Melastoma souladethianum Souvann. & Nob. Tanaka 2026, sp. nov.

Melastoma souladethianum Souvann. & Nob.Tanaka sp. nov. urn:lsid:ipni.org:names:77372122-1 Figs 2, 3 Diagnosis Similar to Melastoma pellegrinianum (H.Boissieu) Karst.Mey., but it differs in having terete young branches (vs. quadrangular in M. pellegrinianum), inflorescences with 2–4 flowers (vs. more than 15), hypanthia sparsely covered in bristles with sparse strigose hairs (vs. bristles without strigose hairs). Etymology The specific epithet honors Dr. Phetlasy Souladeth, curator of the FOF herbarium of the National University of Laos, who led our field trips and is an excellent plant taxonomist. She has greatly contributed to elucidating the flora of Laos and has described more than 30 new species and recorded over 100 species to Laos’ flora. Type material Holotype LAOS • Khammouane Province, Khounkham District, The Rock Viewpoint at Phou Pha Marn, limestone karsts; 455 m a.s.l.; 19 Mar. 2024; Tagane et al. Z1213; holotype FOF [FOF0007123]; isotypes KAG [KAG187615], VNM. Paratypes LAOS • Khammouane Province, Khounkham District, The Rock Viewpoint at Phou Pha Marn, limestone karsts; 455 m a.s.l.; 9 Sep. 2024; Tagane et al. Z1915; FOF [FOF0006984] • same data as for preceding KAG [KAG188567], VNM • same locality as for preceding; 2 July 2022, Souladeth et al. PPM16; FOF; • same locality as for preceding; 454 m a.s.l.; 8 Mar. 2025, S. Tagane et al. Z2396; FOF [FOF0007645], KAG [KAG201118], VNM. Vernacular nameeøນø້ęeoi̊ດຊົuoi̻ En R Phetsomphou (meaning: purple diamond’sMelastoma'). Description Deciduous shrubs, up to 2.5 m tall. Twigs terete, slightly zigzag, older twigs grayish to brown, glabrous; younger twigs green, sparsely covered with bristles and narrowly linear scales of 1–1.5 mm long at the base. Leaves opposite, crowded at the top of twigs, lamina lanceolate, 8–12 × 3–5.5 cm, base slightly cordate to rounded, margin denticulate, apex acuminate; adaxially surface dark green, covered with white strigose hairs which are longer (0.4–1 mm long) and incurved along the margins, abaxially surface light green, sparsely covered with white strigose hairs which are denser on the midrib, longitudinal veins and secondary veins (0.4–1.2 mm long); longitudinal nerves running from base to apex, 4–6 in addition to the midrib, prominent abaxially; secondary veins subparallel, 4–10 mm apart, adaxially inconspicuous, abaxially immersed in the lamina; tertiary veins branching into a conspicuous reticulate network; petioles 2–3.5 cm long, ca 1 mm in diameter, covered by strigose hairs of 0.4–1.4 m long. Inflorescences cluster on terminal twigs with 2–4 flowers. Pedicels slender, 2–6 mm, ca 3 mm in diameter, with dark red to brown fine soft strigose hairs. Bracts rosetted, caducous, lanceolate, 6–10 × 2–3.5 mm, strigose with light green to pink hairs. Hypanthium funnel-shaped, 10–15 × 8–12 mm, sparsely covered with dense bristles and sparsely strigose hairs, bristles 6–12 mm long, up to 0.5 mm wide at the base, apex markedly acuminate, margin entire; sepals 5, broadly triangular to lanceolate, 3–3.5 × 4–6 mm, apex markedly acuminate, pink to light green, with sparse minute bristles on margins and apex. Petals 5, broadly obovate, 28–30 × 20–25 mm, purple, with minute bristles on outside surface, apex acute to obtuse. Stamens anisomorphic comprising 5 longer stamens and 5 shorter stamens; longer stamens 40–45 mm long; filaments 27–35 mm long, yellowish; anthers ca 10–12 mm long, purple; connective strongly incurved, 18–25 mm long, with a basal bifurcate yellow extension, glabrous; shorter stamens 10–15 mm long, filaments yellowish, ca 6–10 mm long, linear-rostrate; anthers dark purple, connective not prolonged, with a yellow basal bifurcate extension, glabrous. Pistil ca 35 mm long; ovary ovoid to ellipsoid, 8–12 × 5–6 mm, greenish, densely covered with bristles from middle part to apex; style filiform, curved apically, 27–32 mm long; stigma truncate. Fruits dry capsule, dehiscent, splitting longitudinally from apex, ellipsoid to cup shaped, 10–14 × 6–8 mm, strigose to glabrescent; seeds numerous, brownish. Distribution, habitat and phenology Melastoma souladethianum sp. nov. is so far known only from the type locality in Phou Pha Marn limestone karst, Khounkham District, Khammouane Province. It grows in open areas on limestone karst, where it is associated with Sterculia sp. (Malvaceae), Phyllanthus mirabilis Müll.Arg. (Phyllanthaceae), Microchirita sp. (Gesneriaceae), Ficus sp. (Moraceae) and Pentacme siamensis (Miq.) Kurz (Dipterocarpaceae). Flowering is from March to April, and fruiting from May to June. The leaves come out in May at the beginning of the rainy season and fall in January in the middle of the dry season. Preliminary conservation status To date, we have observed only one population with fewer than 20 mature individuals. Neither seedlings or saplings could be observed, not even during the rainy season. The area of occupancy (AOO) is 4 km 2 and the extent of occurrence (EOO) follows the area of the Phou Pha Marn Karst, which is 32 218 km 2. This species is preliminarily assessed as Critically Endangered (CR) under categories B2 and D. Notes Melastoma souladethianum sp. nov. blooms from March to May (dry season), before the emergence of its leaves. In contrast, most other species of Melastoma in Laos and the surrounding region flower in the rainy season and into the beginning of dry season from June to the end of October and while in leaf. This specific character might be an adaptation to the survival and reproduction under the severe environment of the limestone karst. Phylogenetic studies including M. souladethianum, sp. nov. with related taxa, as well as ecological studies, including pollination syndromes and seed dispersal strategies, may provide further interesting insights into the reproductive strategy, evolution and speciation of Melastoma.