Abstract: Leaf anatomy characteristics and plasticity of Medicago ruthenica from four populations(Keyouzhongqi and Keshiketengqi,Inner Mongolia;Nanniwan,Shaanxi and Xinglong Mountain,Gansu)treated by different water conditions(dry,moderately dry and wet) were surveyed.Results show that: the soil-water conditions had a significant effect on leaf anatomy characteristics except the wall thickness of the upper and lower epidermis cells.In most populations,palisade tissue thickness,leaf thickness,cell tense ratio,vein protuberant degree,and thickness ratio of palisade tissue and spongy tissue increased with decreasing water,while spongy tissue thickness and ratio decreased with reduction of water.The leaf structure of Medicago ruthenica exhibited plasticity,particularly in the thickness ratio of palisade tissue and spongy tissue,though the leaf thickness remained relatively stable.The order of the mean plasticity index was KY>KQ>NN>XLS.Analysis of variation and Duncan’s multiple comparison showed that overall phenotypic plasticity of the Keyouzhongqi and Keshiketengqi populations was higher than the other two,which is perhaps related with different habitats of the species.This study has practical implications for the selection of high quality forage grass.