Abstract: Cypripedium japonicum, a perennial herb of Orchidaceae, with unique flower shape, brilliant flower color, ornamental and medicinal value. Due to over-harvesting and the environment changes,a large number of wild populations of C. japonicum reduced, and some genetic resources lost. The species is in endangered, and listed as a national endangered plant. Consequently, it is essential to execute studies on floral characteristics and breeding systems of C. japonicum for understanding its extinct mechanism.We conducted a field investigation, and experiments of pollen germination test, emasculation, bagging and artificial pollination, and applied out-crossing index, to study the flowering character, pollen viability, stigma receptivity, breeding system of C. japonicum in natural populations. We also observed surface pattern of pollen and stigma by using an electron microscope. Results showed that: C.japonicum bloomed from April to June and the flowering span was 23 days among populations. The life span of one single flower was approximately 12-16 days. The stigma receptivity started from bloom and lasted for 14 days, and the pollen viability was high in the whole florescence. The stigma exceeded stamen in position and the relative difference in position between the stamen and style remained unchanged in the whole flowering process. The total value of OCI was 4. Seed setting propagation coefficient of C. japonicum was low under field conditions, with only 5%. Based on the results of emasculation, bagging and artificial pollination experiments, C. japonicum is self-compatible and can't self-pollinate autokineticly and has no syngamy. Thus, the breeding system is pollinators depended.

Key words: Cypripedium japonicum, breeding system, pollen viability, stigma receptivity