Effects of gamma irradiation on tissue firmness, some cell wall degrading enzymes and pectic substances of tomato fruit

The firmness of tomato fruit decreased immediately after irradiation reaching the softness of ordinary ripe fruit with the doses of 2.5 kGy or higher. Leakages of K+, sugars and amino acids were higher in 2.5 kGy-irradiated fruit and its tissue section failed to maintain their turgor pressure in lower concentration of sucrose solution while there was no difference between control and 1.0 kGy lots. The activity of pectinmethylesterase (EC 3.1.1.11) was not significantly affected with irradiation immediately and during storage. Dose levels of 1.0 kGy or more suppressed the activity of polygalacturonase (EC 3.2.1.15), and cellulase (EC 3.2.1.4) was stimulated to some extent after irradiation but no uniform pattern was observed during storage. Irradiation reduced the ratio of protopectin to total pectin and increased that of hexametaphosphate soluble pectin (HMP) but did not affect that of water soluble pectin (WSP). The main polymers, polyuronides, in all pectic fractions from irradiated fruits and in irradiated water suspension of alcohol insoluble substances and of fractions of WSP, HMP and the residues were degraded by gamma rays into lower molecular components as estimated by gel filtration chromatography. (author)