Tholen Danny, Zhu Xin-Guang
Chinese Academy of Sciences and Max Planck Society Partner Institute for Computational Biology, Key Laboratory of Computational Biology, Shanghai 200031, People's Republic of China.
Plant Physiol. 2011 May;156(1):90-105. doi: 10.1104/pp.111.172346. Epub 2011 Mar 25.
Photosynthesis is limited by the conductance of carbon dioxide (CO(2)) from intercellular spaces to the sites of carboxylation. Although the concept of internal conductance (g(i)) has been known for over 50 years, shortcomings in the theoretical description of this process may have resulted in a limited understanding of the underlying mechanisms. To tackle this issue, we developed a three-dimensional reaction-diffusion model of photosynthesis in a typical C(3) mesophyll cell that includes all major components of the CO(2) diffusion pathway and associated reactions. Using this novel systems model, we systematically and quantitatively examined the mechanisms underlying g(i). Our results identify the resistances of the cell wall and chloroplast envelope as the most significant limitations to photosynthesis. In addition, the concentration of carbonic anhydrase in the stroma may also be limiting for the photosynthetic rate. Our analysis demonstrated that higher levels of photorespiration increase the apparent resistance to CO(2) diffusion, an effect that has thus far been ignored when determining g(i). Finally, we show that outward bicarbonate leakage through the chloroplast envelope could contribute to the observed decrease in g(i) under elevated CO(2). Our analysis suggests that physiological and anatomical features associated with g(i) have been evolutionarily fine-tuned to benefit CO(2) diffusion and photosynthesis. The model presented here provides a novel theoretical framework to further analyze the mechanisms underlying diffusion processes in the mesophyll.
光合作用受二氧化碳（CO₂）从细胞间隙扩散到羧化位点的传导率限制。尽管内部传导率（g(i)）的概念已为人所知50多年，但该过程理论描述中的缺陷可能导致对其潜在机制的理解有限。为解决这一问题，我们构建了一个典型C₃叶肉细胞光合作用的三维反应扩散模型，该模型涵盖了CO₂扩散途径的所有主要成分及相关反应。利用这个全新的系统模型，我们系统且定量地研究了g(i)的潜在机制。我们的结果表明，细胞壁和叶绿体包膜的阻力是光合作用最显著的限制因素。此外，基质中碳酸酐酶的浓度也可能限制光合速率。我们的分析表明，较高水平的光呼吸会增加对CO₂扩散的表观阻力，而在确定g(i)时，这一效应迄今一直被忽视。最后，我们表明通过叶绿体包膜向外泄漏的碳酸氢盐可能导致在高CO₂浓度下观察到的g(i)下降。我们的分析表明，与g(i)相关的生理和解剖特征在进化过程中经过了精细调整，以利于CO₂扩散和光合作用。本文提出的模型为进一步分析叶肉中扩散过程的潜在机制提供了一个全新的理论框架。
