Abstract: As the annual plants of Ambrosia,Compositae, the dormancy degree of the seeds of Ambrosia artemisiforia and A. trifida varies with the latitude of growing space. The higher latitude, the more proportion of the seeds that lies dormant after maturation and the longer time of cold stratification that required for relieving the dormancy. In Mudanjiang where the latitude is higher, the mature seeds of A. artemisiforia and A. trifida lie dormant wholly and the germination percentage goes beyond 95% after twelve weeks of cold stratification, however, in Nanchang where the latitude is lower, about 10% of seeds of A. artemisiforia and A. trifida don't lie dormant and the germination percentage can go beyond 95% after eight weeks of cold stratification. Part of the dormant seeds can relieve dormancy after being stored at the room-temperature above 15℃ for half a year though they are not treated with cold stratification. The dormancy mechanism perhaps is controlled by the complex of inhibitory substance and accelerative substance. The dormant seeds contain high level of inhibitor. But for the non-dormant seeds, the balance turns favourable to accelevator. The temperature difference in different latitude, cold stratification and the course of respiratory metabolism under long-term storage at room-temperature perhaps have effects on this balance and thus change the dormancy state of seeds.

Key words: Ambrosia artemisiforia, A. trifida, Seed, Dormancy law