DISTRIBUTION OF THE ANIONIC SITES IN THE CELL-WALL OF APPLE FRUIT AFTER CALCIUM TREATMENT - QUANTITATION AND VISUALIZATION BY A CATIONIC COLLOIDAL GOLD PROBE

The ripening and softening of fleshy fruits involves biochemical chang es in the cell wall. These changes reduce cell wall strength and lead to cell separation and the formation of intercellular spaces. Calcium, a constituent of the cell wall, plays an important role in interactin g with pectic acid polymers to form cross-bridges that influence cell wall strength. In the present study, cationic colloidal gold was used for light and electron microscopic examinations to determine whether t he frequency and distribution of anionic binding sites in the walls of parenchyma cells in the apple were influenced by calcium, which was p ressure infiltrated into mature fruits. Controls were designed to dete rmine the specificity of this method for in mure labelling of the anio nic sites on the pectin polymers. The results indicate that two areas of the cell wall were transformed by the calcium treatment: the primar y cell walls on either side of the middle lamella and the middle lamel la intersects that delineate the intercellular spaces. The data sugges t that calcium ions reduce fruit softening by strengthening the cell w alls, thereby preventing cell separation that results in formation of intercellular spaces.