A new species of Argyreia (Convolvulaceae) from Thailand

Abstract Argyreia pseudosolanum Traiperm & Suddee, sp. nov. from the NE region of Thailand is described and illustrated. The new species is remarkable in having a very distinctive corolla shape similar to Solanum, and staminal filament bases glabrous or nearly glabrous with a few multicellular, uniseriate hairs at the attachment point on the corolla tube. Detailed descriptions, illustrations, a summary of the ecology and an IUCN conservation status are provided.


Introduction
Argyreia Lour. is one of the larger genera in the spiny pollen group of Convolvulaceae, and is well characterised by indehiscent fleshy or mealy berries (Staples and Traiperm 2017). The genus is distributed mainly in tropical Asia (Staples and Traiperm 2017), except for four species that are geographically disjunct from Asia and located in Madagascar (Deroin 2001).
A checklist of the genus Argyreia Lour. (Convolvulaceae) has been recently published reporting 135 species and 5 varieties in total (Staples and Traiperm 2017). However, the number of species has been actively increasing and is now up to 140 species due to the fact that two novelties have been discovered in Myanmar (Traiperm et al. 2019) and Thailand (Chitchak et al. 2018), as well as a transfer of Ipomoea nana Collett & Hemsl. into Argyreia (Shalini et al. 2017). Since the account of Thai Convolvulaceae was published (Staples and Traiperm 2010), six more new Thai species of Argyreia have been named to date (Chitchak et al. 2018;Staples 2014, 2016). Up to now, Thailand has 40 species in total (the highest number of species for any country studied so far in tropical Asia) or approximately one-third of all species, making the country a centre of species richness in Argyreia. However, the identification of some species in Thailand is still inconclusive, because the specimens of these species lack reproductive organs, either partially or entirely.
All species of the genus Argyreia have recently been transferred to Ipomoea L. by Muñoz-Rodríguez et al. (2019). We chose, however, not to follow the proposed classification in the present work, as further study of Old World taxa is still required.
During field surveys in Phu Langka National Park, Bueng Kan Province (NE Thailand) by the second author, an unrecognised Argyreia species was discovered. The plant was distinctive for its unusual corolla shape, similar to those found in the family Solanaceae. The specimens were then compared with the type specimens of other Argyreia species that have similar corolla shapes at the three main herbaria in Thailand (BK, BKF and QBG), as well as three major herbaria in the UK (BM, K and K-W) by the first author. This is one of several unknown Argyreia that could not be linked to any previously published names. We therefore describe and illustrate a new Argyreia species from Thailand here.

Materials and methods
Plant materials were collected from Phu Langka National Park in 2018. Morphological measurements were made from dried herbarium specimens. The collected specimens were compared with the type specimens of morphologically similar species at BK, BKF, BM, K, K-W and QBG herbaria, as well as digital images available online from other herbaria (both via JSTOR and the following online collections: G, P, L and NY). Moreover, protologues for similar species were also consulted.
Distribution and ecology. In mixed deciduous forest on a sandstone plateau. Elevation: 530 m.
Etymology. The specific epithet refers to the corolla shape, which is similar to Solanaceae and not found elsewhere in Argyreia.

Conservation status.
Argyreia pseudosolanum is known only from Phu Langka National Park with an estimated area of occupancy around 1 km 2 . The number of mature individuals in the population is less than 50. The species occurs near the large golden stupa on the top of the Phu Langka plateau which has religious activities in the dry season. This area might be disturbed by these activities and this could affect the survival chances of this species. It is assessed here as Critically Endangered, CR B2ab(iii); D, following the IUCN Criteria (2017)