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Abstract
The magnitude of fractionation during photorespiration and the effect on net photosynthetic (13)C discrimination (Delta) were 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)) with discrimination predicted from gas-exchange measurements (Delta(pred)). Photorespiration rates were manipulated by altering the O(2) partial pressure (pO(2)) in the air surrounding the leaves. Contributions from (13)C-depleted photorespiratory CO(2) were largest at high pO(2). 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 Delta(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 Delta was achieved with f = 11.6 per thousand +/- 1.5 per thousand, b = 26.0 per thousand +/- 0.3 per thousand, 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 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 CO(2) exchange of ecosystems into gross fluxes of photosynthesis and respiration.
Highlights
The magnitude of fractionation during photorespiration and the effect on net photosynthetic 13C discrimination (D) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii
In C3 species, leaf level D during photosynthetic gas exchange primarily reflects the balance between CO2 supply by diffusion through stomata and CO2 demand by biochemical reactions in chloroplasts, most importantly catalysis by Rubisco (Farquhar et al, 1982)
The parameters f, b, and gi were treated as unknowns and determined simultaneously for all measurements. 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
Summary
The magnitude of fractionation during photorespiration and the effect on net photosynthetic 13C discrimination (D) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii. D13C data are used to study plant water use efficiency (Hobbie and Colpaert, 2004; Cernusak et al, 2008; Seibt et al, 2008) and respiration and secondary fractionation processes (Ghashghaie et al, 2003; Wingate et al, 2007; Bathellier et al, 2008) and to partition net ecosystem CO2 fluxes between photosynthesis and respiration (Bowling et al, 2001; Ogee et al., 2003; Zobitz et al, 2007) These applications require robust estimates of net 13C discrimination (D) during photosynthesis. F values of 7& (Rooney, 1988), 8& (Gillon, 1997), and 10& to 14& (Igamberdiev et al, 2004) have been reported from a limited number of in vivo experiments on intact leaves, with 11& expected from theory (Tcherkez, 2006)
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