Impatiens subfalcata Soulad. & Tagane, 2021

Souladeth, P., Tagane, S., Suyama, Y., Ishii, N., Nagahama, A. & Souvannakhoummane, K., 2021, IMPATIENS SUBFALCATA (BALSAMINACEAE), A NEW SPECIES FROM LAOS, Edinburgh Journal of Botany 78 (358), pp. 1-10 : 2-8

Impatiens subfalcata Soulad. & Tagane , sp. nov.

Impatiens subfalcata is easily recognised by its slightly falcate and lanceolate or oblanceolate leaves (length-to-width ratio, 4.6–7.6). In Laos, it is similar to Impatiens attopeuensis Hook.f. in its habit, serrate leaf margins and pinkish purple corolla, but distinguished not only by its slightly falcate leaves but also by its more numerous secondary veins (8–10 pairs in I. subfalcata versus 5–7 pairs in I. attopeuensis ), shorter petioles (subsessile to 0.2 cm long versus 0.5–2 cm long), shorter spur (2.1–2.6 cm versus 4–7 cm long), and smaller dorsal petal (6–7 mm versus 12–14 mm).

– Type: Laos, Champasak Province, Dong Hua Sao National Protected Area ( Bolaven Plateau ), 15°04′02.6′′N, 106°12′29.9′′E, 1147 m elevation, 19 ii 2019, Souladeth P., Tagane S., Sengthong A., Nagahama A., Suyama Y., Ishii N. L2492 (holotype FOF! [ FOF0005191 ]; isotype BKF, HNL, KAG! [ KAG128741 ]) . Figure 1 .

Lithophytic, perennial herb, 20–40 cm tall, stems upright, usually solitary, succulent, with slightly swollen, caudiform base, 0.4–1 cm in diameter at the base, light green, glabrous. Leaves alternate, spirally arranged, crowded towards apex of stems; petioles subsessile to 2 mm long, pale green, glabrous; lamina lanceolate, oblanceolate, slightly falcate, 1.7–10.5 × 0.2–1.7 cm, length-to-width ratio 4.6–7.6, thinly chartaceous, base cuneate to acute, slightly decurrent, apex acuminate, margin serrate, adaxially light grey-green, abaxially pale green, glabrous except upper adaxial side of secondary veins hairy, midrib prominent at base, flat, or slightly prominent or slightly sunken towards apex on both surfaces, secondary veins 8–10 pairs, clearly sunken abaxially, tertiary veins invisible on both surfaces. Flowers solitary, axillary, 1.3–1.5 × 1.1–1.3 cm, 2.9–3.1 cm deep, purplish pink in vivo, pale blue-purple in sicco, with white at centre. Bracts narrowly ovate to triangular, c. 2 mm long, apex acute, light green, glabrous on both surfaces, persistent. Pedicel 1.2–2 cm long, 0.6–0.9 mm in diameter, light green, glabrous. Lateral sepals 2, ovate, triangular, c.5 × 2.8 mm, apex acute to shortly acuminate, yellowish brown with green tips at apex, glabrous on both surfaces. Lower sepal navicular, 5–6 × 3–4 mm, c. 1.5 mm deep, apex acute, white, glabrous, abruptly constricted distally into a straight or incurved spur, spur 2.1–2.6 cm long, white with green tip. Petals 5. Dorsal petal transversely broadly elliptic, 6–7 × 7–9 mm, apically slightly recurved, apex obcordate and mucronate stipitate, mucro to 1.2 mm long, base truncate to rounded, purplish pink, except white near base, with green tip, glabrous on both surfaces. Lateral united petals broadly obovate, 9.5–12 mm long, free, purplish pink except white near base, glabrous on both surfaces; upper lobes obovate-elliptic, 11–12 × 4–4.5 mm, apex round to slightly acuminate emarginate with appendage at apex, appendage 0.5–1.5 mm long, base cuneate; lower lobes obovate-elliptic, 9.5–11 × 5.2–6.1 mm, obovate-elliptic, apex slightly obcordate, with appendage at apex, appendage c. 1.2 mm long. Stamens 5, filaments c. 2.5 mm long, flat, pale pink, anthers pale pink. Ovary c. 2 mm long, c. 0.9 mm in diameter, 1 carpel, 4-lobed, green in vivo, reddish brown in sicco, glabrous. Fruits short, fusiform, 1.2–1.8 cm long, 5–6 mm in diameter, green in vivo, reddish brown in sicco, glabrous. Seeds c.20 per capsule, ellipsoid, 2–2.8 mm long, light brown, pilose.

Distribution. Laos, Champasak Province (known only from the type locality).

Habitat and ecology. Impatiens subfalcata grows on dripping rocky walls at a waterfall (see Figure 1A, B ).

Etymology. The species epithet, subfalcata , is derived from the slightly falcate nature of the leaves of this species.

Vernacular name. ທຽນໃບດາບ (thien baidab), suggested here; in Lao, thien means Impatiens species and baidab means ‘falcate leaf’.

Preliminary conservation assessment. Impatiens subfalcata is currently known only from a single population with c.30 individuals at its type locality on the Bolaven Plateau ( Figure 2 ) and may be restricted to dripping-wet, rocky walls. Therefore, the most appropriate initial assessment of the IUCN conservation status for this species is Critically Endangered ( CR) under criteria D ( IUCN, 2012).

Note 1. The possibility that Impatiens subfalcata is simply an extreme variation of I. attopeuensis has been considered. Impatiens attopeuensis is a variable species, and two species, I. longecalcarata Tardieu and I. monotricha Hook.f. , both described from Bolaven, are now treated as synonyms of I. attopeuensis in the checklist of Laos ( Newman et al., 2017 –). However, Impatiens subfalcata is different from these species not only in morphology (as shown in the diagnosis above and the Table) but also in its habitat and elevation range. Whereas Impatiens subfalcata grows on dripping rocky walls in wet evergreen forest at an elevation of 1147 m, I. attopeuensis , including the two synonymised species, grows on rocks or soils near streams or waterfalls at lower elevations, 199– 900 m. The types of Impatiens attopeuensis (a drawing in Hooker’s Icon. Pl. 29: t. 2853; based on Harmand s.n., P), I. longecalcarata (Poilane 15955, P) and I. monotricha (Harmand 1137, P) were collected in Attapeu Province (at 600–900 m for I. longecalcarata and 200–300 m for I. monotricha ).

We collected Impatiens attopeuensis at Tad Tayicseua Waterfall (Souladeth et al. L2573) at an elevation of 634 m ( Figure 3 ) and at Tad Xe Ponglai Waterfall (Souladeth et al. L3222 and Souladeth et al. L3974) at 199 m ( Figure 4 ). These collections are deposited in FOF and KAG. We conducted wide-ranging surveys in the four provinces of Attapeu, Champasak, Salavan and Sekong on the Bolaven Plateau in December 2018, February, July and December 2019, and September 2020, and never encountered Impatiens attopeuensis above 1000 m elevation or observed any morphological intermediates between the two species. Given this, we consider that Impatiens subfalcata is a distinct species. Because Impatiens subfalcata is morphologically most similar to I. attopeuensis , it may have recently originated from it. Further phylogenetic analysis will help to clarify the relationship between these two species.

Note 2. Impatiens subfalcata is also similar to I. notoptera Hook.f. (sometimes treated as a synonym of I. semounensis Hook.f. ), known from Stung Treng, Cambodia, but can be distinguished by its narrower leaf blades (length-to-width ratio 4.6–7.6 in I. subfalcata versus 1.8–2.4 in I. notoptera ), shorter petiole (subsessile to 0.2 cm long versus 2–4 cm long), and larger lateral petals (10–12 mm long versus c. 6 mm long).

Note 3. The recent infrageneric classification of Impatiens by Yu et al. (2016) divided the genus into two subgenera (subg. Clavicarpa S.X.Yu ex S.X.Yu & Wei Wang and subg. Impatiens ), and further divided subgenus Impatiens into seven sections (sect. Fasciculatae X.S.Yu & Wei Wang, sect. Impatiens , sect. Racemosae Hook.f. et Thomson , sect. Scorpioidae S.X.Yu & Wei Wang, sect. Semeiocardium (Zoll.) S.X.Yu & Wei Wang , sect. Tuberosae S.X.Yu & Wei Wang and sect. Uniflorae Hook. f. & Thomson). Impatiens subfalcata belongs to section Uniflorae by its fusiform capsule, conspicuously turgid at the middle, many ovules per locule, and ellipsoid seeds.

The Table summarizes the morphological differences between Impatiens subfalcata , I. attopeuensis and I. notoptera .

Figures

Figure 1. Impatiens subfalcata Soulad. & Tagane. A and B, Habitat (individuals indicated by red arrow); C, habit; D, portion of lower leaf surface; E, leaves; F, flower bud; G, flower (front view); H, flower (lateral view); I, spur; J, petals; K, fruit; L, seeds. Scale bars (yellow): E, 5 cm; F, I and J, 1 cm; L, 2 mm. All photographs from Souladeth et al. L2492. Photographs: S. Tagane.

Figure 2. Distribution of Impatiens subfalcata and I. attopeuensis (including the type locality of I. longecalcarata and I. notoptera, both synonyms of I. attopeuensis) in Laos and Cambodia.

Figure 3. Impatiens attopeuensis at Tad Tayicseua Waterfall, at elevation 634 m, Champasak Province. A, Habit; B, lower leaf surface; C, flower (front view): D, flower (lateral view). All photographs from Souladeth et al. L2573. Photographs: S. Tagane.

Figure 4. Impatiens attopeuensis (form corresponding to I. longecalcarata) at Tad Xe Ponglai Waterfall, at elevation 199 m, Attapeu Province. A and B, Habit; C, individual and flowers (front view); D, flower and leaf (lateral views); E, lower leaf surface; F, flower (back view); G, fruit. A and C from Souladeth et al. L3222; B and D–G from Souladeth et al. L3974. Photographs: A and C, S. Tagane; B and D–G, K. Souvannakhoummane.