Objective  The transition from formation and maturation to germination of Polygonatum cyrtonema Hua seeds were studied to elucidate the dormancy process.

Method Phenotypic, physiological, and biochemical characteristics as well as gene expressions of P. cyrtonema seeds were monitored 135d post-pollination at the formation stage, 180d post-pollination in maturation, and after 4-month stratification for germination. Concurrently at each stage, morphological structure, coat permeability, antioxidant enzyme activities, storage carbohydrates, and gene expressions of the seeds were determined. On germination, both intact seeds and those with 50% coat and endosperm excised were assessed.

Result In transition from formation and maturation to germination of the seeds, the coat color and permeability, endosperm cell arrangement, and embryo morphology differentiated significantly. Meanwhile, the superoxide dismutase activity became significantly higher at the beginning and the end than at the maturation stage (P＜0.05), while the peroxidase and catalase peaked at the germination stage. As the seeds were developing, the content of soluble sugar decreased continuously, whereas those of starch and cellulose rose at first followed by a decline. The transcriptome analysis revealed 1,444 upregulated and 701 downregulated differentially expressed genes (DEGs) were shared at the seed formation and germination stages, but not at maturation. These co-regulated DEGs were primarily enriched in the pathways related to phenylpropanoid biosynthesis, plant hormone signaling, and starch and sucrose metabolism. Among them, BAM1, BAM3, and BAM8 encoding β-amylase (BAM), as well as AGPS1 and AGPP encoding ADP-glucose pyrophosphorylase (AGPase), were significantly upregulated by 2.2 to 32.3-fold at the beginning and germinating stages. For germination, the intact seeds failed to initiate in 45d, but those with 50% coat and endosperm removed started germinating as early as day 3. Additionally, the seeds collected before full maturation had a higher germination rate than the matured counterparts.

Conclusion During the development of P. cyrtonema seeds, the storage carbohydrates (e.g., starch and cellulose) were accumulated from the beginning with low cellular activity as a mechanical barrier around the embryo formed to collectively started dormancy. The subsequent stages of development brought about carbohydrate degradation, cellular metabolism reactivation, and water imbibition that led to germination of the seeds. Hence, an immature seed would germinate earlier than the fully developed, coat-reinforced ones, whereas a half-excised seed would have the barrier partially removed to afford a more effective breaking of dormancy.