ISOTOPIC HETEROGENEITY OF WATER IN TRANSPIRING LEAVES - IDENTIFICATION OF THE COMPONENT THAT CONTROLS THE DELTA-O-18 OF ATMOSPHERIC O-2 ANDCO2

Two direct but independent approaches were developed to identify the a verage delta(18)O value of the water fraction in the chloroplasts of t ranspiring leaves. In the first approach, we used the delta(18)O value of CO2 in isotopic equilibrium with leaf water to reconstruct the del ta(18)O value of water in the chloroplasts. This method was based on t he idea that the enzyme carbonic anhydrase facilitates isotopic equili brium between CO2 and H2O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exc hange in other leaf water fractions. In the second approach, we measur ed the delta(18)O value of O-2 from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O-2 is produced from chloropl ast water irreversibly and without discrimination, the delta(18)O valu e of the O-2 Should be identical to that of chloroplast water. In inta ct, transpiring leaves of sunflower (Helianthus annuus cv. giant mammo th) under the experimental conditions used, the average delta(18)O val ue of chloroplasts water was displaced by 3-10 parts per thousand (dep ending on relative humidity and atmospheric composition) below the val ue predicted by the conventional Craig and Gordon model. Furthermore, this delta(18)O value was always lower than the delta(18)O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the delta(18)O value of w ater in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the delta(18)O values of atmospheric C O2 and O-2, as well as in influencing the delta(18)O of plant organic matter.