The mechanical properties of plant cell walls soft material at the subcellular scale: the implications of water and of the intercellular boundaries

Subcellular mechanical characterization of the cell wall can provide important insights into the cell wall’s functional organization, especially if the characterization is not confounded by extracellular factors and intercellular boundaries. However, due to the technical challenges associated with the microscale mechanical characterization of soft biological materials, subcellular investigations of the plant cell wall under tensile loading have yet to be properly performed. This study reports the mechanical characterization of primary onion epidermal cell wall profiles using a novel cryosection-based sample preparation method and a microelectromechanical system-based tensile testing protocol. At the subcellular scale, the cell wall showed biphasic behavior similar to tissue samples. However, instead of a transition zone between the linear elastic or viscoelastic and linear plastic zones, the subcellular-scale samples showed a plateau-like trend with a sharp drop in the modulus value. The critical ranges of stress (20–40 MPa) and strain (5–12 %) of the plateau zone were identified. A strain energy of 1.3 MJ m−3 was calculated at the midpoint of the critical stress–strain range; this value was in accordance with the previously estimated hydrogen bond energy of the cell wall. Subcellular-scale samples showed very large lateral/axial deformations (0.8 ± 0.13) at fracturing. In addition, investigating the cell wall’s mechanical properties at three different water states showed that water is critical for the flow-like behavior of cell wall matrix polymers. These results at subcellular scale provide new insights into biological materials that possess a structural hierarchy at different length scales; which cannot be obtained from tissue-scale experiments.