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 (δ H) 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 H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ H of leaf sucrose along with the δ H and δ O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ H of sucrose explained 66% of the δ H variation in cellulose (R  = 0.66), which was associated with species differences in the H enrichment of sucrose above leaf water ( : -126% to -192‰) rather than by variation in leaf water δ H itself. was positively related to dark respiration (R  = 0.27), and isotopic exchange of hydrogen in sugars was positively related to the turnover time of carbohydrates (R  = 0.38), but only when was fixed to the literature accepted value of ‰. 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 δ H of sugars is needed.