Low Temperature–ROS–Hormones Co-Regulation of Seed Dormancy Release and Germination in Xanthoceras sorbifolium

Seed dormancy is an important adaptive mechanism in plants. However, some seeds, such as those of Xanthoceras sorbifolium (a valuable economic and medicinal species), exhibit strong dormancy, resulting in a very low natural germination rate. This work investigated the mechanism by which low temperatures (LT: −20 °C storage for 60 days) release seed dormancy and promote germination. This was achieved by examining seed germination conditions, applying scanning electron microscopy (SEM), and measuring physiological indicators of seeds and the hormone levels. Targeted metabolomic analysis of sugar and fatty acid metabolism was also performed. The results were as follows: (1) A high germination rate of 47.3% was observed under LT condition, compared to 32.7% at room temperature (RT: 25 °C). Of four germination methods, direct GMS (germination in moist sand) was the most effective. (2) The two stages of VI (after storage) and VII (on the 7th day of germination) were key stages to breaking the seed dormancy and triggering germination. At both stages, the high integrity of the seed shells and kernels—particularly, the kernels—was observed using SEM. The activities of SOD, CAT, and POD, as well as the levels of IAA and IPA, and the ratios of IAA/ABA and (IAA + GA3 + ZR + IPA)/ABA (tHor/ABA) were found to be higher, especially in stage VII, where tHor/ABA increased by 62.7%, while ABA decreased by 23.2% in the LT treatment compared to the RT treatment. An optimal germination condition (GMS) created a suitable microenvironment, and this could have maintained highly active antioxidant enzymes and kept H2O2 (one of reactive oxygen species, or ROS) within signal transduction levels, and cross-talk to hormones. These changes (enzymes, hormones, ROS, and microenvironment) ensured the seeds reaching an optimal state for dormancy release in the VI stage, and facilitated the seed germination in the VII stage. (3) LT treatment promoted the degradation of starch and fats in the seeds. Significant accumulations of eight soluble sugars, such as glucose, D-fructose, and trehalose, as well as three fatty acids, such as Cis-11,14,17-eicosatrienoic acid (C20-3n3) and γ-linolenic acid (C18-3n6), were observed, while two soluble sugars and one fatty acid decreased under LT conditions. In conclusion, low temperature, as an external signal, together with ROS and GA-ABA-IAA co-regulated the seed dormancy release and germination. The moist-sand microenvironment was also a key factor in awakening the embryos of X. sorbifolium seeds.