Abstract:

【Objective】 Paeonia ostii ‘Feng Dan’ is an edible woody oil crop. Propagation is primarily carried out via seeds. However, under natural conditions, the germination of matured seeds normally takes at least four months, and this strong dormancy is a major restriction on its reproduction. Many studies have been carried out on the dormancy of ‘Feng Dan’ seeds which found that the germination was limited by endogenous inhibitors. However, the effects of the seed coat on dormancy and embryo morphological changes during dormancy breaking have not been studied. In addition, researches on the changes in seed nutrients have rarely been reported. Therefore, we explored changes in the seed coat structure, the morphology of the embryo and the nutrients of ‘Feng Dan’ seeds at different stratification stages to provide a theoretical reference for releasing its dormancy and promoting its germination. 【Method】 The ‘Feng Dan’ seeds were stratified at five different stages and were used as materials for the study. The morphology at each stage was observed by stereomicroscopy. Matured seeds (stage Ⅰ) and seeds stratified for 30 days (stage Ⅱ) were used to observe the seed coat structure in three different regions of the longitudinal section of the seed hilum, the chalazal region, and middle part with the help of an S-3000 scanning electron microscope (SEM). Additionally, during the stratification process, the changes in seed size (transverse and longitudinal diameters) and water absorption were measured. Indexes such as crude fat, soluble protein, soluble sugar, starch content, peroxidase (POD) and acid phosphatase (APA) activity were also studied. 【Result】 The seed coat of ‘Feng Dan’ did not hinder its water uptake. When soaked for 96 h, the water absorption was 54.2%, which was very close to the maximum result of 56.9% (144 h). After stratification for 30 days, the longitudinal diameter of seeds increased significantly from 8.97 mm to 10.44 mm, and the transverse diameter increased significantly from 6.42 mm to 8.87 mm. During the subsequent stratification, the seed size remained almost unchanged. The SEM images showed that the seed coat was mainly composed of three layers: the cuticle, the palisade and the parenchymatous. The seed coat structure of matured seeds showed that the long columnar cells in the palisade layer were arranged neatly and tightly, but some obvious gaps were found in the palisade and parenchymatous layers. After 30 days of stratification, the cells in the palisade layer were loosely arranged, and some small scattered holes between the cuticle and palisade layers were observed. These two obvious changes in seed coat structure may further increase the permeability of ‘Feng Dan’. The embryo of the matured seed was very small. After 30 days of stratification, the cotyledons became slightly larger and the hypocotyl became slightly thicker. When the radicle broke through the seed coat, the cotyledons were further enlarged, the radicle was obviously elongated, and the hypocotyls were thickened, but the seed still needed to be stratified for more than two months for the epicotyl to extend. Therefore, the undeveloped hypocotyl and epicotyl of ‘Feng Dan’ seeds could hinder its germination. During the dormancy breaking procedure, the content of soluble sugar in the seed showed a significant decrease initially (from 63.4 mg/g to 53.8 mg/g), followed by an increase (stages Ⅱ to Ⅲ) and then another decrease. In stage Ⅴ, the content of soluble sugar was 44.6 mg/g, a decrease of 29.8% from stage Ⅰ. The contents of starch decreased by 45.2%, from 26.7 mg/g (stage Ⅰ) to 14.7 mg/g (stage Ⅴ). Soluble proteins decreased by 57.8% from 93.4 mg/g (stage Ⅰ) to 39.3 mg/g (stage Ⅴ), and decreased rapidly during stages Ⅱ and Ⅲ. Before the stratification (stage Ⅰ), the crude lipid content of ‘Feng Dan’ seeds accounted for 32.3% of the fresh weight, which was the main storage substance in the endosperm. In stage Ⅴ, the content decreased by 65.0% to 11.3%, and the most rapid decrease occurred in stages Ⅲ to Ⅳ. During the stratification stages, POD activity increased rapidly, peaking at 52.1 μmol/(min·g) in stage Ⅲ, then decreased slightly (in stages Ⅲ to Ⅳ), followed by a slow increase (stage Ⅳ to stage Ⅴ). APA activity showed an increasing tendency, and the greatest value was 25.1 U/(min·g) in stage Ⅴ. The most rapid increase was during stage Ⅲ and Ⅳ when the radicle enlarged quickly. Overall, these indexes showed different changes during the stratification. The contents of starch, soluble protein and crude lipid showed a gradually decreasing trend coupled with an increase in POD and APA activities. Owing to the higher activities of enzymes, the metabolism in the seed was accelerated, and the nutrition and energy required for germination were produced continuously. 【Conclusion】 The seed of ‘Feng Dan’ had good water permeability and was, therefore, able to take in water successfully during germination; the permeability of the seed coat was not the reason for its dormancy. The embryo size of mature seeds was very small, and the embryo gradually differentiated and developed when stratified at 10 ℃; therefore, the observations of the morphology of embryos indicated that the hypocotyl and epicotyl of ‘Feng Dan’ were the main reasons for its dormancy. During the stratification, the nutrition contents decreased with an increase in the activities of POD and APA. Crude lipids were the main source of energy during dormancy breaking. The dynamic changes in these physiological and biochemical indexes in ‘Feng Dan’ seeds were highly consistent with the morphological changes of the embryo.

Key words: Paeonia ostii ‘Feng Dan’, seeds, seed dormancy, seed coat structure, embryo morphology, physiological and biochemical index