Spiradiclis karstana, a new species of Spiradiclis (Rubiaceae) collected from Yunnan, China, is described for the first time. It is morphologically close to S. jingxiensis, but differs from the latter mainly by its inflorescences with 5–9 flowers, its 1.5–2.4 mm long peduncles, its stipules shorter than 1 mm and the 5–12 pairs of secondary veins. The conservation status is assessed as “Vulnerable” (VU) according to the IUCN Red List Categories and Criteria.

Spiradiclis , Rubiaceae , taxonomy, karst, China

Spiradiclis Blume, a small genus of Rubiaceae as currently circumscribed (Lo 1998, 1999), consists of two subgenera and ca. 50 species. Most of them are widely scattered in southern and south-western China and north-western India, but a few occur in the Indo-China Peninsula (Robbrecht 1988, Deb and Rout 1989, Lo 1999, Chen and Taylor 2011, Wu et al. 2015a, b, 2016, Wang 2016, Pan et al. 2016, Liu et al. 2017). Most representatives of the genus are narrowly distributed in karst regions, generally accompanied by ferns and Begonia L., Elatostema J.R.Forst. & G.Forst. and Gesneriaceae. During flowering, species of Spiradiclis are sometimes confused with Ophiorrhiza L. (Wu et al. 2015a), but they can be distinguished from the latter and other related genera by the linear-oblong or subglobose capsules that dehisce with four valves when mature (Lo 1999, Wu et al. 2015b). In China, nearly 47 species have been found, of which ten species have been published in the last four years (Deng et al. 2014, Wu et al. 2015a, b, 2016, Wang et al. 2015, Wen et al. 2015, Wang 2016, Pan et al. 2016, Liu et al. 2017).

In 2017, a plant lover, Mr. Ming-Feng Long, found a population of Spiradiclis on the cliff of a karst hill in Malipo county, SE Yunnan and contacted the authors for the identification. The individuals were first identified as Spiradiclis jingxiensis R.J.Wang as they shared a similar habitat and morphological characters such as prostrate or decumbent habit, distylous and purple-reddish flowers with slender salverform corollas. After a further comparison of specimens, however, these individuals can be distinguished from Spiradiclis jingxiensis mainly by their puberulent to subglabrous stems, leaves, stipules, peduncles and calyces (vs. dense pubescence), their ovate-triangular stipules, shorter than 1 mm (vs. linear, 1.5–3.0 mm long), their elliptic to oblong leaf blades (vs. ovate to broadly ovate), their 5–12 pairs of secondary veins (vs. 4–5 pairs) and their inflorescences with 5–9 flowers (vs. 1–2 flowers). Therefore, the specimens are assumed to represent an undescribed new taxon, which is here described.

Most materials are deposited at the herbarium of forest plants in the Central South University of Forestry and Technology (CSFI), only one residing at Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and Chinese Academy of Sciences (IBK). Morphological observations of the new species have been carried out, based on field observations as well as on dry specimens. The conservation status of the new species is evaluated in accordance with IUCN guidelines (2016).

Karst ecosystems are known to be distinctive in vegetation and biodiversity and have an extreme and exceptional habitat that can provide the opportunity for speciation and radiation (Myers et al. 2000, Zhu 2007, Biswas 2009). As mentioned above, most representatives of Spiradiclis prefer this unique ecosystem. It may be the reason why this small genus shows fairly complex and diverse habit and morphology. Until now, 11 known species of Spiradiclis, viz., S. danxiashanensis R.J.Wang, S. elliptica Y.M.Shui & W.H.Chen, S. glandulosa L.Wu & Q.R.Liu, S. guangdongensis H.S.Luo, S. hainanensis H.S.Luo, S. jingxiensis R.J.Wang, S. longanensis R.J.Wang, S. pauciflora L.Wu & Q.R.Liu, S. pengshuiensis B.Pan & R.J.Wang and S. umbelliformis H.S.Luo, as well as this new one, are prostrate or decumbent herbs without erect stems and usually with roots at the nodes (Sui and Chen 2006, Chen and Taylor 2011, Wu et al. 2015a, 2015b, Wang et al. 2015, Wen et al. 2015, Wang 2016, Pan et al. 2016). The following key is provided to the prostrate or decumbent Spiradiclis species worldwide.

The authors are grateful to the staff of IBK, IBSC, KUN and PE for permission to examine their specimens. We also thank Dr. Qi Gao (IBK) for help during the course of preparing the paper, Xin-Yi Zeng for her excellent drawing and Ming-Feng Long and Xin-Xin Zhu for their photographs and help in the field. This study was supported by the National Nature Foundation of China (project no. 31600160 and no. 31770213).