Hydrogen isotope fractionation in plants with C 3 , C 4 , and CAM CO 2 fixation.

Measurements of stable isotope ratios in organic compounds are widely used tools for plant ecophysiological studies. However, the complexity of the processes involved in shaping hydrogen isotope values (δ 2 H) in plant carbohydrates has limited its broader application. To investigate the underlying biochemical processes responsible for 2 H fractionation among water, sugars, and cellulose in leaves, we studied the three main CO 2 fixation pathways (C 3 , C 4 , and CAM) and their response to changes in temperature and vapor pressure deficit (VPD). We show significant differences in autotrophic 2 H fractionation (ε A ) from water to sugar among the pathways and their response to changes in air temperature and VPD. The strong 2 H depleting ε A in C 3 plants is likely driven by the photosynthetic H + production within the thylakoids, a reaction that is spatially separated in C 4 and strongly reduced in CAM plants, leading to the absence of 2 H depletion in the latter two types. By contrast, we found that the heterotrophic 2 H-fractionation (ε H ) from sugar to cellulose was very similar among the three pathways and is likely driven by the plant's metabolism, rather than by isotopic exchange with leaf water. Our study offers new insights into the biochemical drivers of 2 H fractionation in plant carbohydrates.