HR: 0830h
AN: B31E-0352    [PDF]
TI: Measurement of Discrimination Against $^{13}$C During Photosynthesis ($\Delta$$^{13}$C) and Quantification of the Short-Term Variability of $\Delta$$^{13}$C Over a Diurnal Cycle
AU: * Prater, J L
EM: jprater@ocean.fsu.edu
AF: Florida State University, Department of Oceanography, Tallahassee, FL 32306-4320 United States
AU: Chanton, J P
EM: chanton@ocean.fsu.edu
AF: Florida State University, Department of Oceanography, Tallahassee, FL 32306-4320 United States
AU: Mortazavi, B
EM: mortazavi@ocean.fsu.edu
AF: Florida State University, Department of Oceanography, Tallahassee, FL 32306-4320 United States
AB: We measured the net fractionation of photosynthesis ($\Delta$$^{13}$C) for a slash pine ($\it{Pinus}$ $\it{elliottii}$) through the analysis of eight consecutive air samples collected from a closed system while foliage contained within the system was undergoing photosynthesis. A model was applied to the series of samples to calculate the isotopic composition of the carbon removed and the resulting $\Delta$$^{13}$C. Raleigh distillation equations were built into the model to correct for the closed-system isotopic-dilution effects, and the results were compared to other techniques for calculating $\Delta$$^{13}$C. This technique was found to have higher resolution than the two-endmember models and provided the necessary tools for investigating short-term variability. Consecutive measurements were made over one daylight cycle to test the hypothesis that the observed variation in $\Delta$$^{13}$C would exceed the variation predicted by the established and frequently modeled relationship between c$_{i}$/c$_{a}$ and $\Delta$$^{13}$C because of the draw down of CO$_{2}$ concentrations at the chloroplast when photosynthetic rates are elevated. The results from this study show a good correlation between predicted $\Delta$$^{13}$C values from the c$_{i}$/c$_{a}$ model and the measured values except when photosynthetic rates were near the daily minimums and maximums, with the predicted value underestimating $\Delta$$^{13}$C when rates were very low and overestimating $\Delta$$^{13}$C when rates were high. Measurements of foliage-respired CO$_{2}$ $\delta$$^{13}$C were also made, and the values are consistently $^{13}$C depleted relative to the fixed carbon „$^{13}$C values, but show a mid-day $^{13}$C enrichment indicating recently fixed carbon as the major substrate for respiration.
DE: 0394 Instruments and techniques
DE: 0400 Biogeosciences
DE: 1694 Instruments and techniques
SC: Biogeosciences [B]
MN: 2003 Fall Meeting