Department of Structural Biology, Weizmann Institute of Science, 234 Herzl Street, Rehovot 7610001, Israel.
Department of Chemical Research Support, Weizmann Institute of Science, 234 Herzl Street, Rehovot 7610001, Israel.
Acta Biomater. 2019 Apr 1;88:111-119. doi: 10.1016/j.actbio.2019.02.023. Epub 2019 Feb 16.
We investigate the structures and mechanical properties of leaf midribs of Ficus microcarpa and Prunus dulcis, which deposit calcium oxalate crystals, and of Olea europaea midribs which contain no mineral deposits, but do contain lignified fibers. The midrib mechanical performance contributes to the leaf's ability to maintain a flat conformation for light harvesting and to efficiently reconfigure to reduce wind drag. We use a novel approach involving 3D visualization of the vein structure during mechanical load. This involves the use of customized mechanical loading devices that fit inside a microCT chamber. We show that the elastic, compression and torsional moduli of the midribs of leaves from the 3 species examined vary significantly. We also observed different modes of fracture and buckling of the leaves during compression. We assess the contributions of the calcium oxalate crystals to the mechanical and fracture properties. In F. microcarpa midrib linear arrays of calcium oxalate crystals contribute to resisting the bending, in contrast to P. dulcis leaves, where the calcium oxalate crystals do not resist bending. In both F. microcarpa and P. dulcis isolated calcium oxalate crystals enable high torsional compliance. The integrated microCT - mechanical testing approach could be used to investigate the structure-mechanics relationships in other complex biological samples. STATEMENT OF SIGNIFICANCE: Leaves need to maintain a flat conformation for light harvesting, but they also need to efficiently reconfigure to reduce wind drag. The leaf central vein (midrib) is a key structural component for leaf mechanicss. 3D visualization of the vein structure under mechanical loads showed that veins can be stiffened by reinforcement units composed of calcium oxalates crystals and lignin. The stiffening units can influence the bending and fracture properties of the midribs, and can contribute to determine if buckling will occur during folding. Mineral stiffening elements could be a widespread strategy to reinforce leaf veins and other biological structures. This structural-mechanical approach could be used to study other complex biological samples.
我们研究了榕树叶和李树叶中存在草酸钙晶体的中脉以及油橄榄叶中脉的结构和力学性能，油橄榄叶中脉不含矿物质，但含有木质化纤维。中脉的机械性能有助于叶片保持平面构象以进行光捕获，并能有效地重新配置以减少风阻。我们采用了一种新的方法，即在机械加载过程中对脉结构进行 3D 可视化。这涉及使用定制的机械加载装置，这些装置可安装在 microCT 室内部。我们表明，所研究的 3 种植物叶片中脉的弹性、压缩和扭转模量差异显著。我们还观察到在压缩过程中叶片的不同断裂和屈曲模式。我们评估了草酸钙晶体对力学和断裂性能的贡献。在榕树叶中脉中，线性排列的草酸钙晶体有助于抵抗弯曲，而在李树叶中，草酸钙晶体不能抵抗弯曲。在榕树叶和李树叶中，分离的草酸钙晶体使叶片具有较高的扭转柔顺性。集成的 microCT-机械测试方法可用于研究其他复杂生物样本的结构-力学关系。
叶片需要保持平面构象以进行光捕获，但也需要有效地重新配置以减少风阻。叶片中央脉（中脉）是叶片力学的关键结构组成部分。在机械载荷下对脉结构进行 3D 可视化表明，由草酸钙晶体和木质素组成的增强单元可以使脉变硬。增强单元可以影响中脉的弯曲和断裂性能，并有助于确定在折叠过程中是否会发生屈曲。矿质增强元素可能是增强叶片脉和其他生物结构的一种广泛策略。这种结构-力学方法可用于研究其他复杂的生物样本。
