Irradiation-induced Changes in Tomato Fruit and Pericarp Firmness, Electrolyte Efflux, and Cell Wall Enzyme Activity as Influenced by Ripening Stage

Abstract

Mature green and pink tomato (Lycopersicon esculentum Mill.) fruit were subjected to ionizing irradiation in the range of 0.7 to 2.2 kGy from gamma-or X-ray sources. Firmness of whole fruit and pericarp tissue, pericarp electrolyte leakage, and pericarp cell wall hydrolase activities were measured following irradiation and during postirradiation ripening at 20 °C. Irradiation-induced softening was evident in mature-green and pink fruit within hours following irradiation, and differences between irradiated and control fruit persisted throughout postirradiation storage. Trends of firmness loss were much more consistent and showed much greater dose dependency in pericarp tissue than whole fruit. Irradiation enhanced electrolyte efflux in fruit of both maturity classes. Fruit irradiated at the mature-green stage softened during postirradiation storage but exhibited an apparently irreversible suppression in polygalacturonase activity, with levels remaining <10% of those of nonirradiated fruit. Polygalacturonase activity was less strongly affected in irradiated pink fruit than in mature-green fruit, but activity remained reduced relative to the controls. Pectinmethylesterase and β-galactosidase activities were significantly enhanced in irradiated fruit of both ripening stages in the early period following irradiation, but reductions were noted after prolonged storage.