Helicteresbinhthuanensis V.S.Dang (Malvaceae, Helicteroideae), a new species from southern Vietnam

Abstract Helicteresbinhthuanensis V.S.Dang, sp. nov. from Ham Thuan Bac District, Binh Thuan Province, Vietnam is described and illustrated. It is morphologically similar to H.angustifolia, which is a common species in mainland southeast Asia, and H.sphaerotheca, which is endemic to the Northern Territory, Australia, but differs from both by several salient characters such as leaf and calyx size, androgynophore length, petal color, and fruit shape. Photographs, a vernacular name, a preliminary conservation assessment, and a table of morphological characters comparing this new species to two closely related species also are provided.


Introduction
Helicteres L. comprises about 60 species in the tropics of Asia and America (Cowie 2011). The genus was included in a broadly-defined Sterculiaceae by Cronquist (1981), but more recently is included in Malvaceae sensu lato based on morphological and molecular evidence (Mabberley 2008;APG IV 2016). The genus is characterized by the sta-mens and pistil forming an androgynophore, sepals united, petals unguiculate, capsules cylindrical, fusiform or ovoid, spiral or rarely straight, pubescent, and seeds wingless due to parenchymal expansion (Cristóbal 2001;Chantaranothai and Poompo 2019 (Hoang et al. 2020).
During fieldwork in July 2019, we collected several specimens of Helicteres in Ham Thuan Bac District, Binh Thuan Province, southern Vietnam ( Figure 1). After morphological study and reviewing the relevant literature from Vietnam and neighboring countries (Tardieu-Blot 1945;Pham 1999;Cristóbal 2001;Phengklai 2001;Nguyen 2003;Tang et al. 2007;Cowie 2011), as well as comparing our material with specimens in several Vietnamese herbaria (HN, VNM, and VNMN) and images available via JSTOR Global Plants, K, and P (all acronyms follow Thiers 2020, continuously updated), we confirm that this material is distinct from previously known taxa. Here, we describe it as a new species, H. binhthuanensis V.S.Dang.

Materials and methods
Our description of the new species is based on observations from living and dried specimens; measurements were made using a ruler accurate to 0.5 mm. Voucher specimens were deposited in HN, VNM, and VNMN. The photographs were taken with a Canon 1000D camera fitted with an EF 100 mm f/2.8 Macro USM lens. The conservation status of the new species was assessed according to the International Union for Conservation of Nature (IUCN 2019).
Phenology. Flowering and fruiting specimens were collected in July. Distribution and habitat. Helicteres binhthuanensis is known only from Ham Thuan Bac District, Binh Thuan Province, southern Vietnam. It grows along roadsides and edges of secondary forests at ca. 80-95 m elevation.
Etymology. The species epithet is derived from the name of the province (Binh Thuan) where the species was discovered. Vernacular names. Tổ kén bình thuận, An xoa bình thuận. Preliminary conservation assessment. Helicteres binhthuanensis was collected from a small population in a habitat that was logged and disturbed. Therefore, we suggest its placement in the Data Deficient (DD) category based on the IUCN Red List Categories (IUCN 2019).  Notes. Morphologically, Helicteres binhthuanensis is most similar to H. angustifolia, which is a common species in mainland southeast Asia, and H. sphaerotheca, which is endemic to the Northern Territory of Australia, between the Mary River and the South Alligator River. It differs from those species in several characters that are summarised in Table 1. 10-13 × 0.8-1.2 cm 3-15 × 1-4 cm 1.5-3.5 × 0.2-0.7 cm Stipule length 3-4 mm long 3.5-6 mm long 1-2 mm long Bract length 1-2 mm long 1.5-2 mm long ca. 1 mm long Calyx tube length 6-8 mm long ca. 6 mm long 3.5-4.5 mm long Calyx lobe length 4-5 mm long 3-4 mm long ca. 0.5 mm long Petal color at anthesis whitish pink or purplish bluish or pink mauve-pink Petal length 12-16 mm long 5-10 mm long 7-9.5 mm long Androgynophore length 10-15 mm long 3-7.5 mm long 4.5-5 mm long Number of stamens 10 15 10 Number of staminodes 5 O 5 Fruit shape ovoid to ellipsoid ovate to oblong globular Seed length 2.5-4 mm long 2-3.5 mm long ca. 2.5 mm long