Aristolochia yachangensis, a new species of Aristolochiaceae from limestone areas in Guangxi, China

Abstract Aristolochia yachangensis B.G.Huang, Yan Liu & Y.S.Huang, a new species from limestone areas in Guangxi, China, is described and illustrated. It is morphologically most similar to A. fangchi Y.C.Wu ex L.D.Chow & S.M.Hwang, A. petelotii O.C. Schmidt and A. championii Merr. & Chun in shape of leaf blade, anther, gynostemium and inflorescence on old woody stems. However, it can be easily distinguished from the latter by shape of inflorescence, length of upper and lower portions of perianth tube, colour of the limb and throat. A table and a key to distinguish the new species from other morphologically similar Aristolochia species are also provided.


Introduction
The genus Aristolochia L. (s. l.) contains 600 species and widely distributes in tropical, subtropical and temperate regions of the world (González 2012;Zhu et al. 2019c). Based on recent studies on molecular phylogeny, chromosome and morphology of Aristolochia, some researchers have suggested that an old genus Isotrema Raf. should be reinstated to accommodate species of Endodeca Raf. and Aristolochia subgen. Siphisia (Duch.) O.C.Schmidt (Zhu et al. 2019a). However, many researchers still advise to use the name Aristolochia rather than Isotrema (Do et al. 2019;Peng et al. 2019;Cai et al. 2020). In this paper, we use the name Aristolochia to describe a new species, because the genus name Isotrema is still controversial.
Currently, there are more than 70 species of Aristolochia known from China, including many new species that have been described from Yunnan, Guangxi, Guangdong, Zhejiang and Hainan in recent years (Gong et al. 2018;Zhu et al. 2018Zhu et al. , 2019bZhu et al. , 2019dLi et al. 2019;Peng et al. 2019;Zhou et al. 2019). As one of the most biodiverse regions of China, Guangxi has 22 Aristolochia species (Peng et al. 2019;Zhu et al. 2019c), including A. bambusifolia C.F.Liang ex H.Q.Wen, A. longlinensis Yan Liu & L. Wu and A. gongchengensis,Y.S.Huang, Y.D.Peng & C.R.Lin, which are endemic in the region (Qin and Liu 2010;Huang et al. 2015;Wu et al. 2015) During a fieldwork in Yachang Orchid National Nature Reserve of north-western Guangxi, China in April 2019, we discovered a special flowering plant of Aristolochiaceae and speculated that it might be a new species of Aristolochia, based on its flower structure. We investigated this species at the same location again and collected specimens of young capsules in May 2019. In order to obtain more detailed morphological data, we came back to the same location once again and collected specimens of mature capsules in July 2019. After consulting Flora of China (Hwang et al. 2003) and other relevant literature (Merrill and Chun 1940;Liang 1975;Chow and Huang 1975;Hwang 1981;Cheng et al. 1988;Ma 1989aMa , 1989bMa and Cheng 1989;Wen 1992;Liu and Deng 2009;Xu et al. 2011;Huang et al. 2013Huang et al. , 2015Wu et al. 2013Wu et al. , 2015Do et al. 2014Do et al. , 2015aDo et al. , 2015bDo et al. , 2016Do et al. , 2019Huong et al. 2014;Zhu et al. 2015Zhu et al. , 2016Zhu et al. , 2017aZhu et al. , 2017bZhu et al. , 2018Zhu et al. , 2019bZhu et al. , 2019dDo and Nghiem 2017;Gong et al. 2018;Li et al. 2019;Peng et al. 2019;Zhou et al. 2019;Cai et al. 2020), as well as comparisons amongst this unknown species and its morphologically most similar species, we confirmed that this species was clearly different from the known Aristolochia species. Hence, it is here described and illustrated as a new species.

Aristolochia yachangensis
Etymology. The specific epithet is derived from the type locality, Yachang Orchid National Nature Reserve, Guangxi, China. The Chinese name is given as "雅长马兜铃".
Distribution and habitat. At present, Aristolochia yachangensis was found only in Yachang Orchid National Nature Reserve of north-western Guangxi, China. It grows on limestone hillside at an elevation of ca. 1340 m. The slope direction is to the southwest, the slope is up to 40°, the tree layer is up to 15 m tall, the canopy cover is 70%, the shrub layer cover is 80% and the herb layer cover is 45%. Its associated species  Conservation status. Thus far, Aristolochia yachangensis has been found only from the type locality. The only subpopulation is located within a protected region and has seven individuals, including two mature ones. Based on the present study, its Extent of   be given a Vulnerable (VU) status, based on the criteria D2 of IUCN. As a newlyfound species, however, it is probable that more subpopulations of A. yachangensis could be found in similar habitats of limestone areas of north-western Guangxi and southern Guizhou, China in the future.
Additional specimens examined (

Discussion
Aristolochia yachangensis is unique in morphology. It is mostly similar to A. fangchi, A. petelotii and A. championii, but can be distinguished from all other Aristolochia species mainly based on the morphological characters of inflorescence, perianth tube, limb and throat. A. yachangensis can be further distinguished from morphologically-close species with the following key.