Abstract: Orinus is an alpine genus with important economic values in the grass family(Poaceae). There are six species in it and all of them have stronger adverse-resistant characteristics, and they are the important resources for improving and applying agricultural breeding and animal husbandry. Meanwhile, the discriminative characters are obscure among these species. We compared and analyzed the main morphological characters and molecular sequence differences among nearest related Orinus longiglumis, O.thoroldii and O.anomala by generalized morphological observation and nuclear ITS region research. There is very slight disparity between O.longiglumis and O.thoroldii in generalized morphological characters, the classification of the two species is very difficult, the differences between O.longiglumis and O.anomala are distinct, and the two taxa are two different species. We further used the ITS sequence variation to tentatively delimit the previously acknowledged species and found that there is no distinct boundary between species and only two distinct lineages were recognized for all three species. A branch is composed of O.longiglumis and O.thoroldii, while another is only made up of O.anomala. The bootstrap support rates of two branches reach as high as 100%. Therefore, O.longiglumis and O.thoroldii should belong to the same species, and O.longiglumis can be reduced to synonymy of O.thoroldii.

Key words: Poaceae, Orinus, Orinus longiglumis, morphological character, ITS sequence