﻿Isotremaputalengense, a new species of Aristolochiaceae from northern Vietnam and two new combinations in Isotrema

﻿Abstract Isotremaputalengense Luu, Q.B.Nguyen & H.C.Nguyen is described as a new species from northern Vietnam. It looks most morphologically like I.wardianum but is distinguishable by a combination of different leafy and floral characters. Morphological comparison between the new plant and closest species is provided. In addition, combinations of two recently described Aristolochia species are made, namely Isotremavuquangense (T.V.Do) Luu, Q.B.Nguyen & H.C.Nguyen and Isotremayachangense (B.G.Huang, Yan Liu & Y.S.Huang) Luu, Q.B.Nguyen & H.C.Nguyen.


Introduction
Aristolochia by having strongly curved perianth, 3-lobed gynostemium, anthers paired on the outer surface of each gynostemium segment, and basipetally dehiscing capsule. This generic concept is followed in many later publications (Li et al. 2019;Zhou et al. 2019;Zhu et al. 2019b, c, d;Cai et al. 2020a;Wang et al. 2020a, b). Although several other authors still prefer assigning their newly described species under Aristolochia subgenus Siphisia (e.g., Cai et al. 2020b;Luo et al. 2020;Zhou et al. 2020;Do et al. 2021), of which Isotrema was accepted as one of the synonyms in the most recent nomenclatural review of Aristolochia-related taxa by Ohi-Toma and Murata (2016), the phylogenetic results by Zhu et al. (2019a) appear to be robust because of their extensive samples of Asian species and combination of molecular, chromosomic and morphological data. Therefore, Isotrema is followed in this paper.

Isotrema putalengense
Phenology. Flowering found in June, fruiting unknown. Etymology. The specific epithet refers to the type locality, Pu Ta Leng Mountain which is part of the Hoang Lien Son Mountain Range and located about 30 km northwest of Vietnam's highest Mt. Fan Si Pan.
Distribution and habitat. The new species is currently only known from Pu Ta Leng Mountain (with its highest peak at 3.049 m elevation), Tam Duong District, Lai Chau Province. It grows on humid fertile soils under a closed broadleaved evergreen forest unexplored botanically. There is no data available on the forest cover of the mountain. Our preliminary notes indicate that this forest is dominated by the Fagaceae, Lauraceae, Theaceae, Ericaceae and Magnoliaceae that are common families on the Hoang Lien Son Mountain Range, which is geographically considered part of the southern extension of the Himalayas and phytogeographically located in the Sikang-Yunnan Province (Averyanov et al. 2003).
Preliminary extinction risk assessment. The plant was recorded in a small population with few scattered individuals in a presently unprotected large forest. It may be found in adjacent similar forests on the Hoang Lien Son Mountain Range. Given this fact, it is provisionally assigned as Data Deficient until more information is recorded (IUCN 2012; IUCN Standards and Petitions Committee 2022).
Discussion. Isotrema putalengense is most morphologically similar to I. wardianum but they have a number of differences as expressed in the diagnosis. Besides, the new species is also close to I. utriforme (S.M.Hwang) X.X.Zhu, S.Liao & J.S.Ma (Hwang 1981;Zhu et al. 2019a) in the shape of leaves and flowers but the latter has glabrous and longer (4-8 cm) petiole, yellow-green flowers borne in axils of leafy shoots, ovatelanceolate bracteoles inserted above middle of peduncle, short upper tube (3-4 mm), convex annulus, saccate limb with ovate-deltate and erect lobes. The shape of flowers in the new species looks like that in I. pseudoutriforme (X.X. Zhu (Hwang 1981;Zhu et al. 2019a, e) but I. pseudoutriforme has ovate to narrowly ovate leaves and plain light yellow flowers, uncurved limb forming obtuse angle with upper tuber and ring-like annulus and I. ovatifolium has ovate leaves and abaxially densely off-white villous, purple-red flowers in axils of leafy shoots. The key morphological differences between the new species and those closest species are presented in Table 1.
The leaves of the new species resemble those of I. cucurbitoides (C.F.Liang) X.X.Zhu, S.Liao & J.S.Ma (Liang 1975;Hwang et al. 2003;Zhu et al. 2019a) and I. yangii X.X.Zhu & J.S.Ma (Zhu et al. 2019e;Wang et al. 2020a) but these two species are readily different in a number of characters: I. cucurbifoides has leaves with Notably, the notch at the bent perianth tube of I. putalengense is obviously Ushaped while it is quite properly V-shaped in the above compared species except I. pseudoutriforme where the U-shaped notch is much narrower than that in the new species. Our field observations provisionally indicate that the notch shape is stable in, and could be typical for, Isotrema species. This character is more representative on longitudinal dissection of the perianth tube. However, its value as a supplemental taxonomic character for species identification has not been paid attention to in former Isotrema studies and needs further examination.

New combinations for some species of Isotrema
As a result of their study, Zhu et al. (2019a) has already transferred almost all species of Aristolochia subgenus Siphisia to Isotrema. Another four combinations were made for later described species (Wang et al. 2020a). Following this generic concept, here we propose new combinations for the other taxa of the subgenus that were described recently.