﻿Petrocodonwui (Gesneriaceae), a new species from Guizhou, China

﻿Abstract Petrocodonwui F.Wen & R.B.Zhang (Gesneriaceae), a typically lithophyte occurring in the Danxia areas of north-western Guizhou, China, is described and illustrated as new to science. The new species shows overall similarity with P.chishuiensis Z.B.Xin, F.Wen & S.B.Zhou, which is also its sister species, based on molecular evidence. The new species can be distinguished from P.chishuiensis by the elongated rhizome, the relatively long indumentum on the peduncle, the shape, size and indumentum of calyx lobes, the location of the stamens in the corolla tube and the shape, size and indumentum of the stigma. We provide a diagnosis, detailed description, photographic images and a table with taxonomic notes to distinguish several other morphologically similar Petrocodon species.

genus are lithophytes (= rock dwelling), either growing in karst topographies or in Danxia landforms. This genus is mainly distributed in karst areas from eastern and south-western China to the northern Indo-China Peninsula, especially in Guangxi of China and North Vietnam (Wei 2018;Wei et al. 2022). Undoubtedly, China is the biodiversity centre of Petrocodon because there are at least 44 species (especially in Guangxi, with 25 species) (Wei 2018). Although many new taxa have been discovered and published in recent years, there were only two new taxa discovered and confirmed from Danxia landforms, namely P. asterocalyx F.Wen, Y.G.Wei & R.L.Zhang (Zhang et al. 2018) and P. chishuiensis Z.B.Xin, F.Wen & S.B.Zhou (Xin et al. 2020). It is well known that all species of Petrocodon are typically lithophilous and usually segregated into unique habitats, for example, karst caves and Danxia gorges, so that most of the species in this genus are narrow endemics (Fu et al. 2022a).
In early August 2021, we conducted a plant diversity survey in Xishui National Nature Reserve in Guizhou Province, China. We noticed an unknown species of Gesneriaceae growing on the surface of Danxia cliff in Niuqingshan, Dabaitang of the Xishui National Reserve. Based on its lithophytic habit and taxonomical characters, we considered it might belong to the genus Petrocodon. Upon closer examination of the flowering specimens in the lab and careful observation of living plants for comparison of vegetative and reproductive organs, we soon discovered several noticeable morphological differences that do not match any known Petrocodon species. Moreover, only two known species of Petrocodon endemic to Danxia landforms were confirmed before this species was discovered. Using morphology or molecular evidence, the new taxon of Petrocodon is recovered as sister to P. chishuiensis, but remarkably different from other species in surrounding cities and counties by some obvious characters. Thus, we concluded it corresponds to a species new to science.

Taxonomic revision
The studied specimens were collected from the type locality and deposited in the Guangxi Institute of Botany Herbarium (IBK) and the Botany Herbarium of Zunyi Normal College (ZY). The macromorphological features were observed on the specimen sheets and taken from field notes and reports from the conservation nurseries at the National Gesneriaceae Germplasm Resources Bank (NGGB) of the Guangxi Institute of Botany (GXIB) and the Gesneriad Conservation Center of China (GCCC). Micromorphological observations were analysed and photographed using a stereomicroscope (Olympus Optical Microscope CX23). The morphological characters were compared with the protologue and type specimens of previously described Petrocodon species (Wang et al. 1990(Wang et al. , 1998Wei et al. 2010;Wei 2018), in particular those involving new taxa of Petrocodon from Guangxi and adjacent provinces (see notes) and herbarium specimens deposited at relevant herbaria (e.g. HITBC, IBK, IBSC, KUN, PE and VMNM).
The description of the new species follows the terminology used by Wang et al. (1998) and Harris and Harris (2001). Assessment of the conservation status of the new species was made according to the Categories and Criteria of the IUCN (IUCN Standards and Petitions Committee 2022).

Phylogenetic analysis
Leaf material of the undescribed species was collected from the type locality in Xishui County (Guizhou, China) and immediately dried in silica gel for DNA extraction (Chase and Hills 1991). The nuclear ribosomal internal transcribed spacer (ITS) region and plastid trnL-F intron spacer region (trnL-F) were used in the study. Primers, DNA extraction, PCR amplification and sequencing followed Yang et al. (2022). To elucidate the phylogenetic affinities of the undescribed species within the genus, we incorporated 39 samples representing 25 species (Table 1), following Yang et al. (2022). The ingroup contained 37 samples from 23 species of Petrocodon. Primulina dryas (Dunn) Mich. Möller & A.Weber and P. pinnata (W.T.Wang) Yin Z. Wang were chosen as outgroups, based on previous phylogenetic analyses (Möller et al. 2009;Weber et al. 2011;Zhang et al. 2018). We performed phylogenetic analyses of the included Petrocodon species, based on the combined dataset of trnL-F and ITS sequences using Maximum Likelihood (ML). We employed IQ-TREE v.2.0.6 (Nguyen et al. 2015) with 1000 bootstrap replicates (Hoang et al. 2018) and default ModelFinder (Kalyaanamoorthy et al. 2017) and found K3Pu+F+G4 as the best fit substitution model. Tree visualisation was carried out in FigTree v.1.4.3 (http://tree.bio.ed.ac.uk/ software/figtree/). Visual comparison of optimal tree topologies of trnL-F and ITS datasets was used to compare topological inconsistencies. Conflicts between tree topologies were considered significant when the inconsistent topologies received bootstrap values ≥ 80% (Fu et al. 2022b). As visual inspection showed no significant topological contradictions for bootstrap support consistency between the trnL-F and ITS datasets (results not shown), the two regions were combined in further analyses. Diagnosis. Petrocodon wui is distinguishable by the elongated rhizome, the shape, size and indumentum of calyx lobes, the two conspicuous rows of orange glands on throat and the abaxial surface of the corolla lip. It morphologically resembles P. chishuiensis, but can be distinguished by having an elongated rhizome up to 30 cm or longer after years of growth (vs. lacking obvious rhizome in P. chishuiensis, following same order); leaf blade oval-oblong (vs. oblong or oblanceolate) and margin conspicuously undulate  Description. Perennial herb, strictly lithophytic. Rhizome brown, abundant fibrous roots, especially at the nodes, rhizome becoming very long and up to 30 cm or longer after years of growth, the lower half of long rhizome usually growing downwards along the surface of rock with lots of fibrous roots, apex of rhizome usually curved and forming a hooked shape, some persistent base of petioles spirally arranged on the surface of rhizome; upper rhizome densely covered with villous multicellular hairs ca. 2 mm long with 4-6 cells. Leaves in whorls of three, 6-15 crowded in a basal rosette or clustered at the top of elongated rhizome after years of growth, but usually some dried leaves persistent below foliage; petiole green, up to ca. 4 cm long, cylindrical, densely white pubescent; leaf blade chartaceous and thinly coriaceous when dried, oval-oblong, 6-10 × 1-3 cm, apex obtuse to acute or subacute, base cuneate, margin entire to inconspicuously or conspicuously undulate and densely ciliate, both surfaces densely white pubescent, lateral veins 4-5-paired; Inflorescences 1-4 or more, axillary, cymose, 4-10-flowered or more; peduncle pale green, 1-4 cm long, ca. 1.5 mm in diameter, densely white villous; bracts 2, opposite, pale green, lanceolate, ca. 10 × 0.5 mm, apex acute, margin entire, both surfaces densely covered with villous multicellular hairs, ca. 1.5 mm long with ca. 3 cells; bracteoles 2, pale green, opposite, narrowly lanceolate, ca. 3 × 0.25 mm, indumentum same as bracts, but hairs on only ca. 2 cells; pedicels pale green, 0.8-2 cm long, indumentum same as peduncle. Calyx 5-sected to near the base, but base slightly united forming calyx tube ca. 1 mm long; lobes equal, pale green to whitish-green, nearly linear, 6-8 mm long, 5-6 mm wide at the base, apex obtuse to rounded, margin entire, outside densely covered with white villous hairs, inside sparsely covered with white villous hairs. Corolla tubular, white, zygomorphic, ca. 2.5 cm long, outside densely white pubescent, inside nearly glabrous, upper part of corolla close to mouth puberulent; corolla tube 1.7-2.2 cm long, ca. 2.5 mm wide at the base of corolla tube and ca. 4.5 mm at the widest part of corolla tube; limb 2-lipped, adaxial lip shorter, 2-lobed to the middle, lobes broadly triangular, ca. 1.5 mm long, ca. 2.5 mm at the bottom of lobe, abaxial lip longer, 3-lobed to the middle or slightly exceeding the middle, lobes ovate, central one longer than lateral ones, ca. 3.5 mm long, lateral ones ca. 2.8 mm long, with two conspicuous rows of orange glands on abaxial lip and corolla throat. Stamens 4, two longer ones adnate to corolla tube ca. 9.5 mm from the base, filaments ca. 4.5 mm long, two shorter ones adnate to corolla tube ca. 8.5 mm from the base, filaments ca. 4 mm long, all filaments linear, straight, but slightly arched at the base and turning into a sheet at the base, white to semi-transparent, densely with brownish-black glands, especially from the middle to the base and glandular-puberulent close to the upper of filament; anthers brownish-purple to dark purple, dorsi-fixed, elliptic to nearly rounded, ca. 1 mm long, ca. 0.9 mm wide, coherent in pairs, thecae confluent at middle, sparsely semi-transparent glands, dehiscing longitudinally. Staminode 1, pale purple, club-like, glabrous, adnate to corolla tube ca. 8 mm from the base. Disc annular, ca. 1 mm high, margin entire. Pistil ca. 2.5 cm long, densely erectly glandular-pubescent; ovary linearcylindrical, ca. 2 cm long, ca. 1.5 mm wide, 1-loculed, placentas 2, parietal; style ca. 6 mm long, ca. 0.8 mm wide; stigma 2, lobes lamellar, rounded to shallowly spatulate, glabrous, ca. 1 mm long, 0.9-1 mm wide. Fruit a capsule, ca. 5.5 cm long, linearcylindrical, 4-valved, pubescent. Seeds appendaged, grain shortly cylindrical, rough, ca. 0.5 mm long, ca. 0.3 mm wide, covered densely verrucate.

Petrocodon wui
Phenology. Flowering occurs in August in the wild; fruiting should occur in October, based on current flowering patterns.
Etymology. We dedicate this new species of Petrocodon to Wu Zheng-Yi (Wu Chengyih) , who devoted over 70 years to the flora of China. The scientific name, "wui", is the latinisation of Wu Zheng-Yi's family name. Coincidentally, a plant enthusiast, Lady Xiang-Hong Wu, took this species' flowering photos in 2017 and sent them to one of the authors (Fang Wen) and her surname is also Wu. Vernacular name. The Chinese name proposed here is "吴氏石山苣苔." Phonetically, it is "Wú Shì Shí Shān Jù Tái".
Distribution and ecology. The new species is endemic to Guizhou Province and known only from the type locality, Xishui National Nature Reserve in Xishui County. It grows on the steep Danxia cliff in an evergreen, broad-leaved forest in a valley of the Danxia landform, at an altitude of 1100-1600 m. The cliff slope faces northwest at an angle of up to 60 to 80 degrees. The tree cover is up to 12 m tall, the canopy cover is 75%, the shrub layer cover is 85% and the herb layer cover is 35%.
Conservation status. Petrocodon wui is known only from the type locality, which is protected by national and local laws and regulations. However, it is clearly scarce, being known from only one very small area of occupancy, estimated at 20 m 2 on a rock surface in a valley of the Danxia landform. Obviously, this area of occupancy of P. wui we found so far is significantly lower than the smallest AOO unit of IUCN which is 4 km 2 (2 × 2 km 2 grid) for Critically Endangered B2. According to the detailed information from our careful field observations on the surroundings of the type area, the known population has about 50 individuals, half of those being mature individuals and half being seedlings. According to the Guidelines for using the IUCN Red List Categories and Criteria (IUCN Standards and Petitions Committee 2022), P. wui is provisionally assessed as "Critically Endangered, CR B2ab(ii) + C2b" because of its limited distribution and vulnerable habitat.
Taxonomic and phylogenetic notes. The aligned matrix of trnL-F and ITS sequences comprised 1594 characters. Of the 370 (23.21%) variable characters, 222 (13.93%) were parsimony informative. The phylogenetic trees revealed that all sampled Petrocodon taxa clustered together as a monophyletic group (BP = 100%), which is consistent with previous studies (Yang et al. 2022). Three strongly-supported clades are attributed to Petrocodon. Of these, the new species belonged in a moderately-supported subclade (BP = 75%) that also includes P. hunanensis X.L.Yu & Ming Li (Weber et al. 2011), P. tongziensis R.B.Zhang & F.Wen and P. chishuiensis (Petrocodon_sp_FW2014) (Fig. 3). This clade, denoted in Zhang et al. (2019), has four fertile stamens as a synapomorphy and our morphological observation of the new species supported this (Fig. 2). Within this clade, the new species is most closely related to P. chishuiensis (BP = 100%) (Fig. 3), whereas it can be easily distinguished from the latter by its rhizome, leaf blade, flowers number per cyme, bracts, bracteoles, calyx, filaments, anthers and staminodes, all of which are presented in Table 2.