Reaumuria soongorica and Zygophyllum xanthoxylum are very important ultra-xeromoph shrubs in the northwest arid desert region
of China. Seeds of the two species were collected at maturity in the Alashan aria
desert grassland of China in 2001 and kept at -5℃ until use. Responses of seed germination to simulated drought stress (germination paper moistened by PEG 6000 solution) and sowing depths were investigated in this study. The drought stress included 10 treatments ranging from 0 to -2.7MPa, and sowing depth comprised 8 treatments ranging from 0 to 4cm in sand soil medium. All tests were conducted at the Herbage Seed Testing Center, Ministry of Agriculture of China ( Lanzhou ) from December 2001 to May 2002. Each test was arranged in 4 replicates with 50 seeds for each replication. Results showed that germinative capacity (3d germination rate) of the test species declined significantly ( P<0.05 ) as the osmotic potential decreased. Compared to control ( 0MPa ), final germination percentages started reducing ( P<0.05 ) at (-0.9MPa) in R. soongorica and at -0.6MPa in Z. xanthoxylum. The lowest thresholds of osmotic potential for seed germination were -1.8MPa in R. soongorica and -1.5MPa in Z. xanthoxylum. Both plumule and radical length of the test species were significantly ( P<0.05 ) reduced as the osmotic potential decreased, except that the radical length increased between 0MPa and -0.3MPa. This result indicated that slight drought stress can promote radical growth in these species. The sowing depth experiment indicated that both test species belong to an epigeal germination type. The maximum seedling emergence percentages were obtained in 0～0.5cm sowing depths for R. soongorica and in 0～2cm for Z. xanthoxylum, showing that emergence of Z. xanthoxylum was more tolerant to the deeper sowing depth. Generally, the plumule length increased and radical length decreased as the sowing depth increased, except that the radical growth of Z. xanthoxylum was inhibited for surface sowing. This may have been because water uptake was limited on the surface for the large seed size of this species. The ratio of plumule and radical length showed a tendency to increase as the sowing depth increased, and this was more evident for Z. xanthoxylum at a sowing depth from 2 to 4cm. Under optimum conditions, germination characters for the test species showed that minimum water uptake rates for germination were 110% and 90%, and initial germination times were 40h and 48h for R. soongorica and Z. xanthoxylum, respectively. R. soongorica seeds showed a lower germination capacity and slower germination speed than those of Z. xanthoxylum. For instance, germination capacities were 28% for R. soongorica and 87% for Z. xanthoxylum. The ecological significance of the relationship between germination and germination conditions is discussed.