Abstract
Stable isotopic compositions of organic hydrogen (δD) have positive correlations (r2 > 0.95) with those of carbon (δ13C) among several compound fractions of terrestrial plant leaves possessing distinctive photosynthetic pathways (C3, C4 and CAM). The δD/δ13C slopes of C3 plants vary from ∼25 to 42, which are larger than those of C4 plants (11 to 12). CAM plants have intermediate δD/δ13C slopes (∼17 to 20) between C3 and C4 plants. Using a δD-δ13C diagram, photosynthetic metabolisms are clearly discriminated, even though they sometimes cannot be distinguished from each other only by carbon isotopes. Relative to bulk organic matter, hydrogen and carbon of lipid fraction are more depleted in 13C than those of pigment fraction, respectively. Furthermore, δD values of lipid and pigment fractions relative to bulk organic hydrogen have negative correlations with δ13C values of corresponding fractions among the three photosynthetic pathways. This isotopic covariance among each fraction may be attributable to kinetically-controlled molecular biosyntheses using similar enzymes but with different isotope fractionations. Or the intermediate molecules for the biosyntheses have isotopically different pools in hydrogen and carbon.
