Leaves are exposed to different light conditions according to their canopy position, resulting in structural and anatomical differences with consequences for carbon uptake. While these structure–function relationships have been thoroughly explored in dense forest canopies, such gradients may be diminished in open canopies, and they are often ignored in ecosystem models. We tested within-canopy differences in photosynthetic properties and structural traits in leaves in a mature Eucalyptus tereticornis canopy exposed to long-term elevated CO₂ for up to three years. We explored these traits in relation to anatomical variation and diffusive processes for CO₂ (i.e., stomatal conductance, g_s and mesophyll conductance, g_m) in both upper and lower portions of the canopy receiving ambient and elevated CO₂. While shade resulted in 13% lower leaf mass per area ratio (M_A) in lower versus upper canopy leaves, there was no relationship between leaf N_mass and canopy gap fraction. Both maximum carboxylation capacity (V_cmax) and maximum electron transport (J_max) were ~ 18% lower in shaded leaves and were also reduced by ~ 22% with leaf aging. In mature leaves, we found no canopy differences for g_m or g_s, despite anatomical differences in M_A, leaf thickness and mean mesophyll thickness between canopy positions. There was a positive relationship between net photosynthesis and g_m or g_s in mature leaves. Mesophyll conductance was negatively correlated with mean parenchyma length, suggesting that long palisade cells may contribute to a longer CO₂ diffusional pathway and more resistance to CO₂ transfer to chloroplasts. Few other relationships between g_m and anatomical variables were found in mature leaves, which may be due to the open crown of Eucalyptus. Consideration of shade effects and leaf-age dependent responses to photosynthetic capacity and mesophyll conductance are critical to improve canopy photosynthesis models and will improve understanding of long-term responses to elevated CO₂ in tree canopies.

中文翻译：

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冠层位置会影响二氧化碳浓度升高的成熟桉树的光合作用和解剖结构。

叶片根据其冠层位置暴露在不同的光照条件下，导致结构和解剖学差异，从而影响碳的吸收。尽管在茂密的森林冠层中已经充分探索了这些结构-功能关系，但在开放冠层中此类梯度可能会减小，并且在生态系统模型中通常会忽略它们。我们在暴露于长期升高的CO ₂长达三年的成熟桉树冠层冠层中，测试了冠层内部叶片的光合特性和结构性状差异。我们探讨了与CO _2的解剖学变异和扩散过程有关的这些特征（即气孔电导，g _s和叶肉电导，g _m）在顶棚的上部和下部都接受周围和升高的CO ₂。而遮阳导致每面积比（13％下叶质量中号_甲在较低与上部顶盖叶），有叶片N之间没有任何关系_的质量和树冠间隙分数。遮荫叶片的最大羧化能力（V _cmax）和最大电子传递（J _max）均降低约18％，并且随着叶片老化而降低约22％。在成熟叶片中，尽管M _A的解剖学差异，我们没有发现g _m或g _s的冠层差异，叶厚度和冠层位置之间的平均叶肉厚度。成熟叶片的净光合作用与g _m或g _s呈正相关。叶肉电导率与薄壁组织平均长度呈负相关，表明长的栅栏细胞可能有助于更长的CO ₂扩散途径和对CO ₂转移至叶绿体的更大抵抗力。在成熟叶片中，g _m与解剖学变量之间几乎没有其他关系，这可能是由于桉树的冠开放而引起的。。考虑阴影效应和叶龄对光合作用能力和叶肉电导的响应对改善冠层光合作用模型至关重要，并将增进对树冠对CO ₂升高长期响应的理解。