Landcare Research, PO Box 40, Lincoln 7640, New Zealand.
Tree Physiol. 2011 Sep;31(9):985-96. doi: 10.1093/treephys/tpr021. Epub 2011 Apr 22.
Mesophyll conductance, g(m), was estimated from measurements of stomatal conductance to carbon dioxide transfer, g(s), photosynthesis, A, and chlorophyll fluorescence for Year 0 (current-year) and Year 1 (1-year-old) fully sunlit leaves from short (2 m tall, 10-year-old) and tall (15 m tall, 120-year-old) Nothofagus solandrii var. cliffortiodes trees growing in adjacent stands. Rates of photosynthesis at saturating irradiance and ambient CO(2) partial pressure, A(satQ), were 25% lower and maximum rates of carboxylation, V(cmax), were 44% lower in Year 1 leaves compared with Year 0 leaves across both tree sizes. Although g(s) and g(m) were not significantly different between Year 0 and Year 1 leaves and g(s) was not significantly different between tree heights, g(m) was significantly (19%) lower for leaves on tall trees compared with leaves on short trees. Overall, V(cmax) was 60% higher when expressed on the basis of CO(2) partial pressure at the chloroplasts, C(c), compared with V(cmax) on the basis of intercellular CO(2) partial pressure, C(i), but this varied with leaf age and tree size. To interpret the relative stomatal and mesophyll limitations to photosynthesis, we used a model of carbon isotopic composition for whole leaves incorporating g(m) effects to generate a surface of 'operating values' of A over the growing season for all leaf classes. Our analysis showed that A was slightly higher for leaves on short compared with tall trees, but lower g(m) apparently reduced actual A substantially compared with A(satQ). Our findings showed that lower rates of photosynthesis in Year 1 leaves compared with Year 0 leaves were attributable more to increased biochemical limitation to photosynthesis in Year 1 leaves than differences in g(m). However, lower A in leaves on tall trees compared with those on short trees could be attributed in part to lower g(m) and higher stomatal, L(s), and mesophyll, L(m), limitations to photosynthesis, consistent with steeper hydraulic gradients in tall trees.
叶肉导度（g(m)）是通过对二氧化碳转移的气孔导度（g(s)）、光合作用（A）和叶绿素荧光的测量来估算的，用于测量 0 年（当年）和 1 年（1 年生）全光照叶片。0 年和 1 年生叶片取自相邻林分中高矮两种不同大小的辐射生长的新几内亚热带南洋杉（Nothofagus solandrii var. cliffortiodes），小树（高 2 米，树龄 10 年）和大树（高 15 米，树龄 120 年）。与 0 年生叶片相比，1 年生叶片在饱和光照和大气 CO2 分压下的光合速率（A(satQ)）低 25%，羧化最大速率（V(cmax)）低 44%。尽管 0 年和 1 年生叶片的 g(s)没有显著差异，大树和小树之间 g(s)也没有显著差异，但大树叶片的 g(m)比小树叶片低 19%。总的来说，当以叶绿体 CO2 分压（C(c)）为基础表示 V(cmax)时，V(cmax)比以细胞间 CO2 分压（C(i)）为基础表示 V(cmax)高 60%，但这与叶片年龄和树木大小有关。为了解释光合作用的相对气孔和叶肉限制，我们使用了一种整合 g(m)效应的整个叶片碳同位素组成模型，生成了所有叶片类型在整个生长季的 A“工作值”表面。我们的分析表明，与大树相比，小树叶片的 A 略高，但较低的 g(m)显然使实际 A 比 A(satQ)显著降低。我们的研究结果表明，与小树相比，大树叶片的 A 较低，但 g(m)较低是导致 1 年生叶片光合作用降低的主要原因，而不是 g(m)的差异。然而，与小树相比，大树叶片的 A 较低可能部分归因于较低的 g(m)和较高的气孔限制（L(s)）和叶肉限制（L(m)），这与大树中更陡的水力梯度一致。
