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

Carbon (δ13C) and hydrogen (δD) 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 δ13C and δD values within a single species. However, most plants show a small but significant systematic difference in both δ13C and δD values with respect to biosynthetically corresponding carbon-numbered compounds. In general, n-alkane and n-alkanol are enriched in 13C (averaging 1.4 ± 1.1‰ and 1.3 ± 1.3‰, respectively) but depleted in D (25 ± 16‰ and 15 ± 12‰, 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 13C- 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.