Internal Conductance to CO₂ Diffusion and $\text{C}{}^{18}\text{O}\text{O}$ Discrimination in C₃ Leaves

18O discrimination in CO2 stems from the oxygen exchange between 18O-enriched water and CO2 in the chloroplast, a process catalyzed by carbonic anhydrase (CA). A proportion of this 18O-labeled CO2 escapes back to the atmosphere, resulting in an effective discrimination against $\text{C}{}^{18}\text{O}\text{O}$ during photosynthesis ($\Delta {}^{18}\text{O}$). By constraining the δ 18O of chloroplast water ($\delta _{\text{e}}$) by analysis of transpired water and the extent of $\text{CO}_{2}-\text{H}_{2}\text{O}$ isotopic equilibrium ($\theta _{\text{eq}}$) by measurements of CA activity ($\theta _{\text{eq}}$ = 0.75-1.0 for tobacco, soybean, and oak), we could apply measured $\Delta {}^{18}\text{O}$ in a leaf cuvette attached to a mass spectrometer to derive the CO2 concentration at the physical limit of CA activity, i.e. the chloroplast surface ($c_{\text{cs}}$). From the CO2 drawdown sequence between stomatal cavities from gas exchange (ci), from $\Delta {}^{18}\text{O}$ ($c_{\text{cs}}$), and at Rubisco sites from $\Delta {}^{13}\text{C}$ ($c_{\text{c}}$), the internal CO2 conductance ($g_{\text{i}}$) was partitioned into cell wall ($g_{\text{w}}$) and chloroplast ($g_{\text{ch}}$) components. The results indicated that $g_{\text{ch}}$ is variable (0.42-1.13 mol m-2 s-1) and proportional to CA activity. We suggest that the influence of CA activity on the CO2 assimilation rate should be important mainly in plants with low internal conductances.