Relationships between primary plant cell wall architecture and mechanical properties for onion bulb scale epidermal cells

This article investigates onion epidermal tissue (Allium cepa) using a combination of mechanical testing, microscopy and modeling and relates tissue mechanical properties to the known structure of the cell walls. Onion epidermal tissue has a simple, regular structure of elongated cells, which have been used to enable the contributions to mechanical properties of cell walls and of higher order structures to be separated and analyzed. Two models of wall behavior were used to explore how Poisson's ratio of cell walls parallel to the plane of the epidermal surface may vary with applied strain. In the first model, cellulose microfibrils can be reorientated in an unrestricted way with the result that the cell wall volume decreases. In the second model the volume of the cell wall remains constant, which controls the reorientation of microfibrils, hence the Poisson's ratio. Measurements made from uniaxially stretched cells show that the data most closely fits model I, therefore, it is concluded that the bulk of the matrix has little influence on the observed mechanical properties (at a test rate of 1 mm/min), allowing cellulose microfibrils to reorient through the matrix in an unrestricted way during uniaxial tests. In its mechanical attributes the primary cell wall resembles more a knitted cloth than a semisolid composite material. When biaxial stretching is applied to tissue, so that there is no re-orientation of microfibrils; the cell wall material is still able to reach surprisingly large elastic strains of up to 12.5% and no plastic deformation was recorded. Current theory suggests that cellulose microfibrils can stretch elastically by a maximum of 7%, therefore further work is required to identify mechanisms that could account for the extra elastic strain.