Partitioning internal diffusion resistances and CO2 isotope exchange in leaves

Background/Question/Methods

The isotopic composition of atmospheric CO₂ (δ¹⁸O and δ¹³C) has the potential to constrain the carbon cycle at a range of scales, in particular partitioning net CO₂ exchanges into component gross fluxes and providing insights to linked C and water fluxes. There are significant limitations, however, to the quantitative application of this approach, in part because of uncertainties associated with the isotope exchange of CO₂ with leaf water and the diffusion of CO₂ inside leaves. The degree of isotopic heterogeneity of leaf water, spatial distributions and activities of carbonic anhydrases, and varying resistances to diffusion of CO₂ from the substomatal cavity to chloroplasts are all key components with important uncertainties. Better constraints on these would yield insights to fundamental aspects of leaf physiology, as well as improve our ability to understand and model the terrestrial C cycle. Using laser spectroscopy, we developed a system that permits the simultaneous measurement of the ¹³C and ¹⁸O isotopic composition of CO₂ and the ¹⁸O of leaf transpiration, while rapidly changing the isotopic composition of the source CO₂.

Results/Conclusions

Our approach should allow key model parameters to be derived directly and simultaneously by taking advantage of constant gas exchange conditions across different source gas isotopic compositions. We utilize the system here to partition internal CO₂ diffusion resistances (total, to the limit carbonic anhydrase activity, and to RUBISCO activity) for three species (Helianthus annuus, Nicotiana tabacum, Arabidopsis thaliana). Measurements were made under constant temperature, CO₂, and O₂ conditions, with similar VPD and under high light. System stability was good, with constant photosynthetic fluxes, stomatal conductances and transpiration across states. While ¹⁸O-derived estimates of internal conductance were similar across the two states and varied significantly across species, ¹³C-derived estimates were less consistent, though species differences were still evident. The results demonstrated that the approach can be used to partition internal diffusional resistances and point to future efforts to better refine the use of CO₂ isotope ratios to study leaf gas exchange.