Swertia hongquanii, a new species of Gentianaceae from Mt. Wuling, southern China

Abstract Swertia hongquanii Jia X. Li, a new species of Gentianaceae, is described and illustrated from Mt. Wuling, southern China. It grows on the tops of steep limestone mountains surrounded by cliffs above an altitude of ca. 1400 m. This species resembles Swertia bimaculata, but differs from the latter by the stem leaf blades 2.0–4.5 × 1.0–2.5 cm, ovate to ovate-cordate, base cordate and subamplexicaul, the seeds irregularly polyhedral and the seed coats minutely thorny and reticulate. Based on morphological traits, i.e. the inflorescence structure and the number and structure of the nectaries, the new species may be a member of series Maculatae. A key to the species of series of section MaculataeOphelia is provided.


Introduction
Mt. Wuling is located at the eastern edge of the Yunnan-Guizhou Plateau in southern China. This plateau has an ancient geological history and complex terrain and geomorphology (Zhu et al. 2015). Due to its long history and changing paleogeography, this area presents a great variety of physiographical characteristics and abundant diverse ecosystems within a short extent of space. Additionally, it has become an important channel for the concentration, diffusion and migration of east Asiatic angiosperms (Chen et al. 2002;Li et al. 2008;Li and Yu 2014;Zhu et al. 2015). Mt. Wuling is rich in flowering plants (4083 species: Chen et al. 2002) and several new taxa have been described from this area recently (e.g. Peng et al. 2007;Li and Yu 2014;Liu et al. 2016). Here we propose another new species of Swertia L. (Gentianaceae), collected from Tianmenshan National Forest Park in this area.
Swertia consists of 3 subgenera, 11 sections and over 160 species (Joshi 2011;Rybczynski et al. 2014;Ho and Liu 2015). The genus is easily recognisable by the rotate corolla and by the presence of coralline nectariferous glands (Suksathan 2001;Ho and Liu 2015). The genus shows a North Temperate and South Temperate disjunctive (Pan-temperate) distribution pattern (Ho and Liu 2015). The majority of species are centred in Asia. South-western China and adjacent regions are diversity centres and initial diversification centres for this genus (Ho et al. 1994;Ho and Liu 2015). In China, approximately 75 species occur, mainly in mountains at an elevation above 1000 m (Ho and Pringle 1995). Moreover, several new species have been described from China since the account of the genus for the Flora of China was published (e.g. Chen et al. 2008;Liu 2010, 2015;Chen et al. 2016).
The new species from Mt. Wuling first came to our attention in September 2007 during our plant investigation in Tianmenshan National Forest Park. It was initially misidentified as Swertia bimaculata (Siebold & Zucc.) Hook.f. & Thomson ex C.B.Clarke due to its lax panicles of cymes and two naked nectaries per corolla lobe. In 2016, in the course of digitising the specimens in CSFI, we found a unique specimen, collected from Mt. Tianmen by Mr. Hongquan Huang (13050406) on 20 September 2013. It was identified as Swertia bimaculata but differs by its petite shape and cordate leaf base. Subsequently, we again visited Tianmenshan National Forest Park to observe this plant in September and November 2017 and collected more flowering and fruiting material. After a morphological comparison, the specimens from Tianmenshan National Forest Park are confirmed to be an undescribed species of Swertia, probably belonging to series Maculatae T.N.Ho & S.W.Liu in section Ophelia (Griseb.) Gilg in subgenus Ophelia (Griseb.) C.B.Clarke.

Materials and methods
During three field expeditions in September 2007, September and November 2017, a total of fifteen flowering and five fruiting individuals from the type locality were collected from Tianmenshan National Forest Park, Zhangjiajie City, Hunan Province, Southwest China. The information and measurements of the new species were taken from live and dried herbarium specimens and from field data. Seeds were examined and imaged with a Leica M205C stereomicroscope attached to a video camera (Leica DFC495). The morphological comparisons with related species, viz., Swertia bimaculata, S. tashiroi Makino, S. oculata Hemsl., S. tozanensis Hayata, S. cordata (Wall. ex G.Don) C.B.Clarke and S. shintenensis Hayata, are based on herbarium specimens (about 2300 specimens) and relevant literature (Ho et al. 1988;Ho and Pringle 1995;Rybczynski et al. 2014;Ho and Liu 2015). Specimens deposited in the following herbaria were examined: CSFI, CSH, CZH, JIU, HTC, IBK, IBSC, LBG, KUN, PE, SYS and WUK (Thiers 2015).
The number of mature individuals was recorded in the field in twenty 1 m 2 sampling plots. We assessed the preliminary conservation status of the new species using our field knowledge and applying the IUCN (2017) criteria. The taxonomic treatment of the genus Swertia follows Ho and Liu (2015).
Phenology. Flowering was observed in September and October. Fruiting was only observed in November, but probably extends till December.
Distribution and habitat. Swertia hongquanii is known only from the type location in Tianmenshan National Forest Park. The climate here is cool, foggy and humid (annual rainfall ca. 1700 mm) and belongs to the subtropical monsoon climate (Li et al. 2008;. The new species grows on the tops of steep limestone mountains surrounded by cliffs above an altitude of ca. 1400 m. The clifftops are covered by mixed evergreen-deciduous forest dominated by species of Fagaceae, Lauraceae, Betulaceae, Ac- eraceae and Ulmaceae (Li et al. 2008 Etymology. The species is named after Mr. Huang Hongquan for his help during our field investigation. He was also the first to collect this new species. Local name. Tianmenshan Zhang Ya Cai Conservation status. Despite several investigations in Mt. Tianmen and the surrounding areas (96 km 2 ), two populations with nearly 500 individuals each (total < 1000) of Swertia hongquanii were found only at the type locality (towering summit terrace with an area of 2 km 2 ). Presently, a tourist plank walkway passes through this location and its habitat could be easily disturbed or destroyed. According to the IUCN (2017) criterion D thresholds (mature individuals < 1000, AOO < 20 km 2 ), the new species could be assessed as VU. However, it grows in the upper part of steep limestone mountains surrounded by cliffs, which previously made it impossible to be encountered until a plank walkway was built across the cliff face for tourists. With limited fieldwork executed at present, it is possible that more populations could be found in similar habitats of the Wuling mountain areas. Therefore, we consider the species DD (Data Deficient).

Discussion
Morphologically, Swertia hongquanii resembles S. bimaculata in possessing two naked gland patches in the middle of each corolla lobe, whereas it is clearly distinguished by stem leaf and seed characteristics (Table 1). During field investigations, we observed that most S. bimaculata flowers were withered while the new species was just beginning to blossom in populations in the same locality (separated by a distance of ca. 300 m). We therefore infer that the morphological differences with S. bimaculata are interspecific rather than intraspecific variations driven by ecological conditions. From its overall vegetative appearance, Swertia hongquanii also resembles S. cordata, with both species having slender and subquadrangular stems with wings on the angles, ovate to ovate-cordate stem leaves with cordate to subamplexicaul bases. Swertia hongquanii is clearly distinguished from S. cordata, however, by the two nectaries in the middle of the corolla lobes; S. cordata, in contrast, has a single nectary on the base of the corolla lobe (Table 1) (Ho and Liu 2015).
According to the classification of Ho and Pringle (1995) and Ho and Liu (1980;, the species of series Maculatae of section Ophelia of subgenus Ophelia are distinguished by their stems being strongly branched, their inflorescences being panicles of cymes and by the presence of one or two nectaries on each corolla lobe reduced to a naked gland patch without raised margin. Six species, viz., S. bimaculata, S. oculata, S. cordata, S. tozanensis, S. shintenensis and S. tashiroi were recognised as members of series. Maculatae, distributed in Asia (Ho and Liu 2015). Based on morphological traits, especially those of inflorescences and nectaries, the new species may also be a member of series Maculatae (Figures 1-3).
The seeds of S. hongquanii are easily distinguished from other species of series Maculatae, as they are irregular polyhedrons with minutely thorny and reticulate seed coats (Figure 3), whereas those of the other species of series Maculatae are ellipsoid to globose with finely warty seed coats (S. tashiroi, S. shintensis, S. bimaculata, S. tozanensis and S. oculata) or with longitudinally and thinly corrugate seed coats (S. cordata) (Ho and Liu 2015).
Key to species of series Maculatae (adapted from Ho and Liu 2015)