Holotype: China, Guangxi Zhuang Autonomous Region. Pingxiang city, Baiyun town, Jiaoguo village, near SinoVietnam border, 22.255854°N, 106.716474°E, 150 m, on moist tufa and rocky surface of limestone cliff, 22 Jul 2013, F. Wen, B. D. Lai & F.Y Nong 20130722-01 (IBK!; isotype: IBK!).

Diagnosis — Primulina heterochroa F. Wen & B. D. Lai differs from its congeners, P. pungentisepala (W. T. Wang) Mich. Möller & A. Weber and P. minutimaculata (D. Fang & W. T. Wang) Yin Z. Wang, by the following combination of characters: peduncle densely covered with spreading reddish purple eglandular and glandular hairs; bracts purple, outside reddish brown pubescent, inside densely purple glandular pubescent; calyx lobes outside densely spreading purple pubescent, inside sparsely shortly pubescent; corolla outside dark reddish purple to purplish brown, densely reddish purple pubescent, lobes inside dark yellow or bronzed yellow with dark brown longitudinal stripes (see also Table 1).

Description — Herbs perennial. Rhizome cylindric, 4–6 cm long, 1–1.5 cm in diam. Leaves 15–18, basal; petiole compressed, 3–4.5 × 0.6–0.8 cm, sparsely shortly strigose; leaf blade dark green with silvery lateral veins, narrowly oblong to oblong-elliptic, 6–10 × 3.5–4 cm, slightly fleshy, thickly chartaceous when dried, lower surface densely strigose and appressed pubescent, upper surface densely appressed pubescent, base attenuate to cuneate, margin entire, apex acute; lateral veins 4 or 5 on each side of midrib, slightly prominent on lower surface, inconspicuously sunken on upper surface. Cymes 5 or more, axillary, 3–5(–12)-flowered or more; peduncle brown, 8–15 cm long, c. 2.5 mm in diam., densely covered with spreading reddish purple eglandular and glandular hairs; bracts 2, opposite, purple, slightly rhombic to broadly lanceolate, 12–15 × 5–6 mm, persistent at flowering, occasionally withered; outside reddish brown pubescent, inside densely purple glandular pubescent, margin entire, apex acute. Calyx 5-parted to base; lobes lanceolate, c. 7 × 2 mm, nearly equal, outside densely spreading purple pubescent, inside sparsely shortly pubescent, margin entire, apex acute. Corolla outside dark reddish purple to purplish brown, throat with 2 bright yellow stripes, upper portion of inside corolla surface with 2 dark purple swollen sparsely short glandular hairy lines, inside of lobes dark yellow or bronzed yellow with dark brown longitudinal stripes, corolla c. 3 cm long, outside densely reddish purple pubescent, inside nearly glabrous; tube nearly tubular, c. 2 cm long, orifice c. 1.1 cm in diam.; limb distinctly 2-lipped; upper lip 2-lobed, lobes triangular, c. 6 mm long; lower lip 3-lobed, lobes oblong, c. 9 mm long, lobe mid-portion often twisted or constricted. Stamens 2, inserted c. 1.5 cm above corolla base; filaments dark purple, c. 6 mm long, geniculate c. 2 mm above insertion, pubescent just below anthers, otherwise glabrous; anthers reniform, constricted at middle, c. 2.5 mm long; staminodes 3, lateral ones inserted 9–10 mm above corolla base, reddish purple, linear, c. 6 mm long, glabrous, apex small capitate, central staminode inserted c. 6 mm above corolla base, punctiform, c. 0.5 mm long. Disk annular, c. 0.6 mm high; pistil c. 2.2 cm long (boundary of ovary and style ambiguous), densely covered with dark purple eglandular and glandular hairs; stigma 2-lobed, c. 3 mm long. Fruit not seen.

Phenology — Flowering specimens were collected in July; the fruiting time is unknown.

Distribution — Primulina heterochroa is known only from a single population at the type locality in Pingxiang city, SW Guangxi Zhuang Autonomous Region, S China, near the border with Vietnam (Fig. 4). It might be eventually discovered in the adjacent county of Longzhou and in Vietnam.

Ecology — Primulina heterochroa grows on shaded moist tufa of limestone cliffs under northern tropical limestone seasonal rain forest on the N-facing slope of a limestone hill at an altitude of about 150 m. The average annual temperature of Pingxiang is 21°C, the average annual precipitation is c. 1400 mm. Dominant plants at the type locality belong to Apocynaceae, Begoniaceae, Euphorbiaceae, Lauraceae, Moraceae, Rubiaceae and Urticaceae.

Conservation status and provisional Red List category — The new species is known only from one population with fewer than 50 mature individuals. We could not find a second population or any individual plants around the type locality despite careful field explorations on five occasions over the past several years. We therefore assess that Primulina heterochroa should be provisionally considered as Critically Endangered: CR B lab(ii,iii,v)+2ab(ii,iii,v); D, according to IUCN Red List categories and criteria (IUCN 2012).

Etymology — The specific epithet “heterochroa” is a Greek-derived compound adjective meaning different-coloured, referring to the remarkable contrasting colours of the corolla.

Vernacular name — Chinese: (yì sè bào chūn j tái). The first two characters mean “different colour” and thus mirror the specific epithet in referring to the corolla colours. The last four characters are the Chinese name for the genus Primulina.

Remarks — Several characters place this new species in the genus Primulina, e.g. stamens 2, stigma 1, capsule linear, straight, significantly longer than the persistent calyx lobes. Primulina heterochroa is very similar to P. pungentisepala and P. minutimaculata from a morphological point of view, but it can be distinguished from the latter two species by certain qualitative and quantitative characters in leaf, inflorescence, bract, calyx, corolla, hairs, etc. The three species share some similar characteristics, e.g. thicker and nearly fleshy leaf blades with silvery lateral veins, and thick pubescent hairs on the peduncles and pedicels. This hints at convergent evolution in three different species adapting to the subtropical monsoon climate with warm year-round temperatures and obviously alternating dry and wet seasons. The morphological differences between the three species are itemized in Table 1 and illustrated in Fig. 3.

Many new taxa of Primulina have been discovered, described and published recently, including this new one. We now know that the biodiversity of Primulina in limestone areas is very dependent on specialized habitats, such as caves, cliffs, crags, overhanging rocks, crevices, damp tufa, and so on, which are on a calcium-rich substrate further enriched from the weathering of the limestone. In exploring for plants in the past, the above-mentioned localities were usually ignored. Thus we expect additional undescribed taxa to be discovered in future field investigations.