Organic hydrogen-carbon isotope signatures of terrestrial higher plants during biosynthesis for distinctive photosynthetic pathways

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.