δ13C and δD relationships among three n-alkyl compound classes (n-alkanoic acid, n-alkane and n-alkanol) of terrestrial higher plants

Carbon (delta C-13) and hydrogen (delta D) isotopic compositions of long-chain n-alkanoic acids, n-alkanes and n-alkanols from plant leaves including C3 angiosperm, C3 gymnosperm, C4 and CAM plants were determined in order to clarify detailed isotopic relationships among the three classes of n-alkyl compounds in terrestrial higher plants. For all plant species, the three classes of n-alkyl compounds have a similar range of delta C-13 and delta D values within a single species. However, most plants show a small but significant systematic difference in both delta C-13 and delta D values with respect to biosynthetically corresponding carbon-numbered compounds. In general, n-alkane and n-alkanol are enriched in C-13 (averaging 1.4 +/- 1.1 parts per thousand and 1.3 +/- 1.37 parts per thousand, respectively) but depleted in D (25 +/- 16 parts per thousand and 15 +/- 12 parts per thousand, respectively) relative to the corresponding carbon-numbered n-alkanoic acid. Moreover, the magnitude of the isotope difference is quite similar in all plants including different types of photosynthesis (C3, C4 and CAM). These results suggest that C-13- and D-discriminations occur at biosynthetic branching points during synthesis of n-alkanoic acid, n-alkane and n-alkanol from a common precursor, and that their fractionation mechanisms are common to all plant species. This systematic isotopic relationship between plant n-alkyl compounds will be useful to compare isotope records between different n-alkyl compounds in various geological samples. (c) 2006 Elsevier Ltd. All rights reserved.