Rhododendron jiucaipingensis Jian Xu & M. T. An sp. nov.

Figs 1, 2

Type.

China • Guizhou: Bijie City, Hezhang County, Xingfa Township, on cliffs, alt. 2,600 m, 27°00'08.86"N, 104°43'47.94"E, 8 May 2021, Jian Xu leg; holotype: GZAC! 20210508 JCP 001 • same data as for holotype; isotype: GZAC! 20210508 JCP 002

Diagnosis.

Rhododendron jiucaipingensis belongs to subgen. Hymenanthes and is similar in morphology to R. hypoglaucum and R. argyrophyllum, but can be distinguished from R. hypoglaucum by its current-year branchlets and young petioles being densely covered with indumentum (vs. glabrous), and its pedicels being densely white-villous (vs. glabrous). It further differs from R. hypoglaucum in having fewer lateral veins (8–11 pairs vs. 10–14 pairs), an ovary sparsely covered with colorless stellate hairs (vs. glabrous), and typically 7 - loculed capsules (vs. 6 - loculed). Compared to R. argyrophyllum, the new species is distinguished by its acuminate leaf apex (vs. obtuse), fewer stamens (10 vs. 12–15), and the presence of colorless stellate hairs on the ovary (vs. white short tomentum). Additionally, R. jiucaipingensis is characterized by its restricted cliff habitat at higher elevations (2,400 –2,700 m).

Description.

Evergreen shrubs, 1.5–3.0 m tall. Branches stout; current-year branchlets green, densely covered with white tomentum; older branches grayish brown, glabrescent. Leaves crowded at branch apices, thickly leathery, oblong-elliptic to oblanceolate-elliptic, 5.5–9.5 cm long, 2.5–4.0 cm wide; apex gradually acuminate, with a short apiculate tip; base gradually narrowed, cuneate; margins revolute; young leaves adaxially sparsely white-tomentose along the midrib, abaxially densely covered with a double-layered white indumentum, the upper layer consisting of long glandular hairs; mature leaves glabrescent adaxially; the superficial indumentum on the abaxial surface shed, leaving only a lower layer of short, brown tomentum; lateral veins 8–11 pairs, faintly visible adaxially and conspicuously raised abaxially; petioles cylindric, 1.0– 1.5 cm long, grooved adaxially, densely covered with long white glandular hairs when young, becoming glabrous at maturity. Inflorescences terminal, umbellate-racemose, with 4–9 flowers; peduncle 1.0– 1.3 cm long, glabrous; pedicels 1.5–2.5 cm long, densely covered with long white villous hairs; calyx triangular, ca. 1.5 mm long, 5 - lobed or with lobes partially connate; corolla funnelform-campanulate, 2.0–3.0 cm long, 2.0–3.0 cm in diameter in the upper part, white, the tube with pink fasciculate blotches and spots, 5 - lobed, lobes orbicular, ca. 1 cm long, ca. 1.5 cm wide, apex rounded and emarginate; stamens 10, 1.5–2.0 cm long, unequal; filaments linear, densely covered with white stiff hairs at the base, anthers oblong-elliptic, 0.2–0.3 cm long; pistil 2.5–3.0 cm long, glabrous; ovary cylindric, sparsely covered with colorless stellate hairs; style ca. 2.5 cm long, glabrous; stigma slightly dilated. Capsules cylindric, curved, 1.2–2.0 cm long, ca. 0.6 cm in diameter, sparsely covered with colorless stellate hairs, usually 7 - loculed, occasionally 6 - or 8 - loculed.

Phenology.

Flowering in May, fruiting in June.

Etymology.

The specific epithet jiucaipingensis refers to the type locality, Xingfa Township (Jiucaiping), Hezhang County, Bijie City, Guizhou, China.

Vernacular name.

The Chinese name is jiǔ cài píng dù juān (韭菜坪杜鹃).

Distribution and habitat.

This species is currently known only from western Guizhou, China (Fig. 2), where it grows on cliffs at elevations of 2,400 –2,700 m. The type specimens were collected from Xingfa Township (Jiucaiping), Hezhang County, Bijie City, Guizhou, China.

Conservation status.

Rhododendron jiucaipingensis is known only from 21 extremely restricted populations in Jiucaiping, Xingfa Township, totaling 103 mature individuals. The species has an extremely restricted Area of Occupancy (AOO <10 km 2, estimated using a 2 × 2 km grid) and occurs at a single location according to IUCN criteria (IUCN Standards and Petitions Committee 2024). Populations may be vulnerable to habitat degradation due to their cliffside habitat and potential anthropogenic activities, including climate change and collection. With such a small number of mature individuals, the species qualifies as Endangered [EN] under Criterion D 1. Although a continuing decline has not been directly observed, the extremely limited AOO, single-location occurrence, projected threats to habitat quality, and small population size meet the IUCN Red List criteria for Endangered [EN] under Criteria B 2 ab (iii) + D 1.

Additional specimens examined.

China • Guizhou: Bijie City, Hezhang County, Xingfa Township, on cliffs, alt. 2,600 m, 27°00'02.48"N, 104°43'54.92"E, 13 May 2021, Jian Xu leg.; paratype: GZBG! XJ 20210513001 • same data as for preceding; paratype: GZBG! XJ 20210513002.

Similar species.

Rhododendron jiucaipingensis is morphologically and phylogenetically most similar to R. hypoglaucum and R. argyrophyllum, which are also distributed in the karst regions of Southwest China. All three species are endemic to the karst regions of Southwest China and share white to pinkish campanulate corollas. However, R. jiucaipingensis is readily distinguished by its densely white-tomentose young branchlets, young petioles with long white glandular hairs, and mature leaf abaxial surfaces with short, brown tomentum. Additionally, the white stiff hairs at the filament base and colorless stellate hairs on the ovary and capsule serve as key diagnostic features. Detailed morphological comparisons are summarized in Table 1.

Molecular phylogenetic evidence.

The ML analysis, based on a comprehensive whole-genome SNP dataset, yielded a highly resolved phylogenetic framework (Fig. 3). Within this topology, R. jiucaipingensis is robustly recovered as a distinct and independent evolutionary lineage within subsect. Argyrophylla. The new species is strongly supported (SH-aLRT = 100 %, UFBoot = 100 %) as sister to a monophyletic clade comprising five species: R. hunnewellianum, R. hypoglaucum, R. argyrophyllum, R. pingianum, and R. ririei. Despite its close phylogenetic affinity to this five-species group, R. jiucaipingensis exhibits significant genetic divergence, occupying a well-supported terminal branch that underscores its long-term evolutionary isolation. This unambiguous molecular evidence, coupled with the clear morphological discontinuities observed (Table 1), provides compelling support for the recognition of R. jiucaipingensis as a distinct new species within subsect. Argyrophylla.