Figure 3.

The δ¹⁸O of chloroplast CO₂ plotted against the δ¹⁸O of water at evaporative sites in R. communis leaves during photosynthesis. In this experiment, air entering the leaf chamber had an average CO₂ partial pressure of 833 μbar, and air exiting the chamber had an average CO₂ partial pressure of 363 μbar. Irradiance varied from 300 to 800 μmol PAR m⁻² s⁻¹, and chamber air temperature varied between 25 and 30°C. The δ¹⁸O of chloroplast CO₂ was calculated as described in the theory section of the main text. The broken line on the graph represents the relationship expected if chloroplast CO₂ were in full oxygen isotope equilibrium with water at the evaporative sites. The δ¹⁸O values are presented relative to VSMOW. The δ¹⁸O of irrigation water fed to the plants was −7.2‰. The arrow on the graph indicates an outlying datum that strongly influenced the slope of the regression between δ_c and δ_e. Excluding this datum resulted in a slope between δ_c and δ_e of 1.10.