Abstract
We investigated the chemical degradation and structural changes of cell walls of watercored tissues induced by GA paste treatment in Japanese pear (Pyrus pyrifolia Nakai) 'Akibae' and 'Housui'. Depolymerization of high molecular-mass trans-1, 2-cyclohexanediamine-N, N, N', N'-tetraacetic acid (CDTA)-soluble pectin occurred in severely watercored fruits, but there were no such metabolic changes in the Na2CO3-soluble fractions in both cultivars. Hemicellulose in 4% KOH soluble fractions, size-fractionated through a Sepharose CL-6B-100, revealed partial molecular-mass downshifts of total sugars in both cultivars, but considerable mol-mass change of xyloglucan only occurred in 'Akibae'. The polymers in 24% KOH soluble fraction showed no detectable alteration of molecular-mass in both cultivars, whereas 'Akibae' exhibited much higher cell wall-bound polygalacturonases and glycosidases activities compared with 'Housui'. The activities of β-galactosidase and β-glucosidase increased as the zone of watercore expanded in 'Akibae'. Total activity of cell wall bound glycanases was higher in 'Akibae' than in 'Housui'. Catabolism of carboxymethyl cellulose by cellulase was not different between sound and watercored tissues in both cultivars, but the activity against xyloglucan was higher in watercored tissue than the sound tissue adjacent to the watercore zones in 'Akibae'. Structural changes observed by light microscopy in sound and severely watercored tissue showed a greater extent of cell separations with the accumulation of pectic materials in the intercellular space of severely watercored tissues in 'Akibae' than in 'Housui'.
