University of New Mexico, Department of Biology, MSC03-2020, Albuquerque, NM 87131, USA.
J Exp Bot. 2010 Jul;61(12):3223-33. doi: 10.1093/jxb/erq137. Epub 2010 May 25.
Mesophyll conductance to CO(2) (g(m)) limits carbon assimilation and influences carbon isotope discrimination (Delta) under most environmental conditions. Current work is elucidating the environmental regulation of g(m), but the influence of g(m) on model predictions of Delta remains poorly understood. In this study, field measurements of Delta and g(m) were obtained using a tunable diode laser spectroscope coupled to portable photosynthesis systems. These data were used to test the importance of g(m) in predicting Delta using the comprehensive Farquhar model of Delta (Delta(comp)), where g(m) was parameterized using three methods based on: (i) mean g(m); (ii) the relationship between stomatal conductance (g(s)) and g(m); and (iii) the relationship between time of day (TOD) and g(m). Incorporating mean g(m), g(s)-based g(m), and TOD-based g(m) did not consistently improve Delta(comp) predictions of field-grown juniper compared with the simple model of Delta (Delta(simple)) that omits fractionation factors associated with g(m) and decarboxylation. Sensitivity tests suggest that b, the fractionation due to carboxylation, was lower (25 per thousand) than the value commonly used in Delta(comp) (29 per thousand) and Delta(simple) (27 per thousand). These results demonstrate the limits of all tested models in predicting observed juniper Delta, largely due to unexplained offsets between predicted and observed values that were not reconciled in sensitivity tests of variability in g(m), b, or e, the day respiratory fractionation.
在大多数环境条件下，CO2 叶肉导度（g(m)）限制碳同化并影响碳同位素分馏（Delta）。目前的工作正在阐明 g(m) 的环境调控，但 g(m) 对 Delta 模型预测的影响仍知之甚少。在这项研究中，使用可调二极管激光光谱仪与便携式光合作用系统耦合，进行了 Delta 和 g(m) 的现场测量。这些数据用于使用 Delta 的综合 Farquhar 模型（Delta(comp)）来测试 g(m) 对预测 Delta 的重要性，其中 g(m) 是使用基于以下三种方法的参数化：（i）平均 g(m)；（ii）气孔导度（g(s)）与 g(m)的关系；和 (iii) 时间（TOD）与 g(m)的关系。与简单的 Delta 模型（Delta(simple)）相比，纳入平均 g(m)、基于 g(s)的 g(m) 和基于 TOD 的 g(m)并不能一致地改善对田间生长的杜松的 Delta(comp)预测，而 Delta(simple)则省略了与 g(m)和脱羧作用相关的分馏因子。敏感性测试表明，由于羧化作用导致的分馏（b）（25 每千）低于在 Delta(comp)（29 每千）和 Delta(simple)（27 每千）中常用的值。这些结果表明，所有测试模型在预测观测到的杜松 Delta 方面都存在局限性，主要是由于预测值和观测值之间存在无法解释的偏差，而这些偏差在 g(m)、b 或 e（日间呼吸分馏）的变异性敏感性测试中无法解决。
