Two new species of Lasianthus Jack (Rubiaceae) from southern Vietnam

Two new species of shrubs in the genus Lasianthus Jack (Rubiaceae), L. naikii V.S.Dang & Vuong sp. nov. and L. sonlangensis V.S.Dang, Vuong & Quan sp. nov. from Kon Chu Rang Nature Reserve of Gia Lai Province in southern Vietnam, are described and illustrated. The similarities and diff erences of two new species with the morphologically closest allies are presented. Descriptions, illustrations, colour plates and vernacular names are provided.


Introduction
The genus Lasianthus Jack is part of the family Rubiaceae Juss. and comprises more than 180 species of shrubs or small trees that grow in the shade of evergreen forests (Robbrecht 1988;Zhu & Taylor 2011;. Species of Lasianthus are distributed mainly in the Old World tropics with 160 species occurring in tropical Asian, 20 species in Africa and four species in tropical America and Australia (Zhu et al. 2012;Naiki et al. 2015). In Vietnam, 43 species and two subspecies have been reported to date (Ha et al. 2020;Dang & Naiki 2020). Among them, 13 species and one subspecies which are found in the shade of evergreen forests and are endemic to Vietnam, were recently described: L. bidoupensis V.S. Dang &

Etymology
The new species is named in honor of Associate Professor Dr Akyo Naiki of University of the Ryukyus, Japan, for his contributions to the study of Rubiaceae in Vietnam.

Distribution, habitat and phenology
Lasianthus naikii sp. nov. is only known from its type locality at 1000-1100 m elevation. It grows under the shade of primary evergreen forest on clay, which is dominated by Garcinia cowa Roxb.

Population and threats
The new species is so far only known from Kon Chu Rang Nature Reserve, Gia Lai Province, southern Vietnam. It was collected in a single population containing less than 50 individuals, occupying an area of less than 15 km 2 .  Table 2 Diagnosis Lasianthus sonlangensis sp. nov. is morphologically similar to L. caeruleus Pitard (Fig. 6) which is endemic to Vietnam in the shape of leaves and inflorescences, but differs from the latter by having secondary veins 5-7 pairs (vs 4-5 pairs), petioles 2-3 mm long (vs 4-5 mm long), bracts present 0.6-1 mm long (vs absent), calyx tube 1.5-2 mm long (vs ca 1 mm long), calyx lobes 0.7-1 mm long (vs ca 0.5 mm long) and corolla lobes 4 (vs 5). For a further comparison see Table 2.

Etymology
The specific epithet is derived from commune ʻSơn Langʼ where the new species was discovered.

Population and threats
Lasianthus sonlangensis sp. nov. is currently known from only one locality with a small population of less than 10 individuals. The forest habitat where we found this species is frequently disturbed by anthropogenic factors, such as spontaneous tourism activities and collection of non-timber forest products (NTFPs).

Discussion
Lasianthus Jack differs from other genera of Rubiaceae by having branches solitary at stem nodes, inflorescences axillary and opposite, inside of corolla tube and inner surface of corolla lobes typically hairy and ovary with 3-9 locules (Zhu et al. 2012;Wong et al. 2019). This genus is especially close to Saprosma Blume, but differs from it by having inflorescences borne on peduncles directly from main stems, these with no more than one reduced basal node (vs inflorescence borne on regular paired axillary short shoots, each short shoot with several stipulate nodes). It is also close to Paralasianthus H.Zhu, but differs from it by having a 4-5-locular ovary (vs 2-locular  (Sy et al. 2015(Sy et al. , 2020Do et al. 2018Do et al. , 2021Bui et al. 2019Bui et al. , 2020Nguyen et al.