Predicting leaf wax n ‐alkane 2 H / 1 H ratios: controlled water source and humidity experiments with hydroponically grown trees confirm predictions of C raig– G ordon model

• Published in: Plant, cell and environment cell and environment, 2015-06, Vol.38 (6), p.1035-1047
• Main Authors: TIPPLE, BRETT J., BERKE, MELISSA A., HAMBACH, BASTIAN, RODEN, JOHN S., EHLERINGER, JAMES R.
• Format: Article
• Language: English
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.12457

The extent to which both water source and atmospheric humidity affect δ 2 H values of terrestrial plant leaf waxes will affect the interpretations of δ 2 H variation of leaf waxes as a proxy for hydrological conditions. To elucidate the effects of these parameters, we conducted a long‐term experiment in which we grew two tree species, P opulus fremontii and B etula occidentalis , hydroponically under combinations of six isotopically distinct waters and two different atmospheric humidities. We observed that leaf n ‐alkane δ 2 H values of both species were linearly related to source water δ 2 H values, but with slope differences associated with differing humidities. When a modified version of the C raig– G ordon model incorporating plant factors was used to predict the δ 2 H values of leaf water, all modelled leaf water values fit the same linear relationship with n ‐alkane δ 2 H values. These observations suggested a relatively constant biosynthetic fractionation factor between leaf water and n ‐alkanes. However, our calculations indicated a small difference in the biosynthetic fractionation factor between the two species, consistent with small differences calculated for species in other studies. At present, it remains unclear if these apparent interspecies differences in biosynthetic fractionation reflect species‐specific biochemistry or a common biosynthetic fractionation factor with insufficient model parameterization. The hydrogen isotope ratios of leaf wax n ‐alkanes were analysed from trees grown hydroponically and under monitored atmospheric conditions. The effects of soil evaporation on the δ2H isotope values of leaf waxes were eliminated, permitting the development of a mechanistic model for the interpretation of δ2H variation in modern and ancient leaf waxes. A relatively stable biosynthetic fractionation was found between leaf water and leaf wax hydrogen isotope values. This experiment demonstrated that δ2H values of leaf wax n ‐alkanes are a proxy for δ2H values of leaf water at the time of wax synthesis.