Morphology and Physiology Response to Stratification during Seeds Epicotyl Dormancy Breaking of <i>Paeonia emodi</i> Wall. ex Royle

<i>Paeonia emodi</i> is a type of wild herbaceous peony with high ornamental and breeding value. Cold stratification is the only method to break its seed epicotyl dormancy to date, however, the key physiological factors during this process are not clear. In this study, rooted seeds of <i>P. emodi</i> were treated with 4 °C stratification, and the changes of the embryo, four nutrients, and two endogenous hormones in the seeds were investigated. The results showed that the plumule elongated at S6 (i.e., ten weeks of cold stratification), and grew to nearly the same length as the cotyledon at S9. Cold stratification increased starch consumption, significantly decreased soluble sugar content in the later stages, and decreased soluble protein content at S9, but it did not influence crude fat content. The activities of α-amylase and β-amylase increased significantly at S4 and S4 to S6, respectively. At S8 and S9, acid protease activity increased, and the increase in lipase activity continued throughout the whole process. At the same time, the ABA content decreased significantly after S6; from this stage, the ABA/GA₃ ratio gradually decreased compared with that of the control, and the difference was significant at S9. Correlation analysis showed that the ABA/GA₃ ratio was significantly correlated with starch content and α-amylase activity. It can be concluded that both carbohydrates and proteins were the energy supply for the epicotyl dormancy breaking of <i>P. emodi</i> seeds, rather than crude fat. Cold stratification promoted substance transformation by increasing the corresponding enzyme activities. The balance of ABA and GA₃ suggested the key stage for the release of dormancy.