Figure 1.

The δ¹⁸O of chloroplast CO₂ plotted against the δ¹⁸O of water at evaporative sites in R. communis leaves during dark respiration. In this experiment, air entering the leaf chamber was free of CO₂, and the CO₂ partial pressure of air exiting the chamber averaged 47 μbar. The δ¹⁸O of chloroplast CO₂ was calculated from measurements of the δ¹⁸O of CO₂ exiting the chamber and gas exchange parameters, 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 evaporative sites. The δ¹⁸O values are presented relative to VSMOW. The δ¹⁸O of irrigation water fed to the plants was −7.2‰.