Leptomischus hiepii, a new species of Rubiaceae from Vietnam

Abstract Leptomischus hiepii, a new species of the tribe Argostemmateae from Son La province, northwestern Vietnam, is described and illustrated. Morphologically it allies to L. wallichii, L. erianthus and L. funingensis by sharing a similar habit, large stipules and similar corolla shape, but it differs by its anisophyllous leaves, 1-flowered inflorescences, homostylous flowers and tubular-campanulate corollas.


Introduction
Leptomischus Drake (Drake 1895) is a poorly known genus of Rubiaceae. It was first described as a monotypic genus with one species, L. primuloides Drake (Drake 1895), occurring in northern Vietnam. Nearly a century later, Lo (1986) published the second species found in southern and southwestern China and northern Vietnam, i.e. L. parviflorus H.S.Lo. Later, in his revision of Leptomischus, Lo (1993) included Indopolysolenia Bennet (Raizada and Bennet 1981) as a synonym of Leptomischus and accommodated a new combination, L. wallichii (Hook.f ) H.S.Lo for Polysolenia wallichii Hook.f. (Bentham and Hooker 1873) from India. He also proposed a new synonym of L. primuloides, i.e. Indopolysolenia burmanica Deb & Rout (Deb and Rout 1990) reported from northern Myanmar. Subsequently, Lo (1998) (Hareesh et al. 2017). To summarize, Leptomischus presently includes seven species distributed in tropical mainland Asia, including northeastern India (2 species), Myanmar (1 species), southern and southwestern China (5 species), and northern Vietnam (2 species).
Despite the fact that the genus Leptomischus is insufficiently known and has previously been studied by few researchers (Lo 1986(Lo , 1993(Lo , 1998Chen and Taylor 2011;Hareesh et al. 2017), this group of plants can be readily distinguished from other genera of Rubiaceae by a combination of the following characters: perennial herbs or shrubs, heterostylous flowers, numerous ovules which are borne on a stipitate placenta near the base of the septum, dry capsular fruits dehiscing through the apical portion or through an operculum, small and numerous seeds with reticulate testa (Lo 1993;Chen and Taylor 2011;Razafimandimbison and Rydin 2019). Leptomischus is considered to belong to the Asian tribe Argostemmateae in subfamily Rubioideae and was found to be sister to all other genera of the tribe (Razafimandimbison and Rydin 2019).
Among the characteristic features mentioned above, the mode of dehiscence of the capsule is the most important, as only two genera of the tribe share such a character (Lo 1993), i.e. Leptomischus and Mouretia Pitard (Pitard 1922). It is said that placenta insertion (near the base of the septum in Leptomischus and near the middle of the septum in Mouretia) is a key character to distinguish Leptomischus from Mouretia (Chen and Taylor 2011;Hareesh et al. 2017). However, the insertion of the placenta is difficult to observe since the ovary is quite small and the placentas are large in comparison with the septum. As a result, this characteristic was not illustrated for any species described (Bentham and Hooker 1873;Drake 1895;Raizada and Bennet 1981;Deb and Rout 1990;Lo 1986Lo , 1993Lo , 1998Chen and Taylor 2011;Hareesh et al. 2017). Moreover, while describing Leptomischus in China, Chen and Taylor (2011) stated that the placentas were "inserted apparently near the middle of septum", and we also found that the placenta of L. funingensis is inserted at the middle of septum (Fig. 4I). To our knowledge, all representatives of Leptomischus have corolla tubes longer than 14 mm (except for 6-6.5 mm long in L. parviflora), anthers 2-3.5 mm long and stigmas 2-5 mm long (Deb and Rout 1990;Lo 1998;Chen and Taylor 2011;Barbhuiya et al. 2014;Hareesh et al. 2017), whereas species of Mouretia have corolla tubes shorter than 4 mm, anthers 0.9-1.4 mm long and stigmas 0.7-2 mm long (Tange 1997;Chen and Taylor 2011).
During an expedition to Muong La Nature Reserve in Son La Province of northern Vietnam, an unusual species of the family Rubiaceae was discovered. It is a perennial herb, mostly glabrous, with anisophyllous leaves, usually 2-lobed stipules, terminal 1-flowered inflorescences, actinomorphic, 5-6-merous, hermaphroditic homostylous flowers, numerous ovules on a stipitate placenta attached to the middle of the septum, fleshy capsular fruits dehiscing through apical portion, small and numerous seeds with reticulate or verrucose testa. It clearly belongs to the tribe Argostemmateae, in which all genera are characterized by hermaphroditic flowers, bilocular ovaries (sometimes 3-5-celled in Mycetia Reinw., capsular fruits, and many small seeds (Rydin et al. 2009;Ginter et al. 2015;Razafimandimbison and Rydin 2019). Because of the corolla tube (3.2-)3.4-3.8(-4) cm long and (1.2-)1.3-1.5(-1.7) cm in diameter at the throat, the anthers 2.2-2.5 mm long, the stigmas 2.5-3 mm in length and the capsular fruits dehiscing through the apical portion, we tentatively place it in the genus Leptomischus.

Material and methods
The specimens of the newly described species are deposited at the following herbaria: Central South University of Forestry and Technology (CSFI), Institute of Ecology and Biological Resource of the Vietnam Academy of Science and Technology (HN), and Komarov Botanical Institute of Russia (LE). Herbarium acronyms follow Thiers (2020). Morphological observations and measurements of the new species are based on living material in the field as well as on dry specimens. The conservation status of the new species is evaluated based on field observations in accordance with IUCN guidelines (2017).
Etymology. The specific epithet honors Dr. Nguyen Tien Hiep, a famous botanist who made significant contributions to the plant taxonomy and nature conservation in Vietnam.
Additional specimens examined (   Habitat, phenology and conservation status. Lithophytic or terrestrial herb growing on rocks in streams and on moist cliffs under primary and secondary evergreen broad-leaved submontane forest on sandstone at elevations of 1300-1400 m a.s.l.. The plants flower in February-March, and bear fruits in July-August. The species was observed as being very common on waterfall cliffs and in humid places; around 300-500 mature individuals occur in Muong La Nature Reserve, spread over a very limited area of approximately 2500 m 2 . In Muong La Nature Reserve, agricultural activities and exploitation for timber and non-timber forest products are prohibited. According to indigenous people, this species is not used as medicinal or ornamental plant, and disturbances to its existence so far have not been recorded. Numerous localities in the mountainous areas of the Hoang Lien Son range, spreading over the provinces of Lai Chau (Phong Tho, Tam Duong and Than Uyen districts) and Lao Cai (Bat Xat, Sa Pa and Van Ban districts) to Yen Bai (Tram Tau, Nghia Lo, Van Yen and Mu Cang Chai districts) and Son La (Muong La and Bac Yen districts), in northwestern Vietnam and the south-western parts of Yunnan province in south-western China, fit the habitat characteristics of this new species. It is therefore expected that more populations of the species will be discovered soon if extensive field surveys are carried out in this region. At present, however, L. hiepii can be considered as an endemic species to Son La province, and is tentatively assessed as "data deficient" (DD) in accordance with the IUCN Red list categories and criteria (2017).
Distribution. Endemic to north-western Vietnam (Son La province, Muong La district, Muong La Nature Reserve).
Discussion. Leptomischus hiepii is a very special member of the genus in several ways, and is inconsistent with most of its congeners. Typical characters of L. hiepii Table 1. Morphological comparison of Leptomischus hiepii, L. wallichii, L. erianthus and L. funingensis (Lo 1993;Chen and Taylor 2011;Hareesh et al. 2017  are anisophyllous leaves, 1-flowered inflorescences, homostylous (5-)6-merous flowers and anthers and stigma positioned at the level of the throat of the corolla tube, while all currently known species of the genus are reported as having isophyllous leaves, sev-eral-flowered inflorescences, heterostylous 5-merous flowers, and anthers positioned much lower than the stigma or vice versa. Anisophyllous and isophyllous leaves are commonly seen in Mouretia, Mycetia and Argostemma Wall. within the tribe Argostemmateae (Chen and Taylor 2011). The 1-flowered inflorescence has not been recorded in Leptomischus but is reported in Argostemma, e.g. in A. bachmaense T.V.Do (Do et al. 2020). Homostylous flowers have hitherto not been recorded in the tribe Argostemmateae but the rare presence of homostylous flowers in otherwise heterostylous genera is not rare in Rubiaceae (Chen and Taylor 2011), e.g., homostylous flowers are reported in Mussaenda campanulata T.T.Duan & D.X.Zhang (Duan et al. 2016) and Ophiorrhiza longifloriformis Schanzer (Schanzer 2005). Flower merosity is variable in the genera Mycetia, Argostemma (Chen and Taylor 2011) and Leptomischus as well, or even within the same inflorescence of a species such as L. erianthus bearing 4-7-merous flowers (Fig. 4B, E). Anthers and stigma positioned at the level of the throat of the corolla tube are a common feature in homostylous flowers of Rubiaceae and also occurs in Neohymenopogon Bennet of the tribe Argostemmateae (Chen and Taylor 2011).