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 CO 2 (p CO 2) 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 p CO 2 in the geological past (i.e. the C3 plant proxy), but there has been little experimental work to establish the relative importance of p CO 2 as a driver of discrimination in more natural settings and over a range of p CO 2 relevant to the deep-time geologic record. Here we report on an experiment exploring the relationship between p CO 2 and Δ13C in Ginkgo biloba , a plant long used to infer past CO 2 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 CO 2 (∼425 ppm) and elevated levels (∼600, ∼800, and ∼ 1000 ppm) while measuring the carbon isotope composition of air (δ13C air) and leaves (δ13C leaf) as well as the ratio of internal to external CO 2 concentration (c i /c a), maximum photosynthetic assimilation rate (A max), C:N ratio, and leaf mass per area (LMA). We found no significant relationship between p CO 2 and Δ13C leaf or c i /c a. We did find a direct correlation of p CO 2 with A max , LMA, and C:N ratio. The lack of increase in Δ13C leaf with rising p CO 2 may result from the lack of change in c i /c a , thicker leaves that slow the rate of diffusion of CO 2 through the leaf to mesophyll cells, higher A max that drives more rapid consumption of intracellular CO 2 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 Δ13C leaf in many different types of C3 plants, suggest that Δ13C leaf does not consistently increase with increasing p CO 2. 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 A max under higher p CO 2. Given the complex relationship between Δ13C leaf and p CO 2 in living plants we consider Δ13C leaf of fossil plants to be an unreliable proxy for paleo-atmospheric p CO 2. [ABSTRACT FROM AUTHOR]