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 and densely ciliate (vs. serrate); cyme 4–10-flowered or more (vs. usually 1–3-flowered); anthers sparsely semi-transparent glands (vs. glabrous) and staminodes pale purple, club-like, glabrous (vs. absent or indistinctive).

Figure 1. 

Petrocodon wui F.Wen & R.B.Zhang, sp. nov. A habit B bracts, showing the abaxial surface C bracteoles D abaxial surfaces of calyx lobe E top view of flower F opened corolla from the dried flower G one of stamens H pistil I seed. Drawings by Tan Deng from the type specimen.

China, Guizhou Province, Zunyi City, Xishui County, Xishui National Nature Reserve, Dabaitang, Niuqingshan, elev. ca. 1100 m, growing on a shaded and moist rock surface on the Danxia cliff in the gorge, Ren-Bo Zhang ZRB2401 (holotype: IBK!, isotypes: ZY!).

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 linear-cylindrical, 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, linear-cylindrical, 4-valved, pubescent. Seeds appendaged, grain shortly cylindrical, rough, ca. 0.5 mm long, ca. 0.3 mm wide, covered densely verrucate.

Flowering occurs in August in the wild; fruiting should occur in October, based on current flowering patterns.

We dedicate this new species of Petrocodon to Wu Zheng-Yi (Wu Chengyih) (1916–2013), 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.

The Chinese name proposed here is “吴氏石山苣苔.” Phonetically, it is “Wú Shì Shí Shān Jù Tái”.

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%.

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 m² 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 km² (2 × 2 km² 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.

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.

Figure 2. 

Petrocodon wui F.Wen & R.B.Zhang, sp. nov. A plants in bloom in natural habitat B plant in flower C upward view of plant showing the abaxial surfaces of leaf blade and petiole D flowering cyme E cymes, calyx and immature capsule F frontal view of corolla G lateral view of corolla and extended pistil H opened corolla I four fertile stamens J calyx and pistil K mature and dehiscent capsules (Photographed by F. Wen and R.B. Zhang).

Figure 3. 

Phylogenetic tree of Petrocodon generated from Maximum Likelihood (ML) of trnL-F and ITS datasets. Numbers on the branches indicate ML bootstrap values (≥ 50%).