国家高新技术研究发展计划（863计划）资助项目（2006AA100202）；中国科学院知识创新工程重要方向资助项目（KZCX3-SW-444 和 KSCX2-YW-N-003）；国家教育部新世纪优秀人才支持计划资助项目
To better understand the drought\|tolerance of alfalfa seedling, we investigated the seeds germination capability and the minimunm water requirment in alfalfa (cultivars: Long-Dong and Algonquin) and sorghum (cultivar: Kang-Si) under water stress mimicked by PEG-6000 solution (ψw=-0.1- -0.5 MPa). Simultaneously, the interspecific difference was obtained. The results showed that the inhibition of seed germination capability was dependent on impairing water efficient uptake under PEG stress, including declines of Germination Ratio, Water Content, Vigor Index and Germination Stress Index and increase of Radicle/Embryo Ratio and so on. Especially, germination capability was more weaked when water stress changed more severe. On the other hand, it must take more time to finish 50％seeds radical emergence, plumule emergence and seedling establishment than that of controls under water stress. The critical water potential at 50% alfalfa seedlings establishment stage was higher than those at 50% seeds radical emergence and plumule emergence stage. It indicated that the stage of seedling establishment was most sensitive to external water potential. Contrasting to alfalfa, the germination capability of sorghum was less affected under the same stress conditions above, and critical water requirments at all stages of germination were lower than those of alfalfa. Therefore, the enduring capability to water stress of alfalfa was weaker than sorghum, and the water requirment at seedling establishment stage was also different between two tested alfalfa cultivars.