Hydrogen isotopic differences between C 3 and C 4 land plant lipids: consequences of compartmentation in C 4 photosynthetic chemistry and C 3 photorespiration

The 2 H/ 1 H 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 C 3 and C 4 plants is still poorly understood. GC‐pyrolysis‐IRMS (gas chromatography‐pyrolysis‐isotope ratio mass spectrometry) measurement of the 2 H/ 1 H ratio of leaf lipids from controlled and field grown plants indicates that the biochemical isotopic fractionation (ε 2 H lipid_biochem ) differed between C 3 and C 4 plants in a pathway‐dependent manner: ε 2 H C4  > ε 2 H C3 for the acetogenic pathway, ε 2 H C4  < ε 2 H 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 CO 2 fixation into C 4 mesophyll (M) and bundle sheath (BS) cells and suppression of photorespiration in C 4 M and BS cells both result in C 4 M chloroplastic pyruvate – the precursor for acetogenic pathway – being more depleted in 2 H relative to pyruvate in C 3 cells. In addition, compartmentation in C 4 plants also results in (i) the transferable H of NADPH being enriched in 2 H in C 4 M chloroplasts compared with that in C 3 chloroplasts for the 1‐deoxy‐D‐xylulose 5‐phosphate pathway pathway and (ii) pyruvate relatively 2 H‐enriched being used for the mevalonic acid pathway in the cytosol of BS cells in comparison with that in C 3 cells.