Isotopic Fractionation of Hydrogen in Plants

The naturally‐occurring stable isotopes deuterium and hydrogen are fractionated by a number of physical and biological processes. Deuterium has a tendency to precipitate out first from a moist air mass. Thus ground water will become isotopically lighter with an increase in latitude, altitude, or distance inland. Water taken up by the plant from the soil undergoes little change until evapotranspiration results in leaf water becoming isotopically heavier. Thus hydrogen isotopes in plants can reveal something of geography (groundwater) and climate. Hydrogen isotopes undergo little fractionation by passage through the food chain, although plant parasites tend to be enriched in D as compared to their hosts, possibly due to higher rates of transpiration in the parasitic plants. The splitting of water in photosynthesis results in the lighter isotope being incorporated into organic matter. An even larger isotopic fractionation results during lipid synthesis and other processes involving the pyruvate dehydrogenase complex. Differences in metabolic pathway between species can be detected by D/H ratios. Hydrogen isotopic differences can be detected between CAM, C 4 , and C 3 species. Within C 4 plants, the NADP‐ME plants are isotopically distinguishable from NAD‐ME and PEP‐CK plants.