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

生态系统呼吸通量可以根据生态系统呼吸二氧化碳的d13C(d13CR；采用基林曲线法)结合微气象测量来计算。最近的研究表明，整个生态系统呼吸的二氧化碳有很大一部分来自植被在过去几天内固定的碳。因此，影响光合作用~(13)C分辨的所有与植被有关的过程，以及与新吸收的有机碳从叶片向根和根际分配相关的碳同位素分馏，都将影响d13CR。直到最近，才能以高分辨率的时间常规测定d13CR。因此，在从几小时到几天的时间尺度内，呼吸底物d13C的变化及其对呼吸二氧化碳d13C的影响，有相当大的可能混淆对基林曲线的解释。这项研究旨在评估可能导致呼吸底物d13C在空间和时间上变化的最重要的植物过程。在田间试验(与桉树)和在受控条件下(与蓖麻树)将解决的中心问题是：1)我们能否利用已知的ci/ca1对有机碳d13C的影响来追踪碳的运输速度，从而估计特定的d13C信号在植物内随时间的分布？2)从地上部到根的运输在多大程度上影响有机碳的d13C，潜在的区别是什么？3)在植物中运输的糖的d13C是否存在昼夜变化？4)在暗呼吸期间是否存在碳同位素分馏以及地上部之间的分馏是否具有可比性。茎和根？此外，植物体内d13C的时空动态与田间条件下d13CR的时间动态有关。

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.