﻿Mitreolaquanruii (Loganiaceae), a new species from a karst region in Guangxi, China

﻿Abstract Mitreolaquanruii, a new species from Guangxi, China, is described and illustrated in this study. It is morphologically similar to M.liuyanii because of the terete stems, creeping and branched at the base, the leaves which are pilose on both surfaces and the bilobed capsules with two erect horns. The new species can be distinguished from M.liuyanii by its taller habit, up to 20–50 cm tall, its linear leaves, 4–18 × 0.3–1 cm with acuminate apex and 8–10 pairs of lateral veins, its narrowly triangular stipules, its linear bracts, ca. 1.0 cm long and glabrous and its glabrous calyx. Mitreolaquanruii is temporarily assessed as data deficient (DD) according to IUCN. The habitat of Mitreolaquanruii is extremely fragile. Therefore, this species deserves close attention and protection.


Introduction
Mitreola L. (Linnaeus 1758) is a genus of the family Loganiaceae with pantropical distribution (Chen 1995).It is generally characterised by cymose inflorescences, 5-merous flowers, cleft styles, half-inferior and bilocular ovaries and bilobed capsules with two erect or incurved horns (Leeuwenberg 1974;Wang 2018;You et al. 2020).The genus comprises about 17 species distributed in Africa, America, Asia, Oceania and the Pacific islands (Leenhouts 1962(Leenhouts , 1972;;Leeuwenberg and Vidal 1972;Leeuwenberg 1974;Li and Leeuwenberg 1996;Islas-Hernández et al. 2019;Li 2020;Liu et al. 2022).Some new species of this genus have been discovered and reported in recent years (Shan et al. 2019(Shan et al. , 2021;;You et al. 2020;Liao and Chen 2021;Liu et al. 2022).Southern and south-western China is the diversification centre of Mitreola and ca.14 species have been recorded in this area so far.Of these, eleven species, most of them found in limestone areas, are endemic to the country (Fang et al. 1995;Li and Leeuwenberg 1996;Ma et al. 2010;Yu et al. 2017;Shan et al. 2019Shan et al. , 2021;;You et al. 2020;Liao and Chen 2021;Liu et al. 2022) Guangxi is located in the south of China.It has geomorphologically diverse landscapes, complex terrain, warm climate, abundant rain, abundant sunshine and other natural conditions, which breed rich and unique plant diversity.A total of 8,892 native plant species have been recorded in Guangxi, including 889 endemic plant species (Wei 2019).As one of the diversification centres of Mitreola, Guangxi has a total of seven species, amongst which five species are endemic to Guangxi and are karst obligate species (Yu et al. 2017;Wei 2019).
During our ethnobotanical field survey in Nandan County, northern Guangxi, in 2020, we collected a peculiar population of Mitreola with linear leaves, which was quite different from known species in the region.In the following three years, the same species was documented flowering and fruiting regularly at the same site.Careful comparison of the morphological and anatomical features of the collected taxon with other Mitreola species led us to believe that the taxon which we collected from Nandan differs from all the previously described species of Mitreola.

Materials and methods
Several specimens were collected under evergreen broad-leaved forests in the hill region of Nandan County, Wuai Town, Tonggong Village from 2020 to 2022 and were deposited in the Herbaria CSFI, GXMI, IBK and KUN.The photographs of the plants were taken with a Panasonic LX100 camera.A detailed comparison with all other heretofore known Mitreola species was undertaken, including specimens deposited at CSFI, GXMG, GXMI, IBK, IBSC, HIB, KUN, PE, SYS and descriptions from botanical websites (e.g.http://www.cvh.ac.cn/, https:// plants.jstor.org/).Herbarium acronyms follow Thiers (updated continuously).The morphological characters are described according to the terminology presented by Li and Leeuwenberg (1996) and the conservation status is assessed according to the IUCN Red List Categories and Criteria (IUCN 2022).Diagnosis.Mitreola quanruii is most similar to M. liuyanii, but can be distinguished from the latter by its larger plant height up to 20-50 cm tall (vs. up to 9 cm), its linear leaves, 4-18 × 0.3-1 cm (vs.leaves oblanceolate, 0.4-5.6 × 0.2-1 cm) with acuminate apex (vs.apex acute to rounded) and 8-10 lateral veins on each side of the mid-rib (vs.4-6 pairs), its narrowly triangular stipules (vs.stipules linear), its linear bracts, ca.1.0 cm long and glabrous (vs.bracts narrowly lanceolate, 2-3 mm long, sparsely pilose on abaxial surface) and its glabrous calyx (vs.abaxial surface of calyx sparsely pilose).
Phenology.Flowering from March to April; Fruiting from May to June.Distribution and habitat.Mitreola quanruii is found growing on a watery stone wall near a rivulet, under evergreen broad-leaved forests in the hill region of Nandan County, Guangxi, China.
Preliminary conservation status.According to currently available data, Mitreola quanruii is only found in its type locality and there are only 63 adult plants and 21 seedlings in an area of ca.450 m 2 (30 × 15 m).Further detailed investigation of similar habitats is needed to give a better understanding of the species' natural distribution and abundance.Mitreola quanruii is temporarily assessed as data deficient (DD) according to IUCN (IUCN 2022).At the type locality of Mitreola quanruii, only a small area of native vegetation remains along the creek, surrounded mostly by plantation forests.The habitat of the Mitreola quanruii is extremely fragile.Therefore, this species deserves close attention and protection.
Etymology.We dedicate this new species of Mitreola to Prof. Quanru Liu for his substantial contributions to botanical research and education in China.
Vernacular name.The Chinese name is proposed as xiàn yè Dù Liáng Cǎo (线叶度量草), which means that the leaves are linear.

Discussion
Karst ecosystems are renowned for their distinct vegetation and high biodiversity, offering exceptional habitats that foster speciation and radiation (Myers et al. 2000;Biswas 2009).The genus Mitreola predominantly comprises limestone obligate and narrowly distributed species (Yu et al. 2017;Shan et al. 2019Shan et al. , 2021;;Wei 2019;You et al. 2020;Liao and Chen 2021;Liu et al. 2022).Notably, recent publications have reported new species of Mitreola exclusively found in the limestone region of southwest China (Shan et al. 2019(Shan et al. , 2021;;You et al. 2020;Liao and Chen 2021;Liu et al. 2022).This region boasts the most extensive karst formations globally and is recognised as one of China's three unique floristic centres, as well as a vital area for global biodiversity conservation efforts (Yu et al. 2017).In recent years, scientists have discovered and documented an increasing number of new plant species, including Annonaceae, Gesneriaceae and Magnoliaceae in this area (Hu et al. 2022;Li et al. 2022;Liu et al. 2022;Yang et al. 2023).Consequently, with further advancements in biodiversity surveys, it is expected that numerous additional species of Mitreola will be identified and published within the limestone regions of southwest China and northern Vietnam, potentially bringing the total count of species within this genus to thirty.This highlights the limestone region's ability to support rich plant diversity and endemism, while providing favourable conditions for Mitreola.Moreover, the unique landforms in this area have likely accelerated the diversification of Mitreola.Consequently, comprehensive surveys and studies on the phylogenetic evolution of Mitreola within the limestone areas of southwest China will yield significant scientific insights into floristic geography and the phylogeny of Mitreola in this particular region.

Figure 1 .
Figure 1.Mitreola quanruii L.Wu & R.C.Hu.A flowering branch B stipule C top view of flower D longitudinally opened corolla showing the position of the stamens and the hair ring in the throat E ovary, calyx, style and stigma F lateral view of fruit.

Figure 2 .
Figure 2. Mitreola quanruii L.Wu & R.C.Hu.A habitat B habit C inflorescence D fruiting branch E leaves F lateral view of flower G lateral view of fruit.