Effect of Gamma Irradiation on Softening Changes and Enzyme Activities During Ripening of Papaya Fruit

The aim of this work was to study the role of cell wall and ethylene related enzymes to papaya fruit firmness. Irradiation treatment was used as an imposed stress to cause changes in firmness. Physiologically mature papaya fruits were irradiated (500 Gy) and allowed to ripen at 22C and 90% RH. Polygalacturo-nase (PG), pectinmethylesterase (PME), βgalactosidase, cellulose and 1-aminocyclopropane-1-carboxylate oxidase (ACC-oxidase) activities were followed during ripening and correlated to changes in firmness, skin and pulp color, respiration and ethylene production. The firmness of irradiated fruits was retained at least 2 days longer than in normal fruits and also had a slower rate of softening. Total soluble solids (d̀Brix), cellulase activity and ethylene production were not altered by irradiation treatment. The activity patterns of PG, PME and β-galactosidase were related to pulp softening and affected by irradiation. ACC-oxidase activity was influenced by irradiation treatment, but its changes were not temporally related to those in firmness. It was concluded that irradiation had no direct effect on firmness but it acted by altering the ripening induced synthesis of cell wall enzymes, mainly PME.