 cm; adaxially emerald, abaxially pale green, apex obtuse, acute to shortly acuminate, base cordate, margin entire or sparsely denticulate; venation acrodromous, with 3 or 5 main veins. Inflorescence terminal or axillary, ascending, simple scorpioid cyme with 2–4 flowers; peduncles 4-winged, 0.8–1.5 cm long, green, glabrous or with few scattered purple hispid, glandular hairs. Flowers 3-merous; pedicels 3–5(10) mm long, pinkish to pale green, glabrous. Hypanthium cylindrical, pale pinkish, yellowish or greenish, sometimes with reddish tint; 5–7(8) mm long, 2–2.3 mm in diameter, with scattered minute yellowish-white or purple, hispid, glandular hairs. Sepals 3, broadly triangular, acute, 1.4–1.5 mm long and wide, with entire margins, abaxially with scattered hispid, yellowish-white to pink, glandular hairs. Petals 3, pale pink-purple, obovate-oblong to elliptic, slightly oblique, 12–14 × 6–7 mm, glabrous. Stamens 3, isomorphic, glabrous, filamentose; filaments almost terete, 12–15 mm long, white to light pinkish; anthers subulate, incurved, 10–12 mm long (appendages included), white, with green tint at the base and with light purplish-pink tint at apex, apically with two pores, at base forked into two pale yellow, short, falcate, lobes, ca. 2 mm long. Ovary ca. 2.5 mm long, 3-locular; ovary crown ca. 1 mm high; ovules numerous. Style filiform, pink to purple, 1.7–1.9 cm long; stigma obscurely capitate. Fruit cylindric to obconoid capsule, often slightly constricted at middle, 8–10 × 4–5 mm.

Distribution: — Vietnam, Son La Province (Van Ho District, Xuan Nha Nature Reserve). Probably stenoendemic to Pha Luong Mountain.

Ecology and phenology: —Terrestrial herb or small undershrub. Primary coniferous and mixed submontane forests with Pinus cernua on brown eroded sandstone at elevations of 1200–1400 m a.s.l., commonly found in shady mossy places on steep slopes, sandstone cliffs, and along ridge edges. Locally common. Flowers and fruits observed in November.

Proposed IUCN conservation status: —The new species is currently known only from a single location (Co Hong Village Area) in Pha Luong Mountain near the Vietnam-Laotian border (Son La Province, Van Ho District, and Houaphanh Province, Sop Bao District). Our two field surveys in November 2013 and September 2016 confirmed the existence of only a single population of about 30–50 mature plants. This fact gives sure evidence that Sonerila phaluongensis , like Pinus cernua Aver., K.S.Nguyen & T.H.Nguyên , represents the strict endemism of Pha Luong Mountain. Though the habitat of the newly discovered species is formally protected by regional authorities from any anthropogenic disturbance, it is provisionally accessed as Critically Endangered according to criteria B1ab(ii,iii,v) + B2ab(ii,iii,v); D.

Note: — Sonerila phaluongensis somewhat resembles S. plagiocardia Diels (1932: 117) in its cylindrical hypanthium, covered with scattered hispid glandular hairs and colour scheme of anthers forked at the base into two prominent lobes. However, it differs from the latter in having dimorphic leavess, shorter petioles, oblanceolate leaf blade, and isomorphic stamens. A detailed comparison of S. phaluongensis and closely related species, S. betongensis and S. plagiocardia is presented in Table 2.

vestiture glabrous densely hispid subglabrous wings 4-winged wing absent 4-winged

Leaf dimorphic dimorphic isomorphic petiole length (cm) 0.5–1.2 (in larger leaf) 1–3.5 (in larger leaf) 2–10 ...continued on the next page

Figures

FIGURE 3. Sonerila phaluongensis A. Flowering plant; B, B'. Portions of leaf, adaxial and abaxial surfaces; C, C'. Flower, frontal and side views; D, D', D''. Stamens, frontal, side and back views; E. Hypanthium. Drawing by Che Wei Lin from the type specimen, L. Averyanov, N.T. Hiep, N.S. Khang, N.D. Thang, L.D. Qui, CPC 7044.

FIGURE 4. Sonerila phaluongensis A. Flowering plant in natural habitat; B. Inflorescence; C. Infructescence; D, E. Flower, frontal and half-side views. All photos by Khang Sinh Nguyen of plants used for the preparation of the type herbarium specimen, L. Averyanov, N.T. Hiep, N.S. Khang, N.D. Thang, L.D. Qui, CPC 7044.