A Transgenic Approach to Understanding the Influence of Carbonic Anhydrase on C¹⁸OO Discrimination during C₄ Photosynthesis
2006
Cousins, Asaph B. | Badger, Murray R. | Von Caemmerer, S (Susanna)
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 ({theta}). 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 (gs) until CA levels were less than 20% of wild type. The values of {theta} determined from measurements of Δ¹⁸O and the ¹⁸O isotopic composition of leaf water at the site of evaporation (δe) were low in the wild-type F. bidentis and decreased in transgenic plants with reduced levels of CA activity. Measured values of {theta} were always significantly lower than the values of {theta} 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 {theta} during C₄ photosynthesis. Furthermore, uncertainties in the isotopic composition of water at the site of exchange may also limit the ability to accurately predict {theta} in C₄ plants.
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