Plant form and fracture mechanics at the plant level

Plants are the staple source of the mammalian herbivore diet. The leaves are generally flat and engineered to capture sunlight for photosynthesis, the primary process that leads to the production of energy, a source that animal subsistence and production is dependent upon. Plants themselves are complex but can be divided into three main morphological organs — roots, stems, and leaves — which are each exposed to environmental forms of mechanical strain. Leaves of monocots are constructed from vascular tissue that forms in parallel strands (veins) extending along the long axis of the leaf. This vascular tissue is supported by mesophyll tissue (i.e., sclerenchyma, storage parenchyma, and clorenchyma cells) and is covered by waxy epidermal tissue that reduces water loss from evaporation. Sclerenchyma is of great interest in materials science because these cells have thick, rigid, nonstretchable secondary walls that confer strength to the plant. There is wide variation in the interveinal distance between plant species, but a small interveinal distance does not necessarily imply that a leaf will be less digestible to livestock [14]. It is the organization of the sclerenchyma bundles that determines fracture properties, and this fact formed the basis of the comment from Wright and Illius [15] that the properties determining digestion were essentially those influencing fracture mechanics. Although fibers may only constitute a very small proportion (5%) of the leaf cross-sectional area, that seemingly small proportion accounts for 90 to 95% of longitudinal stiffness [16].

Animal scientists interested in digestive function have long been interested in the fracture of cellular material as an indicator of its susceptibility to crushing and shearing forces during rumination. As a result, strong working relationships have been forged between animal scientists and plant breeders, with much of the work instigated by plant breeders aimed at selecting for plant traits that increased feeding value (FV), and this has been reflected in the strong focus on screening for low shear strength [17]. Plant fracture properties have also been assessed and related to forage avoidance and/or preference and forage intake [18-21], trampling resistance [22], and plant uprooting ("pulling") [23]. Additionally, the impact of environmental constraints on plant fracture properties have been evaluated [16,24] alongside the relationships with bite force [25,26] and bite dimensions [27,28]. Studies predominantly investigate the fracture mechanics of leaves since leaves are innately the preferred morphological organ. Nevertheless, there have been important contributions from examination of the stem properties of monocotyledons [15] and dicotyledons [18,29,30].

There is a broad range of terminology within the subject of fracture mechanics in plants. In an agricultural context, one of the pioneering studies examining plant fracture properties was the work by Evans [31], but like many other studies, this research has attracted widespread criticism for the inconsistency in adhering to the fundamental engineering principles underlying fracture mechanics. Several published studies and review articles have addressed the confusion in the use of descriptive terms and the units of expression for defining the fracture mechanics in plants. This has generally led to better application of terminology, but incorrect parameters and units still surface in the literature [22,23], largely due to the subjective nature of the experimental objectives.

Briefly, fracture in a test specimen involves both the initiation and propagation of a crack. Cracks can be propagated by three contrasting modes: mode I is by tension (crack opening), mode II is by shear (edge-sliding or in-plane shearing movement), and mode III is caused by tearing (out-of-plane shearing movement) [32]. Where ruminants are concerned, mode I fracture tests best describe the harvesting of forage in a predominantly vertical dimension while mode III fracture represents the mechanisms of fracture that take place when forage is crushed and ground against the molars during chewing.

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FIGURE 5.2 Simulated force-displacement curves for lamina from (a) tensile (mode I) and (b) out-of-plane shear (mode III) fracture tests.

It can also be helpful to be familiar with how materials perform under load. Figure 5.2a and 5.2b illustrate simplified representations of plant lamina when tested under tension (mode I) and out-of-plane shear (mode III) modes, respectively. The triangular-shaped force-displacement curve in Figure 5.2a illustrates the dynamics of a leaf under tension. The curve represents a steady linear increase in force, the slope being an indicator of the leaf's stiffness, until the leaf specimen fractures, at which point the material ceases to be elastic, resulting in a sudden decrease in force to zero immediately after fracture. By contrast, the spiky force-displacement curve in Figure 5.2b illustrates the dynamic relationship of the shearing of a leaf specimen, where the force is constantly changing in a controlled manner as a crack is propagated across the specimen.

The objective of this chapter is to review and discuss the role of plant-animal mechanics in understanding bite procurement. The focus of the remainder of this chapter, therefore, concerns itself with only mode I fracture where relevant. Three plant-based terms of interest in understanding the procurement constraints facing grazing ruminants were summarized by Griffiths and Gordon [33]:

• Fracture force is a measure of the force required to fracture a plant organ under tension and can be assessed from the maximum force recorded on the force-displacement curve that produces fracture.

• Tensile strength is the fracture force under tension per unit of cross-sectional area of the plant specimen.

• Resistance is a plant-based term that has no underlying engineering concept, but it carries importance in the application of plant fracture mechanics to predicting foraging strategies in ruminants. It can be defined as the accumulated force required by the animal to sever all the plant organs encompassed within the bite. It can be argued that tensile strength is a measure of resistance, but in relation to ruminants, we are interested in the resistance of the bite contents to rupture under load, and hence the accumulation of plant material.

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