COMPOUND-SPECIFIC DELTA-C-13 ANALYSES OF LEAF LIPIDS FROM PLANTS WITH DIFFERING CARBON-DIOXIDE METABOLISMS

The stable carbon isotope (deltaC-13) 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 deltaC-13 values of -10 to -16 parts per thousand; the total surface lipid extract deltaC-13 values were on average 8 parts per thousand more depleted. The C3 plants had leaf tissue values of -25 to -30 parts per thousand; total surface lipid extracts were on average 4 parts per thousand. more depleted in C-13. The average n-alkane values for all plants were around 4 parts per thousand more depleted in C-13 than the total surface lipid extracts and were between 1.6 and 13.8 parts per thousand depleted in C-13 relative to total tissue. This depletion relative to the total tissue was on average greater for CAM (I 1.0 parts per thousand) and C4 plants (9.9 parts per thousand) than for C3 plants (5.9 parts per thousand). Variations ranging from 0.1 to 6 parts per thousand were observed between n-alkane 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 deltaC-13 values similar to the n-alkanes. Phytol isolated from C3 plants was, on average, 1.5 parts per thousand enriched in C-13 relative to the average n-alkane deltaC-13 values.