Internal Conductance to CO2 Diffusion and C18OO Discrimination in C3 Leaves

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Abstract
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 C18OO during photosynthesis (Δ18O). By constraining the δ18O of chloroplast water (δe) by analysis of transpired water and the extent of CO2-H2O isotopic equilibrium (θeq) by measurements of CA activity (θeq = 0.75–1.0 for tobacco, soybean, and oak), we could apply measured Δ18O 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 cs). From the CO2 drawdown sequence between stomatal cavities from gas exchange (c i), from Δ18O (c cs), and at Rubisco sites from Δ13C (c c), the internal CO2 conductance (g i) was partitioned into cell wall (g w) and chloroplast (g ch) components. The results indicated that g 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.