Abstract:

Objective: The seed germination of pond cypress is slow, uneven andthus leading to a low seedling rate, which brings certain difficulties to production management. The present study is to identify the causes of seed dormancy of pond cypress by studying the relationship between seed coat and seed dormancy, so as to provide a theoretical basis for the practical application of pond cypress seed and a technical guidance for ecological restoration in the Three Gorges Reservoir area. Method: The relationship between seed coat and seed dormancy of pond cypress was studied through observing the structure of seed and seed coat, germination test, TTC activity test, water permeability test, extraction and bioassay of endogenous inhibitors in different parts of seeds, as well as extraction, separation and bioassay of inhibitors contained in seed coat. Result: 1) The seed coat of pond cypress from outer to inner in order is: epicuticle, sarcotesta, sclerotesta, and endotesta. Compared with the control, the hot water treatment could promote the abscission of epicuticle, but had no obvious effect on the structure of seed coat, and the seed coat permeability had no obvious change. After 15 minutes of acid treatment, corrosion cracks and pits appeared on the seed surface, the structure of seed coat was damaged, the seed coat became thinner and the seed coat permeability increased; After 30 minutes of acid treatment, a large number of corrosion cracks and pits appeared on the surface of seed coat, which further damaged the seed coat structure while enhancing the seed permeability. 2) The viability of newly harvested pond cypress seeds was at 80.95%, but the seeds did not germinate and had significant dormancy. 3) The seed coat of pond cypress has a certain degree of water permeability, and the kernel can absorb part of the water. Although the acid treatment is conducive to promoting the seed kernel to absorb water, the hard seed coat hinders the water absorption and expansion of the kernel to a certain extent, which makes the kernel absorb water slowly, and may produce mechanical resistance to the elongation and growth of the radicle. 4) The seeds of pond cypress contain endogenous inhibitors. The inhibition intensity of methanol extracts from different parts on seedling and root growth was as follows: seed coat > kernel. The result showed that the inhibitive substances mainly existed in the seed coat instead of seed kernel. 5) The methanol extract from seed coat of pond cypress was separated and bioassayed by systematic separation method. The result showed that the methanol extracts from seed coat inhibited the growth of Brassica chinensis seedlings at different degrees. When the concentration of each separation phase was 1 g ·mL^-1, the inhibition intensity of Brassica chinensis seedling height was petroleum ether phase > ethyl acetate phase > ether phase > methanol phase > aqueous phase, and the inhibition intensity of Brassica chinensis root length was petroleum ether phase > methanol phase > water phase > ethyl acetate phase > ether phase. Petroleum ether phase and methanol phase are the main phases with inhibitory effect. Conclusion: The water permeability of seed coat of pond cypress may not be the main factor causing seed dormancy, but the mechanical restraint of seed coat may lead to slow seed germination. The inhibitory substances in seed coat of pond cypress are mainly concentrated in petroleum ether phase, whose components are characterized with smaller polarity molecules. Therefore, the structure of seed coat and endogenous inhibitors are the main causes of seed dormancy. The seed of pond cypress may take on a comprehensive dormancy (combinational dormancy, PY+PD) with both physical and physiological dormancy traits.

Key words: Taxodium distichum var. imbricatum, seed dormancy, scanning electron microscope, seed coat permeability, endogenous inhibitory substance