Background: Water uptake is essential for seed germination. However, Gleditsia sinensis seeds have a water-impermeable seed coat, which is beneficial for its adaption to the environment, but prohibits its germination without treatment. This feature may be associated with the structure of the seed coat. Thus, the aim of this research was to identify and describe the initial water uptake site and water movement and to determine the relationship between seed coat structure and water absorption. Results: A water temperature of 80 °C was optimal to break the hardseededness of G. sinensis seeds. Scanning electron microscopy (SEM) images revealed that the seed coat consisted of a palisade layer and light line that can hinder water entry into the seed. Also, a structure of vascular bundles existed in the hilar region. Hot water treatment caused the tightly closed micropyle to open and the cavity beneath it expanded; the layer of palisade cells in the lens was raised. The embryo dye-tracking tests showed that the radicle tip was the initial region to be stained red. After staining for 24 h, the red-stained area on the vascular bundle side of cotyledon was more extensive than that on the other side. Further studies by MRI maps indicated that the micropyle was the initial site for water imbibition. Some water then migrated along the space between the seed coat and the endosperm to the chalazal; simultaneously, the rest of the water reached the embryonic axis and spread into cotyledons. The maps of 20–24 h showed that water was unevenly distributed within the cotyledons in a way that the edge parts were more hydrated than the center. Blocking tests showed that the hilar region was the initial and an important region during seed imbibition. The medial region and chalazal portion were capable of imbibing water when the hilar region was blocked, but water absorption was later and slower than that through the hilar region. Conclusion: MRI technology provides a promising and non-invasive technique to identify the water gap and the path of water movement in the seed. Combined with the results of SEM, the relation between seed coat and its imbibition can be inferred.

Quercus texana seeds are recalcitrant and thus highly sensitive to desiccation, which makes storage difficult. For practical seed handling, it is important to define their safe water content and to understand how water is distributed during dehydration. The present study utilized magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) technologies to investigate the migration and phases of water, respectively, revealing the underlying reasons for the recalcitrance of Q. texana seeds. The water content of fresh Q. texana seeds was found to be 39.6% and the germination percentage was 93.3%. As the water content decreased, the germination percentage decreased continuously, reaching 0% at a water content of 13.0%. At 20.0% water content, the germination percentage was 71.7%. MRI showed that water was primarily stored in the embryo axis and cotyledon center in fresh Q. texana seeds. Water loss occurs in the following order during seed dehydration: embryo axis, cotyledon center, cotyledon periphery, and cotyledon end. However, water in the radicle region persisted until seed water content decreased to 15.0%, at which point no signal was detected. The NMR T₂ relaxation spectrum indicated the presence of bound water (T₂₁ = 0.01–5.44 ms) and free water (T₂₂ = 7.19–1401.93 ms) in the seeds. During the dehydration process, most of the water was lost as free water, and the T₂₂ shifted to longer times. Concurrently, the bound water shifted to shorter T₂₁ times. Overall, for practical purposes, seed water should be maintained at or above 20.0%. MRI further showed that water loss from the radicle plays a decisive role in the decline of seed germination, and that protecting the region of radicle and the cupule scar can effectively retard water loss. Furthermore, the bound-water content is positively correlated with seed germination.

Sapium sebiferum is a tree with socioeconomic, environmental, and medicinal value. S. sebiferum seeds possess physiological dormancy, which is induced by endogenous inhibitors and can be broken by cold stratification. However, the putative spatiotemporal changes in inhibitor activity are currently unknown, which can hinder the propagation of S. sebiferum seeds. The objective of this study was to investigate the spatiotemporal dynamics of inhibitor activity and its effect on germination during the cold stratification of S. sebiferum seeds. An extractant consisting of 80% methanol was used to extract the inhibitory substance from the seed coat and endosperm at different stages of cold stratification. The extract was then applied to both the Chinese cabbage seeds and excised embryos of S. sebiferum. The germination percentage and germination index were used to assess the inhibitor activity of S. sebiferum seeds. The germination of non-stratified S. sebiferum seeds was completely inhibited (0% germination). As the stratification duration was extended, the germination percentage of S. sebiferum seeds gradually increased. However, dormancy persisted until the stratification duration reached 120 d; at this point, the mean germination was 81.3%. The germination test on Chinese cabbage seeds revealed a significant increase from 10.0% (stratified for 0 d) to 91.2% (stratified for 120 d) when treated with endosperm extracts. The germination indexes also increased from 0.0 to 40.3, indicating a decrease in the inhibitory activity of endosperm extracts. The seed coat extracting solution showed varying dynamic changes. The lowest germination was observed after 60 d of stratification, with no significant differences among the results of 0 d, 30 d, and 60 d. However, after stratification for 90 d, the germination percentage of Chinese cabbage seeds increased. The germination percentage of excised embryos exhibited similar changes to those of Chinese cabbage seeds. This study discovered that endogenous inhibitors were present in both the seed coat and endosperm of S. sebiferum seeds, and the inhibitor activity was higher in the endosperm. The spatiotemporal patterns of inhibitor activity suggest that the endogenous inhibitors move from the endosperm to the seed coat during early cold stratification stages, aiding in the physiological dormancy release of S. sebiferum seeds. These findings enhance our understanding of seed biology in S. sebiferum and will facilitate high-efficiency seed propagation.