Ethylene-induced tissue breakdown in fruit of watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) (senescence, climacteric, ripening, ultrastructure, cell wall)
Elkashif, Mohamed Elhag.
Thesis of Ph.D., University of Florida, 1985, 127 pages
1985
บทคัดย่อ
Although watermelon Citrullus lanatus (Thunb.) Matsum and Nakai fruit have been classified as climacteric, there are reports that they exhibit cellular breakdown when exposed to ethylene gas. This study comprises an investigation of the postharvest behavior of watermelon fruit under normal conditions and in response to ethylene or propylene. Parameters measured included the effects of ethylene on ripening and respiratory activity, electrolyte leakage, cell wall ultrastructure, and cell wall hydrolases. All experiments included a comparison of the changes which occurred during both ethylene-induced breakdown and natural ripening of fruit.
Respiratory rates in harvested melons showed little change throughout ripening. Respiratory activity was enhanced in the presence of ethylene but returned to normal levels upon removal of the gas. Endogenous ethylene production was not initiated by exposure of fruit to propylene and was detected only in fruit exhibiting symptoms of decay. The results supported the hypothesis that watermelon fruit was nonclimacteric.
Ethylene-treated tissue exhibited extensive cell separation, high activity of D-galacturonase, and enhanced electrolyte leakage. These ethylene responses did not appear to represent merely an acceleration of normal ripening since they were not observed in fruit not exposed to ethylene.
The sequence of events which resulted in watermelon tissue breakdown emphasized the predominant role of the cell wall. The increase in D-galacturonase activity observed after the first day of ethylene treatment coincided with the development of a distinct zone of separation in the middle lamella of the cell wall. Following longer periods of ethylene treatment, extensive pectin degradation was apparent from the increased quantities of polymers fractionating on Ultrogel AcA 34. Ultrastructural studies revealed that cell walls started to show signs of deterioration after the first day of ethylene treatment.
The membrane system did not seem to play a major role in initiating watermelon tissue breakdown, since no change in electrolyte leakage was noted during the first 3 days of ethylene treatment. The increase in electrolyte leakage observed after the third day of ethylene treatment was apparently due to cell wall degradation and weakening which led to the rupture of the membrane.