Ranunculus maoxianensis (Ranunculaceae), a new species from Jiuding Shan in Maoxian county, northwestern Sichuan province, China, is here illustrated and described. The species is morphologically similar to R. chongzhouensis, a species also occurring in Sichuan, in having reniform leaves and puberulous receptacles, carpels and achenes, but differs by having leaves adaxially puberulous with shorter appressed hairs 0.16‒0.28 mm long (vs. longer appressed hairs 0.55‒0.85 mm long), larger flowers (1.8‒2 cm vs. 1.4‒1.6 cm in diameter), larger (8‒10 × 5.5‒6.5 mm vs. 6‒7 × 4.5‒5 mm) and widely obovate petals (vs. obovate), more numerous stamens (35‒55 vs. 12‒18), and subglobose gynoecium and aggregate fruit (vs. ellipsoid). The two species are also different in chromosome number and chromosome morphology. Ranunculus chongzhouensis has a karyotype of 2n = 2x = 16 = 10m + 6sm while R. maoxianensis has a karyotype of 2n = 4x = 32 = 16m + 16sm. An emended description of R. chongzhouensis is provided, and its geographical distribution is largely extended.

Asia, buttercups, chromosome number, Ranunculus sect. Ranunculus

Ranunculus L., comprising approximately 600 species, is the largest genus in the Ranunculaceae and is widely distributed in all continents (Tamura 1995; Hörandl et al. 2005; Paun et al. 2005; Hörandl and Emadzade 2012). In China, one of the centers of species diversity in Ranunculus, more than 150 species and 30 varieties are currently recognized in the genus (Wang 1995a, b, 1996, 2007, 2008, 2013, 2015, 2016, 2018, 2019a, b, 2022; Yang 2000; Wang and Gilbert 2001; Wang and Liao 2009; Luo and Zhao 2013; Wang and Chen 2015; Wang et al. 2016; Yuan and Yang 2017a, b, c; Zhang et al. 2020; Fei et al. 2022, 2023a, b). Many taxa in the genus occur in the Hengduan Mountains region in southwestern China, including mainly southeastern Gansu, eastern Qinghai, western Sichuan, southeastern Xizang (Tibet) and northwestern Yunnan, one of the most important biodiversity hotspots worldwide (Zhang et al. 2009; Sun et al. 2017). In total, 56 species and nine varieties of Ranunculus are known from that region (Wang 1993, 1995a, b, 2008, 2013, 2015, 2022; Yang 2000; Wang and Gilbert 2001; Wang and Liao 2009; Wang and Chen 2015; Yuan and Yang 2017c).

During a botanical expedition to the Hengduan Mountains region from June to August 2022 for the first author’s Ph.D. dissertation project, we discovered an unusual population (Figs 1, 2) of Ranunculus on Jiuding Shan in Maoxian county, northwestern Sichuan province, China. The plants are somewhat similar to R. chongzhouensis W.T. Wang (Figs 3–10), a species also occurring in Sichuan, in having reniform leaves and puberulous receptacles, carpels and achenes, but differ by an array of characters, such as the length of hairs on the adaxial side of leaf blades, size of flowers, size and shape of petals, number of stamens, and shape of the gynoecium and aggregate fruit. Moreover, our chromosomal examination revealed that the Maoxian population in question has a chromosome number of 2n = 16 (Fig. 11A), while R. chongzhouensis has a chromosome number of 2n = 32 (Fig. 11B). Therefore, we determined that this population represents a hitherto undescribed species, which we describe as R. maoxianensis below. Furthermore, we found that R. chongzhouensis, the putative closest ally of R. maoxianensis, is much more widely distributed than documented before and that its original description is not complete, lacking a description of root and floral characters, with the description of indumentum of leaf blades, receptacles and achenes being also incorrect. Based on our critical observations of herbarium specimens and living plants in the wild, the description of this species is here emended. We also largely extended its geographical distribution.

For morphological comparison, we critically examined specimens or high-resolution specimen images of Ranunculus at CDBI, KUN, PE, and WCSBG (acronyms according to Thiers 2022). We also observed living plants in three populations of R. chongzhouensis from Sichuan and one population of the new species R. maoxianensis (Table 1) at flowering and fruiting time (June to July). We observed characters of roots, stems, leaves, pedicels, flowers, receptacles, petals, stamens, gynoecium, carpels, aggregate fruit and achenes, paying special attention to the indumentum of basal leaves, size of flowers, size and shape of petals, number of stamens, and the shape of gynoecium and aggregate fruit.

For scanning electron microscopy (SEM), dry leaves were taken from herbarium specimens (Table 1) and mounted directly onto stubs using double-sided sellotape, gold-coated, and then observed and photographed under a JSM-6360LV scanning electron microscope.

For chromosomal examination, living plants of Ranunculus chongzhouensis from Xiling Xue Shan in Dayi, Sichuan, and R. maoxianensis from its type locality, i.e., Jiuding Shan in Maoxian, Sichuan (Table 1), were cultivated in pots in the experimental garden of South China Botanical Garden, Chinese Academy of Sciences. Root tips were pretreated in 0.1% colchicine for 2.5 h, fixed in Carnoy I (glacial acetic acid: absolute ethanol = 1: 3), then macerated in 1 M HCl at 37 °C for 45 min, and stained and squashed in Carbol fuchsin. Karyotype formulas were based on the data of measurements of mitotic-metaphase chromosomes of three cells taken from photographs. We followed the acronyms proposed by Levan et al. (1964) to describe the karyotypes.

Our critical observations on herbarium specimens and living plants in the wild indicate that Ranunculus maoxianensis (Figs 1, 2) is morphologically similar to R. chongzhouensis (Figs 3–10) in having reniform leaves and puberulous receptacles, carpels and achenes, but differs by having leaves adaxially puberulous with shorter appressed hairs 0.16‒0.28 mm long (vs. longer appressed hairs 0.55‒0.85 mm long) (Fig. 12A, C), larger flowers (1.8‒2.0 cm vs. 1.4‒1.6 cm in diameter), larger (8‒10 × 5.5‒6.5 mm vs. 6‒7 × 4.5‒5 mm) and widely obovate petals (vs. obovate), more numerous stamens (35‒55 vs. 12‒18), and subglobose gynoecium and aggregate fruit (vs. ellipsoid). The difference in the indumentum of leaves between the two species is further confirmed by our SEM results (Fig. 12B, D). A detailed morphological comparison between R. maoxianensis and R. chongzhouensis is given in Table 2.

Our chromosomal examination reveals that Ranunculus maoxianensis is a diploid species with 2n = 2x = 16 = 10m + 6sm (Fig. 11A, C), while R. chongzhouensis is a tetraploid with 2n = 4x = 32 = 16m + 16sm (Fig. 11B, D). This result lends strong support to the description of R. maoxianensis as a new species.

Our literature consultation and critical observations on herbarium specimens and living plants in the wild reveal that Ranunculus chongzhouensis, the putative closest ally of R. maoxianensis, lacks the description of root and floral characters, with the description of indumentum of leaf blades, receptacles and achenes by Wang (2015) being also incorrect. Wang (2015) described this species based on a single specimen, Z.B. Feng, D.H. Zhu & X.J. Li 4171 (PE; Fig. 3A), from Jiguan Shan in the Anzihe Nature Reserve in Chongzhou city, Sichuan province, China. The three plants on the sheet are all fruiting, lacking roots and flowers. Therefore, the number of basal leaves and floral morphology were not mentioned in the original description of R. chongzhouensis. We traced two isotype sheets of R. chongzhouensis from WCSBG (Fig. 3B, C), which Wang (2015) did not see when he described this species as new. Wang (2015) described the basal leaves of R. chongzhouensis as adaxially glabrous and abaxially appressed puberulous, and the receptacles and achenes as glabrous. However, the basal leaves of this species are adaxially appressed puberulous (Fig. 3D, E) and abaxially glabrous (Fig. 3F, G), and the receptacles (Fig. 3H) and achenes (Fig. 3H, I) are puberulous. These results are further confirmed by our observations of living plants in three populations of R. chongzhouensis, respectively, from Chongzhou (type locality), Dayi, and Xiaojin, all in Sichuan province (Figs 4–9). In addition, we found that the indumentum on the abaxial side of the leaf blades of R. chongzhouensis is somewhat variable between populations. The leaf blades are often abaxially glabrous (Figs 4E, 6E), but sometimes abaxially puberulous (Fig. 8E).

From our survey of herbarium specimens and fieldwork, we found that Ranunculus chongzhouensis is much more widely distributed than reported by Wang (2015). In addition to its type locality, this species is also distributed in Baoxing, Dayi, Heishui, Luding, Songpan, and Xiaojin counties. Selected specimens from Baoxing (Fig. 10A), Heishui (Fig. 10B), Luding (Fig. 10C) and Songpan (Fig. 10D) are shown in Fig. 10.

Ranunculus maoxianensis is readily assigned to R. sect. Ranunculus due to its swollen achenes with a distinct beak and receptacles hardly enlarged after anthesis. Wang (2015) also included R. chongzhouensis in this section. We accept the sectional placement of this species.

We are grateful to one anonymous reviewer, Dr. Marco Pellegrini, and Dr. Liang Zhao, for their valuable comments on the manuscript. We thank the curators of CDBI, KUN, PE, and WCSBG for allowing us to use their scanned images of specimens and for research facilities. This work was supported by the National Natural Science Foundation of China (grant nos. 31870184, 31770218, and 31970210).