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I.B. Ferguson and C.B. Watkins

Apple fruit (Malus domestics Borkh. cv. Cox's Orange Pippin) were harvested in four orchards from trees growing under the same conditions but differing in crop load. Regardless of fruit size, apples from light-cropping trees had lower Ca and higher K concentrations and more bitter pit than did fruit from trees with heavy crop loads. The inverse relationship between Ca concentration in the fruit and the incidence of bitter pit also varied according to crop load and could affect the ability to predict incidence of bitter pit from Ca measurements. Differences in fruit maturity that would influence bitter pit incidence were not associated with crop load. The enhanced susceptibility to storage disorders, such as bitter pit, in fruit of all sizes from light-cropping trees suggests the need to handle fruit from such trees differently for postharvest storage.

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C.B. Watkins and F.W. Liu

The storage potential of ‘Empire’ apples [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] in controlled atmosphere storage has been studied. Fruit were treated with a range of partial pressures of CO2 (pCO2) from 0 to 5 kPa at storage temperatures of 0, 0.5, and 3 °C. The predominant storage disorders that developed were external CO2 injury, flesh browning (chilling injury), senescent breakdown (soft flesh browning), and core browning. All disorders except external CO2 injury increased with longer storage periods. The incidence of external CO2 injury was usually greater with higher storage temperature, whereas flesh browning was worst at lower storage temperatures and senescent breakdown was higher at warmer storage temperatures. The effect of storage temperature on core browning was not consistent. External CO2 injury, flesh browning, and core browning incidences were higher with increasing pCO2, especially above 2 kPa. Flesh firmness was lowest at warmer storage temperatures and in the absence of CO2. Orchard to orchard variation for all factors was high. Relationships of disorders with mineral concentrations were specific to disorder and storage conditions. The results suggest that ‘Empire’ should be stored at 1 to 2 °C, reflecting a compromise between risk of flesh browning at 0 °C and risk of senescent breakdown and unacceptably soft fruit at 3 °C and that pCO2 should be maintained below 2 kPa and closer to 1 kPa.

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C.B. Watkins and J.F. Nock

The inhibitor of ethylene binding, 1-methylcyclopropene (1-MCP) has been applied to `Gala', `Cortland', `McIntosh', `Empire', `Delicious', `Jonagold', and `Law Rome' apples under air and/or controlled atmosphere (CA) storage conditions. 1-MCP gas concentrations ranged from 0 to 2 mL·L–1. Effects of 1-MCP were greater in CA than air storage. A dose response of internal ethylene concentrations and flesh firmness to 1-MCP was found in cultivars such as `McIntosh' and `Law Rome', whereas in others, such as `Delicious' and `Empire', ripening was generally prevented by all 1-MCP concentrations. We have further investigated the effects of 1-MCP on `McIntosh' by increasing rates of the chemical to 50 mL·L–1, and confirming that fruit of this cultivar respond poorly if fruit have entered the climacteric prior to 1-MCP application. Efficacy of 1-MCP is affected by cultivar and storage conditions, and that successful commercial utilization of the chemical will require understanding of these relationships.

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C.B. Watkins, K.J. Silsby and M.C. Goffinet

The histology of external CO2 injury of the skin of `Empire' apples and postharvest factors affecting occurrence of injury were investigated. Injury was greater in a 5% CO2/2% O2 atmosphere than in 2% CO2/2% O2, but incidence was affected by orchard source. Susceptibility to injury was highest during the first 4 weeks of storage, while a postharvest treatment with diphenylamine prevented the disorder. Ethanol reduced injury, but ascorbic acid increased incidence of the disorder. Keeping fruit in air cold storage for 10 days before application of CO2 markedly reduced incidence of CO2 injury. Histological studies showed that external CO2 injury begins at the hypodermis—cortex boundary and spreads outward into the upper hypodermis and inward into outer cortex cells, although the cuticle and epidermis appear unaffected and unbroken. Radial walls of affected cells collapse and become pleated, so that the skin surface sinks below nearby normal regions. Other cellular events include loss of cytoplasmic integrity, coagulation of the protoplast, loss of organelle structure, and cell wall separation. Nondigested starch can be found in cells of affected fruit at the hypodermis—cortex boundary. We conclude that several factors affect fruit susceptibility to CO2 injury, including orchard, antioxidant treatment, and delays before application of CO2.

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A. Rugkong, J.K.C. Rose and C.B. Watkins

Tomato fruit (Solanum lycopersicon L.) can develop mealiness and enhanced softening when exposed to chilling temperatures during storage, but the involvement of cell wall-associated enzymes in chilling injury development is not well understood. To study this aspect of injury development, we have exposed breaker stage tomato cv. Trust fruit to a chilling temperature of 3 °C for 0, 7, 14, and 21 days followed by storage at 20 °C for 12 days. Ethylene production was not affected by storage except after 21 days, where production was greater at 20 °C. Exposure of fruit to chilling temperatures delayed the ripening-related color change (chroma and hue) and initially increased compression values, but percentage of extractable juice was not affected consistently. Increased polygalacturonase activity during ripening was reduced by about 50% after 7 days at 3 °C, and further inhibited with increasing storage periods. In contrast, the activities of pectin methylesterase and α-galactosidase were not significantly affected by the cold treatments. β-Galactosidase activity was greater in all chilled fruit compared with fruit ripened at harvest, whereas endo-β-1,4-glucanase activity was lower after 21 days at 3 °C. These results will be compared with equivalent changes in the activities of cell wall enzymes that are associated with wooliness development in chilling-injured peach fruit.

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Adirek Rugkong, Jocelyn K.C. Rose and Chris B. Watkins

Tomato fruit (Solanum lycopersicum L.) can develop mealiness and enhanced softening when exposed to chilling temperatures during storage, but the involvement of cell wall-associated enzymes in chilling injury development is not well understood. To study this aspect of injury development, we have exposed breaker-stage `Trust' tomato fruit to a chilling temperature of 3 °C for 0, 7, 14, and 21 days followed by storage at 20 °C for 12 days. Ethylene production was not affected by storage except after 21 days where production was greater at 20 °C. Exposure of fruit to chilling temperatures delayed the ripening-related color change (chroma and hue) and initially increased compression values, but percent extractable juice was not affected consistently. Increased polygalacturonase (PG) activity during ripening was reduced by about 50% after 7 days at 3 °C, and further inhibited with increasing storage periods. In contrast, the activities of pectin methylesterase (PME) and α-galactosidase were not significantly affected by the cold treatments. β-Galactosidase activity was greater in all chilled fruit compared with fruit ripened at harvest, whereas endo-β-1,4-glucanase activity was lower after 21 days at 3 °C. In chilled fruits, transcript accumulations for PG, PME (PME1.9), and expansin (Expt.1) were lower during storage at 20 °C compared with those of nonchilled fruits. Transcript accumulation for β-galactosidase (TBG4) was affected only at 14 days of cold storage, when transcript accumulation decreased. Cold treatment increased transcript accumulation of endo-β-1,4-glucanase (Cel1) after 12 days at 20 °C and decreased transcript accumulation after 7 days and 21 days at 21 °C. Cell wall analyses to investigate relationships among enzyme activities and cell wall disassembly are ongoing.

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J.P. Fernandez-Trujillo, J.F. Nock and C.B. Watkins

Several strawberry fruit cultivars were exposed to air or CO2 at 2 °C for up to 9 days. Concentrations of fermentation products and organic acids, and activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH), were measured. Acetaldehyde, ethanol, and ethyl acetate concentrations accumulated in CO2-treated fruit of `Honeoye' and `Kent', but not in `Cavendish' or `Annapolis'. We classified the former group of cultivars as intolerant to high CO2 and the latter group as tolerant to high CO2. Activities of PDC and ADH were higher in CO2-treated than air-treated fruit of the tolerant cultivars but not in the intolerant cultivars. Succinate accumulated in fruit of all cultivars, but concentrations were highest in the tolerant than in the intolerant cultivars. These results will be discussed in relation to mechanisms of CO2 action on fruit metabolism.

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D.M. Dawson, C.B. Watkins and L.D. Melton

Cell wall changes in `Fantasia' nectarines [Prunus persica (L) Batsch var. nectarina (Ait) maxim] were determined after storage at 0C with or without intermittent warming (at 20C at 2-week intervals) and after ripening. For comparison, fruit were examined at harvest and after ripening without storage. Fruit stored continuously at 0C for 6 weeks became mealy during ripening, whereas fruit subjected to intermittent warming ripened normally. Ripening immediately after harvest was associated with solubilization and subsequent depolymerization of pectic polymers and a net loss of galactosyl residues from the cell wall. No solubilization of pectic polymers from the cell wall occurred during storage of fruit at 0C. Mealy fruit, ripened after continuous storage at 0C, showed only limited solubilization of pectins and depolymerization, high relative molecular weight (M) polymers being predominant. During ripening after storage, pectic polymer solubilization was not as extensive in intermittently warmed fruit as in fruit undergoing normal ripening but solubilized polymers were depolymerized, low M uronic acid-rich polymers becoming predominant. Intermittent warming of fruit resulted in significant softening during storage, alleviating the development of mealiness by promotion of cell wall changes associated with normal ripening.

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F R Harker, C B Watkins, B A Cregoe, P L Brookfield and W J Bramlage

The apple growing districts of New Zealand are spread across a wide range of latitudes. Differences in growing conditions associated with these various districts may influence the way fruit mature on the tree. In this study, the relationships between background colour and physiological maturity of Royal Gala apples have been compared in four major production areas. Royal Gala apples were strip picked from trees in three orchards during the commercial harvest period Hawkes Bay, Canterbury, Nelson and Otago. The maturity of these fruit was assessed, and fruit stored at 0°C for 12 weeks. Following removal from “storage, the quality of the fruit was assessed paying particular attention to -greasiness. Results from this trial indicate that the relationship between background colour and fruit maturity is not consistent. Indeed, the maturity of apples of a particular background colour may differ according to district and harvest date. Greasiness of fruit was related to harvest maturity in Hawkes Bay. However, fruit from Canterbury and Otago became severely greasy even when harvested at early maturities.

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D.M. Dawson, L.D. Melton, D.M. Dawson and C.B. Watkins

Nectarine fruit (Prunus persica (L) Batsch) cv. Fantasia, were ripened immediately after harvest (normal ripening), or stored for 6 weeks either continuously at 0°C or were intermittently warmed (IW) for 48 h at 20C after 2 and 4 weeks, and then ripened. Fruit subjected to IW ripened normally, whereas the continuously stored fruit developed mealiness during ripening. Normal ripening was associated with solubilization and depolymerization of pectic polymers and a net loss of galactose. Only limited pectic solubilization and removal of side chains occurred during ripening of mealy fruit. Pectic polymer polymerization occurred at each IW occasion continued during ripening after storage, but was not as extensive as in normally ripened fruit. Mealy fruit had high autolytic capacity, probably as a result of insoluble pectic polymers in the cell wall that were not solubilized during ripening. The release of uronic acid suggests that cool storage temperatures do not irreversibly inhibit polygalacturonase activity.