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Isotopic heterogeneity of water in transpiring leaves: identification of the component that controls the δ 18 O of atmospheric O 2 and CO 2
Author(s) -
YAKIR D.,
BERRY J. A.,
GILES L.,
OSMOND C. B.
Publication year - 1994
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.1994.tb00267.x
Subject(s) - chloroplast , photosynthesis , sunflower , chemistry , helianthus annuus , relative humidity , composition (language) , botany , environmental chemistry , horticulture , biology , biochemistry , meteorology , physics , gene , linguistics , philosophy
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.