Pectinolytic Enzyme Activity and Changes in Water Potential Components Associated with Internal Breakdown in ‘McIntosh’ Apples1

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Abstract
A tissue macerating factor and pectinesterase activity were found in extracts from ‘McIntosh’ apple (Malus svlvestris, L.) during development of internal breakdown. Pectinesterase activity increased significantly and activity of the tissue macerating factor decreased significantly accompanying development of internal breakdown; concentrations of both remained low throughout development of the disorder. With loss of fruit firmness, fruit tissue water potential decreased as a result of decreasing solute potential and decreasing pressure potential. The first component of fruit firmness to decrease, tissue rigidity, is believed to decrease as a result of hydrolysis of intercellular pectins. The second component, cell turgor, decreased due to an increase in permeability of cell membranes to water in the later stages of development of internal breakdown. Decrease in both components of fruit firmness appear to contribute to tissue softening accompanying development of internal breakdown.

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