Fractionation of oxygen isotopes by root respiration:: Implications for the isotopic composition of atmospheric O2

The ratio of O-18/O-16 in atmospheric oxygen depends on the isotopic composition of the substrate water used in photosynthesis and on discrimination against O-18 in respiratory consumption. The current understanding of the composition of air O-2 attributes the magnitude of the respiratory fractionation to biochemical mechanisms alone. Thus the discrimination against O-18 is assumed as 18 parts per thousand in normal dark respiration and 25 parts per thousand to 30 parts per thousand in cyanide resistant respiration. Here we report new results on the fractionation of O-2 isotopes in root respiration. The isotopic fractionation was determined from the change in delta O-18 of air due to partial uptake by roots in closed containers. The discrimination in these experiments was in the range of 11.9 parts per thousand to 20.0 parts per thousand with an average of 14.5 parts per thousand. This average is significantly less than the known discrimination in dark respiration. A simple diffusion-respiration model was used to explain the isotopic discrimination in roots. Available data show that O-2 concentration inside roots is low due to slow diffusion. As a result, due to diffusion and biological uptake at the consumption site inside the root, the overall discrimination is small. Root respiration is an important component of the global oxygen uptake. Our new result that the discrimination against O-18 is less than generally thought indicates that the mechanisms affecting delta O-18 of atmospheric oxygen should be re-evaluated. Copyright (C) 2001 Elsevier Science Ltd.