Perennial species with the C 4 pathway hold promise for biomass-based energy sources. We have explored the extent that CO 2 uptake of such species may be limited by light in a temperate climate. One energetic cost of the C 4 pathway is the leakiness (f) of bundle sheath tissues, whereby a variable proportion of the CO 2 , concentrated in bundle sheath cells, retrodiffuses back to the mesophyll. In this study, we scale f from leaf to canopy level of a Miscanthus crop (Miscanthus 3 giganteus hybrid) under field conditions and model the likely limitations to CO 2 fixation. At the leaf level, measurements of photosynthesis coupled to online carbon isotope discrimination showed that leaves within a 3.3-m canopy (leaf area index 5 8.3) show a progressive increase in both carbon isotope discrimination and f as light decreases. A similar increase was observed at the ecosystem scale when we used eddy covariance net ecosystem CO 2 fluxes, together with isotopic profiles, to partition photosynthetic and respiratory isotopic flux densities (isofluxes) and derive canopy carbon isotope discrimination as an integrated proxy for f at the canopy level. Modeled values of canopy CO 2 fixation using leaf-level measurements of f suggest that around 32% of potential photosynthetic carbon gain is lost due to light limitation, whereas using f determined independently from isofluxes at the canopy level the reduction in canopy CO 2 uptake is estimated at 14%. Based on these results, we identify f as an important limitation to CO 2 uptake of crops with the C 4 pathway.

No abstract