The effect of CO2 concentration on carbon isotope discrimination during photosynthesis in Ginkgo biloba: implications for reconstructing atmospheric CO2 levels in the geologic past

Some experiments and observations of free-living plants have found that increasing atmospheric concentration of CO2 (pCO2) is directly correlated with increasing discrimination against 13C during photosynthesis (Δ13C) in C3 plants. The inverted form of this correlation has been used to estimate pCO2 in the geological past (i.e. the C3 plant proxy), but there has been little experimental work to establish the relative importance of pCO2 as a driver of discrimination in more natural settings and over a range of pCO2 relevant to the deep-time geologic record. Here we report on an experiment exploring the relationship between pCO2 and Δ13C 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 pCO2 (∼425 ppm) and elevated levels (∼600, ∼800, and ∼ 1000 ppm) while measuring the carbon isotope composition of air (δ13Cair) and leaves (δ13Cleaf) 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 pCO2 and Δ13Cleaf or ci/ca. We did find a direct correlation of pCO2 with Amax, LMA, and C:N ratio. The lack of increase in Δ13Cleaf with rising pCO2 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 Δ13Cleaf in many different types of C3 plants, suggest that Δ13Cleaf does not consistently increase with increasing pCO2. 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 pCO2. Given the complex relationship between Δ13Cleaf and pCO2 in living plants we consider Δ13Cleaf of fossil plants to be an unreliable proxy for paleo-atmospheric pCO2.