Abstract:

【Objective】 The hardness of the seed coat seriously hinders water absorption in Cercis canadensis seeds. The structure of the seed coat was studied to determine how to break the hardness and reveal how these seeds uptake water. 【Method】 Seeds were soaked in hot water, then the structure of the seed coat was assessed using scanning electron microscopy (SEM). Seeds of C. canadensis were stained with aniline blue and sealed with Vaseline to determine the relationship between the seed coat and water absorption. 【Result】 Soaking in hot water at various temperatures for 5 min, followed by gradual cooling for 24 h broke the hardness of C. canadensis seeds and improved rate of imbibition. However, seed viability decreased with increasing water temperature. The SEM images of the seed coat revealed some fissures in the hilar region. Soaking in hot water dissolved the wax between the hilum and vascular bundle, and caused the wax in the fissures to disappear. The SEM images of longitudinal sections showed that the seed coat comprised a cuticle, palisade, and sclerenchyma layers. However, unique structures such as light lines, counter-palisade layers, and vascular bundles were found in the hilar region. After being soaked in hot water, sclerenchyma cells and a palisade layer near the micropyle separated and a bulge formed. Blocking various parts of the seeds with Vaseline revealed that the hilar region absorbed water more rapidly than the other blocked regions when soaked in water for 12 h. After soaked in water for 96 h, the middle part of the seed coat began to absorb water. However, the bottom of the cotyledon did not start absorbing water until the seeds were soaked for 120 h. The micropyle was the first to be stained by aniline blue (2 h), followed by the hilum (3 h). Water then entered the seed along the edge of the seed coat, which was the space separating the sclerenchyma cells and the palisade layer (4 h). When soaked in water for 9 h, the vascular bundle was stained blue. 【Conclusion】 Soaking in hot water at 80 ℃ for 5 min was optimal for breaking the hardness of C. canadensis seeds without compromising seed viability. Blocking with Vaseline showed that the hilar region is crucial for water absorption. The results of aniline blue staining further indicated that the micropyle was the initial site of water absorption and that the vascular bundle was an important structure for water movement. Aniline blue staining also showed that sclerenchyma cells could restrict water uptake at the early stage of soaking.

Key words: Cercis canadensis, seed coat structure, mechanism of water absorption, seed hardness