Strobilanthes bolavenensis K. Yamaz., Tagane & Soulad. 2021, sp. nov.

Strobilanthes bolavenensis K. Yamaz., Tagane & Soulad., sp. nov. Figs. 2 & 3.

Type:— LAOS. Champasak Province, Dong Hua Sao National ProtectedArea [Bolaven Plateau], Paksong District, near Nong Luang Village; in lower montane forest, 15°04′22.14″N, 106°12′20.71″E, 1167 m, 18 December 2019, Souladeth P., Tagane S., Kongxaysavath D., Rueangruea S., Somran S., Suyama Y., Suzuki E. L3452 [fl. & young fr.] (holotype FOF!, isotype: KAG [KAG155816!]).

Strobilanthes bolavenensis is most similar to S. guangxiensis endemic to Guangxi, China in having purple corolla and hairy simple spike inflorescence but distinguished by its smaller plant height up to 30 cm tall (vs. ca. 1 m tall), rachis of inflorescence sparsely puberulent (vs. densely pubescent), bracts and bracteoles covered with short appressed yellowish brown hairs near apex adaxially (vs. glabrous), larger calyx length 1.4–1.6 cm long (vs. 1 cm long), and calyx acute at apex (vs. acuminate).

Perennial herb, 30 cm tall. Stems erect, terete, ca 5 mm in diam. at base, dull brownish yellow to dark yellow-green, densely covered with short appressed brown hairs when young, glabrescent, lenticellate. Leaves opposite, subequal or sometimes slightly anisophyllous; blades papery, lanceolate to ovate, or narrowly elliptic, (1.3–)3.8–13 × (0.7–) 2–4 cm, length/width ratio (1.9–)2.4–2.9, apex acute to acuminate, margin subentire to slightly crenate, recurved when dry, base cuneate, symmetric, adaxially dark green, glabrous, with linear cystoliths 0.1–0.2 mm long (visible under stereoscopic microscope), abaxially light pale green, covered with short brown hairs, especially denser on midrib and secondary veins, midrib prominent abaxially, secondary veins 4–9 pairs, prominent abaxially, tertiary veins scalariformreticulate; petioles (0.2–) 0.4–1.6 cm long, covered with short appressed brown hairs. Inflorescence spike, on terminal and on old stems behind leaves, 3.4–8.9 cm long, 4–10-flowered, peduncle subsessile, rachis sparsely puberulent, internode 0.8–1 cm long; inflorescence bracts oblong-lanceolate, ca. 1 × 0.4 cm, apex acuminate, margin ciliate, adaxially covered with short appressed yellowish brown hairs near apex, covered with linear cystoliths, abaxially covered with dark brown glands and 2 types yellowish brown hairs, the longer hairs ca. 0.6 mm long, the shorter ones ca. 0.1 mm long; bracts persistent, spathulate-lanceolate to elliptic-lanceolate, 1–1.5 × 0.2–0.4 cm, apex acuminate, margin ciliate, adaxially covered with short appressed yellowish brown hairs near apex, covered with linear cystoliths, abaxially covered with glands and 2 types of yellowish brown hairs; bracteole 2, oblong-lanceolate, 1–1.5 × 0.2–0.3 cm, margin ciliate, adaxially covered with linear cystoliths, near apex part with short appressed yellowish brown hairs, abaxially densely covered with glands and 2 types yellowish brown hairs. Calyx 1.4–1.6 cm long, 5-lobed almost to base, outside covered with 2 types of yellowish-brown hairs, glabrous inside; tube 2–3 mm long; lobes subequal, linear, 1.2–1.3 cm, accrescent in fruit, up to 1.9 cm long, apex acute. Corolla purple, funnel-shaped, ca. 4.1 cm long, ca. 0.9 cm in diam., membranous, glabrous outside, pilose to throat inside, 5-lobed; lobes broadly elliptic, ca. 6 mm long, apex emarginate. Stamens 4, didynamous; the shorter pairs of filaments ca. 2 mm long, glabrous; the longer pairs ca. 5 mm long, glabrous inner side and pilose only outer side, anthers ca. 2.8–3 mm long, dorsifixed, attached at lower 1/3. Ovary narrowly obovoid, 3.7–4 mm long, ca. 0.9 mm in diam., densely hairy at apex, style 2.5 cm long, pilose except near base, stigma, ca. 1 mm long, glabrous. Capsule (immature) narrowly ellipsoid-ovoid, 10 mm long, ca. 2.5 mm in diam., sparsely covered with glangular hairs. Seeds (immature) 4, suborbicular, 1.5–2 mm in diam., strongly fattened, light yellow brown, pubescent.

Distribution and habitat:— LAOS, known only from the type locality. Strobilanthes bolavenensis was found in hill evergreen forests at 1167 m elevation, where it grows with Hymenasplenium excisum (C. Presl) S. Linds. (Aspleniaceae), Cornopteris opaca (D. Don) Tagawa (Athyriaceae), Cyclosorus ciliatus (Wall. ex Benth.) Panigrahi (Thelypteridaceae), Canscora andrographioides Griff. ex C. B. Clarke (Gesneriaceae), Sonerila yunnanensis Jeffrey ex W. W. Sm. (Melastomataceae).

Etymology:— This specific epithet “ bolavenensis ” refers to the type locality of this species.

Vernacular name:— Hom Bolaven [ຫ້ອມບໍລະເວນ].

Phenology:— Flowering specimens were collected in December.

Preliminary conservation assessment:— Critically Endangered (CR). Strobilanthes bolavenensis is known only from a single population with ca. 10 mature individuals in Bolaven Plateau, at elevation of 1167 m, though we had intensive floristic inventories in the area seven times in 2018–2020 (e.g. Nagahama et al. 2019, Souladeth et al. 2020, Suddee et al. 2020, Tagane et al. 2021a, b, Yamazaki et al. 2021). This situation satisfies the CR (critically endangered) status in criterion D of IUCN Red List Categories (IUCN 2012).

Note:— In Laos, 27 species of Strobilanthes have been known, among which S. bolavenensis is easily distinguished from them by a combination of small plant habit (30 cm tall), abbreviated simple spike inflorescence 3.4–8.9 cm long, bracts adjacent to rachis, and bracts and bracteoles densely covered with yellowish brown hairs abaxially.

Strobilanthes bolavenensis is phenotypically most similar to S. guangxiensis in having lenticellate twigs, purple corolla, simple spike inflorescence with flowers paired and on two sides of spike, and bracts imbricate in spike. However, it is clearly distinguished from S. guangxiensis by the diagnosis mentioned above. In addition, S. bolavenensis is different from S. guangxiensis in having smaller cystoliths on adaxial side of lamina (0.1–0.2 mm long vs. 0.2–0.5 mm long in S. guangxiensis), inflorescence on terminal or on old stem behind leaves (vs. terminal and in axils of leaf base), and larger ovary 3.7–4 mm long (vs. ca. 3 mm long). For detail comparison, see Table 1.