JXB Advance Access originally published online on July 14, 2009
Journal of Experimental Botany 2009 60(13):3809-3817; doi:10.1093/jxb/erp222
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© 2009 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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RESEARCH PAPER |
Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.)
1School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602, USA
2College of Natural Resources, University of Idaho, Moscow, Idaho 83844, USA
* To whom correspondence should be addressed: E-mail: mmcguire{at}uga.edu
Previous reports have shown that CO2 dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO2, the internal transport of respired CO2 at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of 13CO2 in sunlight. Simultaneously, leaf net photosynthesis and CO2 efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for 13C content to determine the quantity of transported 13CO2 label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the 13CO2 label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported 13CO2 label by woody tissues averaged 6% of the assimilation of CO2 from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of 13C differed from rates calculated from measurements of CO2 efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO2 transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO2 may be an important means by which trees reduce the carbon cost of respiration.
Key words: Corticular photosynthesis, CO2 recycling, dissolved CO2, woody tissue respiration, xylem
Received 5 March 2009; Revised 31 May 2009 Accepted 22 June 2009