The post-storage quality of fruits depends on the ultrastructure of the outer tissues and on the fruit mineral composition (mostly on calcium). Pectins and their bonds with calcium play a central role in the tissues during ripening.
During senescence the middle lamella disappears from the cell walls, the fruits become soft and cell wall reticulation can be seen. There are changes within the cells too; plasmalemma separates from the wall, tonoplast from the cytoplasm, and plastids, mitochondria and nuclei are disorganized.
The ultrastructure of the skin and flesh of apples and pears were studied as a function of treatments (calcium, irradiation, calcium combined with irradiation), storage time and varieties. The texture and calcium content of the different parts of fruits were determined.
We established that low dose (1 kGy) irradiation induced softening in the fruit, dissolution of middle lamellae, wrinkling of cell membranes which generally remained intact, and retention of starch by plastids of the skin. Calcium treatment preserved the cell membranes and middle lammellae. The combined treatment preserved the cell compartments the best (with a lot of starch in the plastids; the cytoplasm remained essentially unchanged). At the same time this treatment could not prevent the breakdown of the middle lamellae in irradiated tissue.
Kovacs, E.; Keresztes, A.; and Kovacs, J.
"The Effects of Gamma Irradiation and Calcium Treatment on the Ultrastructure of Apples and Pears,"
Food Structure: Vol. 7
, Article 2.
Available at: http://digitalcommons.usu.edu/foodmicrostructure/vol7/iss1/2