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  • Author or Editor: Yunxia Qiu x
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Over ripe and abnormally soft fruits occur often during papaya shipments to the mainland U.S.A. Calcium fertilization to the soil did not always increased Ca concentration in the mesocarp. Calcium plus K treatment was more effective at increasing the Ca concentration in the mesocarp than Ca treatment alone. Calcium and K fertilization did not affect the fruit color development. There was a positive correlation between mesocarp Ca concentration and ripe fruit firmness, with no relationship between K or Mg concentration and ripe fruit firmness. Vacuum infiltration with CaCl2, MgCl2, KCl to mesocarp plugs in vitro showed that Ca significantly delayed softening and reduced C2H4 production, and that MgCl2 and KCl also slowed the softening. Use of the chelating agent sodium citrate increased the rate of softening, probably, by removing Ca from the cell wall. We conclude that Ca is an important factor in fruit firmness and that the increase of Mg and K by infiltration has different effects on fruit firmness from that by soil fertilization.

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The uptake of Ca by `Sunset' papaya (Carica papaya L.) fruit and its role in ripening was studied. The highest mesocarp Ca uptake rate occurred in fruit that were <40 days postanthesis when fruit transpiration was probably highest. Ca uptake rate by the mesocarp was low, from 60 to 80 days postanthesis when fruit fresh and dry weight increased. Mesocarp Ca uptake rate increased again from 100 to 140 days postanthesis when fruit fresh weight growth rate declined and dry weight growth rate increased. Mesocarp Ca concentration did not significantly differ from the peduncle to the blossom end. although Ca was significantly higher in the outer than inner mesocarp at the fruit equator. Mesocarp Ca concentration fluctuated significantly throughout the year ranging from 68 to 204 μg·g-1 fresh weight (FW). Soil Ca application did not always increase fruit mesocarp Ca concentration, while K and N fertilization decreased mesocarp Ca concentration. Attempts to increase mesocarp Ca concentration by spraying CaCl2 onto papaya fruit during growth and by postharvest vacuum infiltration and dipping of the cut peduncle into CaCl2 were unsuccessful. Mesocarp Ca concentration was positively correlated to the firmness of ripe papaya fruit and the rate of softening of mesocarp plugs. Less correlation was found between fruit firmness and the ratio of Ca concentration to K or Mg concentration, or to Mg plus K concentrations. Mesocarp Ca concentration of 130 μg·g-1 FW or above was associated with slower fruit softening rate than fruit with a lower concentration.

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