Morphological and structural study of seed pericarp of Opuntia ficus-indica prickly pear fruits

The morphological study of pericarp of Opuntia ficus-indica (OFI) seeds showed that the cells were mainly made up of spindle-shaped sclerenchyma fibers. The chemical composition of the pericarp revealed a significant amount of polysaccharides, with cellulose (35%) and xylan (27%). The structure of xylan and cellulose, both in isolated form and as a component of seed pericarp of OFI were studied by X-ray and CP/MAS 13C NMR spectroscopy. The supramolecular structure of xylan is very sensitive to the surrounding environment, in particular to the presence of water and of cellulose fibers. The cellulose fibers presented X-ray diagrams typical of secondary wall cellulose but they were sensitive toward NaOH since they started to be converted into cellulose II at a NaOH concentration as low as 8%. In seed pericarp, cellulose fibers interact with xylan polymers, causing these to adopt a conformation different to the one observed for xylan both in dry or hydrated form, suggesting that xylans were probably present as composites with cellulose fibers.