[An attempt to interpret the kinetics of isotope exchange between C(18)O 2 and the water of a leaf: Experiments in the dark].

The interpretation of photosynthesis experiments using (18)O as a tracer becomes very difficult on account of the exchange rate between oxygen atoms of carbon dioxide and water. The mathematical analysis of the kinetics of isotope exchange between CO2 highly enriched with (18)O and the water of an aerial leaf kept in darkness allows the determination of two velocity coefficients, k e and k. k e, related to the gaseous CO2 diffusion, makes it possible to evaluate the CO2 flow between the gaseous phase and the leaf, as well as the diffusion resistance of the boundary layer and the leaf stomata. k, related to the CO2 hydration, gives the in situ enzymic activity of carbonic anhydrase. Measurements carried out on a great number of leaves show that k e and k vary in the same direction.In addition, a simple relation exists between the isotopic composition of the gaseous phase and the corresponding values of the C(18)O2 inside the leaf. Thus it is possible to determine the conditions for a leaf that may be favourable for light experiments designed to ascertain the origin of photosynthetic oxygen by use of (18)O.