Naturally low carbonic anhydrase activity in C₄ and C₃ plants limits discrimination against C¹⁸OO during photosynthesis

The ¹⁸O content of CO₂ is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO₂ and ¹⁸O-enriched leaf water and retrodiffusion of most of this CO₂ back to the atmosphere, leaves effectively discriminate against ¹⁸O during photosynthesis. Discrimination against ¹⁸O (Δ¹⁸O) is expected to be lower in C₄ plants because of low c_i and hence low retrodiffusing CO₂ flux. C₄ plants also generally show lower levels of carbonic anhydrase (CA) activities than C₃ plants. Low CA may limit the extent of ¹⁸O exchange and further reduce Δ¹⁸O. We investigated CO₂–H₂O isotopic equilibrium in plants with naturally low CA activity, including two C₄ (Zea mays, Sorghum bicolor) and one C₃ (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ¹⁸O in C₄, as well as in some C₃, plants. Variations in CA activity and in the extent of CO₂–H₂O isotopic equilibrium (θ_eq) estimated from on-line measurements of Δ¹⁸O showed large range of 0–100% isotopic equilibrium (θ_eq= 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO₂ concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ¹⁸O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ_eq< 1 (incomplete isotopic equilibrium) on ¹⁸O of atmospheric CO₂ can be similar for C₃ and C₄ plants and in both cases it increases with natural enrichment of ¹⁸O in leaf water.