Reproduction isolation is the basis of species preservation and divergence,but little is known about the breeding system of Rehmannia.We studied the floral syndroma and reproductive characteristics of the five Rehmannia species (R.chingii,R.piasezkii,R.henryi,R.glutinosa,R.solanifolia) through investigating the following indices in field:flowering dynamics of natural populations,floral morphologies of flower organs,nectar volume and sugar concentration,out-crossing index,pollen-ovule ratio,pollination and bagging experiment.Our results indicate that:(1)Rehmannia species flowers are hermaphroditic.The flowering duration of a single florescence was 5-7 days,and the whole population flowering duration was 40-60 days.There are distinct difference among the five Rehmannia species in inflorescence types,floral characteristics,flower color,pollen amounts and P/O values.We found that R.solanifolia were pollen abortion.(2)The evidence of the out-crossing index,pollen-ovule ratio and hybridization experiment supported that the breeding system of Rehmannia was belonged to cross pollination system,partially self-compatible,with no apomictic ability,and pollinators required.We also found there was vegetative reproduction in Rehmannia,especially in R.glutinosa and R.solanifolia.(3)There are nectaries on the base of Rehmannia corolla and they can produce nectar with higher concentration of sugar,and this also supports the existence of pollinators.However,we did not observe any effective pollinators in Rehmannia species except in R.glutinosa with bees,which might be due to the scattered individual,isolated habitats and the bad weather conditions during pollination observation.Longer flowering duration could compensate for the insufficiency of pollinator visits.According to our results,we suppose that the different flowers display of five Rehmannia species possibly attracting distinct pollinators,together with genetic,geographic and ecological isolation,results in the differentiation of the breeding system in Rehmannia species.