Carbon allocation in plants - transport of newly assimilated carbon, carbon isotope fractionation and the relation to delta13C in respiratory CO2

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Ecosystem respiratory fluxes can be calculated from d13C of ecosystem respiratory CO2 (d13CR; by the Keeling plot approach) combined with micrometeorological measurements. Recent studies showed that a large fraction of the CO2 respired from the whole ecosystem originates from carbon fixed within the last few days by the vegetation. As consequence, all vegetation-related processes that affect photosynthetic 13C discrimination as well as carbon isotope fractionation associated with allocation of newly assimilated organic carbon from the leaves to the roots and the rhizosphere will influence d13CR. Until recently d13CR could not be determined routinely with a high resolution in time. Consequently, variation in d13C of substrates for respiration within a time scale from hours to days and its influence on d13C of respired CO2 has considerable potential to confound interpretation of Keeling plots. This study is aimed at assessing the most important processes in plants that may cause spatial and temporal variation in d13C of respiratory substrates. The central questions that will be addressed in field experiments (with Eucalyptus delegatensis) and under controlled conditions (with Ricinus communis) are: 1) Can we use the known influence of Ci/Ca1 on d13C of organic carbon to trace carbon transport velocity and hence estimate the time-dependent distribution of a particular d13C signal within the plant? 2) To what extent does the shoot-to-root transport influence d13C of organic carbon and what are the underlying metabolic processes of a potential discrimination? 3) Are there day-night-variations in d13C of sugars transported in the plant? 4) Is there carbon isotope fractionation during dark respiration and is the fractionation comparable among shoots, stems and roots? In addition, the spatial and temporal dynamics in d13C within the plant will be related to the temporal dynamic of d13CR under field conditions.