ISOTOPE COMPOSITION (O-18, H-2) OF TRANSPIRED WATER-VAPOR - CONTROLLED EXPERIMENT

Delta-H-2 and delta-O-18 measurements were performed to study the relations between the transpiration process and the increased enrichment observed (i) in the remaining water in the leaves exposed to the light and (ii) in the water vapour provided by transpiration (where simultaneous measurements were made on quantitatively condensed water and gaseous phase). This study was performed under controlled and monitored environment conditions (C2 3A climatic chamber, Department of Radioagronomy, CEA, Cadarache, France). The atmosphere of the climatic chamber was supplied with vapour originated from the vegetation. When the leaf water reached an isotopic steady-state, the transpiration flux produces an isotopic enrichment in the vicinity of the leaf by returning to the atmosphere a vapour of which the isotopic composition is identical to that of the source water. As a result, the isotopic composition of the vapour in the surrounding environment of the canopy becomes progressively different from that of the regional vapour generated by the evaporation in the oceanic area.