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Plant, Cell and Environment (2014) 37, 1494–1498
doi: 10.1111/pce.12346
Commentary
Advances in measurements and models of photosynthetic carbon isotope discrimination in C3 plants The scientific community has long recognized that the study of atmosphere–biosphere interactions is vital to understanding fields as varied as agriculture, ecology and global change. Measurements of CO2 and H2O exchange are potentially enhanced by measurements of the minor isotopologues of those species (e.g. 13C16O2), because of the extra information revealed about mechanisms limiting the fluxes. In the past, it was difficult to add the minor isotopologues to these ecosystem fluxes because samples had to be analysed in an isotope ratio mass spectrometer in a laboratory setting. Nowadays, optical isotope analysers (lasers and Fourier transform infrared spectroscopy) allow for rapid measurement of isotope ratios with sufficient precision, and in field settings. This has motivated a series of innovative studies both in the laboratory and in the field that combine measurements and models of photosynthetic fluxes and isotope exchange. Absorption spectroscopy methods have increased the rate at which measurements can be taken in the laboratory, allowing us to gain a better understanding of the dynamic responses of mesophyll conductance (Tazoe et al. 2011; Evans & von Caemmerer 2013), value for photorespiratory fractionation (Evans & von Caemmerer 2013), respiratory effects and their impact on photosynthetic discrimination calculations (Barbour et al. 2007; Stutz et al. 2014) and calculations of leakiness in C4 species (Ubierna et al. 2011). Field applications that traditionally have used isotope records from atmospheric flask samples or tree ring chronologies will benefit from the large data sets that are starting to become available with optical isotope analysers. Example of these are isotope-constrained carbon budgets (Ballantyne et al. 2010), partitioning of oceanic and terrestrial fluxes (Ciais et al. 1995; Fung et al. 1997), or confirmation of simulations of 13C discrimination by the terrestrial biosphere (Suits et al. 2005). Some other examples of field applications that have already used optical analysers include partitioning of net ecosystem exchange (Bowling et al. 2001; Zobitz et al. 2008) and the testing of the predictive ability of different discrimination models (Wingate et al. 2007; Bickford et al. 2009). In this issue of Plant, Cell & Environment, Gentsch et al. (2014) collect the largest available field data set of instantaneous measurements of observed discrimination (60 d of records with a half-hour interval) and use it to test different formulations of the 13C photosynthetic discrimination model (Δ13C). They demonstrate that both simple and ‘comprehensive’ models of Δ13C were able to predict flux-weighted discrimination equally well. Herein, we build upon the work of Gentsch et al. Correspondence: G. D. Farquhar. Fax: +61 2 6125 4919; e-mail: [email protected] 1494
(2014), comment on the errors that may result when using different formulations of Δ13C and give some recommendations about what formulation should be chosen for different applications.
SIMPLIFIED VERSUS ‘COMPREHENSIVE’ MODELS OF DISCRIMINATION One common debate in these kinds of studies is the degree of complexity that should be included in the mathematical models of 13C photosynthetic discrimination (Δ13C).The original model for Δ13C (Farquhar et al. 1982b),extended to include ternary effects (Farquhar & Cernusak 2012), includes several parameters that are difficult to measure or estimate and therefore often sacrificed for easier formulations (Fig. 1a). In the last few years, as the reliability of data and of parameter values has improved, detailed studies have shifted from the use of the very simplified formulation to the use of the full equation. To illustrate the impact of Rubisco fractionation, stomatal conductance, mesophyll conductance, respiration and photorespiration on total Δ13C, we divide the ‘comprehensive’ (thus far) photosynthetic discrimination model for C3 plants into the following components:
Δ com = Δ b − Δ gs − Δ gm − Δ e − Δ f
(1)
where the detailed equations for Δb, Δgs, Δgm, Δe and Δf are given in Fig. 1b. This division is different from what was applied in Evans & von Caemmerer (2013), which lumped together the first two terms Δb − Δgs. We prefer to keep them separated because then the magnitude of the stomatal (Δgs) and mesophyll (Δgm) contributions to Δcom are directly comparable. We calculated these five components of discrimination in four scenarios resulting from the combination of high or low photosynthetic rate (A = 20 or 3 μmol m−2 s−1) with large or small mesophyll conductance (gm = 0.5 or 0.1 mol m−2 s−1; Fig. 2). Stomatal conductance (gs), day respiration (Rd) and transpiration (E) rates were kept constant at 0.3 mol m−2 s−1, 1 μmol m−2 s−1 and 4 mmol m−2 s−1, respectively. All other parameters involved in the calculation of Δ13C from theory were either derived from these previous values or from known constants. Figure 2 illustrates that the largest moderators of Rubisco fractionation are Δgs and Δgm. When both A and gm were large, the diminutions of Δb (Rubisco fractionation) by gs and gm were 8 and 3‰, respectively. Under current ambient conditions [21% O2, [CO2] = 400 μmol mol−1], photorespiration represents a constant contribution of 1.1‰ at 25 °C. The effect of day respiration is modulated by the ratio © 2014 John Wiley & Sons Ltd
Photosynthetic carbon isotope discrimination in C3 plants Rd/(A + Rd). Thus, the respiratory contribution to discrimination is larger whenever Rd is large in proportion to A. The estimated value of fractionation during day respiration, e, varies between 0 and −5‰ (Tcherkez et al. 2004, 2010, 2011). In our example with e = −5‰, Δe reached a contribution to total Δ13C of −1.1‰ when A was low. Occasionally, a much larger apparent respiratory fractionation can occur when the substrate for respiration has a very different δ13C value from that of recent assimilates.This typically occurs in experiments that use a depleted tank for gas exchange/isotope measurements or when there is a shift in respiratory substrates, for example, at dusk or dawn (Wingate et al. 2007; Gentsch et al. 2014; Stutz et al. 2014). Nevertheless when weighted by carbon assimilation, under most conditions the respiratory contribution to daytime discrimination will be small and negligible. Notice (Fig. 2) that photorespiration and respiration contributions to total discrimination partially cancel each other out because of the opposite sign of the fractionation factors f (positive) and e (negative). If the terms Δb–Δgs are lumped together and ternary and boundary layer conductance effects are considered negligible, it results in the familiar, simple expression of Δsim [= as + (b − as)Ci/Ca]. If the value of b in Δsim is reduced (let us call it b ) to account for the drop in discrimination by Δgm, then Δcom is essentially approximated by Δ sim − b , with the remaining error (
Plant, Cell and Environment (2014) 37, 1494–1498
doi: 10.1111/pce.12346
Commentary
Advances in measurements and models of photosynthetic carbon isotope discrimination in C3 plants The scientific community has long recognized that the study of atmosphere–biosphere interactions is vital to understanding fields as varied as agriculture, ecology and global change. Measurements of CO2 and H2O exchange are potentially enhanced by measurements of the minor isotopologues of those species (e.g. 13C16O2), because of the extra information revealed about mechanisms limiting the fluxes. In the past, it was difficult to add the minor isotopologues to these ecosystem fluxes because samples had to be analysed in an isotope ratio mass spectrometer in a laboratory setting. Nowadays, optical isotope analysers (lasers and Fourier transform infrared spectroscopy) allow for rapid measurement of isotope ratios with sufficient precision, and in field settings. This has motivated a series of innovative studies both in the laboratory and in the field that combine measurements and models of photosynthetic fluxes and isotope exchange. Absorption spectroscopy methods have increased the rate at which measurements can be taken in the laboratory, allowing us to gain a better understanding of the dynamic responses of mesophyll conductance (Tazoe et al. 2011; Evans & von Caemmerer 2013), value for photorespiratory fractionation (Evans & von Caemmerer 2013), respiratory effects and their impact on photosynthetic discrimination calculations (Barbour et al. 2007; Stutz et al. 2014) and calculations of leakiness in C4 species (Ubierna et al. 2011). Field applications that traditionally have used isotope records from atmospheric flask samples or tree ring chronologies will benefit from the large data sets that are starting to become available with optical isotope analysers. Example of these are isotope-constrained carbon budgets (Ballantyne et al. 2010), partitioning of oceanic and terrestrial fluxes (Ciais et al. 1995; Fung et al. 1997), or confirmation of simulations of 13C discrimination by the terrestrial biosphere (Suits et al. 2005). Some other examples of field applications that have already used optical analysers include partitioning of net ecosystem exchange (Bowling et al. 2001; Zobitz et al. 2008) and the testing of the predictive ability of different discrimination models (Wingate et al. 2007; Bickford et al. 2009). In this issue of Plant, Cell & Environment, Gentsch et al. (2014) collect the largest available field data set of instantaneous measurements of observed discrimination (60 d of records with a half-hour interval) and use it to test different formulations of the 13C photosynthetic discrimination model (Δ13C). They demonstrate that both simple and ‘comprehensive’ models of Δ13C were able to predict flux-weighted discrimination equally well. Herein, we build upon the work of Gentsch et al. Correspondence: G. D. Farquhar. Fax: +61 2 6125 4919; e-mail: [email protected] 1494
(2014), comment on the errors that may result when using different formulations of Δ13C and give some recommendations about what formulation should be chosen for different applications.
SIMPLIFIED VERSUS ‘COMPREHENSIVE’ MODELS OF DISCRIMINATION One common debate in these kinds of studies is the degree of complexity that should be included in the mathematical models of 13C photosynthetic discrimination (Δ13C).The original model for Δ13C (Farquhar et al. 1982b),extended to include ternary effects (Farquhar & Cernusak 2012), includes several parameters that are difficult to measure or estimate and therefore often sacrificed for easier formulations (Fig. 1a). In the last few years, as the reliability of data and of parameter values has improved, detailed studies have shifted from the use of the very simplified formulation to the use of the full equation. To illustrate the impact of Rubisco fractionation, stomatal conductance, mesophyll conductance, respiration and photorespiration on total Δ13C, we divide the ‘comprehensive’ (thus far) photosynthetic discrimination model for C3 plants into the following components:
Δ com = Δ b − Δ gs − Δ gm − Δ e − Δ f
(1)
where the detailed equations for Δb, Δgs, Δgm, Δe and Δf are given in Fig. 1b. This division is different from what was applied in Evans & von Caemmerer (2013), which lumped together the first two terms Δb − Δgs. We prefer to keep them separated because then the magnitude of the stomatal (Δgs) and mesophyll (Δgm) contributions to Δcom are directly comparable. We calculated these five components of discrimination in four scenarios resulting from the combination of high or low photosynthetic rate (A = 20 or 3 μmol m−2 s−1) with large or small mesophyll conductance (gm = 0.5 or 0.1 mol m−2 s−1; Fig. 2). Stomatal conductance (gs), day respiration (Rd) and transpiration (E) rates were kept constant at 0.3 mol m−2 s−1, 1 μmol m−2 s−1 and 4 mmol m−2 s−1, respectively. All other parameters involved in the calculation of Δ13C from theory were either derived from these previous values or from known constants. Figure 2 illustrates that the largest moderators of Rubisco fractionation are Δgs and Δgm. When both A and gm were large, the diminutions of Δb (Rubisco fractionation) by gs and gm were 8 and 3‰, respectively. Under current ambient conditions [21% O2, [CO2] = 400 μmol mol−1], photorespiration represents a constant contribution of 1.1‰ at 25 °C. The effect of day respiration is modulated by the ratio © 2014 John Wiley & Sons Ltd
Photosynthetic carbon isotope discrimination in C3 plants Rd/(A + Rd). Thus, the respiratory contribution to discrimination is larger whenever Rd is large in proportion to A. The estimated value of fractionation during day respiration, e, varies between 0 and −5‰ (Tcherkez et al. 2004, 2010, 2011). In our example with e = −5‰, Δe reached a contribution to total Δ13C of −1.1‰ when A was low. Occasionally, a much larger apparent respiratory fractionation can occur when the substrate for respiration has a very different δ13C value from that of recent assimilates.This typically occurs in experiments that use a depleted tank for gas exchange/isotope measurements or when there is a shift in respiratory substrates, for example, at dusk or dawn (Wingate et al. 2007; Gentsch et al. 2014; Stutz et al. 2014). Nevertheless when weighted by carbon assimilation, under most conditions the respiratory contribution to daytime discrimination will be small and negligible. Notice (Fig. 2) that photorespiration and respiration contributions to total discrimination partially cancel each other out because of the opposite sign of the fractionation factors f (positive) and e (negative). If the terms Δb–Δgs are lumped together and ternary and boundary layer conductance effects are considered negligible, it results in the familiar, simple expression of Δsim [= as + (b − as)Ci/Ca]. If the value of b in Δsim is reduced (let us call it b ) to account for the drop in discrimination by Δgm, then Δcom is essentially approximated by Δ sim − b , with the remaining error (
