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WHOLE PLANT AND ECOPHYSIOLOGY

Carbon Isotope Fractionation during Photorespiration and Carboxylation in Senecio^1,[W],[OA]

Physiological Ecology Group, Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom

The magnitude of fractionation during photorespiration and the effect on net photosynthetic ¹³C discrimination () were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii. We determined the contributions of different processes during photosynthesis to by comparing observations (_obs) with discrimination predicted from gas-exchange measurements (_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 phosphoenolpyruvate carboxylase (b), and mesophyll conductance (g_i) were determined simultaneously for all measurements. Instead of using _obs data to obtain g_i and f successively, which requires that b is known, we treated b, f, and g_i as unknowns. We propose this as an alternative approach to analyze measurements under field conditions when b and g_i are not known or cannot be determined in separate experiments. Good agreement between modeled and observed was achieved with f = 11.6 ± 1.5, b = 26.0 ± 0.3, and g_i of 0.27 ± 0.01, 0.25 ± 0.01, 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 by about 1.2 on average under field conditions. In addition, diurnal changes in 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.

² Present address: Teagasc, Johnstown Castle Environmental Research Centre, Wexford, Ireland.

³ Present address: RPS Group, Willowmere House, Compass Point Business Park, Stocks Bridge Way, St. Ives PE27 6JL, United Kingdom.

⁴ Present address: UMR Bioemco, 78850 Thiverval-Grignon, Université Pierre et Marie Curie, Paris 6, France.

The author responsible for the 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: Ulli Seibt (useibt{at}dge.stanford.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.130153

^* Corresponding author; e-mail useibt{at}dge.stanford.edu.

Received September 20, 2008; accepted October 12, 2008; published October 15, 2008.

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