Relationship between LRU and 18O discrimination (18Δ) in plants with different levels of CA activity (antisense lines). A, Mean values in C3 species (tobacco) with moderate and low levels of CA activity (percent of wild type). B, Data of C4 species (F. bidentis) with moderate CA activity. Values are based on light response experiments under high light intensity (1,889 μmol photon m−2 s−1) and all other conditions as in Figure 2. C, Schematic summary of the relationships between LRU and 18Δ values, indicating the possible factors underlying the observed changes based on data from A and B. CA deficiency eliminates COS uptake (LRU approaches zero) and reduces 18O discrimination (18Δ) in both C3 and C4 plants. Compared to C3 wild-type plants, the C4 wild-type (WT) species show a small decrease in LRU (lower As/Ac due to high Ac values in these plants) but a large decrease in 18Δ (PEPC in these plants limits 18O exchange and CO2-water isotopic equilibrium).