Why are non-photosynthetic tissues generally 13C enriched compared with leaves in C3 plants? Review and synthesis of current hypothesesLucas A. Cernusak A N , Guillaume Tcherkez B , Claudia Keitel C , William K. Cornwell D , Louis S. Santiago E , Alexander Knohl F , Margaret M. Barbour G , David G. Williams H , Peter B. Reich I , David S. Ellsworth J , Todd E. Dawson K , Howard G. Griffiths L , Graham D. Farquhar C and Ian J. Wright M
A Charles Darwin University, School of Environmental and Life Sciences, Darwin, NT 0909, Australia.
B Plateforme Métabolisme-Metabolome IFR87, Batiment 630, IBP CNRS UMR8618, Université Paris-Sud XI, 91405 Orsay cedex, France.
C Environmental Biology Group, Research School of Biological Sciences, Australian National University, Canberra, ACT 2601, Australia.
D Biodiversity Research Group, University of British Colombia, Vancouver, BC V6T 1Z4, Canada.
E Department of Botany and Plant Sciences, University of California Riverside, Riverside, CA 92521, USA.
F Institute of Plant Sciences, ETH Zurich, Zurich 8092, Switzerland.
G Landcare Research, PO Box 40, Gerald Street, Lincoln 7640, New Zealand.
H Department of Renewable Resources, University of Wyoming, Laramie, WY 82071, USA.
I Department of Forest Resources, University of Minnesota, St Paul, MN 55108, USA.
J Center for Plant and Food Sciences, University of Western Sydney, Penrith, NSW 1797, Australia.
K Department of Integrative Biology, University of California Berkeley, Berkeley, CA 94720, USA.
L Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK.
M Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia.
N Corresponding author. Email: firstname.lastname@example.org
Functional Plant Biology 36(3) 199-213 http://dx.doi.org/10.1071/FP08216
Submitted: 6 August 2008 Accepted: 18 January 2009 Published: 2 March 2009
Non-photosynthetic, or heterotrophic, tissues in C3 plants tend to be enriched in 13C compared with the leaves that supply them with photosynthate. This isotopic pattern has been observed for woody stems, roots, seeds and fruits, emerging leaves, and parasitic plants incapable of net CO2 fixation. Unlike in C3 plants, roots of herbaceous C4 plants are generally not 13C-enriched compared with leaves. We review six hypotheses aimed at explaining this isotopic pattern in C3 plants: (1) variation in biochemical composition of heterotrophic tissues compared with leaves; (2) seasonal separation of growth of leaves and heterotrophic tissues, with corresponding variation in photosynthetic discrimination against 13C; (3) differential use of day v. night sucrose between leaves and sink tissues, with day sucrose being relatively 13C-depleted and night sucrose 13C-enriched; (4) isotopic fractionation during dark respiration; (5) carbon fixation by PEP carboxylase; and (6) developmental variation in photosynthetic discrimination against 13C during leaf expansion. Although hypotheses (1) and (2) may contribute to the general pattern, they cannot explain all observations. Some evidence exists in support of hypotheses (3) through to (6), although for hypothesis (6) it is largely circumstantial. Hypothesis (3) provides a promising avenue for future research. Direct tests of these hypotheses should be carried out to provide insight into the mechanisms causing within-plant variation in carbon isotope composition.
Additional keywords: diel cycle, heterotrophic tissue, PEP-carboxylase, refixation, respiration.