Two new species of Sonerila Roxb. (Melastomataceae) from Laos

Two new species of Sonerila Roxb. (Melastomataceae), S. erectifolia Phonep., Soulad. & Tagane sp. nov. from southern Laos, and S. souvannii Phonep. & Soulad. sp. nov. from central Laos, are described and illustrated. Comparisons with morphologically similar species are presented, along with ecological information and preliminary conservation status. A key to the species of Sonerila in Laos is also provided.


Introduction
Sonerila Roxb. is a large plant genus in the family Melastomataceae Juss. consisting of about 180 accepted species (POWO 2020). The genus is distinguished from morphologically similar genera in Melastomataceae by the presence of scorpioid cymes and trimerous fl owers . Recent molecular phylogenetic studies have indicated that Sonerila is monophyletic (Zhou et al. 2019). The genus is distributed throughout tropical South and Southeast Asia, from India and Sri Lanka to the Indo-Pacifi c (Suddee et al. 2014;Dang et al. 2016;Shin et al. 2020). The most recent taxonomic treatment of Sonerila in Indochinese region is provided by the Flore générale de l'Indo-Chine (Guillaumin 1913), which included ten species. However, this number has since risen to 15, with 11 species now recognised in Vietnam (Ho 2003;Dang et al. 2016), six in Cambodia (Hansen 1989;Chen & Renner 2001;Cho et al. 2015;Shin et al. 2020), and seven in Laos, namely, S. bolavenensis Soulad., Tagane & Suddee, S. lecomtei Guillaumin, S. neodriessenioides C.Hansen, S. plagiocardia Diels, S. tenera Royle, S. vatphouensis Munzinger & C.V.Martin, and S. yunnanensis Jeff rey ex W.W.Sm. (Newman et al. 2017 onwards;Souladeth et al. 2021). In other countries of the region, seven species have been found in China (Chen & Renner 2001;Lin 2015) and 15 in Thailand Sae Wai & Hu 2020).
During botanical surveys in Vientiane Capital in central Laos and on the Bolaven Plateau in southern Laos from 2019 to 2020, two unknown species of Sonerila were collected and found to be morphologically diff erent from all the other species of the genus known in Laos, as enumerated by Newman et al. (2017 onwards). After critically comparing this material with specimens from herbaria, the fi rst of the two unknown species was found to be similar to Sonerila violifolia Hook.f. ex Triana from Myanmar and Thailand. The second species was considered to be most similar to S. cardamomensis S.H.Cho from Cambodia, but was also found to share some characters with S. dongnathamensis Suddee, Phutthai & Rueangr. and S. tuberosa C.Hansen from Cambodia and Thailand. However, this examination also led to the identifi cation of distinct morphological characters with which the two taxa could be readily diff erentiated from these species, and we thus describe and illustrate them as new species here. We also provide information on their ecology and preliminary conservation status. Consequently, the number of species of Sonerila known in Laos is increased to nine, for which we provide an identifi cation key.

Material and methods
Field surveys were carried out in Naxaythong District of Vientiane Capital and on the Bolaven Plateau during August 2019 and September 2020. The specimens were photographed and deposited in the herbarium of the National University of Laos (FOF), Herbier National du Laos (HNL), the Forest Herbarium Bangkok (BKF), the Royal Botanic Garden Edinburgh Herbarium (E) and The Kagoshima University Museum (KAG); acronyms of herbaria follow Index Herbariorum (Thiers continuously updated). The descriptions and illustrations presented here are based on an analysis of these herbarium specimens. Comparisons of diagnostic characters were based on Lao specimens, as well as digital images of specimens held at AAU, AUH, BHL, BKF, BM, E, K, P and PE that are available online, and on relevant taxonomic literature (Hansen 1989;Munzinger & Martin 2000;Chen & Renner 2001;Renner et al. 2001;Suddee et al. 2014;Cho et al. 2015;Lin 2015;Shin et al. 2020). The terminology follows Renner et al. (2001). The specifi c epithets and the authors' names are in line with the International Plant Names Index (IPNI continuously updated). The preliminary conservation assessments followed the Guidelines for Using the IUCN Red List Categories and Criteria Version 14 (IUCN 2019).

Diagnosis
Sonerila erectifolia sp. nov. is similar to Sonerila violifolia in its scorpioid or subumbelliform infl orescence, but it diff ers in the length of stem (ca 1 cm long in S. erectifolia sp. nov. vs 10-18 cm long in S. violifolia), the venation of the leaves (2 veins running from the base of lamina vs pinnately veined with 2-3 lateral primary veins), hairiness of leaves (sparsely covered with whitish hairs on veins abaxially vs glabrous), the length of the petiole (6-10 cm long vs 1-7 cm long), and number of fl owers per infl orescence (5-12(-18) fl owers vs 1-9 fl owers). For further comparison see Table 1.

Etymology
The specifi c epithet refers to the plant having erect leaves.

Preliminary conservation status
Sonerila erectifolia Phonep., Soulad. & Tagane sp. nov. is found only at the top of the Bolaven Plateau (near Nong Luang Village). The number of mature individuals is estimated to be fewer than 250, which together generate an area of occupancy of less than 10 km 2 . The area is a well-known tourist attraction with associated habitat disturbance that could aff ect the survival of this species. Therefore, we assess the species as Critically Endangered (CR) B2ab(iii) (IUCN 2019). Diagnosis Sonerila souvannii sp. nov. is similar to S. cardamomensis, described from Cambodia, in its acaulescent habit and in the shape of its leaves, but it diff ers in the shape of its rhizome (cylindrical in S. souvanii vs bulbous in S. cardamomensis), larger leaves (8-10 × 6-9 cm vs 1-4 × 0.9-2.7 cm), petiole length and color (8-15 cm long and light green vs 0.4-2.5 cm long and reddish brown), greater number of fl owers per cyme (20-25 vs 5-11), and petal length and pubescence (9-10 × 6-7 mm and glabrous vs 5.5-6 × 3.5-4 mm with glandular trichomes on midveins abaxially). Sonerila souvannii sp. nov. is also similar to S. dongnathamensis from Thailand in the shape of its infl orescences, but diff ers in the rhizome shape (cylindrical vs globose), the shape of the leaf base (cordate vs obtuse to rounded), pedicel length (10-25 mm long vs 1-4 mm long), the number of fl owers per cyme (20-25 vs 5-12), and in the length and outline of the petals (9-10 × 6-7 mm, acute vs 3-5 × 3-4 mm, acuminate with long seta). The new species is also related to S. tuberosa from Cambodia, but distinguished in the rhizome shape (cylindrical vs slightly bulbous), the shape of the leaf base (cordate vs very broadly ovate to orbicular), pedicel length (10-25 mm long vs 5-6 mm long), the number of fl owers per cyme (20-25 vs 4-8), and in the length and outline of the petals (9-10 × 6-7 mm, acute vs 5 × 2.7 mm, elliptic, apiculate, pink) (

Etymology
The specific epithet honours Keooudone Souvannakhoummane (Centre for Development and Environment), a plant taxonomist who has greatly contributed to our understanding of plant diversity in Laos through the description of many new species in various families for the fl ora of Laos.

Preliminary conservation status
We found three small sub-populations of S. souvannii sp. nov. with each comprising only 5-6 individuals, about 20 m apart. The species has not yet been found anywhere else. Considering the species grows in a very narrowly restricted area where human activity, such as collection of non-timber forest products (NTFPs) and the logging of wood for local uses, can be seen, we need to pay attention to its conservation. As the number of mature individuals is less than 50, we assess this species as Critical Endangered (CR) criteria D (IUCN 2019).

Discussion
This paper describes two additional species of the genus Sonerila, bringing the total diversity of this genus in Laos to nine species. A taxonomic key to the nine species is provided. Since Guillaumin (1913), seven additional species have been added to the Indochinese fl ora. However, intensive surveys of particular places are needed in order to understand the true species diversity of this genus in this region. We expect to fi nd more new species and new records of Sonerila in Laos as compared with the number reported from neighbouring countries. Recently, several new species have been described from the Bolaven Plateau, e.g., Coleus bolavenensis Suddee, Tagane & Rueangr. (Lamiaceae Martinov), Gentiana bolavenensis Nagah., Tagane & Soulad. (Gentianaceae Juss.), and Sonerila bolavenensis Soulad., Tagane & Suddee (Nagahama et al. 2019;Suddee et al. 2020;Souladeth et al. 2021), suggesting that with further fl oristic surveys in this area and also another National Protected Areas in Laos, more new species will be added to the fl ora of Laos.