ROLE OF CALCIUM IN REDUCING POSTHARVEST CELL WALL DEGRADATION IN `GOLDEN DELICIOUS' APPLE FRUIT

in HortScience
Authors:
S. Roy1Horticultural Crops Quality Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705
2Electron Microscope Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

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W.S. Conway1Horticultural Crops Quality Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

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A.E. Watada1Horticultural Crops Quality Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

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G. Gillen3Surface Miroanalysis Science Division, NIST, Gaithersburg, MD 20899

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W.P. Wergin2Electron Microscope Laboratory, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

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Calcium is an important constituent of the cell wall and plays roles in maintaining firmness of fruit and reducing postharvest decay. The modification of the cell wall is believed to be influenced by calcium that interacts with acidic pectic polymers to form cross-bridges. Infiltrating apples with CaCl2 has been suggested as an effective postharvest treatment for increasing the calcium content. Three different methodologies were used to analyze the effects of calcium on the cell walls: 1) nickel staining of polygalacturonate on free-hand sections, 2) cationic gold labeling of anionic binding sites in the cell walls, and 3) analytical detection of calcium ions (40Ca, 44Ca) using a secondary ion mass spectrometry. The combination of these methods allowed us to directly visualize the cellular features associated with the infiltration of calcium. Treatment resulted in significant enrichment in the cell wall of the pericarp, transformed the acidic pectins in calcium pectates, and resulted in new calcium cross-bridges. Evidence now suggests that exogenously applied calcium affects the cell wall by enhancing its strength and reinforcing adhesion between neighbor cells; therefore, calcium infiltration delays fruit degradation.

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