ISOTOPIC HETEROGENEITY OF WATER IN TRANSPIRING LEAVES - IDENTIFICATION OF THE COMPONENT THAT CONTROLS THE DELTA-O-18 OF ATMOSPHERIC O-2 AND CO2

Two direct but independent approaches were developed to identify the average delta(18)O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the delta(18)O value of CO2 in isotopic equilibrium with leaf water to reconstruct the delta(18)O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO2 and H2O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exchange in other leaf water fractions. In the second approach, we measured the delta(18)O value of O-2 from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O-2 is produced from chloroplast water irreversibly and without discrimination, the delta(18)O value of the O-2 Should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average delta(18)O value of chloroplasts water was displaced by 3-10 parts per thousand (depending on relative humidity and atmospheric composition) below the value 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 water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the delta(18)O values of atmospheric CO2 and O-2, as well as in influencing the delta(18)O of plant organic matter.