Fig. 5
Fracture biomechanics of the pericarp and the effect of fungal degradation on the mechanical resistance of distinct Lepidium didymum fruit valve regions. a Comparative puncture force analysis of the distal and proximal pericarp regions. Fungal activity caused a drastic decrease by ~52% in the mechanical resistance (breaking strength) of the distal pericarp (p-value < 0.001) where the pericarp rupture is initiated and the radicle will emerge. A smaller but significant decrease by fungal activity of ~30% was evident in the proximal (p-value = 0.04) and dorsal (Supplementary Fig. 2c) pericarp regions. To conduct the biomechanical analyses, fruit valves of accession KM2423 were incubated at standard conditions for the time indicated either untreated (fungi colonise the pericarp) or surface-sterilised (no fungi). No decrease in the mechanical resistance was observed upon surface-sterilisation (distal p-value = 0.89; proximal p-value > 0.99). Mean values ± SE (N ≥ 26). b Comparative force–displacement curves revealing distinct fracture biomechanical properties, namely, sudden complete failure (fatal ‘brittle’ failure) for the distal pericarp, and slower gradual failure (‘composite’ failure) for the proximal pericarp. This breaking behaviour clearly identifies this distal region mechanically as the PBZ crack initiation zone (iPBZ) for pericarp rupture, which upon mechanical failure causes the fruit valve to split into half. In contrast to this, in the proximal pericarp the measuring needle was driven through the proximal pericarp, layer by layer until the fruit valve finally split in half. Examples presented are from surface-sterilised fruit valves (35 days); the same difference in breaking behaviours between distal and proximal pericarps were evident for the other conditions. c Light microscopy of a full centric longitudinal fruit valve section exhibiting the distinct pericarp layers (toluidinblue histostaining). Arrows indicate force directions for the biomechanical analyses
