Data from: The evolution of photosynthetic anatomy in Viburnum (Adoxaceae)

Premise of research: Leaf mesophyll is often differentiated into a palisade layer with tightly packed, elongated cells (I-cells) and a spongy layer with loosely packed, complex shaped cells. An alternative palisade type, composed of branched H-cells, has evolved in a number of plant lineages. Viburnum (Adoxaceae) possesses both types of palisade, providing an opportunity to assess the significance of evolutionary switches between these forms. Methodology: An anatomical survey of 80 species spanning the Viburnum phylogeny permitted an analysis of palisade differences in relation to other characters. A geometric model of leaf mesophyll surface area for CO2 absorption correlated well with measured photosynthetic capacity in a subset of species, allowing us to infer shifts in photosynthetic function. Pivotal results: Ancestrally, viburnums probably produced a palisade with one layer of H-cells. Multiple transitions to two layers of H-cells (H2) and to one or two layers of I-cells (I1, I2) occurred. These shifts were correlated with increases in photosynthetic capacity, and H2 appear functionally equivalent to I1 with respect to CO2 absorption. Conclusions: Photosynthetic anatomy H2 and I1 palisade may represent alternative evolutionary solutions for increasing leaf CO2 absorption. Additionally, H-cells and I-cells might perform differently with respect to light absorption and/or drought tolerance. The evolution of I-palisade cells may thus have tracked movements into open environments, while H2 could increase photosynthetic capacity in the forest understory.

研究背景：植物叶肉通常分化为两类组织：一类为栅栏组织，由排列紧密的长条形I型细胞（I-cells）构成；另一类为海绵组织，由排列疏松、形态复杂的细胞构成。另有一类由分枝状H型细胞（H-cells）构成的栅栏组织亚型，已在多个植物演化支系中独立演化出现。荚蒾属（Viburnum，五福花科Adoxaceae）同时具备这两类栅栏组织，为探究这两种结构间演化转换的生物学意义提供了理想研究体系。

研究方法：本研究对覆盖荚蒾属系统发育（phylogeny）的80个物种开展解剖学调查，以此分析栅栏组织差异与其他性状的关联。针对部分物种构建了用于评估CO₂吸收效率的叶肉表面积几何模型，该模型与实测的光合能力（photosynthetic capacity）拟合度极佳，借此可推断光合功能的演化变迁。

关键结果：荚蒾属的祖先类群大概率仅具单层H型细胞栅栏组织。后续多次发生演化过渡，分别形成两层H型细胞栅栏组织（H2型）以及单层或双层I型细胞栅栏组织（I1、I2型）。这些结构转换均与光合能力的提升显著相关，且在CO₂吸收效率方面，H2型栅栏组织与I1型功能等效。

研究结论：H2型与I1型栅栏组织或代表了提升叶片CO₂吸收效率的两类独立演化解决方案。此外，H型细胞与I型细胞在光吸收效率和/或耐旱性（drought tolerance）方面可能存在功能分化。I型栅栏组织的演化或与植物向开阔生境的迁移同步，而H2型栅栏组织则可能提升森林林下生境中的光合能力。