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WHOLE PLANT AND ECOPHYSIOLOGY

Bundle Sheath Leakiness and Light Limitation during C₄ Leaf and Canopy CO₂ Uptake^1,[W],[OA]

Physiological Ecology Group, Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, United Kingdom (J.K., H.G.); Biobased Products, Agrotechnology and Food Science Group, Wageningen University and Research Center, 6708 PD Wageningen, The Netherlands (H.E.S.); Botany Department, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland (F.A., N.F., F.C., M.B.J.); Teagasc, Oak Park Crops Research Centre, Carlow, Ireland (J.F.); and Teagasc, Johnstown Castle Environmental Research Centre, Wexford, Ireland (G.J.L.)

Perennial species with the C₄ pathway hold promise for biomass-based energy sources. We have explored the extent that CO₂ uptake of such species may be limited by light in a temperate climate. One energetic cost of the C₄ pathway is the leakiness () of bundle sheath tissues, whereby a variable proportion of the CO₂, concentrated in bundle sheath cells, retrodiffuses back to the mesophyll. In this study, we scale from leaf to canopy level of a Miscanthus crop (Miscanthus x giganteus hybrid) under field conditions and model the likely limitations to CO₂ 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 = 8.3) show a progressive increase in both carbon isotope discrimination and as light decreases. A similar increase was observed at the ecosystem scale when we used eddy covariance net ecosystem CO₂ 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 at the canopy level. Modeled values of canopy CO₂ fixation using leaf-level measurements of suggest that around 32% of potential photosynthetic carbon gain is lost due to light limitation, whereas using determined independently from isofluxes at the canopy level the reduction in canopy CO₂ uptake is estimated at 14%. Based on these results, we identify as an important limitation to CO₂ uptake of crops with the C₄ pathway.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Johannes Kromdijk (wk229{at}cam.ac.uk).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.129890

^* Corresponding author; e-mail wk229{at}cam.ac.uk.

Received September 14, 2008; accepted October 23, 2008; published October 29, 2008.

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