﻿Argostemmaehuangzhangense (Rubiaceae), a new species from Guangdong, China

﻿Abstract Argostemmaehuangzhangense, a new Rubiaceae species from E’huangzhang Nature Reserve, Guangdong Province, China, is here described and illustrated. A morphological comparison between the new species and its putative relatives, A.lamxayanum, A.laotica and A.verticillatum, is presented. The new species is mostly similar to A.laotica, but they can be distinguished from each other since Argostemmaehuangzhangense presents solitary flower (vs. 2-flowered inflorescences), flower lobes 4 (vs. 5) and anthers opening by longitudinal slits (vs. apical pores). In a preliminary IUCN Red List status of Argostemmaehuangzhangense this species is assigned as Vulnerable (VU).


Introduction
The genus Argostemma Wall. (Wallich 1824) belongs to the coffee family, Rubiaceae (subfamily: Rubioideae), and in its own tribe Argostemmateae (Bremer 1987;Bremer and Manen 2000). Argostemma is a large genus of more than 160 species and is widely distributed in the Old World tropics with most species occurring in SE Asia and two species in west tropical Africa (Bremer 1989;Sridith and Puff 2000;Mabberley 2017;Lanorsavanh et al. 2020). In China, six species of Argostemma were recorded (Chen and Taylor 2011). Key morphological characters of Argostemma are (i) opposite or verticillate leaves that are slightly to markedly anisophyllous, (ii) 4-or 5-merous flowers without nectaries, (iii) white and rotate corollas, (iv) Inner surface of corolla tube glabrous, (v) free anthers or coherent into a tube, (vi) anthers with opening by longitudinal slits or apical pore, and (vii) sometimes connective prolonged at the apex (Puff et al. 1995;Chen and Taylor 2011).
An unknown Argostemma species was discovered during recent field surveys conducted between April 2017 and May 2018 at the E'huangzhang Nature Reserve, Yangchun City, Guangdong Province. The flowers of the unknown Argostemma species are clearly 4-merous and it differs from all known Argostemma species in China. In addition, we compared the unknown Argostemma from E'huangzhang Nature Reserve against Argostemma species occurring in Southeastern Asian which presented morphological characters divergent from those in our specimen. Thus, it was concluded that the unknown Argostemma from E'huangzhang Nature Reserve is a new species, which is hereby described and illustrated here.

Materials and methods
This study was conducted based on living and dried herbarium specimens collected from nearby Wufu Waterfall in the E'huangzhang Nature Reserve. Herbarium specimens at IBSC, KEP and KUN, as well as types at IBSC, K and NY, were examined for morphological comparisons (acronyms follow Thiers 2022). Taxonomic literature of the genus for Thailand (Sridith and Puff 2000;Sridith and Larsen 2004;Sridith 2007Sridith , 2009Sridith , 2012, China (Chen and Taylor 2011), Vietnam (Vu et al. 2020), Laos (Lanorsavanh and Chantaranothai 2013, 2016, 2019Lanorsavanh et al. 2020), and Myanmar (Kress et al. 2003;Tanaka et al. 2010) were consulted.
Diagnosis. Argostemma ehuangzhangense is similar to A. lamxayanum and A. laotica in its habit and pseudo-verticillate leaves, but differs in having terminal solitary 4-merous flowers, short anthers (1.2-1.5 mm long) opening by longitudinal slits.
Phenology. This species is recorded flowering in March-May and fruiting in May-September.
Distribution. Argostemma ehuangzhangense is endemic to E'huangzhang Nature Reserve, southwestern Guangdong Province. It is currently known only from two populations recorded in county of Dianbai and city of Yangchun.
Habitat. Growing along river on wet sandstone under the evergreen broad-leaf forest at 400-750 m a.s.l.
Etymology. The specific epithet 'ehuangzhangense' is derived from the type locality, E'huangzhang Nature Reserve of the Guangdong Province, in China. This area is the oldest geological platform in the Guangdong Province, in which many endemic species occur (Wang et al. 2003;Ding et al. 2018). The new species is also expected to occur in the Yunkaishan National Nature Reserve, Maoming City, because this area shares a similar tectonic history with E'huangzhang. Therefore, we proposed for the vernacular name of the species as yuèxīxuěhuā (粤西雪花).
Preliminary conservation status. The species is endemic to the Guangdong Province. According to our field survey, there are only two localities in which Argostemma ehuangzhangense is recorded and each population consists of 100-250 individuals. The number of mature individuals of Argostemma ehuangzhangense is more than 400, but less than 1000. Thus, we believe that Argostemma ehuangzhangense would be considered VU (Vulnerable) in an official IUCN Red List assessment (IUCN Standards and Petitions Subcommittee 2022)