Species variation in the hydrogen isotope composition of leaf cellulose is mostly driven by isotopic variation in leaf sucrose

Experimental approaches to isolate drivers of variation in the carbon-bound hydrogen isotope composition (δ²H) of plant cellulose are rare and current models are limited in their application. This is in part due to a lack in understanding of how ²H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ²H of leaf sucrose along with the δ²H and δ¹⁸O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ²H of sucrose explained 66% of the δ²H variation in cellulose (R² = 0.66), which was associated with species differences in the ²H enrichment of sucrose above leaf water ((Formula presented.) : −126% to −192‰) rather than by variation in leaf water δ²H itself. (Formula presented.) was positively related to dark respiration (R² = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R² = 0.38), but only when (Formula presented.) was fixed to the literature accepted value of (Formula presented.) ‰. No relation was found between isotopic exchange of hydrogen and oxygen, suggesting large differences in the processes shaping post-photosynthetic fractionation between elements. Our results strongly advocate that for robust applications of the leaf cellulose hydrogen isotope model, parameterization utilizing δ²H of sugars is needed.