Isotopic heterogeneity of water in transpiring leaves: identification of the component that controls the δ¹⁸O of atmospheric O₂ and CO₂

Two direct but independent approaches were developed to identify the average δ 18 O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the δ 18 O value of CO 2 in isotopic equilibrium with leaf water to reconstruct the δ 18 O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO 2 and H 2 O 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 δ 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 δ 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 δ 18 O value of chloroplasts water was displaced by 3—10 % (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig & Gordon model. Furthermore, this δ 18 O value was always lower than the δ 18 O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the δ 18 O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the δ 18 O values of atmospheric CO 2 and O 2 , as well as in influencing the δ 18 O of plant organic matter.