Homeostatic response of Aptian gymnosperms to changes in atmospheric CO2 concentrations

where a is the kinetic isotope effect associated with the diffusion of CO2 into stomatal pores (4.4 parts per thousand), b is the kinetic isotope effect caused by the enzyme rubisco (27 parts per thousand), and Ci is the leaf intercellular space CO2 concentration. Whereas some studies assume that Ci/Ca is constant, others state instead that 13 Delta leaf could serve as a proxy for Ca (Schubert and Jahren, 2012). For geological materials, 13 Delta leaf can be determined from the delta 13C of plant leaves (delta 13Cleaf) and atmospheric CO2 (delta 13Ca): The sensitivity of plant carbon isotope fractionation (13 Delta leaf) to changes in atmospheric CO2 concentrations (Ca) is the subject of heavy debate, with some studies finding no sensitivity, while others show a strong dependency. We tested the hypothesis of photosynthetic homeostasis by using delta 13C of n-alkanes, cuticles, and bulk organic matter of gymnosperm-rich rocks (Arundel Clay) from two sites deposited during the Aptian, a time that experienced significant Ca variations. Our results show no effect of Ca on 13 Delta leaf, and a relatively constant Ci/Ca (0.64 +/- 0.04, 1 sigma; i-intercellular space), a value that is similar to that of modern gymnosperms. These results suggest that Aptian gymnosperms used homeostatic adjustments with rising Ca, probably involving increased carbon assimilation and/or stomatal closure, a response also found in modern gymnosperms. The similarity between Aptian and modern gymnosperms suggests that the processes responsible for regulating CO2 and water vapor exchange during photosynthesis have remained unaltered in gymnosperms for the past 128 m.y.

Automated summary

The study explores the sensitivity of plant carbon isotope fractionation to changes in atmospheric CO2 concentrations, with some studies finding no effect while others show a strong dependency. Results suggest that Aptian gymnosperms utilized homeostatic adjustments with rising Ca, possibly involving increased carbon assimilation and/or stomatal closure. The similarity between Aptian and modern gymnosperms indicates that the processes regulating CO2 and water vapor exchange during photosynthesis have remained unchanged in gymnosperms over the past 128 million years.