Mechanical Defences to Herbivory

Abstract

The two major mechanical defences of plants are toughness and hardness. These have different material causes and ecological functions. In any non-metal, high toughness is achieved by composite construction (i.e. by an organized mixture of components). The primary source of toughening in plants is the composite cell wall (cellulosic microfibrils set in a hemicellulose and, sometimes, lignin matrix), with a toughness of 3.45kJm−2, which is ten-times the probable toughness of its individual components if they could be isolated. The toughness of most plant tissues is roughly proportional to the volume fraction of tissue occupied by cell wall (Vc) and, compared to animal tissues and non-biological composites, is very low. High toughness in plant cells is not produced by the walls themselves, but by their plastic intracellular collapse. This is a truly cellular toughening mechanism, one of the most potent ever discovered by materials scientists, depending on an elongate cell shape with microfibrils directed uniformly at a small angle to the cellular axis. Only ‘woody’ cells, tracheids and fibres, have this framework and only in the S2 layer of their secondary wall. Despite this non-optimum configuration, toughness is elevated by this mechanism ten-times above that due to cell wall resistance alone. The effectiveness of toughness in preventing herbivory is indisputable, but largely indirect due to confusion over a false equivalence between nutritional ‘fibre content’ and toughness. In contrast, generalized hardness requires high density. If hardness is due to high Vc, this conflicts with ‘woody’ toughness because there is then no lumen for cell walls to collapse into. Thus, dense seed shells may be brittle (i.e. low toughness) even if built from fibres. However, solid cell wall is not very hard. Instead, high hardness in plants is associated with amorphous silica and is always localized. The efficacy of hardness is more difficult to evaluate than toughness because some animals specialize in coping with it.