Department of Life Science, Tokyo University of Pharmacy and Life Sciences, Hachioji, Tokyo 192-0392, Japan.
Department of Biological Sciences, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Ann Bot. 2022 Sep 19;130(3):265-283. doi: 10.1093/aob/mcac100.
Plants invest photosynthates in construction and maintenance of their structures and functions. Such investments are considered costs. These costs are recovered by the CO2 assimilation rate (A) in the leaves, and thus A is regarded as the immediate, short-term benefit. In photosynthesizing leaves, CO2 diffusion from the air to the carboxylation site is hindered by several structural and biochemical barriers. CO2 diffusion from the intercellular air space to the chloroplast stroma is obstructed by the mesophyll resistance. The inverses is the mesophyll conductance (gm). Whether various plants realize an optimal gm, and how much investment is needed for a relevant gm, remain unsolved.
This review examines relationships among leaf construction costs (CC), leaf maintenance costs (MC) and gm in various plants under diverse growth conditions. Through a literature survey, we demonstrate a strong linear relationship between leaf mass per area (LMA) and leaf CC. The overall correlation of CC vs. gm across plant phylogenetic groups is weak, but significant trends are evident within specific groups and/or environments. Investment in CC is necessary for an increase in LMA and mesophyll cell surface area (Smes). This allows the leaf to accommodate more chloroplasts, thus increasing A. However, increases in LMA and/or Smes often accompany other changes, such as cell wall thickening, which diminishes gm. Such factors that make the correlations of CC and gm elusive are identified.
For evaluation of the contribution of gm to recover CC, leaf life span is the key factor. The estimation of MC in relation to gm, especially in terms of costs required to regulate aquaporins, could be essential for efficient control of gm over the short term. Over the long term, costs are mainly reflected in CC, while benefits also include ultimate fitness attributes in terms of integrated carbon gain over the life of a leaf, plant survival and reproductive output.
植物将光合作用产物投资于其结构和功能的构建和维护。这些投资被视为成本。这些成本由叶片的二氧化碳同化率（A）回收，因此 A 被视为直接的、短期的收益。在进行光合作用的叶片中，二氧化碳从空气中扩散到羧化部位受到几个结构和生化屏障的阻碍。胞间空气空间到叶绿体基质的 CO2 扩散受到叶肉阻力的阻碍。相反的是叶肉导度（gm）。各种植物是否实现了最佳的 gm，以及实现相关 gm 需要多少投资，仍然没有解决。
本综述考察了在不同生长条件下，各种植物的叶片构建成本（CC）、叶片维护成本（MC）和 gm 之间的关系。通过文献调查，我们证明了叶面积与叶片 CC 之间存在很强的线性关系。CC 与 gm 之间的整体相关性在植物系统发育群中较弱，但在特定群体和/或环境中存在明显的趋势。增加 CC 是增加 LMA 和叶肉细胞表面积（Smes）所必需的。这使得叶片能够容纳更多的叶绿体，从而增加 A。然而，LMA 和/或 Smes 的增加通常伴随着其他变化，如细胞壁增厚，这会降低 gm。确定了使 CC 和 gm 的相关性难以捉摸的因素。
为了评估 gm 对 CC 回收的贡献，叶片寿命是关键因素。gm 与 MC 的关系，特别是在调节水通道蛋白所需的成本方面的关系，对于短期有效地控制 gm 可能是至关重要的。从长远来看，成本主要反映在 CC 上，而收益还包括叶片整个生命周期内综合碳增益、植物生存和繁殖输出等最终适应属性。
