Photosynthetic C18OO fractionation is related to within- and between-species variations in photosynthetic traits

The C¹⁸O¹⁶O/C¹⁶O¹⁶O fractionation during photosynthesis (Δ¹⁸O_A) carries rich information about plant physiology and environmental conditions, which is crucial for plant physiological, ecological, and biogeochemical studies. Δ¹⁸O_A is mainly determined by the isotopic fractionation during diffusion and the CO₂-leaf water oxygen exchange reaction. Therefore, Δ¹⁸O_A is believed to relate to leaf physiological parameters such as the evaporative ¹⁸O enrichment of leaf water (Δ_e), CO₂ influx and efflux. Based on current mechanistic understanding of Δ¹⁸O_A, oxygen isotope composition of atmospheric CO₂ (δ_a) can be used to estimate global gross primary productivity and to separate photosynthetic and respiratory CO₂ fluxes at the ecosystem scale. However, there is uncertainty about the key physiological factors responsible for changes in Δ¹⁸O_A and whether there is a difference in Δ¹⁸O_A between C₃ and C₄ plants.

In this study, we investigated the response of Δ¹⁸O_A to short-term changes in CO₂ levels in three C₃ species (Helianthus annuus, Vigna unguiculata and Triticum aestivum) and one C₄ species (Cleistogenes squarrosa) grown under different levels of vapour pressure deficit (VPD) and nitrogen supply. Utilising a new mass-balance equation that distinguishes metabolic (mitochondrial and photo-respiratory CO₂) and purely diffusive (retro-diffusive CO₂) CO₂ fluxes, we assessed the effect of the gross CO₂ efflux from leaves.

We found a significant CO₂ effect on Δ¹⁸O_A for C. squarrosa, but not for the C₃ species. Δ¹⁸O_A was not significantly correlated with Δ_e of the C₃ species, and Δ¹⁸O_A of C₄ species was not sensitive to changes in Δ_e driven by VPD. The gross CO₂ efflux and Δ¹⁸O_A were significantly correlated for both C₃ and C₄ species, demonstrating its role in regulating Δ¹⁸O_A. We also found that the C₃ species had significantly higher Δ¹⁸O_A than the C₄ species, due to the lower ratio of intercellular to atmospheric CO₂ (C_i/C_a) in the latter. Our study reveals the distinct difference in Δ¹⁸O_A between C₃ and C₄ species and the remarkable relationship between Δ¹⁸O_A and physiological parameters, which provides new insights into how Δ¹⁸O_A can be used to infer carbon cycle processes from leaf to ecosystem scales.