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Title: The effect of calcium on papaya fruit softening 
Author: Qiu, Yunxia
Date: 1992
Abstract: Papaya (Carica papaya L. "Sunset") fruit growth had 2 main peak periods of growth. The highest mesocarp Ca uptake occurred in fruits less than 60 days post-anthesis possibly related to a high transpiration rate from fruit with a high surface area to weight ratio. Water and nutrition which were supplied predominantly through the phloem may result in low Ca uptake by the mesocarp 60 to 80 days post-anthesis when the mesocarp rapidly increases its FW and DW. Mesocarp Ca uptake rate increased again 100 to 140 days post-anthesis when mesocarp FW growth rate reduced and DW growth rate increased. Dramatic increases of sucrose in the mesocarp during this period may reduce water potential, which may bring xylem water and Ca into the mesocarp. The Ca concentration in the mesocarp was not different from the peduncle to the blossom end probably because the vascular bundles carrying Ca pass longitudinally through the whole tissue. Soil Ca application sometimes increased mesocarp Ca concentration. Factors such as soil type, levels of other minerals in the soil, root growth, and rainfall might affect the uptake of Ca by papaya plants. Mesocarp Ca concentration fluctuated significantly throughout the year. Potassium and N (urea) fertilization decreased mesocarp Ca concentration. Attempts to increase mesocarp Ca content by spraying CaCl2 onto papaya during growth and development and by postharvest dipping of peduncle into CaCl2 were unsuccessful. Mesocarp Ca concentration was positively correlated to the firmness of ripe papaya fruit. There were no correlations between the firmness of ripe fruits and Mg concentration, or K concentration, but there were correlations between fruit firmness and the ratio of Ca concentration to Mg concentration, or to K concentration, or to Mg+K concentrations. The involvement of K or Mg appeared to be related to Ca, rather than by the effects of these minerals. Mesocarp Ca content of 150 µg g-1 FW or above gave maximum delay in softening and firmest flesh. Both the proportion of different extractable fractions and molecular size range of papaya pectin and hemicellulose were altered during fruit ripening. The pectin molecular size declined and the solubility of pectin in Ca chelating agent CDTA and Na2C03 solutions increased. The molecular size range of hemicellulose also changed with an increase of solubility of hemicellulose in KOH fractions. These changes may lead to disruption and loosening of the papaya fruit cell wall structure during fruit ripening. These results suggested that pectin hydrolysis and the modification of hemicellulose were involved in papaya fruit softening and may be influenced by Ca content. Infiltrating mesocarp tissue with CaC12 effectively slowed the softening rate and reduced the rates of respiration and ethylene production. Treatment with Ca chelating agent EGTA hastened softening rate and increased rates of respiration and ethylene production. Calcium probably slowed the mesocarp softening through strengthening the cell wall structure, inhibiting cell wall degrading enzyme activity, and/or reducing the metabolic rate of ripening process. Infiltration of mesocarp tissue with MgC12 and KCl had no effect on softening.
Description: Thesis (Ph. D.)--University of Hawaii at Manoa, 1992. Includes bibliographical references (leaves 149-171) Microfiche. xii, 171 leaves, bound ill. 29 cm
URI: http://hdl.handle.net/10125/9473
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.

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