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 relate d enzymes to papaya fruit firmness. Irradiation treatment was used as an im posed stress to cause changes in firmness. Physiologically mature papaya fr uits were irradiated (500 Gy) and allowed to ripen at 22C and 90% RH. Polyg alacturonase (PQ), pectinmethylesterase (PME), beta -galactosidase, cellula se and 1-aminocyclopropane-1-carboxylate oxidase (ACC-oxidase) activities w ere followed during ripening and correlated to changes in firmness, skin an d pulp color, respiration and ethylene production. The firmness of irradiat ed fruits was retained at least 2 days longer than in normal fruits and als o had a slower rate of softening. Total soluble solids (degrees Brix), cell ulase activity and ethylene production were not altered by irradiation trea tment. the activity patterns of PG, PME and beta -galactosidase were relate d to pulp softening and affected by irradiation. ACC-oxidase activity was i nfluenced by irradiation treatment, but its changes were not temporally rel ated 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.