The toughness of plant cell walls

Abstract

All plant tissues and plant-based materials are composites and therefore, during standard fracture mechanics tests, cracks within them tend to arrest and deflect because of crack-stopping mechanisms at cell boundaries or in air-spaces. Due to this change of direction cracks do not cross the toughest structures, frustrating both their measurement and the understanding of the cracking process. Accordingly, there are no accurate values for the toughness of plant cell walls. We have attempted to solve this problem here by driving the crack with blades. We show from cutting experiments on twenty individual plant tissues and plant-based materials that the intrinsic toughness of plant cell wall, independent of cell form, is between 3.4-4.2 kJ m$^{-2}$; for any tissue it is directly proportional to the volume fraction that the cell wall occupies. Plastic work, which is dependent on cellular geometry, can increase toughness to a value of at least 30 kJ m$^{-2}$ in woody tissues, but this capacity is probably not linearly related to cell wall volume fraction.