Measurement and interpretation of the oxygen isotope composition of carbon dioxide respired by leaves in the dark

We measured the oxygen isotope composition (delta(18)O) of CO2 respired by Ricinus communis leaves in the dark. Experiments were conducted at low CO2 partial pressure and at normal atmospheric CO2 partial pressure. Across both experiments, the delta(18)O of dark-respired CO2 (delta(R)) ranged from 44parts per thousand to 324parts per thousand. (Vienna Standard Mean Ocean Water scale). This seemingly implausible range of values reflects the large flux of CO2 that diffuses into leaves, equilibrates with leaf water via the catalytic activity of carbonic anhydrase, then diffuses out of the leaf, leaving the net CO2 efflux rate unaltered. The impact of this process on delta(R) is modulated by the delta(18)O difference between CO2 inside the leaf and in the air, and by variation in the CO2 partial pressure inside the leaf relative to that in the air. We developed theoretical equations to calculate delta(18)O of CO2 in leaf chloroplasts (delta(c)), the assumed location of carbonic anhydrase activity, during dark respiration. Their application led to sensible estimates of delta(c), suggesting that the theory adequately accounted for the labeling of CO2 by leaf water in excess of that expected from the net CO2 efflux. The delta(c) values were strongly correlated with delta(18)O of water at the evaporative sites within leaves. We estimated that approximately 80% of CO2 in chloroplasts had completely exchanged oxygen atoms with chloroplast water during dark respiration, whereas approximately 100% had exchanged during photosynthesis. Incorporation of the delta(18)O of leaf dark respiration into ecosystem and global scale models of (COO)-O-18 dynamics could affect model outputs and their interpretation.