An updated checklist of Begonia (Begoniaceae) in Laos, with two new species and five new records

Abstract Two new species of Begonia L. (Begoniaceae), B. laotica (sect. Parvibegonia) and B. hypoleuca (sect. Reichenheimia), from north Laos are described and illustrated. Begonia augustinei, B. dryadis, B. lancangensis, B. sizemoreae and B. sillentensis subsp. mengyangensis were newly recorded taxa in Laos. Furthermore, an updated checklist of Begonia of Laos is also compiled.


Introduction
Begonia Linnaeus (1753: 1056) is the sixth largest genera of angiosperms and the number of accepted species of Begonia currently stands at 1947 (Hughes et al. 2015) and is likely to rise to well over 2000 (Moonlight et al. 2018;Tian et al. 2018). According to the annotated checklist of south east Asia Begonia by Hughes (2008) and recent taxonomic publications (Hughes 2007;Newman et al. 2007;De Wilde et al. 2011;Averyanov and Nguyen 2012;Souvannakhoummane et al. 2016Souvannakhoummane et al. , 2018Yang et al. 2018;Averyanov et al. 2019), 19 species of Begonia have been described from Laos, so far.
As part of the botanical inventory of China-Laos transboundary biodiversity conservation, we carried out floristic surveys in the Phou Hin Phee National Biodiversity Conservation Area, Oudomxay Province, the Nam Ha National Biodiversity Conservation Area in Luang Namtha Province and the Phou Dean Din National Bio-Diversity Park, Phongsaly Province of northern Laos. During the fieldwork from 2018 to 2019, some interesting species of Begonia have been collected. After reviewing the literature and herbarium specimens, we described and illustrated two species as new to science and five species as new records for the flora of Laos. We provided an updated total of 26 species for the Begonia flora of Laos.

Begonia laotica
Phenology. Flowering in October-November and fruiting in December. Distribution. Endemic to the type locality, Oudomxay Province, Maung Xai, Phou Hin Phee National Biodiversity Conservation Area, Laos.
Ecology. Restricted to the shaded base of limestone cliff at ca. 1378 m elevation. Etymology. The specific epithet 'laotica' refers to the type locality in Laos. Conservation status. Data Deficient (DD) (IUCN 2017). The new species was only collected from the limestone cliff of Hin Phee National Biodiversity Conservation Area. Due to its remote habitat, we suggested that the species may not suffer from strong human disturbance. According to the available information, we proposed the preliminary conservation status of this new species as Data Deficient (DD) (IUCN 2017).
Phenology. Flowering and fruiting in October-November. Distribution. The species is known only from the type locality, Luang Namtha Province, Nam Ha National Biodiversity Conservation Area, Laos.
Ecology. The species was collected on shaded moist limestone rock surfaces of stone forest at 656 m elevation.
Etymology. The specific epithet 'hypoleuca' refers to the lower surface of its paler or silver leaf.
Conservation status. Data Deficient (DD). Begonia hypoleuca has been collected on only one occasion from the type locality. However, the type locality is located in the Nam Ha National Biodiversity Conservation Area and we did not discover any strong pressure on the species. Thus, the species preliminarily has been assigned in the Data Deficient (DD) category as being appropriate according to the guidelines for using the IUCN Red List Categories and Criteria (IUCN 2017).
Notes. Begonia hypoleuca is most similar in morphological characteristics to B. henryi Hemsl. under the section Reichenheimia. It can, however, be distinguished by the following characteristics, including reddish linear red dots on petiole (vs. without), sparsely pubescent on petiole (vs. brown villous), slightly denticulate leaf margin (vs. crenate), glabrous leaves (vs. pubescent), leaf upper surface green (vs. green with dark green patches), lower surface pale green (vs. pale green with reddish patches), male flower tepals broadly ovate (vs. oblateorbicular), apex acute (vs. rounded), stamens number fewer 10-15 (vs. up to 30).  Note. This species is characterised by elongate rhizomes, extremely asymmetric leaves, base cordate to deeply cordate, margin irregularly serrulate (Hemsley 1900;Ku 1999;Huang and Shui 2006;Ku et al. 2007).   Note. This species was first discovered and reported in the forest understorey, by streams in a valley of South Yunnan, China. The discovery of this species in Laos shows the geographical linkage between two type localities. The species is differentiated from the allied species by its short, stout rhizome and the presence of cauline leaves. (Irmscher 1951;Ku 1999;Huang and Shui 2006;Ku et al. 2007). Note. The species is characterised by its dioecious and erect stems with ovate or ovate-oblong leaves and fleshy berry-like fruits Huang 1999, Ku et al. 2007). Additionally, B. handelii Irmscher (Irmscher 1921: 24) and B. acetosella Craib (Craib 1912: 153) have been discovered in the same locality. We suspect that this species may be a natural hybrid species of them.

Distribution. Vietnam, Laos.
Note. Begonia sizemoreae was originally described in north Vietnam. Now, its distribution in Laos has been confirmed. This species is very similar to B. rex Putz. (Putzey 1857: 141) from Assam, India, in its leaf shape, fine variegated leaves and flower structure, but it is distinct in its leaf margin with very hairy upper surface of the lamina with hairs 5-10 mm long and its conspicuous deep crimson tertiary venation in the centre and outer part of the lower leaf surface (Kiew 2004(Kiew , 2007. Note. The species is characterised by its dioecious and prostrate stems with large broad leaves and fleshy berry-like fruits (Tebbitt and Guan 2002;Ku et al. 2007).