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The oxygen isotope composition of atmospheric CO2 is an important signal that helps distinguish between ecosystem photosynthetic and respiratory processes. In C4 plants the carbonic anhydrase (CA)-catalyzed interconversion of CO2 and bicarbonate (HCO3  −) is an essential first reaction for C4 photosynthesis but also plays an important role in the CO2-H2O exchange of oxygen as it enhances the rate of isotopic equilibrium between CO2 and water. The C4 dicot Flaveria bidentis containing genetically reduced levels of leaf CA (CAleaf) has been used to test whether changing leaf CA activity influences online measurements of C18OO discrimination (Δ18O) and the proportion of CO2 in isotopic equilibrium with leaf water at the site of oxygen exchange (θ). The Δ18O in wild-type F. bidentis, which contains high levels of CA relative to the rates of net CO2 assimilation, was less than predicted by models of Δ18O. Additionally, Δ18O was sensitive to small decreases in CAleaf. However, reduced CA activity in F. bidentis had little effect on net CO2 assimilation, transpiration rates (E), and stomatal conductance (g  s) until CA levels were less than 20% of wild type. The values of θ determined from measurements of Δ18O and the 18O isotopic composition of leaf water at the site of evaporation (δ  e) were low in the wild-type F. bidentis and decreased in transgenic plants with reduced levels of CA activity. Measured values of θ were always significantly lower than the values of θ predicted from in vitro CA activity and gas exchange. The data presented here indicates that CA content in a C4 leaf may not represent the CA activity associated with the CO2-H2O oxygen exchange and therefore may not be a good predictor of θ during C4 photosynthesis. Furthermore, uncertainties in the isotopic composition of water at the site of exchange may also limit the ability to accurately predict θ in C4 plants.

A Transgenic Approach to Understanding the Influence of Carbonic Anhydrase on C18OO Discrimination during C4 Photosynthesis