Laboratoire Ecologie, Systématique et Evolution, UMR 8079, Université Paris-Sud XI, 91405 Orsay CEDEX, France.
Tree Physiol. 2010 Jul;30(7):818-30. doi: 10.1093/treephys/tpq039. Epub 2010 May 26.
The present study examines the impact of the C source (reserves vs current assimilates) on tree C isotope signals and stem growth, using experimental girdling to stop the supply of C from leaves to stem. Two-year-old sessile oaks (Quercus petraea) were girdled at three different phenological periods during the leafy period: during early wood growth (Girdling Period 1), during late wood growth (Girdling Period 2) and just after growth cessation (Girdling Period 3). The measured variables included stem respiration rates, stem radial increment, delta(13)C of respired CO(2) and contents of starch and water-soluble fraction in stems (below the girdle) and leaves. Girdling stopped growth, even early in the growing season, leading to a decrease in stem CO(2) efflux (CO(2R)). Shift in substrate use from recently fixed carbohydrate to reserves (i.e., starch) induced (13)C enrichment of CO(2) respired by stem. However, change in substrate type was insufficient to explain alone all the observed CO(2R) delta(13)C variations, especially at the period corresponding to large growth rate of control trees. The below-girdle mass balance suggested that, during girdling periods, stem C was invested in metabolic pathways other than respiration and stem growth. After Girdling Period 1, the girdle healed and the effects of girdling on stem respiration were reversed. Stem growth restarted and total radial increment was similar to the control one, indicating that growth can be delayed when a stress event occurs early in the growth period. Concerning tree ring, seasonal shift in substrate use from reserves (i.e., starch) to recently fixed carbohydrate is sufficient to explain the observed (13)C depletion of tree ring during the early wood growth. However, the inter-tree intra-ring delta(13)C variability needs to be resolved in order to improve the interpretation of intra-seasonal ring signals in terms of climatic or ecophysiological information. This study highlighted, via carbohydrate availability effects, the importance of the characterization of stem metabolic pathways for a complete understanding of the delta(13)C signals.
本研究通过实验环割来阻止叶片向茎供应 C，以考察 C 源（储备与当前同化物）对树木 C 同位素信号和茎生长的影响。实验选择了两年生的栓皮栎（Quercus petraea），在叶片生长期内的三个不同物候期进行环割：早期木质部生长期间（环割期 1）、晚期木质部生长期间（环割期 2）和生长停止后不久（环割期 3）。测量的变量包括茎呼吸速率、茎径向增量、呼吸 CO2 的 δ13C 和茎（环割下）和叶片中淀粉和水溶性部分的含量。环割即使在生长季节早期也会停止生长，导致茎 CO2 排放（CO2R）减少。底物利用从最近固定的碳水化合物向储备（即淀粉）的转变导致呼吸 CO2 的 δ13C 富集。然而，单独改变底物类型不足以解释所有观察到的 CO2R δ13C 变化，特别是在与对照树生长速率较大的时期。环割下的质量平衡表明，在环割期间，茎 C 被投入到呼吸和茎生长以外的代谢途径中。环割期 1 后，环割愈合，环割对茎呼吸的影响被逆转。茎生长重新开始，总径向增量与对照相似，表明当生长期间早期发生胁迫事件时，生长可以被延迟。关于树木年轮，从储备（即淀粉）到最近固定的碳水化合物的季节性底物利用转变足以解释早期木质部生长过程中观察到的树木年轮 δ13C 耗竭。然而，需要解决树木内年轮之间的 δ13C 变异性，以便提高对气候或生态生理信息的季节内环信号的解释。本研究通过碳水化合物可用性的影响，强调了对茎代谢途径进行特征描述对于全面理解 δ13C 信号的重要性。
