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

The O-18 content of CO2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO2 and O-18-enriched leaf water and retrodiffusion of most of this CO2 back to the atmosphere, leaves effectively discriminate against O-18 during photosynthesis. Discrimination against O-18 (Delta O-18) is expected to be lower in C-4 plants because of low ci and hence low retrodiffusing CO2 flux. C-4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C-3 plants. Low CA may limit the extent of O-18 exchange and further reduce Delta O-18. We investigated CO2-H2O isotopic equilibrium in plants with naturally low CA activity, including two C-4 (Zea mays, Sorghum bicolor) and one C-3 (Phragmites australis)species. The results confirmed experimentally the occurrence of low Delta O-18 in C-4, as well as in some C-3, plants. Variations in CA activity and in the extent of CO2-H2O isotopic equilibrium (theta (eq)) estimated from on-line measurements of Delta O-18 showed large range of 0-100% isotopic equilibrium (theta (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 Delta O-18 as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of theta (eq) <1 (incomplete isotopic equilibrium),on O-18 of atmospheric CO2 can be similar for C-3 and C-4 plants and in both cases it increases with natural enrichment of O-18 in leaf water.