Carbonic anhydrase (CA,EC 4.2.1.1),catalyzing reversible hydration of CO2 to HCO3-,plays an important role in turbo-charge CO2 supply to photosynthetic organisms in plants.In order to understand the effect of CA on photosynthesis,four rice cultivars with different photosynthetic characteristics,i.e.,R49,N22,TXZ-25 and ITA312,were selected in the experiment,and their flag leaves at the filling stage (in the 5 DAF,15 DAF,and 25 DAF rice flag leaves) were used to study at the transcriptional,physiological and biochemical levels.The relative quantitative expression of OsCA gene,enzyme activity of CA protein and net photosynthetic rate were analyzed to reveal the relationship of CA and photosynthesis.Results showed that a significant correlation was found between the gene expression and enzyme activity of rice CA and the net photosynthetic rate.The CA enzyme activity and its gene expression quantity existed a cultivar-specific difference,and the corresponding values of R49 were highest among the four cultivars at the same stage.the CA enzyme activity showed a similar dynamic trend of a rise first followed by a decline with grouting process,of which the value reached a maximum at the middle filling stage (15 DAF); and the changes on net photosynthetic rate of the different cultivars were consistent,showing a trend of gradual decline.Therefore,the changes of OsCA gene transcripts and enzyme activity values can be considered as an indicator for physiological status to test the stability of leaf and/or to identify cultivar-specific difference in rice.