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Abstract

Light-reflectance measurements at 648-740 and 674-740 nm decreased as chlorophyll was lost during the maturation and degreening of citrus fruits. The difference between these measurements changed as the chlorophyll level declined. This change was shown as an initial decrease followed by an increase in 648-674 nm measurements. Analyses of rind samples revealed changes in the relative concentration of chlorophyll a and b and consequent decreases in the a/b ratio as total chlorophyll levels decreased. Formulas were developed to convert light-reflectance readings at 674-740 and 648-740 nm to concentration of chlorophyll a and b in the tissue. The greater resistance of chlorophyll b to degradation during color development may explain the difficulty of satisfactorily degreening some fruit and may serve as a basis in selecting for improved coloring characteristics.

Abstract

Abscisic acid (ABA) introduced through the stem of ‘Pineapple’ and ‘Valencia’ oranges stimulated cellulase activity in the separation zones. ‘Pineapple’ orange was affected more than ‘Valencia’. Cellulase activity was greatest under normal atmospheres in which ethylene accumulated in the treated fruit. Under one-fifth atmosphere with little ethylene accumulation, cellulase was 2 to 2.5 times greater in separation zones of treated fruit than in non-treated fruit. ABA introduced through the stem was more effective than spray applications. ABA sprayed on the fruit was partly absorbed but did not increase ethylene production or cellulase activity, or decrease fruit removal force.

Abstract

Healing of minor injuries to ‘Valencia’ orange (Citrus sinensis (L) osbeck) occurs rapidly at 30°C and 96-98% relative humidity. It is manifested by a marked increase in the activity of phenylalanine ammonia-lyase (PAL) and deposition of lignin. PAL activity is confined mainly to injured flavedo while uninjured peel and albedo tissues exhibit very little activity. Inoculation of injured flavedo, immediately after injury, with P. digitatum sacc. disrupts the increase in PAL activity. When acetone powder from decayed peel was mixed with that of healed injured fruit and enzyme preparation was made from the mixture, there was no apparent reduction in phenylalanine ammonia-lyase activity, except that cuased by dilution. The synthesis of both PAL and lignin in mature fruit is strongly inhibited by cycloheximide, but not by actinomycin D.

Abstract

Strains of Penicillium digitatum (Sacc.) and P. italicum (Wehmer) resistant to thiabendazole and benomyl were isolated from decaying citrus fruits obtained from the Rotterdam, Netherlands, terminal market and originating from 18 countries. Significantly more Penicillium sp isolates with resistance to thiabendazole and benomyl were collected from grapefruit and lemons than from oranges. Significantly more isolates of P. digitatum than P. italicum grew on agar plates with 4, 10, or 40 ppm thiabendazole. A greater percentage of P. digitatum than P. italicum isolates grew on 4 and 10 ppm benomyl-agar plates, but a greater percentage of P. italicum than P. digitatum isolates grew on 40 and 80 ppm benomyl-agar plates. Both species were more resistant to thiabendazole than to benomyl, and often showed cross-resistance to the fungicides. Resistant Penicillium sp isolates produced larger colonies on 4 and 10 ppm thiabendazole and 40 and 80 ppm benomyl.

`Eureka' lemons [Citrus limon (L.) Burro. f.] treated for commercial storage were held for 6 months at 13C. One-half of the fruits were individually sealed in high-density polyethylene (HDPE) plastic film and half not sealed. The HDPE-seaIed lemons showed little change in the water relations characteristics, while unsealed lemons lost weight and decreased in water potential throughout the storage period. The maturity indices in the two treatments were generally similar during the first 3 months of storage, after which maturation of wrapped fruit was slower than that of the control. The overall marketable quality of the fruit was higher in HDPE-sealed lemons than in unsealed. From these results, it appears feasible to introduce seal packaging in packing lines where lemons will be placed in extended storage.

Several citrus varieties, including `Navel' oranges, `Marsh' grapefruit and `Fallglo' tangerines are prone to develop postharvest peel pitting at nonchilling temperatures. The disorder is characterized by depressions in flavedo that ultimately affect oil glands. Increasing evidence indicates that changes in peel water status during postharvest handling of fruit plays a major role in the appearance of the disorder. Peel pitting was triggered when fruit were transferred from low to high relative humidity (RH) consistently in several citrus growing areas. A transient increase in fruit ethylene production and ABA content was observed within the first 24 hours after transfer from low to high RH. Water potential decreased with storage at low RH in flavedo and albedo, and recovered faster in flavedo than in albedo cells upon transfer to high RH. The differential recovery in water potential between flavedo and albedo is postulated to cause collapse of external albedo layers and pitting. The effect of climatic conditions in the field at harvest was also examined. Harvesting fruit at low RH induced more severe pitting after storage than harvesting at high RH. In addition, increasing hours of low RH storage prior to storage at high RH resulted in increased pitting. The results demonstrate that change in peel water status is a major factor leading to the development of postharvest peel pitting in citrus.