| Summary: | 碩士 === 國立中興大學 === 園藝學系所 === 96 === Skin freckles on papaya fruits, as a rule, appear in spring and winter and thus affect fruit appearance seriously. The objectives of this study were to investigate the cause of this disorder and to reduce the occurrence of freckles. In the experiment of nitrogen treatments, four concentrations of ammonium sulphate (NH4)2SO4 were applied to ‘NTU 10’ and ‘Tainung No.2’ papaya trees (11.9、25.0、38.2、51.2 g/tree) at intervals of every two weeks. Compositions of leaves and fruits were measured seven months after fertilization and again one month after stop applying fertilizer. The result of the experiment showed that 38.2 g/tree was the best concentration used among treatments. The rate of fruit decay was a serious problem from high nitrogen fertilization. Peel color change of ‘Tainung No.2’ papaya was somewhat delayed, and the total soluble solids were also lower than other treatments. Increasing N/Ca ratio of leaves and fruits was found with increased nitrogen fertilizer, and thus indirectly affected potassium accumulation. Treatment with 11.9g ammonium sulphate per tree resulted in fruits and leaves nitrogen deficiency, especially in leaves. However, accumulation of total phenolic compound was found. Before ripening the freckle incidence area of ‘NTU 10’ and ‘Tainung No.2’ papaya fruits were similar, but ‘Tainung No.2’ was more serious after ripening. There was no significant correlation between N fertilization and the appearance of freckles. ‘Tainung No.2’ was consistently more susceptible than ‘NTU 10’ papaya fruits.
In season of high and low skin freckles occurrence during fruit growth and development, chemical analysis of phosphorus, potassium, magnesium concentrations in the peel showed the tendency of gradual increase. Concentration of calcium and zinc in peel leveled off in November while freckle appeared. This seemed to indicate that freckles were not related to the mineral deficiency. It is assumed that the hardening of the cell wall and increasing in turgor pressure during the ripening of fruits could lead to the degeneration of wall structure and the occurrence of freckles. At the later stage of fruits development, the phosphorus, calcium, magnesium, iron, manganese, zinc, copper in pulp decreased progressively and stabilized until complete turning of pulp color. Potassium content change very little during entire period of fruit growth. Total phenolic compound and phenylalanine ammonia-lyase activity of fruit increased during the period of peel and pulp color change.
The main cause of the skin freckle was related to the low-temperature. Wrapping fruits in bags to reduce the temperature fluctuation improved the situation. In Nantou County more skin freckles occurred than in Kaohsiung due to the differences in temperature in the two areas. Cluster wrapping of fruits with Tyvek material before the appearance of freckles, while still attached to the tree, could reduce the physiological disorder after bagging for 96 days. Tyvek material was better than Non-woven material for the purpose. Moreover, wrapping increased brightness and chroma of the peel, but reduced fruit weight at harvest. No significant differences in water content, dry matter, total soluble solids, and latex volume of fruits were found between Tyvek and non-wrapping treatments. The potassium, calcium and magnesium of pulp were less after 48 days wrapping with cluster Tyvek than non-wrapping treatment. With increasing in time of bagging, only potassium, manganese concentrations were decreased. Total phenolic compound and phenylalanine ammonia-lyase activity were less after cluster wrapping with Tyvek. In conclusion, skin freckles occurrence seems to be associated with browning of fruits and environmental stress.
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