Cell walls of loosening skin in post-veraison grape berries lose structural polysaccharides and calcium while accumulate structural proteins

Changes of mechanical properties, ultrastructure, cell wall components and wall-bound peroxidase in grape skin were studied prior to and after veraison in cv. Golden Muscat (Vitis vinifera × V. labrusca). Skin loosened continuously after veraison. Ultrastructural changes of the loosening skin included wall swelling in the epidermis and subepidermis cells, degradation of the middle lamella in the inner cells, and wall surfaces becoming “wavy”. Apoplast acidification took place in post-veraison berries. Contents of wall structural components, e.g. insoluble pectin, hemicellulose, cellulose and wall-bound calcium in the skin decreased, while both content of SDS-insoluble amino acids (especially hydroxyproline) and the activity of wall-bound peroxidase increased significantly after veraison. The activity of the enzyme was found localized mainly in the walls of the outermost four layers of cells including the epidermis and subepidermis tissues. The results indicated that, during loosening of skin, the cell walls in the skin constantly lose structural polysaccharides and calcium. Meanwhile, structural protein incorporation and phenolic cross-linkage formation became active especially in the walls of epidermis and subepidermis cells.