Begonia guangdongensis, a new species of Begonia (Begoniaceae) from Guangdong, China

Abstract A new species of Begonia section Coelocentrum, B. guangdongensis W.H. Tu, B.M. Wang & Y.L. Li from Guangdong Province, China, is described and illustrated here. Morphologically, the new species is most similar to B. biflora T. C. Ku and B. longistyla Y. M. Shui & W. H. Chen, but differs from B. biflora by its rugose leaves and glabrous capsules and from B. longistyla by its glabrous stipules without ciliate margin, densely hirsute-pilose leaves and obtuse apex of bracts. Additionally, it is also somewhat similar to B. chongzuoensis Yan Liu, S. M. Ku & C.-I Peng, but there are significant distinctions in their stipules, leaves and bracts. The conservation status of B. guangdongensis is assessed as Critically Endangered (CR), according to the IUCN Red List Categories and Criteria.


Introduction
The genus Begonia L. (Begoniaceae), consisting of ca. 1900 species, is one of the ten most species-rich flowering plant genera and is widely distributed in the tropical and subtropical areas of the world (Frodin 2004;Hughes et al. 2015). Most Begonia species are narrowly distributed, especially those in limestone karsts (Tebbitt et al. 2006;Ku et al. 2007;Hughes and Hollingsworth 2008). According to the recent taxonomic revision of the genus Begonia in Flora of China, nearly 200 species, with 141 local endemics, are reported and represent seven sections (Ku 2007;Ku et al. 2007). The Begonia sect. Coelocentrum, comprising of more than 70 species, is a typical limestone group confined to the Sino-Vietnamese karst areas and most species circumscribed within the section are rare and known only from a single collection or population Peng et al. 2014). Although the section has been shown to be paraphyletic, based on phylogenetic analyses, this section is morphologically well delimited by its parietal placentation and rhizomatous perennation . Species within this section differ from one another by leaf texture, pubescence and stipule, inflorescence and fruit morphology .
During a plant diversity survey around Yangchun City in Guangdong Province in October 2019, we discovered a species of Begonia with parietal placentation and rhizomatous perennation on the slope of a limestone hill, which was identified as a member of Begonia sect. Coelocentrum. After critical reviewing the type specimens and protologues of relevant species of this section described from the Sino-Vietnamese karst regions, it was concluded that the species is new to science. Herein, we describe and illustrate it, as well as assess its conservation status.  (Peng et al. 2012) by sharing obliquely ovate asymmetric leaves, hairy petioles, a glabrous peduncle, 2-3 times branched dichasial cyme and glabrous trigonous-ellipsoid capsules. However, it can be easily distinguished from B. biflora by its stipules with aristate apex and without ciliate margin (vs. aristate and ciliate apex and ciliate margin), rugose leaves (vs. flat) and glabrous capsules (vs. pubescent); it differs from B. longistyla by its abaxially glabrous stipules without ciliate margin (vs. abaxially hairy stipules with ciliate margin), leaves densely hirsute-pilose (vs. densely tuberculate-based pilose-setula) and inflorescence equal or slightly shorter than petioles (vs. much longer) and it can be distinguished from B. chongzuoensis by its persistent stipules and bracts (vs. caducous), rugose leaves (vs. nearly flat), leaves densely white hirsute-pilose (vs. moderately to sparsely whitish-hyaline or reddish setulose) and an unequally 3-winged capsule (vs. equal or subequal). (Table 1).
Habitat. This new species grows on the slope of a limestone hill in evergreen forests at an elevation of 80-100 m (Fig. 3).
Distribution. Only one population of this new species was discovered in Guangdong Province in China.
Conservation Status. Critically Endangered (CR). Limestone areas in Chunwan Town have been searched for this new species, but Begonia guangdongensis is known only from one population consisting of ca. 100 mature individuals. The area of occupancy (AOO) of the species is estimated to be less than 4 km 2 , which indicates the species belongs in the Critically Endangered category under criterion B2, according to the IUCN Red List Categories and Criteria (IUCN 2019). Since the species grows on a limestone hill near two cement factories, the species is threatened by the limestone quarrying. Its habitat will likely be destroyed since the area is undergoing a continuing decline. Based on the current information (one location with area in continuing decline and AOO less than 10 km 2 ), the new species can be assessed as Critically Endangered [B2ab(iii)] (IUCN 2019). Discussion. Begonia guangdongensis, belonging to Begonia sect. Coelocentrum, is a very distinctive species in having leaf features, such as rugose and densely hirsutepilose leaves and an obtuse apex of the capsules. Although it is more or less similar to B. biflora, B. longistyla and B. chongzuoensis in their obliquely ovate asymmetric leaves and glabrous trigonous-ellipsoid capsules, it differs from B. biflora by its sparsely hairy and smooth rhizomes (vs. rough rhizomes with many membranous scales), leaves with densely hirsute-pilose and depressed veins on adaxial surface (vs. with sparsely setula and veins not depressed) and stipule and capsule features discussed in the above diagnostic description. However, B. guangdongensis is quite different from B. longistyla, being distinguished by its oblong bracts with obtuse apex (vs. ovate bracts with a tip apex), stipule and leaf pubescence, length of inflorescence and capsules features. B. guangdongensis is also markedly distinct from B. chongzuoensis by its stipules, leaf and bract and capsule features. In addition, their distribution range is different (Fig. 3). Both B. biflora and B. longistyla are distributed in Yunnan Province and B. chongzuoensis in Guangxi Province, whereas B. guangdongensis occurs in Guangdong Province. Additionally, B. guangdongensis flowers in September to October, while B. biflora flowers in May, B. longistyla in April to June and B. chongzuoensis in May to September. Thus, even if they were growing together, they would be genetically isolated in time.