Carbon Isotope Fractionation during Photorespiration and Carboxylation in Senecio.

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 (Δ_obs) with discrimination predicted from gas-exchange measurements (Δ_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 Δ_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 Δ 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^-1 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. [ABSTRACT FROM AUTHOR]