This species is distinct from all currently known congeneric species and could be easily distinguishable by its annual habit, single, erect and unbranched stems, long petiolate leaves with truncate to broadly cuneate base and grayish green to silver gray lower surface, terminal racemes up to 35 cm long, white corolla with the palate densely covering conspicuous clavate gland-like hairs and having no stolons and basal leaves.

China. Jiangxi Province: Guixi City, Liukou town, under the cliffs of Danxia mountains, alt. 75m a.s.l., 12 June 2021, Bin Chen CB05735 (holotype CSH!, barcode CSH0186434; isotypes CSH!, barcode CSH0186431, CSH0186433, CSH0118470); in the same location of holotype, 24 August 2021, Xingui Le & Lin Xu CSH42465 (paratype CSH!, barcode CSH0188116).

Annual herbs, 15–65 cm tall, without stolons. Primary roots thick and strong; adventitious roots numerous, shotting from the stem base, white and slightly freshy. Stems single, erect, unbranched, terete; old stems purplish brown, sparsely puberulent; young stems grayish green, densely villous and sparsely glandular hairy. Leaves all cauline, numerous, alternate, long petiolate, larger at middle of stem; petioles densely puberulent to subglabrous, 1.5–4.5 cm long; leaf blade broadly ovate to suborbicular, membranous, 2.5–5.3 × 2.3–4.8 cm, adaxially green, subglabrous to sparsely puberulent, abaxially grayish green to silver gray, subglabrous, densely villous on veins, apex obtuse to rounded, base truncate to broadly cuneate, margin crenate, teeth apices callous, sometimes with 1 or 2 pairs of lobes near base; veins conspicuous on both surface, elevated abaxially, fluted adaxially. Racemes terminal or occasionally axillary on the top 1–3 nodes, shortened when young but elongated up to 35 cm long when fruiting, lax, multiflowered; pedicels slender, 0.8–2.5 cm long, densely villous and glandular hairy. Calyces broadly campanulate, 3.0–4.0 mm long, 5-veined, densely villous and glandular hairy outside, subglabrous inside; lobes 5, ovate-triangular, longer than the tube, apex acute, midrib conspicuous, lateral veins inconspicuous. Corolla white, dotted yellow on palate, 0.9–1.2 cm long, sparsely minutely puberulent to glabrous outside, tube cylindric, 0.4–0.6 cm long, exserted from calyx; limb 2-lipped, upper lip bilobed, upwarp, lobes lanceolate; lower lip trilobed, middle lobe narrowly ovate, ca. 1.5 mm long, smaller than lateral lobes, lateral lobes spreading away from middle lobes, broadly ovate to rectangular; palate comprising 2 longitudinal elevations extending from point of filament fusion to base of lower lobes, densely covered by gland-like hairs, hairs clavate and conspicuous, ca. 0.7 mm long, white to transparent. Stamens 4, didynamous, glabrous, inserted at the same level in distal part of tube, included; anterior pair longer, curved, appressed to corolla tube, posterior pair spreading; anthers bithecal, positioned adjacent to corolla tube on upper lip; filaments filiform, glabrous. Styles 0.4–0.5 cm long, included, exserted beyond anthers, stigma 2-lamellate. Capsule globose, ca. 2.5 mm in diam, apex rounded, included by persistent calyx.

Flowering was observed from early June to late August and fruiting from June to late September.

The species is currently known only from the type locality of Danxia mountains in Liukou Town of Guixi City, eastern Jiangxi Province, China (Fig. 4), and grows under shaded cliffs and near the edges of subtropical evergreen broad-leaved forests, at an elevation about 75 m a.s.l. (Figs 1A, 5).

Figure 4. 

Distribution map of Mazus danxiacola sp. nov.

Figure 5. 

Danxia landform of the type locality of Mazus danxiacola sp. nov. (left) and the habitat of the new species under a cliff (right). Arrows show where the population could be found.

The specific epithet “danxiacola” refers to the species inhabiting in Danxia landform.

Simplified Chinese: 丹霞通泉草; Chinese pinyin: Dān Xiá Tōng Quán Căo.

Based on our special botanical surveys for Danxia landforms in Jiangxi Province in 2021, M. danxiacola has been discovered only from one single locality so far in Liukou Town of Guixi City in Jiangxi Province, China, and 2 populations were found in the locality, which totally occupied ca. 200 m². In these populations, a total of ca. 80 fruiting individuals were counted in August 2021 and there were a lot of seedlings in each of the population when we firstly encountered the species in June 2021 (Fig. 1B, C), indicating that the species has a well-developed reproductive strategy in the habitat of Danxia landform. However, the locality is close to downtown of Guixi City, has not been projected to a nature reserve yet and all populations are obviously facing man-made interferences, such as deforestation, touring and grazing, we thus propose to categorize the species as critically endangered (CR) under criteria B and D following IUCN Red List Categories (IUCN 2022).

Morphologically, M. danxiacola bears a series of rare traits which are not common in Mazus, such as annual habit, single erect unbranched stems, without basal leaves, stem leaves many and alternate, extremely long petioles up to 4.5 cm, abaxial leaf surface grayish green to silver gray, and palate of corolla densely covered by conspicuously clavate gland-like hairs. The combination of these traits makes M. danxiacola distinct from all other congeneric taxa. Our molecular phylogenetic analyses based on cpDNA dataset indicated that M. fauriei may be closely related to M. danxiacola (Fig. 2), but M. fauriei is a perennial herb with all leaves basal and rosulate and petioles broadly winged (Hong et al. 1998), which is apparently different from M. danxiacola that has only cauline leaves and long unwinged petioles. In the nrITS trees, M. pumilis and M. gracilis were shown as possible alliances of M. danxiacola, however, cauline leaves of the former two species are always opposite, and their basal and cauline leaves are all decurrent to form short petioles (Hong et al. 1998), clearly differing from those alternate and long petiolate leaves of M. danxiacola. It is worth mentioning that there are obvious conflicts between the cpDNA and nrITS phylogenies which have been discovered and discussed in a previous study (Xiang et al. 2021). In fact, the available molecular data of Mazus were not sufficient enough to represent all known species of the genus, thus it is hard to definitely confirm the closest relatives of M. danxiacola at the moment through molecular phylogenetics. Future molecular studies including more species at population level and using more DNA markers may shed light on the determination of specific relationships within Mazus.

So far, M. danxiacola is the first species of Mazus that was found to be endemic to Danxia landform. Danxia landform is a unique type of petrographic geomorphology found in southeast, southwest, and northwest China with a high level of floral endemism (Liu et al. 1999; Liu and Liu 2003; Luo et al. 2010). In southeast China, Danxia landforms are well developed in Guangdong, Fujian, Jiangxi, and Hunan provinces, and the special environment, including deep valleys, grooves, moist caves, cliffy rocks, dry cliff-tops and shaded rock bottoms (Fig. 5), has significant effects on the growth of special plants (Chen et al. 2008). Just in the last ten years, a lot of new taxa have been continuously discovered from Danxia mountains of these provinces, i.e., Danxiaorchis J.W. Zhai, F.W. Xing & Z.J. Liu (Zhai et al. 2013), Spiradiclis danxiashanensis R.J. Wang (Wang et al. 2015), Viola hybanthoides W.B. Liao & Q. Fan (Fan et al. 2015), Begonia danxiaensis D.K. Tian & X.L. Yu (Tian et al. 2019), Phyllostachys danxiashanensis N.H. Xia & X.R. Zheng (Zheng et al. 2019), Semiaquilegia danxiashanensis L. Wu, J.J. Zhou, Qiang Zhang & W.S. Deng (Zhou et al. 2019), Lespedeza danxiaensis Q. Fan, W.Y. Zhao & K.W. Jiang (Zhao et al. 2021), Asplenium danxiaense K.W. Xu et al. (2022), etc., indicating that it is valuable to strengthen the flora investigations in Danxia landforms and uncover the biodiversity.