Abstract:
The ratio of 18O/16O in atmospheric oxygen depends on the isotopic composition of the substrate water used in photosynthesis and on discrimination against 18O in respiratory consumption. The current understanding of the composition of air O2 attributes the magnitude of the respiratory fractionation to biochemical mechanisms alone. Thus the discrimination against 18O is assumed as 18%% in normal dark respiration and 25%% to 30%% in cyanide resistant respiration. Here we report new results on the fractionation of O2 isotopes in root respiration. The isotopic fractionation was determined from the change in δ18O of air due to partial uptake by roots in closed containers. The discrimination in these experiments was in the range of 11.9%% to 20.0%% with an average of 14.5%%. 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 O2 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 18O is less than generally thought indicates that the mechanisms affecting δ18O of atmospheric oxygen should be re-evaluated.