Abstract: In order to study mechanism of seed germination thermoinhibition，the mature Brassica rapa ssp. chinensis seeds were used to investigate the response of seed germination to different temperatures，the effect of after-ripening，cold stratification and phytohormones to seed germination，and the relationship between seed germination and different treatments and degrading enzymes of cell walls. The results showed that seed germination was significantly decreased at ≥ 20 ℃ and in darkness，but the thermoinhibited seeds did not exhibit secondary dormancy. After-ripening，stratification and GA₃ treatment effectively decreased thermoinhibition of seed germination，in contrast ABA treatment notably increased thermoinhibition. Activities of endo-β-mannanase，β-mannosidase and α-galactosidase increased during seed germination. Compared with freshly-collected seeds，treatments of after-ripening，stratification and GA₃ increased the activities of above three enzymes，ABA treatment increased endo-β-mannanase activity and did not alter β-mannosidase and α-galactosidase activities. There was no direct correlation between seed germination and change in endo-β-mannanase and α-galactosidase activities of seeds imbibed at different temperatures for 48 h. The results indicated that freshly-collected B. rapa seeds had a typical germination thermoinhibition，which can be broken by after-ripening，stratification and GA₃ treatments in an increasing the degrading enzyme activity of cell walls.

Key words: Brassica rapa ssp. chinensis, seed, germination thermoinhibition, phytohormone, after-ripening, stratification；degrading enzyme of cell walls