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BIOENERGETICS AND PHOTOSYNTHESIS

A Transgenic Approach to Understanding the Influence of Carbonic Anhydrase on C¹⁸OO Discrimination during C₄ Photosynthesis¹

Molecular Plant Physiology Group (A.B.C., M.R.B., S.v.C.) and ARC Centre of Excellence in Plant Energy Biology (M.R.B.), Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia

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 (CA_leaf) has been used to test whether changing leaf CA activity influences online measurements of C¹⁸OO discrimination (¹⁸O) and the proportion of CO₂ in isotopic equilibrium with leaf water at the site of oxygen exchange (). The ¹⁸O 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 ¹⁸O. Additionally, ¹⁸O was sensitive to small decreases in CA_leaf. 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 determined from measurements of ¹⁸O and the ¹⁸O isotopic composition of leaf water at the site of evaporation (_e) were low in the wild-type F. bidentis and decreased in transgenic plants with reduced levels of CA activity. Measured values of were always significantly lower than the values of 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 during C₄ photosynthesis. Furthermore, uncertainties in the isotopic composition of water at the site of exchange may also limit the ability to accurately predict in C₄ plants.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Susanne von Caemmerer (susanne.caemmerer{at}anu.edu.au).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.085167

^* Corresponding author; e-mail asaph.cousins{at}anu.edu.au; fax 61–2–61255075.

Received June 15, 2006; accepted August 1, 2006; published August 18, 2006.

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