The effect of Gamma radiation (60Co) on the postharvest physiology of Musa acuminata collar cv. Dwarf Cavendish.
Strydom, Gerhardus Johannes.
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Bananas are climacteric fruits and show climacteric patterns with respect to both their ethylene production and respiration rates during the ripening process. Treatment with ethylene accelerated the ripening process, evidenced by colour changes. Chlorophyllase apparently synthesised chlorophyll in the 0.2 kGy irradiated fruit during the post-climacteric period. Gamma irradiation of green bananas with doses of 0.4 kGy or greater resulted in extensive tissue damage, undesirable colour development, and drastic changes in respiration and ethylene production. Irradiation at 0.2 kGy, did however, effectively delay ripening (without causing any phytotoxicity) of the fruit apparently by decreasing its sensitivity to its own endogenous ethylene for a period up to 28 days when kept at room temperature, provided that the relative humidity was maintained at 75 % to prevent desiccation. Irradiation at this dose did not prevent the onset of ripening when subjected to high concentrations of exogenous ethylene. Ultrastructurally total cell collapse due to cell wall, middle lamella and membrane damage was evident at doses of 0.4, 0.6 and 1.0 kGy. Irradiation with a dose of 0.2 kGy did not cause cell wall softening and membrane breakdown and thus provided a banana fruit of excellent postharvest quality with respect to ultrastructural features. Glucose and fructose occurred probably directly from sucrose that may not accumulate because it is transient after the banana fruit is harvested. The accumulation of sucrose glucose and fructose was not stimulated by ethylene in non-irradiated and irradiated fruit. These results indicate a clear temporal separation between those biochemical processes that appear to be stimulated by ethylene (respiration, fruit softening and colour change) from those that are not (sugars). Pectin methyl esterase resulted in the demethylation of pectin-like material in the cell walls of ripening bananas.