Abstract:The study used seeds of Paeonia rockii treated with different concentrations of gibberellic acid (0,100,300,500 mg/L) and different temperatures (normal 25,30,40 and 50 ℃). The change of seed rooting rate of each treatment was investigated, and the ultrastructure of seed embryo at different rooting stages was observed by transmission electron microscopy(TEM). The ultrastructural changes of seed embryo in the process of dormancy release of Paeonia rockii seeds and the correlation between the changes and dormancy release were explored. Result show that: (1) GA3 and warm water treatment significantly advanced the rooting time, and 300 mg/L GA3 and 40 ℃ warm water treatment had the best rooting effect, 14.7 and 16.0 d earlier than the control, and the rooting rate was 62.33% and 67.00%. (2) The results of optical microscope showed that GA3 and warm water treatment had no significant effect on the shape and arrangement of the upper and lower epidermal cells of the cotyledons, but the thickness of the cotyledons was thinner and the vascular bundle structure was obvious. The best rooting effect was 300 mg/L GA3 and 40 ℃. (3) TEM showed that the lipid in the cotyledons of the seeds of Paeonia rockii was degraded in the early stage of germination and the lipid bodies were distributed around the cell wall in different sizes. The number of lipid bodies was obviously reduced. The nucleus was clear, a small number of mitochondria appeared, protein began to accumulate in the cytoplasm, and around the vacuole formation of protein deposits. In the later stage of seed germination, the matter in the cotyledons of seed embryo is dense, the lipid bodies have been degraded and fused into a larger mass, the nutrients stored in the cells are basically degraded completely, endoplasmic reticulum, golgi body, mitochondria and other organelles appeared. The structure is more complete, which indicates that the material metabolism activity in the cells is strengthened at this time and the seed dormancy is relieved. The results showed that the germination rate of seeds could be significantly improved by soaking seeds in 300 mg/L GA3 and 40 ℃ warm water, and the effect of soaking seeds in 40 ℃ warm water was better, so it was better to use more warm water soaking method in actual production.