Oxalisxishuiensis (Oxalidaceae), a new species from Danxia landforms in Guizhou, China

﻿Abstract Oxalisxishuiensis, a new species of Oxalidaceae from Danxia landforms of Xishui County, Guizhou, China, is described and illustrated. It is morphologically similar to O.wulingensis by the two lateral leaflets arranged at about 180° angle and oblong pink petals with lilac veins, but clearly differs from the latter by leaflets almost as long as wide, obliquely obcordate lateral leaflets, shorter peduncles, longer capsule (1.2–1.5 cm vs. 0.5–0.7 cm) and alveolate seeds.

During field surveys to Xishui County, north Guizhou Province, China, in November 2022, a population of Oxalis with special morphological characteristics attracted our attention.To conduct further detailed observation, we transplanted PhytoKeys 239: 267-273 (2024), DOI: 10.3897/phytokeys.239.119046 Yan-Bing Yang et al.: A new species of Oxalis is described and illustrated in the study five individuals in the greenhouse of the Guizhou Academy of Forestry and three individuals were made into herbarium specimen after flowering.After careful morphological examination and comparison with morphologically similar species in Oxalis, it is confirmed as an undescribed new species of Oxalis.Here, we formally describe this new species.

Materials and methods
Morphological characteristics were observed and measured from the living plants.The comparison with morphologically similar species was based on the digital specimens from the online database CVH (https://www.cvh.ac.cn/) and JSTOR Global Plants (https://plants.jstor.org/),as well as the descriptions from relevant literature (Liu and Watson 2008;Deng et al. 2013;Aoki et al. 2019;Tian et al. 2020).Diagnosis.The new species is most morphologically similar to Oxalis wulingensis, but differs from the latter by its leaf blade ca. as long as wide, obliquely obcordate lateral leaflets (vs.long obtriangular), shorter peduncle (ca.3-4 cm long, shorter than leaves vs. 10-12 cm long, longer than leaves), longer capsule (1.2-1.5 cm long vs. 0.5-0.7 cm long) and alveolate seeds (vs.only with longitudinally ridge).
Phenology.Chasmogamous flowers from February to March; Cleistogamous flowers from May to June.Fruiting from February to July.
Etymology.The species epithet, xishuiensis, refers to the type locality of the new species.
Vernacular name.习水酢浆草 (xí shuǐ cù jiāng cǎo) Conservation status.Currently, only one population of the new species with approximately 60 individuals has been found.Danxia landforms are widely distributed in this area, so we speculate that there may be other populations of this new species.Due to insufficient field investigations, the natural range of this species in the wild is unclear.According to the IUCN Red List Categories and Criteria (IUCN 2022), we recommend this species placement in the 'Data Deficient' (DD).

Discussion
In the Oxalis, there are eight native species in China, but only two native species in Guizhou (Liu and Watson 2008;Deng et al. 2013;Tian et al. 2020).Discovery of Oxalis xishuiensis adds to the native local floras.
According to the classifications by Lourteig (2000) and Aoki et al. (2019), Oxalis xishuiensis should be classified into Oxalis subgen.Oxalis sect.Oxalis subsect.Oxalis.Oxalis xishuiensis is characterised by the obliquely obcordate lateral leaflets arranged at about 180° angle, shorter peduncles and alveolate seeds.This unique combination of morphological characteristics distinguishes O. xishuiensis from all other species of subsect.Oxalis (Liu and Watson 2008;Deng et al. 2013;Aoki et al. 2019;Tian et al. 2020).We made a detailed morphological comparison between O. xishuiensis and its relatives (Table 1).Oxalis xishuiensis morphologically is most similar to O. wulingensis by the two lateral leaflets arranged at about 180° angle and oblong pink petals with lilac veins, whereas O. xishuiensis leaflets are almost equal in length and width, two lateral leaflet blade shapes are asymmetric (obliquely obcordate) and smaller in size than the leaflet blades in the middle, mature leaf blades abaxially pale green (vs.purple in O. wulingensis), peduncles shorter than leaves, longer capsule 1.2-1.5 cm long (vs.0.5-0.7 cm in O. wulingensis) and seeds with alveolate on both surfaces.Furthermore, the new species was discovered only from Danxia landforms hills, which is completely different from O. wulingensis growing in limestone habitat.Oxalis xishuiensis resembles O. acetosella in the obcordate leaf blades, but differs in leaf blades ca. as long as wide, two lateral leaf blades asymmetric and arranged at about 180° angle, peduncles shorter than leaves and petals oblong, pink with lilac veins (vs.obovate, white, lilac to pinkish veined in O. acetosella).

Figure 1 .
Figure 1.Oxalis xishuiensis Y.B. Yang, M.T.An & H. Li A, B habitat C plants D rhizome E upper surface of leaves F lower surface of leaves G flowering plant H lateral view of the flower I frontal view of the flower J dissected flower K bract L capsule M seeds.

Figure 2 .
Figure 2. Oxalis xishuiensis Y.B. Yang, M.T.An & H. Li A habit B lower surface of leaves C upper surface of leaves D lateral view of the flower E frontal view of the flower F petals G stamens H chasmogamous capsule I cleistogamous capsule J seeds.

Table 1 .
Morphological comparison of species of Oxalis xishuiensis and its relatives.