Department of Ecology and Evolutionary Biology, Brown University, 80 Waterman Street, Box G-W, Providence, RI 02912, USA.
Curr Biol. 2013 Apr 22;23(8):722-6. doi: 10.1016/j.cub.2013.03.029. Epub 2013 Apr 11.
Succulent water storage is a prominent feature among plants adapted to arid zones, but we know little about how succulence evolves and how it is integrated into organs already tasked with multiple functions. Increased volume in succulent leaves, for example, may result in longer transport distances between veins and the cells that they supply, which in turn could negatively impact photosynthesis. We quantified water storage in a group of 83 closely related species to examine the evolutionary dynamics of succulence and leaf venation. In most leaves, vein density decreased with increasing succulence, resulting in significant increases in the path length of water from veins to evaporative surfaces. The most succulent leaves, however, had a distinct three-dimensional (3D) venation pattern, which evolved 11-12 times within this small lineage, likely via multiple developmental pathways. 3D venation "resets" internal leaf distances, maintaining moderate vein density in extremely succulent tissues and suggesting that the evolution of extreme succulence is constrained by the need to maintain an efficient leaf hydraulic system. The repeated evolution of 3D venation decouples leaf water storage from hydraulic path length, facilitating the evolutionary exploration of novel phenotypic space.
肉质水分存储是适应干旱地区的植物的一个显著特征，但我们对肉质的进化方式以及它如何融入已经承担多种功能的器官知之甚少。例如，肉质叶片体积的增加可能导致叶脉和它们供应的细胞之间的运输距离延长，这反过来又可能对光合作用产生负面影响。我们量化了 83 种密切相关物种的水分存储，以研究肉质和叶片脉络的进化动态。在大多数叶片中，随着肉质程度的增加，叶脉密度降低，导致水分从叶脉到蒸腾面的路径长度显著增加。然而，最肉质的叶片具有独特的三维（3D）脉络模式，在这个小谱系中进化了 11-12 次，可能通过多种发育途径。3D 脉络“重置”了内部叶片距离，在极度肉质组织中保持适度的叶脉密度，这表明极端肉质的进化受到维持高效叶片水力系统的需要的限制。3D 脉络的反复进化将叶片水分存储与水力路径长度解耦，促进了新颖表型空间的进化探索。
