Graduate Thesis Or Dissertation
 

Physiology of ripening in pears

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  • Investigations were conducted to study in detail some biochemical and physiological aspects of ripening in pears. Anjou pears picked at different degrees of maturity were treated with Ethrel (2-chloroethane phosphonic acid), 4000 ppm, and ethylene, 500 ppm, for different lengths of time. Respiration rate was measured twice daily and changes in protein nitrogen, soluble pectin, flesh firmness and permeability were determined at frequent periodic intervals. Treatment with Ethrel or for short periods of time with ethylene resulted in Anjou pears attaining full ripeness without a concomitant change in respiratory activity. Decrease in flesh firmness, increase in protein nitrogen and soluble pectin occurred, even though the fruit remained in the preclimacteric condition, as shown by a positive response in respiration to ethylene treatment when fully ripe. Fruits treated continuously with ethylene developed the climacteric rise. These fruits, however, did not ripen faster than fruits given short treatments which caused ripening but failed to induce a climacteric. These data suggest that the mechanisms concerned with respiration and the initiation of ripening respond differently to ethylene and probably occupy different sites in the cell. Initiation of ripening apparently requires only a low initial concentration, while development of the climacteric rise in respiration requires the presence of a higher and possibly continuous amount. With increase in fruit maturity, ripening, was initiated by shorter treatments. Samples picked at 61 and 73 percent of full maturity ripened after 24 hours' treatment while only 12 hours were required at 82 percent of maturity. When treated with Ethrel, which liberates ethylene when absorbed by the tissues, a climacteric rise in respiration developed only in fruits with more than 82 percent of full maturity. This would indicate that the ethylene producing capacity of Anjou pears increases with advance in maturity. Initiation of ripening was accompanied by an increase in protein nitrogen and soluble pectin content. The maximum content of soluble pectin was reached at the fully-ripe stage, followed by a decline thereafter. Chlorophyllase activity in Bartlett pears was associated with the green to yellow color change and increased during ripening. Permeability changes in cellular membranes during ripening were studied by leakage of electrolytes and water flux from fruit tissues. In both Anjou and Bartlett pears, ethylene treatment did cause changes in permeability but were detectable only after the changes in firmness and increase in protein nitrogen had occurred.
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