Abstract
Plant leaves commonly exhibit a thin, flat structure that facilitates a high light interception per unit mass, but may increase risks of mechanical failure when subjected to gravity, wind and herbivory as well as other stresses. Leaf laminas are composed of thin epidermis layers and thicker intervening mesophyll layers, which resemble a composite material, i.e. sandwich structure, used in engineering constructions (e.g. airplane wings) where high bending stiffness with minimum weight is important. Yet, to what extent leaf laminas are mechanically designed and behave as a sandwich structure remains unclear. To resolve this issue, we developed and applied a novel method to estimate stiffness of epidermis- and mesophyll layers without separating the layers. Across a phylogenetically diverse range of 36 angiosperm species, the estimated Young's moduli (a measure of stiffness) of mesophyll layers were much lower than those of the epidermis layers, indicating that leaf laminas behaved similarly to efficient sandwich structures. The stiffness of epidermis layers was higher in evergreen species than in deciduous species, and strongly associated with cuticle thickness. The ubiquitous nature of sandwich structures in leaves across studied species suggests that the sandwich structure has evolutionary advantages as it enables leaves to be simultaneously thin and flat, efficiently capturing light and maintaining mechanical stability under various stresses.
Highlights
The primary function of plant leaves is recognized as photosynthesis and has been studied intensively from various points of view (Lambers et al, 2008; Blankenship, 2014).it is much less recognised that a large fraction (i.e.14–77%) of leaf dry mass is in structural components i.e. cell walls (Onoda et al, 2011)
The relationship was clearly different from the 1:1 relationship (EB/ET=2.59 ± 0.48, n=36) and close to the theoretical maximum (EB/ET=3), meaning that leaf laminas of these plant species behaved as nearly ideal sandwich structures
The thin cell walls of mesophyll and the presence of intercellular airspaces are primarily important to facilitate a high rate of CO2 diffusion for photosynthesis (Parkhurst, 1977; Terashima et al, 2001), but our study suggests that air spaces can contribute to higher bending stiffness by increasing the second moment of area (I) in the sandwich structure
Summary
The primary function of plant leaves is recognized as photosynthesis and has been studied intensively from various points of view (Lambers et al, 2008; Blankenship, 2014).it is much less recognised that a large fraction (i.e.14–77%) of leaf dry mass is in structural components i.e. cell walls (Onoda et al, 2011). Leaves typically have a flat, thin structure, which is associated with a large leaf surface area per unit biomass, and is ideal for efficient light interception (Givnish, 1986; Braybrook and Kuhlemeier, 2010), but are concomitantly prone to.
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