Carbonic anhydrase and its influence on carbon isotope discrimination during C4 photosynthesis.: Insights from antisense RNA in Flaveria bidentis

In C-4 plants, carbonic anhydrase (CA) facilitates both the chemical and isotopic equilibration of atmospheric CO 2 and bicarbonate (HCO3-) in the mesophyll cytoplasm. The CA-catalyzed reaction is essential for C-4 photosynthesis, and the model of carbon isotope discrimination (Delta(13)C) in C-4 plants predicts that changes in CA activity will influence Delta(13)C. However, experimentally, the influence of CA on Delta(13)C has not been demonstrated in C-4 plants. Here, we compared measurements of Delta(13)C during C-4 photosynthesis in Flaveria bidentis wild-type plants with F. bidentis plants with reduced levels of CA due to the expression of antisense constructs targeted to a putative mesophyll cytosolic CA. Plants with reduced CA activity had greater Delta(13)C, which was also evident in the leaf dry matter carbon isotope composition (delta(13)C). Contrary to the isotope measurements, photosynthetic rates were not affected until CA activity was less than 20% of wild type. Measurements of Delta(13)C, delta(13)C of leaf dry matter, and rates of net CO2 assimilation were all dramatically altered when CA activity was less than 5% of wild type. CA activity in wild- type F. bidentis is sufficient to maintain net CO 2 assimilation; however, reducing leaf CA activity has a relatively large influence on Delta(13)C, often without changes in net CO2 assimilation. Our data indicate that the extent of CA activity in C-4 leaves needs to be taken into account when using Delta(13)C and/or delta(13)C to model the response of C-4 photosynthesis to changing environmental conditions.