Compound-specific δ 13C analyses of leaf lipids from plants with differing carbon dioxide metabolisms

The stable carbon isotope (δ 13C) values of (1) total leaf tissue (2), total surface lipid extracts and (3) individual n-alkanes isolated from leaves were determined for plants utilizing the C3, C4, and Crassulacean Acid (CAM) pathways of carbon fixation. The C4 and CAM species had leaf tissue δ 13C values of −10 to −16‰; the total surface lipid extract δ 13C values were on average 8‰ more depleted. The C3 plants had leaf tissue values of −25 to −30‰; total surface lipid extracts were on average 4‰ more depleted in 13C. The average n-alkane values for all plants were around 4‰ more depleted in 13C than the total surface lipid extracts and were between 1.6 and 13.8‰ depleted in 13C relative to total tissue. This depletion relative to the total tissue was on average greater for CAM (11.0‰) and C4 plants (9.9‰) than for C3 plants (5.9‰). Variations ranging from 0.1 to 6‰ were observed between n-alkalene homologues for the plants studied, emphasizing the need for caution when interpreting small variations in the isotopic compositions of individual compounds isolated from sedimentary environments. n-Aldehydes from C3 plants had carbon-isotopic compositions similar to the n-alkanes from the same plants, suggesting that other biosynthetically related n-alkyl compounds (e.g. n-acids and n-alcohols) will have δ 13C values similar to the n-alkanes. Phytol isolated from C3 plants was, on average, 1.5‰ enriched in 13C relative to the average n-alkane δ 13C values.