A new species (Begonia giganticaulis ) of Begoniaceae from southern Xizang (Tibet) of China Dai-Ke Tian 1,2 * , Wen-Guang Wang 3 * , Li-Na Dong 4 * , Yan Xiao 1,2 , Min-Min Zheng 1,2,5 , Bin-Jie Ge 1,2 1 Shanghai Chenshan Plant Science Research Center, Chinese Academy of Sciences, 3888 Chenhua Road, Songjiang, Shanghai 201602, China 2 Shanghai Key Laboratory for Plant Functional Genomics and Re- sources, Shanghai Chenshan Botanical Garden, 3888 Chenhua Road, Songjiang, Shanghai 201602, China 3 Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, 666303, Yunnan, China 4 Guangxi Key Laboratory of Plant Conservation and Restoration Ecology in Karst Terrain, Guangxi Institute of Botany, Guangxi Zhuang Autonomous Region and Chinese Academy of Sciences, Guilin 541006, Guangxi, China 5 University of Chinese Academy of Sciences, Beijing 10049, China Corresponding author: Dai-Ke Tian (dktian@cemps.ac.cn) Academic editor: J. Wieringa | Received 27 September 2021 | Accepted 22 November 2021 | Published 30 December 2021 Citation: Tian D-K, Wang W-G, Dong L-N, Xiao Y, Zheng M-M, Ge B-J (2021) A new species (Begonia giganticaulis) of Begoniaceae from southern Xizang (Tibet) of China. PhytoKeys 187: 189–205. https://doi.org/10.3897/ phytokeys.187.75854 Abstract Begonia giganticaulis, a huge new species in Begonia sect. Platycentrum of Begoniaceae from southern Xizang (Tibet) of China, is described. Morphologically, it is mostly similar to B. longifolia and B. aceto- sella, but clearly difers from the former mainly by its dioecious and taller plants, sparse hairs on abaxial veins, longer inforescence, unique shape of fruits, and difers from the latter mainly by its late and longer fowering time, 6-tepals of female fower and 3-loculed ovary. Te phylogenetic analyses also support the separation of the new species from other taxa. Based on the current data, its conservation status is assigned to Endangered (B2a) according to the IUCN Red List Categories and Criteria. Keywords Conservation status, molecular evidence, morphology, southern Tibet, taxonomy * Te authors contributed equally. Copyright Dai-Ke Tian et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. PhytoKeys 187: 189–205 (2021) doi: 10.3897/phytokeys.187.75854 https://phytokeys.pensoft.net RESEARCH ARTICLE Launched to accelerate biodiversity research A peer-reviewed open-access journal