Leaf scale quantification of the effect of photosynthetic gas exchange on Δ47 of CO2

Abstract

The clumped isotope composition (Δ47, the anomaly of the mass 47 isotopologue relative to the abundance expected from a random isotope distribution) of CO2 has been suggested as an additional tracer for gross CO2 fluxes. However, the effect of photosynthetic gas exchange on Δ47 has not been directly determined and two indirect/conceptual studies reported contradicting results. In this study, we quantify the effect of photosynthetic gas exchange on Δ47 of CO2 using leaf cuvette experiments with one C4 and two C3 plants. The experimental results are supported by calculations with a leaf cuvette model. Our results demonstrate the important roles of the Δ47 value of CO2 entering the leaf, kinetic fractionation as CO2 diffuses into, and out of the leaf and CO2–H2O isotope exchange with leaf water. We experimentally confirm the previously suggested dependence of Δ47 of CO2 in the air surrounding a leaf on the stomatal conductance and back-diffusion flux. Gas exchange can enrich or deplete the Δ47 of CO2 depending on the Δ47 of CO2 entering the leaf and the fraction of CO2 exchanged with leaf water and diffused back to the atmosphere, but under typical ambient conditions, it will lead to a decrease in Δ47.

Exploring the potential of Δ17O in CO2 for determining mesophyll conductance

“…Our estimates of g m18 for sunflower are in good agreement with values reported in previous studies ( Shrestha et al 2019 ; Adnew et al 2021 ). For maize, g m18 = 0.31 mol m −2 s −1 bar −1 is within the wide range of 0.169 to 0.9 mol m −2 s −1 bar −1 reported in the literature ( Flexas et al 2012 ; Ubierna et al 2017 , 2018 ; Kolbe and Cousins 2018 ; Adnew et al 2021 ; Crawford and Cousins 2021 ). However, Barbour et al (2016) and Gillon and Yakir (2000a) reported even higher g m18 values for maize, 1.78 mol m −2 s −1 bar −1 and 1.0 mol m −2 s −1 bar −1 , respectively.…”

“…For g m18 , the P -value between LL and HL experiments is 0.493, 0.897, and 0.286 for sunflower, maize, and ivy, respectively. This is similar to our earlier results in Adnew et al (2021) , where g m18 of sunflower did not show a light intensity effect between 200 and 1,500 µ mol m −2 s −1 and Ogée et al (2018) where g m18 did not show a significant ( P > 0.5) change with an irradiance change from 150 to 1,500 µ mol m −2 s −1 for Flaveria bidentis and tobacco ( Nicotiana tabacum ) leaves.…”

“…Both Δ A 18 O and Δ A Δ 17 O are not intrinsic properties of plant CO 2 uptake, but are strongly dependent on the difference between the oxygen isotope composition of leaf water and of the CO 2 entering the leaf ( Cousins et al 2006 ; Holloway-Phillips et al 2019 ; Adnew et al 2020 ). The fractionation associated with the initial fixation by the enzyme RubiscO (ribulose-1,5-bisphosphate carboxylase-oxygenase) or PEP (phosphoenolpyruvate) has no/negligible effect on the δ 18 O and Δ 17 O value of CO 2 ( Farquhar and Lloyd 1993 ; Adnew et al 2021 ). As a result, Δ A 18 O and Δ A Δ 17 O can be positive, 0, or negative depending on the oxygen isotope composition of the CO 2 entering the leaf and the isotope composition of leaf water ( Adnew 2020 ).…”

“…g m cannot be measured directly and its indirect determination is challenging ( Warren 2006 ; Pons et al 2009 ). Several techniques are used to estimate g m indirectly, including the variable J method ( Fabre et al 2007 ; Flexas et al 2007 ), the leaf anatomical method ( Tomás et al 2013 ), the curve-fitting method ( Ubierna et al 2017 ), the 13 C photosynthetic discrimination method ( Evans et al 1986 ), and the 18 O photosynthetic discrimination method ( Gillon and Yakir 2000a ; Barbour et al 2016 ; Adnew et al 2021 ). Details on these g m measurement techniques can be found in Pons et al (2009) and Cousins et al (2020) .…”

Temperature dependence of isotopic fractionation in the CO2‐O2 isotope exchange reaction

“…The CO 2 ‐O 2 exchange technique has been used in different laboratories to study the carbon cycle 1 , 2 , 3 , 4 , 6 , 22 , 23 and precise measurement of Δ′ 17 O is used to correct for biases in clumped isotope measurements of CO 2. 24 , 25 , 26 , 27 However, the fractionation factor in CO 2 ‐O 2 isotope exchange is not well established, and different values have been reported by different laboratories.…”