An experimental evaluation of the use of C3 dC plant tissue as a proxy for the paleoatmospheric dCO2 signature of air

[1] Previous work suggests that the relationship between the carbon isotope composition of air (dCa) and plant leaf tissue (dCp) can be used to track changes in the carbon isotope composition of paleo-atmospheric CO2. Here we test this assertion in a series of experiments using the model plant Arabidopsis thaliana grown under a range of atmospheric CO2 concentrations relevant to geologic time (380, 760, 1000, 1500, 2000 and 3000 ppm). Nested within these CO2 experiments water availability was controlled (giving two sets of experimental plants; low and high water treatment at each CO2 concentration) to manipulate stomatal opening, a key process governing carbon fixation and isotope discrimination. Results show a highly significant relationship between dCa and d Cp under both experimental water treatments. To test the utility of d Cp to predict dCa we compare calculated d Ca to measured values of d Ca. These data show that although there is a significant relationship between calculated and measured dCa, there is disparity between the two values of dCa and a large difference between calculated values under different water treatments even when grown in a common CO2 concentration. These results demonstrate that environmental factors that alter stomatal opening can severely impact on the use and reliability of dCp to predict d Ca and as such, results should be interpreted with caution.