Phyllanthodendron can be readily morphologically distinguished from Glochidion, but recent molecular evidence showed that Phyllanthodendron is paraphyletic due to Glochidion being nested within it. In this study, a new species of the former Phyllanthodendron is described and illustrated as Glochidion yangchunense Z.Q. Song & Gang Yao from the limestone areas of South China. This is a peculiar new species and morphologically distinguished by its discoid flowers, T-shaped disc segments, and glabrous flowering branches. A key to Glochidion yangchunense and related species in China is provided here.

Karst, Malpighiales, Phyllantheae, Phyllanthodendron, Taxonomy

Phyllanthodendron Hemsl. was previously considered as a distinct genus (Croizat 1942; Li 1987, 1994; Li and Gilbert 2008; Xia and Tong 2018), or treated as a section or a subgenus of the genus Phyllanthus L. (Beille 1927; Chantaranothai 2007; Webster and Carpenter 2008; Bouman et al. 2018). However, several molecular studies revealed a highly supported sister relationship between Phyllanthodendron and the genus Glochidion J.R. Forst. & G. Forst. (Kathriarachchi et al. 2006; Pruesapan et al. 2012; van Welzen et al. 2015; Luo et al. 2017; Pornpongrungrueng et al. 2017). A more comprehensive molecular phylogenetic study has recently shown that Phyllanthodendron is a paraphyletic group with Glochidion nested within (Bouman et al. 2021). In a result, Phyllanthodendron was formally united with Glochidion (Bouman et al. 2022). Currently, Glochidion is the largest genus of the family Phyllanthaceae and comprises 300–350 species of shrubs or trees distributed in Asia and Pacific (Yao et al. 2020; Bouman et al. 2022). In the Plants of the World Online (https://powo.science.kew.org/; POWO 2023), 328 accepted specific names are listed under the genus. Within Glochidion, three subgenera were elected, i.e. subg. Glochidion, subg. Phyllanthodendron (Hemsl.) R.W. Bouman and subg. Pseudoactephila (Croizat) R.W. Bouman. The two latter subgenera correspond to the previous Phyllanthodendron and consist of 19 Asian species, which can be readily distinguished from the subgenus Glochidion by the presence of a floral disc and apiculate anthers (Bouman et al. 2022).

During our field plant investigations in Yangchun City, Guangdong Province, South China, we found an interesting monoecious shrub in a limestone hill. The plant has five (male flowers) or six (female flowers) sepals, three stamens, connate filaments, apiculate anther connectives, 3-locular ovaries, unlobed floral disc segments, and inflated capsules (Fig. 1). These characters are consistent with those of the previous Phyllanthodendron and the plant may be a member of Glochidion subg. Pseudoactephila for lacking the specialized floriferous branchlets, as circumscribed in Bouman et al. (2022). After being compared with all known related species, this plant is proposed as new to science. A full description, color photographs, phenology, conservation status, morphological comparison, and a distribution map of the species are provided here.

Figure 1. 

Glochidion yangchunense Z.Q. Song & Gang Yao A habit B detail of the apex of branches C fruiting branches D, E leaves, adaxial and abaxial surfaces, shapes and sizes F, G pistillate flower, front and lateral view H, I staminate flower, front and lateral view J branches with staminate and pistillate flowers as well as fruit K fruits and seeds. Arrows indicate disc segments.

All specimens of the previous Phyllanthodendron (now as Glochidion subg. Phyllanthodendron and G. subg. Pseudoactephila) kept in the Herbaria CANT, GXMG, GXMI, HITBC, IBK, IBSC, KUN, PE and SYS have been carefully examined by visiting these herbaria, and the images of Phyllanthodendron specimens deposited in the Herbaria A, BM, E, K, M, MO, P, US, WU and SZG were also studied. Acronyms for the herbaria follow the Index Herbariorum (Thiers 2023). We also observed the living status of relevant taxa through field investigations and accessing some websites such as Plant Photo Bank of China (https://ppbc.iplant.cn/), Chinese Field Herbarium (https://www.cfh.ac.cn/), and Chinese Union of Botanical Gardens (https://image.cubg.cn/). The distribution map was made by the software ArcGIS 10.2.

In China, Phyllanthodendron was usually accepted as a distinct genus, and 10 species were recorded in the limestone area (Li 1987, 1994; Wei 2005; Li and Gilbert 2008; Xia and Tong 2018). Yao et al. (2021) synonymized Phyllanthodendron orbicularifolium P.T. Li under P. petraeum P.T. Li, because they found a continuous variation in the characters that have been used to distinguish them through examination of herbarium specimens and field observations. Ding et al. (2023) newly reported Phyllanthus mirabilis Müll. Arg. (= Glochidion mirabile (Müll. Arg.) R.W. Bouman) from China. Thus, 11 species (including G. yangchunense) of the previous Phyllanthodendron are distributed in China. Due to these changes, a new key for the 11 species in China is provided here.

Morphologically, in the previous Phyllanthodendron, the sepals of most species are concave inside and form a cup-shaped or urceolate shape, and the shape of disc segments is usually described as linear, oblong, or ligulate (Li and Gilbert 2008; Bouman et al. 2022; also see Fig. 3). But the new species described in this study, Glochidion yangchunense, is a peculiar species and it has the sepals jointly to a discoid shape, and T-shaped disc segments (Fig. 1F, H). The new species resembles Glochidion anthopotamicum, a species widely distributed from southwestern China to southeastern China (Li and Gilbert 2008). However, the new species differs greatly from the latter by a series of morphological characters that can be referenced from the above chapter “Diagnosis”. Additionally, the new species usually has larger leaves at the upper part of branches and smaller leaves at the lower part of branches (Fig. 1C, E), and sepals acute to slightly acuminate at apex (Fig. 1F, H). While the species G. anthopotamicum usually has smaller leaves at the upper part of branches and larger leaves at the lower part of branches, and sepals caudate-acuminate at apex (Fig. 3C–F). The new species is also similar to G. dunnianus (H. Lév.) R.W. Bouman in having evident lateral veins in sepals (especially in female flowers; Figs 1F, H, 3K, L), but it differs from the latter by its terete branches (Fig. 1C, E), outer sepals much narrower than inner ones (Fig. 1F, H), anther connectives with an apiculate apex (Fig. 1I), T-shaped disc segments (Fig. 1F, H), and smaller fruits (ca. 8–10 mm in diameter; Fig. 1K). In contrast, G. dunnianus has 2-winged branches (Fig. 3H, I), equivalent or sub-equivalent sepals (Fig. 3K, L), anther connectives with a narrowly subulate apex (Fig. 3L), linear-shaped disc segments (Fig. 3K, L), and larger fruits (ca. 10–15 mm in diameter).