Life-history strategy is the significant research area of plant population ecology. Seed germination is the critical stage of plant life history. Therefore, research on seed germination rule and strategy helps to know and elucidate plant evolution and ecological adaptation traits. Plants in Songnen grassland area were used to test the responses of seed germination to constant temperature and diurnal increased or decreased temperature, the effects of global warming simulated by alternating temperature regimes on seed germination of C3 and C4 species, plant germination pattern and the changes under different constant temperature. The mechanisms of germination responses to NaCl and PEG and the interaction between salt and temperature were also studied. Photosynthetic contribution of cotyledons to seedling growth was evaluated as well.According to the results of constant temperature effects, we proposed the modified thermal time model on the basis of thermal time model and supplemented an equation describing germination rate constant. Germination rate of C3 species was lower than C4 species. According to thermal time model, the average base temperature of C3 species was lower than C4 species, but not significantly. The average thermal time constant of C3 species was higher than C4 species, which approached significant difference. In this study, plant seed germination pattern was divided into five categories: quick germination, delayed germination, steady germination, normal germination and delayed-quick germination. Most annual species belonged to quick germination pattern. Perennial C3 species, which distributed to meadow, belonged to delayed-quick germination pattern, normal germination pattern or steady germination pattern. Seed germination pattern of most species changed with temperature, which was an adaptive strategy to environments.Under global warming simulated by alternating temperature regimes conditions, germination traits of C3 and C4 species changed. C3 species was sensitive to high temperature and C4 species was sensitive to low temperature. By comparison of germination results in all the alternating temperature regimes, we found that high or low temperature, not amplitude acted an important role in the effects of global warming on seed germination. Field experiment showed that more plants germinated in spring, not autumn in Songnen grassland. Germination experiment exhibited that significant differences in germination between diurnal increased and decreased temperature in the spring and autum low ranges 5-10 and 10-15oC, with the former higher. Germination in spring is an adaptive strategy by natural selection. The relationship between germination rate and salinity under different temperatures of C. virgata and D. sanguinalis was curvilinear. We thought there may be some physiological mechanisms, which reduced osmotic stress in the middle and high salinities. This led to the obviously increasing of the germination rate. Further experiments showed that compared with PEG treatment, seeds can germinate in lower water potential induced by NaCl and germinate faster. Seed dry weight decreased with the time. Water content, seed sodium concentration and seed solution sodium concentration of NaCl treatments were significantly higher than the isotonic PEG treatments. Seed sodium concentration and seed solution sodium concentration of germinating and un-germinating seeds in NaCl treatments increased as the water potential decreased. Water content, seed sodium concentration and seed solution sodium concentration increased linearly with time.According to the results, we proposed salinity model:θS = (Sm– S) t, in whichθS is salinity constant, Sm is the maximum salinity above which seed can't germinate, S is the external salinity, and t is germination time. The germination response of seed to salt was divided into four stages. The first stage mainly involved negative osmotic effect. In the second stage, ionic effect and osmotic effect existed together, with ionic effect stonger. In the third stage, no seed germinated in PEG treatment and ionic positive effect was largely higher than osmotic effect. In the fourth stage, ionic effect gradually began to harm the seed.Cotyledon photosynthetic contribution experiments showed that loss of cotyledons in seed imbibition stage, first leaf development did not proceed following early loss of cotyledons, but the reason did not appear to be an excess of respiration over assimilation, so a hormonal type signal may be involved; loss of cotyledons prior to cotyledon expansion, and so presumably before any substantive photosynthesis, induced seedling ontogenetic compensation (increase in shoot:root ratio loss of cotyledons at the first leaf stage of seedling growth delayed seedling development without changing seedling ontogeny, indicating an ongoing photosynthetic contribution of cotyledons. The data highlighted the way in which almost total reliance on the photosynthetic role of cotyledons in small seeded species confered the possibility to achieve a very high relative growth rate during early seedling development, and raised the possibility that quantitative determination of the ratio between the photosynthetic contribution of cotyledons to seedling development and total (photosynthetic plus reserve) contribution may provide a useful indicator of species competitive strategy and fitness for different habitats.This study has theoretical significances and practical value for predicting plant germination and emergence, testing effects of global warming on plant distribution characteristics and developing and utilizing wild plant resources.