The importance of phellogen cells and their structural characteristics in susceptibility and resistance to excoriation in immature and mature potato tuber (Solanum tuberosum L.) periderm

Potato tuber (Solanian tuberosum L.) periderm maturation is an important physiological process that directly affects the susceptibility and development of resistance to costly excoriation (skinning-type wounds) at harvest. The objectives of this research were to identify the specific types of cells and the cellular changes associated with susceptibility and resistance to tuber excoriation in immature and mature tubers respectively. Epifluorescent microscopic examination of immature tuber periderm (phellem, phellogen and phelloderm cells) from several genetically diverse cultivars has shown that the cellular damage resulting from excoriation occurs within the phellogen (cork cambium), a meristematic layer of cells that gives rise to neighbouring phellem and phelloderm cells. Tuber excoriation is the result of the fracture of radial phellogen cell walls linking the skin (phellem) to the phelloderm. As the tuber periderm. matures, phellogen cells become inactive and the radial walls of these cells become more resistant to fracture; resistance to excoriation develops. Ultrastructural studies of immature tuber periderm show that radial walls of active phellogen cells are thin and fragile. During periderm maturation, both radial and tangential phellogen cell walls thicken as they strengthen and become resistant to fracture, thereby providing resistance to excoriation. These results refute previous theories of the physiological changes responsible for the onset of resistance to tuber skinning injury. The combined results establish a paradigm whereby the thickening and strengthening of tuber phellogen cell walls upon periderm maturation are the determinant for resistance to tuber excoriation.