Fig. 6
Microscopy of the L. didymum fruit valve reveals functional–morphological and micromechanical distinct endocarp regions. a Schematic drawing of the fruit valve to illustrate the orientation of imaged sections. The seed is always positioned inside the cavity with the radicle (embryonic root) directed towards the distal pericarp region adjacent to the ‘Natural Pericarp Opening’ (NPO) where the pericarp rupture initiates (iPBZ, crack initiation zone). b Scanning electron microscopy (SEM) top view onto the inside wall and the smooth breaking edge along the PBZ. The pericarp rupture spreads from the iPBZ, along the distal and dorsal pericarp as indicated (arrow). c SEM top view onto the smooth breaking edge of the lateral pericarp region. d SEM view onto the inside valve endocarp at the PBZ. e SEM top view onto the pericarp at the distal NPO border. f Light microscopy (LM) cross section of the pericarp at the distal NPO border. Red safranin staining indicates intense lignification of the dead thick-walled endocarp cells. Astrablue staining indicates non-lignified primary cell walls of the living parenchymatic exocarp and mesocarp cells. g SEM top view onto the pericarp at the proximal NPO border. h LM cross section of the pericarp at the proximal NPO border. i LM longitudinal section (safranin-astrablue histostain) of the distal pericarp with the iPBZ (crack initiation zone). Reduced lignification of the iPBZ endocarp compared to the adjacent endocarp is evident. j, k Fluorescence microscopy of the distal pericarp with the iPBZ. j Red fluorescence in the iPBZ due to binding of wheat germ agglutinin (WGA, conjugated with Alexa Fluor 633 nm) indicates distinct glycoprotein or hemicellulose composition of the iPBZ endocarp compared to the adjacent endocarp. k Autofluorescence (control without WGA) supports reduced lignification of the iPBZ. c, e–h, Red arrows, parallel oriented long endocarp cells. Yellow arrows, cross-sectioned endocarp cells
