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 (δ2 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 2 H-fractionations in carbohydrates differ between species. We analysed, for the first time, the δ2 H of leaf sucrose along with the δ2 H and δ18 O of leaf cellulose and leaf and xylem water across seven herbaceous species and a starchless mutant of tobacco. The δ2 H of sucrose explained 66% of the δ2 H variation in cellulose (R2 = 0.66), which was associated with species differences in the 2 H enrichment of sucrose above leaf water ( ε sucrose : -126% to -192‰) rather than by variation in leaf water δ2 H itself. ε sucrose 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 ε sucrose was fixed to the literature accepted value of - 171 ‰. 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 δ2 H of sugars is needed.

Estimation of Photorespiratory Fluxes by Gas Exchange.

Photorespiratory fluxes can be easily estimated by photosynthetic gas exchange using an infrared gas analyzer and applying the Farquhar, von Caemmerer, and Berry (Farquhar et al. Planta 149:78-90, 1980) photosynthesis model. For a more direct measurement of photorespiratory CO2 release from glycine decarboxylation, infrared gas analysis can be coupled to membrane-inlet mass spectrometry, capable of separating the total CO2 concentration into its 12CO2 and 13CO2 components in a continuous online fashion. This chapter discusses how to calculate rates of photorespiration from Rubisco kinetics and describes in detail a method for measuring the CO2 release from glycine decarboxylation using 13CO2.