IMPATIENS SUBFALCATA (BALSAMINACEAE), A NEW SPECIES FROM LAOS

Impatiens subfalcata Soulad. & Tagane, a new species of Balsaminaceae from southern Laos, is described and illustrated. The new species is morphologically similar to Impatiens attopeuensis and I. notoptera in having succulent stems, serrate leaf margins and pinkish flowers, but distinguished from these two by its falcate lanceolate or oblanceolate leaves, more numerous secondary veins, shorter petioles and smaller dorsal petals.


Introduction
Impatiens L. is a genus of Balsaminaceae comprising more than 1000 species. The genus is mainly distributed in tropical and subtropical Africa and Eurasia, with most species found in five hotspots, namely Southeast Asia and southwestern China, eastern to central Himalaya, southern India, tropical Africa, andMadagascar (Grey-Wilson, 1980, 1989;Chen et al., 2007;Cho et al., 2016;Ruchisansakun et al., 2018).
During our recent floristic surveys on the Bolaven Plateau, southern Laos, in 2017-2019, a species of Impatiens belonging to section Uniflorae Hook.f. & Thomson was collected 2 A new species of Impatiens from Laos at Tad Seua Waterfall ( Figure 1A). It is most similar to Impatiens attopeuensis Hook.f. from Attapeu Province of southern Laos, but a comparison with the type specimens and protologue of I. attopeuensis as well as those of the other species in Laos and surrounding countries revealed that it differs from any of the previously known species. Therefore, we describe it as a new species, Impatiens subfalcata Soulad. & Tagane, the thirty-sixth Impatiens species from Laos. The measurements in the following description are based on the herbarium materials we collected.
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.
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 ( (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

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A new species of Impatiens from Laos 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 3. The recent infrageneric classification of Impatiens by Yu et al. (2016)  . Impatiens subfalcata belongs to section Uniflorae by its fusiform capsule, conspicuously turgid at the middle, many ovules per locule, and ellipsoid seeds.