Abstract: [Objective] The seeds of Albizia julibrissin, collected from Suqian of Jiangsu Province, were used to identify the main water entry sites during imbibition. The structure of seed coat of A. julibrissin and the relationship with water uptake were studied to explore the mechanism of dormancy breaking of A. julibrissin seeds.[Method] Taking the healthy full seeds without pest and disease as material, the structure of seed coat of A. julibrissin was observed by a stereo microscopy and a scanning electron microscopy (SEM). The effect of seed coat structure on water absorption was studied by dye-tracking and vaseline sealing experiments.[Result] The seed coat of A. julibrissin is impervious to water. Soaking treatment with hot water at different temperatures (60℃,70℃,80℃) was able to efficiently break the hardness of the seeds, however the seed viability decreased with the increase of hot water temperature. Structural characteristics of the seed coat were also examined. A. julibrissin seeds were irregular flat ellipse and had a hard and opaque seed coat with a yellowish-brown colour. There was a protruding oval-shaped edge which was a large fissure in the seed coat parallel to the macroaxis of seed. SEM images showed that from exterior to the interior the seed coat consisted of five layers:the epidermal layer, the palisade layer, the osteosclereid layer, the stereid layer, and the parenchymal cell. There were many cracks with different size, shape and depth in the seed coat. Micropyle and lens were closed and its hilum was covered by wax, vascular bundles extended parallel to the surface of the seed coat from the hilum. Morphological changes during dormancy breaking were also evaluated. Micropyle opened, the thickness of wax layer in hilum reduced, and a large crack appeared in the lens after the hot water treatment. Blocking experiments showed that all parts of treated seeds could absorb water, but the quantity of water uptake at the end of cotyledon was always the lowest. After incubation for 4 h, the most of water was imbibed by the hilum region. Subsequently, water uptake in the middle of seeds quickly increased. It was notable that water absorption was not significantly different between treatments exposing the hilum region and the middle part of seeds, but these two treatments had significantly higher water absorption values than the treatment exposing the cotyledons extremities after 12 h steeping conditions. The difference trend remained until the water absorptivity of A. julibrissin seeds reached saturation. Aniline blue staining showed that the seed coat could absorb water in 3 ways:1) crack at lens, 2) gaps in the micropyle and hilum, and 3) the epidermis layer of the seed. Water first entered the seed coat from the lens and moved along the vascular bundles within it. Then there was water entering the seed through micropyle and hilum. After that, water also permeated through the epidermal layer which was the outermost layer of seed coat. However, palisade layer hindered the further entry of water into the seed. The first red staining appeared in the radicle after 2 h of soaking in TTC solution, then water moved to the end of cotyledon. In horizontal direction, water penetrated from the edge of the cotyledons to the middle.[Conclusion] Treated in water at 70℃ for 5 minutes was the optimum method to break dormancy of A. julibrissin seeds. The series experiments indicated that nondormant seeds could absorb water throughout the entire seed coat, but the initial site of water absorption was the lens followed by the micropylar and hilum. After water entered the embryo it moved from the radicle to the terminal cotyledon. The hardness of seeds possibly relates to the structures of palisade layer, light line, parenchymal cell and the wax which covered the hilum and filled in vascular bundle.

Key words: Albizia julibrissin, mechanism of water uptake, seed coat structure, TTC staining, aniline blue staining