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Published on October 15, 2008; 10.1104/pp.108.130153

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Received September 20, 2008
Accepted October 12, 2008

Carbon isotope fractionation during photorespiration and carboxylation in Senecio

Gary J. Lanigan , Nicholas Betson , Howard Griffiths , and Ulli Seibt *

Physiological Ecology Group, Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK

* Corresponding author; email: useibt{at}dge.stanford.edu.

The magnitude of fractionation during photorespiration, and effect on net photosynthetic 13C discrimination ({Delta}), was investigated for three Senecio species, S. squalidus, S. cineraria and S. greyii. We determined the contributions of different processes during photosynthesis to {Delta} by comparing observations ({Delta}obs) to discrimination predicted from gas exchange measurements ({Delta}pred). Photorespiration rates were manipulated by altering the O2 partial pressure (pO2) in the air surrounding the leaves. Contributions from 13C depleted photorespiratory CO2 were largest at high pO2. The parameters for photorespiratory fractionation (f), net fractionation during carboxylation by Rubisco and PEPc (b), and mesophyll conductance (gi) were determined simultaneously for all measurements. Instead of using {Delta}obs data to obtain gi and f successively, which requires that b is known, we treated b, f and gi as unknowns. We propose this as alternative approach to analyse measurements under field conditions when b and gi are not known, or cannot be determined in separate experiments. Good agreement between modelled and observed {Delta} was achieved with f = 11.6±1.5{per thousand}, b = 26.0±0.3{per thousand}, and gi of 0.27, 0.25, and 0.22±0.01 mol m-2 s-1 for S. squalidus, S. cineraria, and S. greyii, respectively. We estimate that photorespiratory fractionation decreases {Delta} by about 1.2{per thousand} on average under field conditions. In addition, diurnal changes in {Delta} are likely to reflect variations in photorespiration even at the canopy level. Our results emphasize that the effects of photorespiration must be taken into account when partitioning net CO2 exchange of ecosystems into gross fluxes of photosynthesis and respiration.




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