Other pages about wound healing:
- The Process of Wound Healing
- Healing Factors
- Cutting Seed Tubers
- Handling Seed Pieces
- Wounding at Harvest
- Healing in Storage
Wound healing is accomplished by a series of distinct processes.
First, a break such as a cut occurs through the skin barrier. This results in a layer of broken and collapsed cells at the surface (Diagram 1) and opens an area through which pathogens causing diseases such as soft rot can enter. Below this layer are storage cells that contain starch and are called the parenchyma, large rectangular cells. Between these cells are spaces (intercellular space) which contain water carrying nutrients, proteins and hormones. When a break in the skin occurs, this intercellular fluid leaks out resulting in water loss (shrinkage) from the tuber.
Depending on conditions, in the first several hours, lignin and pectin are produced and form cross-links in the intercellular space as a temporary barrier (Diagram 2). These cross-links form between the outermost layers of parenchyma cells below the break. Under good conditions and at room temperature (65 to 70 F), this process takes less than eight hours. Formation of the cross-links inhibits the entry of bacteria and the occurrence of soft rot (Nolte et al., 1987).
The next process is the deposition of suberin in the cell walls of these parenchyma cells forming a suberized layer (Diagram 3). The suberization of these cells inhibits the water loss from the tuber and inhibits fungal infection such as dry rot (Nolte et al., 1987). Under good conditions and at room temperatures, this process can take no more than a day but under cool temperatures such as 50 F, it can up to two weeks.
The last process in wound healing is the formation of the phellogen layer (Diagram 4). This layer of long, rectangular cells occurs from cell division of the parenchyma cells just below the suberized parenchyma cells (the phellum) or suberized layer. This process takes several days to several weeks depending on temperature and at room temperatures may take three to five days under good conditions but can take over two weeks under 50 F and several weeks at even colder temperatures (Wiggington, 1974).
Together these layers form the new skin or periderm of the tuber. Under optimal conditions, the whole process takes a week. The periderm then allows control of movement between the inside of the tuber and its environment. It is clear that temperature and relative humidity are major factors affecting the speed of wound healing (Widdington, 1974); see panel "Healing Factors"). It is important to note that as temperature increases, there is also a promotion of microbial growth and that if relative humidity is too high (>97%), cell proliferation at the surface can occur as well as growth of anaerobic bacteria such as those causing soft rot.
To summarize (Diagram 5), a healed cut would consist of an outer surface of collapsed cells below which is an area of suberized cells, then some corky cells and a layer of new cells forming a new barrier or the phellogen layer. This comprises the periderm or new skin. Below this new skin lies the storage parenchyma cells. For more detailed cellular description of the three layers of the periderm refer to Reeves et al., 1969; for a cellular analysis of the difference between normal periderm and wound periderm, refer to Sabba and Lulai, 2002, and for a description of the role to tuber maturity, refer to Lulai and Freeman, 2001.
Lulai, E.C. and Freeman, T.P. 2001. The importance of phellogen cells and their structural characteristics in susceptibility and resistance to excoriation in immature and mature potato tubers (Solanum tuberosum L.) periderm. Annals Bot 88:555-561.
Nolte, P., G.A. Secor, and N.C. Gudmestad. 1987. Wound healing, decay and chemical treatment of cut potato tuber tissue. Amer J Potato Res 64:1-9.
Reeves, R.M., Hautala, E., and Weaver, M.L. 1969. Anatomy and composition variation within potatoes I. Developmental histology of the tuber. Amer Potato J 46:361-373.
Sabba, R.P. and Lulai, E.C. 2002. Histological analysis of the maturation of native and wound periderm in potato (Solanum tuberosum L.) tuber. Annals Bot 90:1-10.
Wiggington, M.J. 1974. Effects of temperature, oxygen tension and relative humidity on the wound-healing process in the potato tuber. Potato Res 17:200-214.