title: Assimilation of xylem-transported CO2 is dependent on transpiration rate but is small relative to atmospheric fixation.
abstract: The effect of transpiration rate on internal assimilation of CO2 released from respiring cells has not previously been quantified. In this study, detached branches of Populus deltoides were allowed to take up (13)CO2-labelled solution at either high (high label, HL) or low (low label, LL) (13)CO2 concentrations. The uptake of the (13)CO2 label served as a proxy for the internal transport of respired CO2, whilst the transpiration rate was manipulated at the leaf level by altering the vapour pressure deficit (VPD) of the air. Simultaneously, leaf gas exchange was measured, allowing comparison of internal CO2 assimilation with that assimilated from the atmosphere. Subsequent (13)C analysis of branch and leaf tissues revealed that woody tissues assimilated more label under high VPD, corresponding to higher transpiration, than under low VPD. More (13)C was assimilated in leaf tissue than in woody tissue under the HL treatment, whereas more (13)C was assimilated in woody tissue than in leaf tissue under the LL treatment. The ratio of (13)CO2 assimilated from the internal source to CO2 assimilated from the atmosphere was highest for the branches under the HL and high VPD treatment, but was relatively small regardless of VPD×label treatment combination (up to 1.9%). These results showed that assimilation of internal CO2 is highly dependent on the rate of transpiration and xylem sap [CO2]. Therefore, it can be expected that the relative contribution of internal CO2 recycling to tree carbon gain is strongly dependent on factors controlling transpiration, respiration, and photosynthesis.
authors.affiliation: Laboratory of Plant Ecology, Department of Applied Ecology and Environmental Biology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, 9000 Gent, Belgium. Jasper.Bloemen@UGent.be
authors.forename: Jasper
authors.initials: J
authors.lastname: Bloemen
authors.forename: Mary Anne
authors.initials: MA
authors.lastname: McGuire
authors.forename: Doug P
authors.initials: DP
authors.lastname: Aubrey
authors.forename: Robert O
authors.initials: RO
authors.lastname: Teskey
authors.forename: Kathy
authors.initials: K
authors.lastname: Steppe
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism* / Q000502:physiology; D018515:Plant Leaves / Q000378:metabolism / Q000502:physiology; D018521:Plant Physiological Phenomena; D018547:Plant Stems / Q000378:metabolism / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology; D032107:Populus / Q000378:metabolism / Q000502:physiology; D014197:Trees / Q000378:metabolism / Q000502:physiology; D052584:Xylem / Q000378:metabolism* / Q000502:physiology
chemical_list: D002245:Carbon Dioxide
keywords: carbon budget; carbon isotope; internal CO2 transport; leaf mesophyll; transpiration; woody tissue photosynthesis; xylem.