Hydrogen isotopic differences between C₃ and C₄ land plant lipids: consequences of compartmentation in C₄ photosynthetic chemistry and C₃ photorespiration

The ²H/¹H ratio of carbon-bound H in biolipids holds potential for probing plant lipid biosynthesis and metabolism. The biochemical mechanism underlying the isotopic differences between lipids from C₃ and C₄ plants is still poorly understood. GC-pyrolysis-IRMS (gas chromatography-pyrolysis-isotope ratio mass spectrometry) measurement of the ²H/¹H ratio of leaf lipids from controlled and field grown plants indicates that the biochemical isotopic fractionation (ε²H_{lipid_biochem}) differed between C₃ and C₄ plants in a pathway-dependent manner: ε²H_C4 > ε²H_C3 for the acetogenic pathway, ε²H_C4 < ε²H_C3 for the mevalonic acid pathway and the 1-deoxy-D-xylulose 5-phosphate pathway across all species examined. It is proposed that compartmentation of photosynthetic CO₂ fixation into C₄ mesophyll (M) and bundle sheath (BS) cells and suppression of photorespiration in C₄ M and BS cells both result in C₄ M chloroplastic pyruvate – the precursor for acetogenic pathway – being more depleted in ²H relative to pyruvate in C₃ cells. In addition, compartmentation in C₄ plants also results in (i) the transferable H of NADPH being enriched in ²H in C₄ M chloroplasts compared with that in C₃ chloroplasts for the 1-deoxy-D-xylulose 5-phosphate pathway pathway and (ii) pyruvate relatively ²H-enriched being used for the mevalonic acid pathway in the cytosol of BS cells in comparison with that in C₃ cells.