Carbon isotope fractionation during photorespiration and carboxylation in Senecio

Abstract

The magnitude of fractionation during photorespiration and the effect on net photosynthetic 13C discrimination (Δ) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii. We determined the contributions of different processes during photosynthesis to Δ by comparing observations (Δobs) with discrimination predicted from gas-exchange measurements (Δpred). 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 (gi) were determined simultaneously for all measurements. Instead of using Δobs 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 an alternative approach to analyze measurements under field conditions when b and gi are not known or cannot be determined in separate experiments. Good agreement between modeled and observed Δ was achieved with f=11.6‰ ± 1.5‰, b=26.0‰ ± 0.3‰, and gi 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 Δ by about 1.2‰ on average under field conditions. In addition, diurnal changes in Δ 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. © 2008 American Society of Plant Biologists.