A substantial portion of locally respired CO2 in stems can be assimilated by chloroplast-containing tissues. Woody tissue photosynthesis (Pwt ) therefore plays a major role in the stem carbon balance. To study the impact of Pwt on stem carbon cycling along a gradient of water availability, stem CO2 efflux (EA ), xylem CO2 concentration ([CO2 ]), and xylem water potential (Ψxylem ) were measured in 4-year-old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well-watered conditions, local Pwt decreased EA up to 30%. Axial CO2 diffusion (Dax ) induced by distant Pwt caused an additional decrease in EA of up to 25% and limited xylem [CO2 ] build-up. Under drought stress, absolute decreases in EA driven by Pwt remained stable, denoting that Pwt was not affected by drought. At the end of the dry period, when transpiration was low, local Pwt and Dax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of Pwt for an adequate interpretation of EA measurements and (b) homeostatic Pwt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited.

中文翻译：

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木质组织的光合作用使茎部CO2的外流减少了一半，并且不受幼小杨树的干旱胁迫的影响。

茎中的大部分局部呼吸的CO2可以被含叶绿体的组织吸收。因此，木质组织的光合作用（Pwt）在茎碳平衡中起主要作用。为了研究Pwt对沿水可利用量梯度的茎碳循环的影响，在4岁的Populus tremula中测量了茎CO2外排量（EA），木质部CO2浓度（[CO2]）和木质部水势（Ψxylem） L.树木暴露于干旱胁迫和木质组织在不同光照条件下的排斥。在浇水条件良好的情况下，本地Pwt将EA降低了30％。遥远的Pwt引起的轴向CO2扩散（Dax）导致EA进一步降低最多25％，并限制了木质部[CO2]积累。在干旱胁迫下，由Pwt驱动的EA绝对降低保持稳定，这表明Pwt不受干旱影响。在干燥期结束时，当蒸腾量较低时，局部Pwt和Dax每天分别抵消茎干呼吸的20％和10％。这些结果突显了（a）Pwt对于充分解释EA值的重要性，以及（b）沿干旱胁迫梯度的稳态Pwt，当限制叶片碳的吸收和韧皮部的运输时，这可能在促进茎部新陈代谢方面起着至关重要的作用。