﻿Rediscovery of Primulabrachystoma (Primulaceae), a rare species endemic to Gaoligong Mountain of Chinese-Burma frontier

﻿Abstract The rare Primulabrachystoma W.W.Sm. is an endemic species confined to Gaoligong mountain of Chinese-Burma frontier, which has been rediscovered from the same region after nearly 100 years. In total, 11 specimens from Gaoligong Mountain have been found in the herbaria worldwide, since its first collection in 1920 by Farrer, Reginald John. Previously, this species was described as homostylous but our finding shows the species also exhibited heterostyly. A complete description of the species, the distribution, morphological comparison and identification key from closely related species are provided here. An assessment of its conservation status suggests that the species is ‘Endangered’ (EN).

Primula elegans Forrest var. maculosa H.Chuang. However, a comparison with the type specimen of P. brachystoma indicates that all these specimens belong to this species; therefore, our observation suggests that P. brachystoma is also distributed in China.
Primula brachystoma was originally described as homostylous by Smith (Smith and Forrest 1923). During a botanical expedition in the regions of Gaoligong Mountain in May 2015, we found a homostylous Primula with an acute leaf apex and regularly denticulate at margin, campanulate calyx and yellow corolla with annulus marked, on the western slopes of the Gaoligong Mountains near Dulong River. After a comparison with type specimens, the plant matched P. brachystoma (Fig. 1A-D). Another heterostylous Primula with an acute leaf apex and regularly denticulate at margin, campanulate calyx and yellow corolla with annulus marked, was found in Shibali and Yaping of Fugong county during a botanical expedition in Gaoligong Mountain of Fugong Xian, Yunnan, China in 2018. After comparing with the type specimens, excluding the heterostylous flowers, all its other characteristics match the description of P. brachystoma. Considering that some species in the genus Primula have both homostylous and heterostylous flowers such as Primula chungensis Balf.f. & Kingdon-Ward, Primula oreodoxa Franch., Primula polonensis Kingdon-Ward and Primula sinensis Sabine ex Lindl. etc (Hu 1990;Bawri et al. 2015), the plant from Shibali and Yaping could be P. brachystoma ( Fig. 1E-I). Therefore, we believe that P. brachystoma, having both homostylous and heterostylous flowers, is similar to its closely allied species P. polonensis and P. chungensis in the same section. This paper provides a complete morphological description, distribution, morphological comparison and identification key from closely related species. The threat status of P. brachystoma through field surveys and review of type specimens, as well as color photographs, are also provided to facilitate proper identification of the species (Figs 1, 3).

Materials and methods
We collected fresh material and specimens of P. brachystoma from Gongshan county, Yunnan on the way to the Dulong River in May 2015 while travelling from Yaping and Shibali of Fugong county in July 2018. The identity of our plant collection has been confirmed by consulting the original description and online images of the type specimens from key Herbaria (BM, E, K). Complete morphological characters of the species were measured using a vernier calliper. The voucher specimens are stored at KUN. For comparison purposes, specimens of closely related species, P. polonensis, P. prenantha subsp. prenantha, P. prenantha subsp. morsheadiana, P. serratifolia Franch., Primula melanodonta W.W.Sm. from the key herbaria of China (IBSC, KUN, PE), type specimens' images online of the closely related species from BM, E, K, P, and relevant literature (Smith et al. 1977;Hu 1990;Hu and Kelso 1996) were also consulted. The conservation status of P. brachystoma was assessed using the guidelines for IUCN Red List categories and criteria (IUCN 2022; IUCN Standards and Petitions Committee 2022).

Description.
A perennial herb, completely glabrous and efarinose, with numerous robust roots. Leaves forming a rosette; leaf blade oblanceolate to sublanceolate, 6-12 cm long, 2-4 cm broad, acute and shortly apiculate at the apex, petiole very short or as long as 1/3 of the blade, base attenuate with broadly winged petiole, margin regularly fine dentate, teeth triangular, apex acute and subulate, leaf abaxially with prominent midrib and conspicuous lateral veins, and inconspicuous mesh vein. Scapes slightly slender, 15-25 cm long; umbels 1 (or rarely 2), 2-7 flowered. Bracts linear-lanceolate, 0.6-0.8 cm long. Pedicel slightly recurved when flowering, erect when fruiting, 0.8-1 cm long in flowering, up to 1.5 cm long in fruiting. Calyx campanulate, green, 5-7 mm long, with 5 ribs, splitting slightly less than 1/3 of its full length, lobes triangular, apiculate at the apex. Flowers homostylous or heterostylous in different populations, corolla funnel-shaped, yellow, tube 8-10 mm long, with a marked annulus, limb 1-1.5 cm wide, lobes sub-quadrangular to obovate, 4-5 mm long, shallowly notched; homostylous flowers: the stamens are inserted in the middle of the corolla tube, filament ca. 0.5 mm long, anther ca. 0.8-1 mm long, yellowish white, the style reaches to the level of the stamens; heterostylous flowers: in long-styled flowers the style nearly reaches the annulus and the stamens are inserted towards the base of the corolla, in short-styled flowers the stamens are inserted slightly below the annulus and the style is shorter than the calyx, the filament and anther as those in homostylous flowers. Capsule globose, green in young fruiting time and pale purple in mature fruiting time, 5-6 mm long, as long as or slightly shorter than calyx, 4-5 mm in diameter. Distribution and habitat. It is found on Gaoligong Mountain on the border between China and Burma. One homostylous population is at the type locality Shing Hong of Burma and two homostylous populations are at Sandui and Tsuga of Gongshan county, China; the other two heterostylous population are at Sibali and Luodigolu, Yaping of Fugong county, China. It grows along moist streams or on wet grassy slopes at forest margins, 2500-3000 m above sea level. Provisional conservation status. Primula brachystoma is neither listed in the IUCN Red List (IUCN 2022), nor in the threatened Species List of China's Higher Plants (Qin et al. 2017). The authors have conducted field surveys in the regions of Gaoligong mountains many times and discovered only three populations of Primula brachystoma in Fugong county and Gongshan county. Surveys from other plant hunters also didn't find more populations in this area. We estimated the extent of occurrence of the species to be less than 1000 km 2 , and the adult individuals as fewer than 1000. The sites where the known populations grow are also places for grazing, so they face a strong threat from human activities. Accordingly, we evaluate the species as Endangered (EN B1ab(iii)), considering the IUCN standards (IUCN Standards and Petitions Committee 2022).

Diagnosis
Morphologically, P. brachystoma is similar to P. polonensis, P. prenantha subsp. prenantha, P. prenantha subsp. morsheadiana, P. serratifolia, P. melanodonta in P. sect. Proliferae Pax. Among these species, P. brachystoma and P. polonensis have both homostyly and heterostyly flowers, longer leaves with inconspicu-ous mesh vein on abaxial surface, and short corolla tube (8-10 mm) with a marked annulus; P. brachystoma differed from the latter by its leaf blade being acute and shortly apiculate, calyx splitting slightly less than 1/3 of its full length ( Fig. 2A, B). P. brachystoma differed from the homostylous species P. prenantha subsp. prenantha and P. prenantha subsp. morsheadiana by the leaf blades of the latter two are both rounded at the apex, the abaxial surface of leave blade with conspicuous mesh vein, corolla tube more elongated ( Fig. 2A, C, D;  Fig. 3A-E). P. brachystoma differed from the heterostylous species P. serratifolia and P. melanodonta by the leaf blades of P. serratifolia and P. melanodonta are both rounded to obtuse at the apex, the abaxial surface of leave blade with conspicuous mesh vein (Figs 2A, E, F; 3A, F-I). The main morphological difference between P. brachystoma and its allies is summarized in Table 1, and the following Keys.

Funding
No funding was reported.