Cellular changes and physiological responses to tuber pressure-bruising

Tubers located at lower depths in bins of stored potatoes suffer from pressure-bruising which often develops into an internal grayish or blackspot-like defect. However, little is known about key physiological and cellular changes that constitute pressure-bruising and resulting defects that degrade potato marketability and profits. We determined the changes in periderm water vapor permeability and some of the changes in cellular integrity that may occur in pressure-bruised areas compared to non-pressure-bruised areas of tubers after eight months of storage. Porometric measurements of pressure-bruised areas revealed elevated water vapor loss, which is conducive to blackspot formation, even though there were no apparent breaks in the periderm of pressure-bruised areas. Pressurebruised areas did not appear to readily wound-heal, because the elevated water vapor loss was shown to continue, without decline, when the tubers were porometrically measured again 4 days after removal from the pressure-bruising environment. Analysis of changes in cellular integrity of pressure-bruised regions showed that the tough, rigid cells of the suberized phellem could be damaged or completely flattened by this form of stress. A layer of what appeared to be crushed cells was often detected beneath the pressure-bruised, but partially intact, phellem. Autofluorescence of the crushed layer appeared to be derived from cell walls of collapsed phelloderm cells, starch granules of neighboring cells and material accumulated as a physiological response to the cellular damage incurred from pressure-bruising. These results help explain why the blackspot-like defect sometimes occurs at pressure-bruise sites.