Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis
The 18 O content of CO 2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO 2 and 18 O-enriched leaf water and retrodiffusion of most of this CO 2 back to the atmosphere, leaves effectively discriminate against 18 O during photosynthesis. Discrimination against 18 O (Δ 18 O) is expected to be lower in C 4 plants because of low c i and hence low retrodiffusing CO 2 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 18 O exchange and further reduce Δ 18 O. We investigated CO 2 -H 2 O 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 Δ 18 O in C 4 , as well as in some C 3 , plants. Variations in CA activity and in the extent of CO 2 -H 2 O isotopic equilibrium (θ eq ) estimated from on-line measurements of Δ 18 O 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 CO 2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ 18 O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ eq < 1 (incomplete isotopic equilibrium) on 18 O of atmospheric CO 2 can be similar for C 3 and C 4 plants and in both cases it increases with natural enrichment of 18 O in leaf water.