Saveyn An, Steppe Kathy, Lemeur Raoul
Applied Ecology and Environmental Biology, Ghent University, Coupure Links 653, Ghent, 9000, Belgium.
Ann Bot. 2007 Mar;99(3):477-85. doi: 10.1093/aob/mcl268. Epub 2007 Jan 4.
Daytime CO2 efflux rates (FCO2) from tree stems are often reported to be lower than expected from the exponential relationship between temperature and respiration. Explanations of daytime depression in FCO2 have focused on the possible role of internal CO2 transport in the xylem. However, another possible cause that has been overlooked is the daily dynamics of the water status in the living stem tissues and its influence on stem growth rate and thus respiration. The objective of this study was to assess the daily dynamics of stem water status and growth rate and to determine the extent to which they may be linked to daily variations in stem FCO2.
FCO2 of young beech and oak stems were measured under controlled conditions. Relative stem turgor pressure (Psi(p)), obtained from simulations with the 'RCGro' model, was used as an indicator of the water status in the living stem tissues. Daily dynamics of stem growth were derived from Psi(p): growth was assumed to occur when Psi(p) exceeded a relative threshold value.
There was a strong correspondence between fluctuations in FCO2 and simulated Psi(p). The non-growth conditions during daytime coincided with depressions in FCO2. Moreover, FCO2 responded to changes in Psi(p) in the absence of growth, indicating also that maintenance processes were influenced by the water status in the living stem tissues.
Daytime depressions in stem FCO2 correlate with the daily dynamics of turgor, as a measure of the water status in the living stem tissues: it is suggested that water status of tree stems is a potentially important determinant of stem FCO2, as it influences the rate of growth and maintenance processes in the living tissues of the stem.
通常报道树木茎干白天的二氧化碳排放速率（FCO₂）低于根据温度与呼吸作用之间的指数关系所预期的值。对FCO₂白天降低现象的解释主要集中在木质部内部二氧化碳运输可能发挥的作用上。然而，另一个被忽视的可能原因是活茎组织中水分状况的日动态变化及其对茎干生长速率进而对呼吸作用的影响。本研究的目的是评估茎干水分状况和生长速率的日动态变化，并确定它们与茎干FCO₂日变化的关联程度。
在受控条件下测量年轻山毛榉和橡树茎干的FCO₂。通过“RCGro”模型模拟得到的相对茎干膨压（Ψ(p)）被用作活茎组织中水分状况的指标。茎干生长的日动态变化由Ψ(p)推导得出：当Ψ(p)超过相对阈值时假定发生生长。
FCO₂的波动与模拟的Ψ(p)之间存在很强的对应关系。白天的非生长条件与FCO₂的降低同时出现。此外，在没有生长的情况下，FCO₂对Ψ(p)的变化有响应，这也表明维持过程受活茎组织中水分状况的影响。
茎干FCO₂的白天降低与膨压的日动态变化相关，膨压可作为活茎组织中水分状况的一种度量：表明树木茎干的水分状况可能是茎干FCO₂的一个重要决定因素，因为它影响茎干活组织中的生长速率和维持过程。
