The effect of CO2 concentration on carbon isotope discrimination during photosynthesis in Ginkgo biloba: implications for reconstructing atmospheric CO2 levels in the geologic past. (15th November 2022)

• Record Type: Journal Article
• Authors: Scher, Mason A.; Barclay, Richard S.; Baczynski, Allison A.; Smith, Bryton A.; Sappington, James; Bennett, Lily A.; Chakraborty, Suvankar; Wilson, Jonathan P.; Megonigal, J. Patrick; Wing, Scott L.
• Is Part Of: Geochimica et cosmochimica acta. Volume 337(2022)
• Page Start: 82
• Page End: 94
• Publication Date: 2022-11-15
• Subjects: Carbon isotopes -- Discrimination -- Atmospheric CO2 concentration -- Paleo-pCO2 proxy; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2022.09.033
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Some experiments and observations of free-living plants have found that increasing atmospheric concentration of CO2 ( p CO2 ) is directly correlated with increasing discrimination against 13 C during photosynthesis (Δ 13 C) in C3 plants. The inverted form of this correlation has been used to estimate p CO2 in the geological past (i.e. the C3 plant proxy), but there has been little experimental work to establish the relative importance of p CO2 as a driver of discrimination in more natural settings and over a range of p CO2 relevant to the deep-time geologic record. Here we report on an experiment exploring the relationship between p CO2 and Δ 13 C in Ginkgo biloba, a plant long used to infer past CO2 levels because of the strong similarity of extant to fossil Ginkgo and the abundance of Ginkgo fossils with preserved cuticle from late Mesozoic and Cenozoic periods of warm global climate. We grew Ginkgo biloba plants for three years under ambient p CO2 (∼425 ppm) and elevated levels (∼600, ∼800, and ∼ 1000 ppm) while measuring the carbon isotope composition of air (δ 13 Cair ) and leaves (δ 13 Cleaf ) as well as the ratio of internal to external CO2 concentration ( ci /ca ), maximum photosynthetic assimilation rate ( Amax ), C:N ratio, and leaf mass per area (LMA). We found no significant relationship between p CO2 and Δ 13 Cleaf or ci /ca . We did find a direct correlation of p CO2 with Amax, LMA, and C:N ratio. The lack of increase in Δ 13 Cleaf with rising p CO2 may result from the lack of change in ci /ca, thicker leaves that slow the rate of diffusion of CO2 through the leaf to mesophyll cells, higher Amax that drives more rapid consumption of intracellular CO2 and/or changes in the relative proportions of starches, lipids or other compounds that have distinct isotopic compositions. Our results, along with a compilation of data from the literature on Δ 13 Cleaf in many different types of C3 plants, suggest that Δ 13 Cleaf does not consistently increase with increasing p CO2 . Rather, there is a diversity of responses, both positive and negative, that are not clearly related to taxonomic group or growth form but may reflect changes in leaf structure, stomatal response and Amax under higher p CO2 . Given the complex relationship between Δ 13 Cleaf and p CO2 in living plants we consider Δ 13 Cleaf of fossil plants to be an unreliable proxy for paleo-atmospheric p CO2 .