ummary. The vapor pressure difference botween H~ 1so and tt~ l~O is the reason for the accumulation of the heavy molecule in transpiring leaves. Since photosynthcsis on land is the main sourco of atmospheric oxygen, this mechanism is important for the remarkable enrichment of iso in atmosphoric O 3 (Dole effect). Using a simple box model for transpiring lcaves a quantitative understanding of the isotope fractionation is possible which is woll con-firme4 by the results of model experiments as well as by measurements on trees. Maximum enrichment of H 2 iso in the water of leaves (relative to seil water) is 25 7~ (theoretically, for dxy air) and was foun4 untier natural conditions to be 21 ~ (for 28 % relative humidity); minimum theoretical enrichment is zero (observed 2.5 %°). IntroduetionThe atmospheric reservoir of free oxygen is involved in the global biocyele on land: O S is liberated from water during photosynthesis, and consumed by the respirative proeesses. Both reactions are seleetive, to some degree, for the isotopie masses of oxygen which leads to a stationary excess of the heavy isotope, 1sO, in atmospherie 0 3. A quantitative understanding of this phenomenon (Dole effeet [i2]) is of aetual interest in the context of global environmental studies, e.g. the gas exchange between atmosphere and ocean [t8].The isotope fraetionation oeeurring during respiration has been studied by Lane and Dole [~2], but is not suffieient to aeeount for the 180 exeess actually present. The other prineipal eontribution has been overlooked a long time, sinee the mechanism is somewhat peculiar: it is not the aet of photosynthesis itself whieh is responsible for the isotope fraetionation, but the vapor pressure differenee of isotopie water moleeules in transpiring leaves --the most common place of land photosynthesis [7,9].The purpose of the presënt paper is a first approaeh to a quantitative understanding of this contribution to atmospherie 1so enriehment on the basis of a box model for transpiring leaves. Computation of the isotopie eomposition of oxygen from land photosynthesis, on the global average, will be presented in a following paper.1. Theoretieal Considerations (G. Dongmann, K. Wagener) A Model el the H 2180 Enrichment in the Water o/LeavesFor the water transport through leaves the compartment model in Fig. i is eonsidered.

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