The Mechanism Of Water Absorbing And Dormancy In Cercis Chinensis Seeds

In this study, we observed the seed development of Cercis chinensis during the process of maturity and also investigated the causes and methods for breaking seed dormancy. Besides, we studied the mechanism of water uptake by obseving the seed coat structure. The main results were as follows.1. The developmental process of C. chinensis seed has three semi-independent phases which were rapid growth, reserve accumulation and seed maturation stages. The morphological development of seed embryo was almost completely at the end of rapid seed growth stage. The germination rate of isolated embryo was increased with the physiological maturity of C. chinensis seed and increased to 74% before the early stage of reserve accumulatio.SEM images showed that during the early stage of maturity some pores appeared at the surface of seed coat, and then disappeared with the subsequent development of seeds. At a later stage, waxy of seed coat appeared and became thicker as the seed matures, and also micropyle and hilar groove cracked gradually. By the longitudinal section of seed coat, we found that the palisade layer near hilum formed earlier than the middle part of seed coat and the palisade cells in this area was monolayer and zigzagged in the middle. After flowering 105 days, a water entry from the hilum extending along the seed coat could be observed. A reticular tracheidal bar from hilar groove to the opposite end was found in the water entry pathway while flowering for 135 days. After seed maturation, from the exterior to the interior, the seed coat of C. chinensis consisted of three layers: the waxy cuticle, the palisade layer, and the parenchymatous layer. The cuticle was composed of wax and the other covering. The palisade layer consisted of a monolayer macrosclereids with thick walls elongated longitudinally. The macrosclereids were tightly arranged. The waxy cuticle and the palisade layer may be the main barrier controlling the entrance of water through seed coat.By the TEM, we found that at the early seed-filling stage, the endosperm cells were characterized by a large central vacuole with a small quantity of lipid droplets and density electron substances. As the maturity of seed, mitochondria, endoplasmic reticulum and Golgi apparatus formed and then degraded gradually. Meanwhile, the quantity of lipid droplets and density electron substances were increased continuously. It indicated that with the development and maturity of seed, the storage macromolecules accumulated and the metabolism decreased continuously, after that the seed turned to dormancy.3. The aniline blue dye-tracking test can be used to locate the site of water entry and the Vaseline was used to make the seed coat partially impermeable to measure the water absorption capability of different seed parts. After that, we could determine the early site of water entry and the changes of the sites of water uptake after the breaking of its hardness. The results showed: hilum region was the early site of water absorption, which may result from the crack in hilum and the presence of large amount of tracheid group inside the crack. Changes of any sites of water entry could be made by hot water treatment and water entered through any sites of C. chinensis seed after the breaking of its hardness, while the micropyle still played important roles in water imbibition.4. The seed coat structure appeared to be a barrier to water uptake and was one of the main reasons for seed dormancy. The embryo of C. chinensis seed didn't show physiological dormancy, but the endosperm of C. chinensis seed had the mechanical stress to the embryo growth. Meanwhile, the inhibitor substances present in the seed coat and endosperm played a role in dormancy of C. chinensis seed.5. We evaluated the effects of hot water pre-treatment, mechanical scarification, and the chemical scarification immersed in concentrated sulphuric acid on breaking seed dormancy. All of the treatments had a positive effect on seed germination. The preferable methods for hot water treatment was to begin with 80 ?and soaking in water for 24 h, while it was better for seeds soaking in concentrated sulphuric acid for 2min. Gibberellic acid could not increase seed germination before breaking the hardness of seed. When the hardness was broken, Gibberellic acid could have a positive effect on seed germination but it has no direct contact with the concentration of Gibberellic acid. Combined with practical production, the highest germination frequency was achieved by dipping seed in hot water, then soaking the seeds in 500 mg·L-1 GA3 for 24 hours prior to cold stratification at 5°C for 80 days.