Taxonomic Revision Of Populus Linnaeus From Pan-Himalaya

As a cosmopolitan woody family of Malpighiales,Salicaceae is drawn broadly in APG system,including Samydaceae and Scyphostegiaceae.Circumstance of Salicaceae is discussed in this study.Morphological and molecular evidence are given supporting the tranferring of Flacourtia turbinata to Scyphostegioideae as an independent genus.This study reinforces the 3-subfamilies classification for Salicaceae,i.e.Samydoideae,Scyphostegioideae and Salicoideae,and Salicaceae would be represented by 2 subfamilies and 10 genera from Pan-Himalaya.Populus Linnaeus is a valuable woody genus of Salicaceae,and is widely distributed to temperate zone of the Northern Hemisphere.With rare taxonomic characters and high level of variation within species,Populus is one of the most difficult taxon of angiosperm for classification.Pan-Himalaya is one of the most important distribution centre of Popalus,however,no comprehensive taxonomic study exists at present.Because of numerous taxonomic problems such as missing names,synonyms and unidentified types,Populus from Pan-Himalaya needs a detailed revision.In this study,124 scientific names that have been validly published are researched textually and more than 4000 specimens from 30 herbaria all over the world were checked.Specimens of 620 gatherings are collected from Xizang,Sichuan,Yunnan,Qinghai and Gansu.Morphological characters are evaluated based on observation among populations.Terminal bud presence or absence,disc deciduous or persistent,winter bud viscid or not,shape of ovary and types of placenta are found to be significant in distinguishing subgenera and sections.Shape of leaf blade,type of hair,petiole,bract,disc and carpel are effective in specific division.Flow cytometry is carried out to detect the ploidy level for several species.Most of these taxa are found to be diploid,while a population of triploid P.davidiana,with remarkable larger leaf blade than diploid,is verified.As the leaf size is one of key characters to distinguish P.davidiana from P.rotundofolia,the borders between them are blured by triploid variation.Previous phylogeny study fails to resolve relationship among sections,and morphological boundary between some sections is not clear.Thus maternal relationships among sections is discussed based on chloroplast gemones.As a result,Populus is divided to 2 subgenera and 4 sections,i.e.Subgen.Turanga and Subgen.Populus(including Sect.Leucoides,Sect.A igeiros and Sect.Populus).Eventually,22 species(including 2 artificial hybrid species)and 2 variaties are confirmed from Pan-Himalaya,and three names that are lack of materials would be left for further research.Twenty-nine names are disposed as new synonymsTaxonomic comments,distribution maps and illustrations are apposed to most species.