Premna bhamoensis (Lamiaceae, Premnoideae), a new species from Kachin State, northeastern Myanmar

Abstract In the present study, we describe and illustrate a new species, Premna bhamoensis Y. T. Tan & B. Li (Lamiaceae), from Myanmar. In the 1980s, this species was transplanted from Bhamo County in northeastern Myanmar to the Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences. The species shows striking morphological similarity to P. menglaensis B. Li, and thus, has been misidentified as the latter for a long period of time. However, morphological comparison revealed that P. bhamoensis is distinct from P. menglaensis in many aspects. Moreover, literature survey and specimen examinations also indicated that P. bhamoensis is undoubtedly different from all seven known congenetic species recorded from Kachin State, Myanmar, and a key for their identification has been provided in this paper.


Introduction
The genus Premna L. is one of the largest woody genera belonging to the mint family, consisting of approximately 200 species distributed mainly in the Old World tropics and subtropics (Verdcourt 1992, Harley et al. 2004). The genus was first described by Linnaeus (1771), on the basis of two species, P. serratifolia L. and P. integrifolia L., which are now treated as a single species (de Kok 2013). It was traditionally placed in the subfamily Viticoideae Briq. (Briquet 1897, Chen and Gilbert 1994, Harley et al. 2004), but was recently transferred to the newly established subfamily Premnoideae B. Li, R.G. Olmstead & P.D. Cantino (Li et al. 2016).
With 46 species recognized in China, Premna is the fifth largest genus in Lamiaceae flora of China (Chen andGilbert 1994, Li andHedge 1994). In June 2011, a field survey was carried out to investigate the biodiversity of Premna in Yunnan Province, southwestern China. When visiting the Xishuangbanna Tropical Botanical Garden (XTBG), Chinese Academy of Sciences (CAS), the authors found a rare Premna shrub in the fruiting stage (Figure 1), which was being cultivated in the C20 region of the garden, and was labeled as "Premna laevigata C. Y. Wu" (≡ P. menglaensis B. Li, after Li et al. 2013). Superficially, the plant strongly resembles P. menglaensis in having a climbing habit, ovate-oblong to elliptic leaves, and a congested pyramidshaped thyrse (Figure 2), but differs noticeably in having densely pubescent branchlets and petioles, and lips of fruiting calyces distinctly 2-or 3-lobed. Many more differences between this plant and P. menglaensis were discovered during the flowering stage, whose observations were taken during the month of May 2012. This analysis indicated that the plant probably represented a new species. In order to verify the information about the origin of this putative new species, we examined the XTBG introduction records and found that this plant was introduced from Bhamo County, Kachin State of northeastern Myanmar in the 1980s. However, precise location data and the collection date had not been recorded. During the period from 2011 to 2016, the first author has visited Kachin State many times, but the plant was not found in this area. Further examination of literature and specimens revealed that seven Premna species have been recorded from Kachin State, viz., P. barbata Wall. ex Schauer, P. bengalensis C.B. Clarke, P. khasiana C.B. Clarke, P. pinguis C.B. Clarke, P. pyramidata Wall. ex Schauer, P. racemosa Wall. ex Schauer, and P. scandens Roxb. (Kress et al. 2003). However, none of them is morphologically similar to the putative new species. Therefore, it is confirmed that this species of Premna is new to science, and thus, we describe and illustrate it in this study.

Methods
Morphological observations of the new species were carried out based on living plants as well as dry specimens, during the period from 2011 to 2016. Measurements were made using a ruler and a micrometer. Both herbarium and fresh specimens of P. menglaensis were examined under a stereo dissecting microscope (StereoZoom® Leica S8 APO, © Leica Microsystems 2017). The conservation status of the new species was evaluated based on the guidelines of the International Union for Conservation of Nature (IUCN 2012). A distribution map was prepared using data obtained from our field observations, herbarium specimens and relevant literature (extrapolated or approximated with respect to a few old or vaguely specified localities). Diagnosis. The species is most similar in morphology to P. menglaensis B. Li, but differs from the latter in having branchlets and petioles densely tomentose (vs. glabrous or glabrescent), leaf blades papery with minute pubescence (vs. leathery and glabrous), flowers green to greenish yellow (vs. red flowers), calyces slightly 2-lipped with five equal lobes (vs. calyces distinctly 2-lipped with entire or minute emarginate lips), and stamens exserted from corolla (vs. included).
Phenology. Flower buds were observed in early April. Flowering was observedfrom mid-May to early June and fruiting from late May to late June. Distribution. Per the introduction record, P. bhamoensis is originally collected from northeastern Myanmar, but currently known only from the cultivated type in the Xishuangbanna Tropical Botanical Garden (Figure 4). Based on our experience in examination of Asian Premna specimens, we suspect that the species is probably endemic to Kachin State of Myanmar and distributed in a very small area.
Etymology. The specific epithet of this new species, "bhamoensis", is derived from the name of the locality, Bhamo County, from where the species was originally collected.
Preliminary conservation status. Since we have neither rediscovered the wild population of P. bhamoensis in Myanmar, nor identified any other specimens in the herbarium, very few details about its natural distribution and/or population status are currently known. Therefore, the information is inadequate to assess the species' risk of extinction, whether directly or indirectly. In accordance with the IUCN Red List Categories (IUCN 2012), we propose to temporarily list the species as a taxon under Note. Among the seven Premna species recorded in the flora of Kachin State, Myanmar (Kress et al. 2003), P. pyramidata (= P. tomentosa Willd.) and P. bengalensis have  dense stellate hairs on branchlets, leaves, and petioles, P. racemosa (= P. interrupta Wall. ex Schauer) has spikelike inflorescences, and P. scandens is a large and glabrous vine. Thus, these four species are quite distinct from P. bhamoensis. P. barbata and P. khasiana both have bracts at the base of branchlets, whereas such bracts are absent in P. bhamoensis. P. pinguis differs from P. bhamoensis in having ovate leaves with strongly serrulate margins, and branches without interpetiolar ridges. All these differences make P. bhamoensis a distinct Premna species in Myanmar. A key to the Premna species in Kachin State of Myanmar is provided below.
Among the Asian Premna, P. menglaensis, as the introduction label indicated, is the species showing the maximum level of similarity to P. bhamoensis. Both are climbing shrubs with ovate-oblong to elliptic leaves, and congested pyramid-shaped inflorescences. However, P. bhamoensis can be easily distinguished from P. menglaensis on the basis of the differences observed in a number of traits, e.g., branchlets and petioles (densely pubescent vs. glabrous or glabrescent), leaf blades (papery and mi-nutely pubescent vs. leathery and glabrous), flower color (green to greenish yellow vs. red), calyx shape (slightly 2-lipped with five equal lobes vs. distinctly 2-lipped with entire or minute emarginate lips), stamens length (exserted from corolla vs. included) (Figure 2). P. bhamoensis also resembles P. fulva Craib in having a climbing habit, dense indumentum on branchlets and petioles, green to greenish yellow flowers, and calyces with five lobes, but clearly differs in leaf shape (ovate-oblong to elliptic with entire margins vs. ovate to subrounded with serrate margins) and inflorescence type (congested pyramid-shaped thyrse vs. flat-topped corymbose cyme) (Chen andGilbert 1994, Tan andLi 2016). Besides, branchlets, petioles, leaf blades, and inflorescences of P. fulva are densely covered with long, spreading, golden-brown hairs, which are different from the hairs found on P. bhamoensis.
A key to the species of Premna in Kachin State of Myanmar