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.
(2-chloroethyl)phosphonic acid (ethephon) applied as a preharvest spray at rates of 200, 300, and 500 ppm induced significant on-the-tree degreening of fruit of ‘Robinson’, ‘Lee’ ‘Nova’, and ‘Dancy’ tangerines and ‘Hamlin’ oranges. Greatest degreening occurred 2 to 6 days following application and subsequent to peak-ethylene evolution. Fruit which were partially or totally degreened on the tree required less postharvest degreening and showed less decay in storage than untreated fruit. Ethephon applied at 200 to 500 ppm induced varying degrees of fruit loosening and, often, fruit drop. Generally, less than 10% ofthe leaves abscised on all cultivars with rates under 200 ppm and on ‘Nova’ and ‘Dancy’ tangerines and ‘Hamlin’ oranges with rates under 500 ppm. Considerable leaf abscission occurred on ‘Robinson’ and ‘Lee’ tangerines treated with 300 and 500 ppm ethephon.
Vacuolar acidification was investigated in `Palestine' sweet (Citrus limmetioides Tanaka) and `Persian' acid lime [(Citrus aurantifolia (Christm.) Swingle] (vacuolar pHs of 5.0 and 2.1, respectively) using tonoplast vesicles isolated from juice cells. The ATPase activity of tonoplast-enriched vesicles from sweet limes was strongly inhibited by bafilomycin A1 and NO3-, but was unaffected by vanadate. In contrast, the ATPase activity in acid lime membranes was only slightly inhibited by bafilomycin A1 and NO3- and was strongly inhibited by high concentrations of vanadate. The vacuolar origin of the acid lime vesicles was confirmed by immunoblotting. After solubilization and partial purification of the two enzymes by gel filtration, their inhibitor profiles were largely unchanged. Based on equal ATPase activities, vesicles from sweet and acid limes were able to generate similar pH gradients. However, in tonoplast vesicles from sweet limes, the maximum ΔpH was reached four times faster than in those from acid limes. Addition of ethylenediamine tetraacetic acid (EDTA) to chelate Mg+2 after the maximal ΔpH was attained resulted in collapse of the pH gradient in vesicles from sweet limes, whereas no change in ΔpH was observed in vesicles from acid limes, indicating a less H+ permeable membrane. Vacuolar ATPases from both cultivars exhibited identical pH optima and showed similar Mg+2 dependence, but only the acid lime ATPase activity was inhibited by Ca+2. These data confirm that the vanadate-sensitive form of the V-ATPase found in lemon and acid limes is specific to hyperacidifying tissues rather than to citrus juice cells. Sweet lime vacuoles bear the classical V-ATPase also found in vegetative plant tissues.
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.
`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.
Rind color of citrusfruit is an important cosmetic preference of consumers when purchasing citrusfruit who generally prefer a deep orange rind color ( Krajewski, 1997 ). As citrusfruit mature, changes in rind color are the result of decreased
The external color is an important attribute of citrusfruit quality. Consumers usually relate the external fruit color to its internal maturity, although in some circumstances, these factors are unrelated. This fact significantly influences the
phytotoxicity using 1-MCP when ethephon is used to cause loosening in mature citrusfruit.
LEAVES AND MATURE FRUIT DIFFERENTIALLY RESPOND TO 1-METHYLCYCLOPROPENE + ETHEPHON IN THE FIELD
Ethephon is frequently a starting point for most screening programs
As part of a larger study to improve rind color of citrusfruit, this initial study was conducted to determine the concentration of various gibberellin-biosynthesis inhibitors required to elicit a biological response in citrus trees as measured by