Lectotypification of the name Melastoma candidum f. albiflorum and its taxonomic status

Abstract A nomenclatural and taxonomic treatment of the name Melastoma candidum f. albiflorum (Melastomataceae) is presented. A lectotype is designated for this name, with an updated morphological description based on fresh material. The name Melastoma candidum f. albiflorum is proposed as a heterotypic synonym of Melastoma candidum.


Introduction
Melastoma L. comprises species mainly distributed in the Southeast Asia and extends to India, South China, Japan, northern Australia, and Oceania (Meyer 2001). Melastoma is taxonomically difficult, and the recognized number of species in this genus remains controversial due to rapid adaptive radiation and extensive natural hybridization (Renner and Meyer 2001;Liu et al. 2014;Zhou et al. 2017). It was claimed that this genus comprises about 100 species (Chen 1984), but only 22 species were recognized in the recent taxonomic revision (Meyer 2001;Chen and Renner 2007). However, it was estimated that this genus might include 80-90 species based on a scientific investigation of the island of Borneo (Wong 2016).
In China, nine Melastoma species were recorded in the south of the Yangtze River by Chen (1984), yet only five of them were recognized in more recent publications (Meyer 2001;Chen and Renner 2007). This revised treatment has incorporated M. affine, M. normale and M. candidum into M. malabathricum, but this has not been accepted by several plant taxonomists (Huang et al. 2018).
In this study, we are in agreement with the recognition of M. candidum as a distinct species (Liu et al. 2014;Ng et al. 2017). Melastoma candidum is a relatively common species with purple flowers that occurs in southern China and northern Vietnam (Chen 1984). Melastoma candidum f. albiflorum J. C. Ou, a form of M. candidum, was first described from Taiwan Island (Ou 1976 During a recent field survey, we collected an unidentified specimen of Melastoma in Fujian province, China, which we believed has not been recorded in mainland China. It closely resembles M. candidum in morphology, but has white flowers. After a comprehensive morphological comparison, we propose that this specimen be conspecific with M. candidum f. albiflorum. Here we designate a lectotype for this taxon name and discuss its taxonomic status.

Materials and methods
Morphological data for identification and description of this taxon were based on observations of specimens in the field (ten individuals) and the herbarium. The voucher specimens are deposited in the Herbarium of Sun Yat-sen University (SYS), Guangzhou, China. Lectotypification and taxonomic treatment of this taxon is presented according to the "International Code of Botanical Nomenclature" (McNeill 2014; Turland et al. 2018), and a full description is provided.
Distribution and habitat. The white-flowered form of M. candidum was first reported only from Hsinchu Hsien and Ilan Hsien (Taiwan). This form has also been reported to occur in the Ryukyu Island (Hatusima and Amano 1994), but without exact specimen information. The individuals in Fujian, China represent the first known occurrence of this form outside of Taiwan Island (Fig. 2). It occurs only in lowland evergreen forest margins at an elevation of approximately 150-300 in Pinghe County, Fujian. They occur in evergreen forests dominated by Blechnum orientale, Dicranopteris pedata, Miscanthus floridulus and Rhodomyrtus tomentosa.
Notes. The key character of the white form of M. candidum is its white-colored flowers, which can be easily distinguished from the purple form of M. candidum. Whereas the purple form has a relatively wide range of distribution in northern Vietnam and southern China (Liu et al. 2014), the white-flowered individuals have been found across a narrow region. Individuals in Taiwan and mainland China may represent independent origins of white petals from local populations of M. candidum, since breakdown of the anthocyanin synthesis pathway in plants is relatively common (Smith et al. 2012;Zheng et al. 2019).
The flowers of the white form of M. candidum have been used in folk-medicine for the treatment of hypertension, dysentery, diarrhea and antibacterial (Chou and Liao 1982). During our survey, we also learned that the white form has been cultivated as a medicinal herb by the local people in Pinghe County, Fujian. They believe that it is highly effective for the treatment for nephritis, and has led to the exploitation of natural populations, threatening its survival in the wild. Due to its narrow geographical range and small population size, effective conservation effort is required. Paratype