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`Anna' and `Granny Smith' apples (Malus domestics Borkh.) that were held at 38C for 4 days before storage at 0C not only were firmer than controls upon removal from storage, but also softened more slowly during shelf life at 17C. Skin yellowing and loss of acidity attendant upon the heat treatment were not prevented by dipping fruit in 2% CaCl2 before heating. Both heat-treated and control fruit softened at the same rate upon exposure to ethylene at 100 μl·liter-1 upon removal from storage. The insoluble pectin content of cortical tissues was higher in heat-treated fruit than in controls after 10 days at 17C, while soluble pectin levels were lower. Arabinose and xylose levels were lower in cell walls from heat-treated cortical tissue, but the treatment had no effect on loss of galactose residues during shelf life.

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The concentrations of water-soluble polyuronides in apples [Malus domestica Borkh.) and pears (Pyrus communis L.) increased, but those of EDTA- and HCl-soluble polyuronides decreased during softening. Total polyuronide content decreased slightly during softening in both fruits. Depolymerization of polyuronides was observed only in the water-soluble fraction in pear fruit during softening, concomitant with an increase in polygalacturonase (PG) activity. No detectable depolymerization was observed in any of the polyuronide fractions during softening of apple fruit nor was any PG activity detected. The polyuronide fractions extracted from pear and apple cell walls contained various amounts of methoxyl groups. Polyuronides with a high degree of methoxylation were preferentially lost from EDTA- and HCl-soluble polyuronides during softening of both fruit. The water-soluble polyuronide had a lower degree of methoxylation than those lost in the EDTA- and HCl-soluble fractions. These results suggest de-esterification of polyuronides with a high degree of methoxylation rather than the depolymerization of polyuronides in the solubilization of polyuronides during ripening of apples and pears.

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Effects of the synthetic auxin 2,4-DP on fruit ripening of `La France' pear (Pyrus communis L.) on `Quince C' (Cydonia oblonga Mill. rootstock) were investigated. A solution of 2,4-DP at 90 μL·L-1 was applied 143, 151, and 159 days after full bloom (DAFB) to whole trees and compared with nonstored nontreated fruit and stored nontreated fruit (harvested 165 DAFB). Internal ethylene concentration in 2,4-DP-treated fruit increased more than in nonstored nontreated fruit and the level was higher the earlier the application time. Fruit firmness decreased earliest for fruit treated with 2,4-DP at 143 DAFB, followed by 151 DAFB-treated fruit and then 159 DAFB-treated fruit. In the nonstored nontreated fruit, firmness also showed a slight decrease with time. In all 2,4-DP treatments, water-soluble polyuronide (WSP) increased with ripening and hexametaphosphate-soluble polyuronide (HMP) and HCl-soluble polyuronide (HP) concentrations decreased. Most notably, WSP concentration increased earliest in fruit treated with 2,4-DP at 143 DAFB. Total concentration of neutral sugars from cell walls in each treatment decreased with time, and the levels in fruit treated with 2,4-DP at 143 DAFB were lowest at each sampling time. Arabinose concentrations were high compared with other neutral sugars throughout fruit ripening for each treatment, while glucose concentrations were high in nonripened fruit. At 193 DAFB, ≈85% of the fruit treated with 2,4-DP at 143 DAFB reached edible condition (firmness not more than 0.4 N·mm-2) on the tree. Furthermore, ≈85% of the fruit treated with 2,4-DP at 151 DAFB reached edible condition on 200 DAFB and close to 100% of the fruit treated with 2,4-DP at 159 DAFB on 207 DAFB. When ripened in a controlled room at 20 °C and 90% relative humidity after 2,4-DP treatment, the fruit treated earliest reached edible condition the soonest. Results demonstrate that 2,4-DP treatment can be used as an effective method of producing good quality fruit ripened on the tree, and that 2,4-DP may be an adequate replacement for cold storage conditioning to induce ripening capacity. Chemical name used: 2,4-dichlorophenoxy-propionic acid (2,4-DP).

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Modifications to solubilized cell wall polyuronides of sweet persimmon (Diospyros kaki L. `Fuyu') were examined during development of chilling injury (CI) during storage and in response to heat treatments that alleviated CI. Storage at 0 °C caused the solubilization of a polyuronide fraction that possessed a higher average molecular mass than polyuronide solubilized during normal ripening. The viscosity of this fraction was 30-times that of normally ripened fruit. Fruit heat-treated before or following storage contained a soluble polyuronide fraction with a markedly lower average molecular mass and decreased viscosity than in chilling injured fruit. Heat treatment also impeded an increase in viscosity of the cell wall material if applied before storage. CI (gelling) was related to the release of polyuronide from the cell wall during storage and its lack of subsequent degradation. Heat treatments retarded polyuronide release but promoted degradation of solubilized polyuronides.

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Changes in tissue water relations, cell wall calcium (Ca) levels and physical properties of Ca-treated and untreated `Golden Delicious' apples (Malus×domestica Borkh.) were monitored for up to 8 months after harvest. Pressure infiltration of fruit with CaCl2 solutions at concentrations up to 0.34 mol·L-1 reduced both fruit softening and air space volume of fruit in a concentration-dependent manner. Turgor potential-related stress within the fruit persisted during storage and was higher in Ca-treated than in untreated fruit. Fruit that were pressure infiltrated with CaCl2 solutions between 0.14 and 0.20 mol·L-1 and then waxed to reduce water loss during storage showed no peel injury. Calcium efflux patterns from apple tissue disks indicated two distinct Ca compartments having efflux kinetics consistent with those for cell wall Donnan-phase bound and water free space soluble Ca. At Ca concentrations up to 0.20 mol·L-1, cell wall bound Ca approached saturation whereas soluble Ca showed a linear dependence. At higher external Ca concentrations, only soluble Ca in the tissue increased. During 8 months of cold storage, cell wall Ca-binding capacity increased up to 48%. The osmotic potential of apples harvested over three seasons ranged between-1.32 and -2.33 MPa. In tissue disks, turgor potential changes caused by adjusting the osmolality of the incubation solution with CaCl2 or sorbitol were accompanied by changes in the osmotic and water potentials of the tissue. In CaCl2 solutions up to 0.34 mol·L-1, turgor potential was ≥0.6 MPa in tissue incubated in 0.14 or 0.17 mol·L-1 solutions of CaCl2 and was more than 3 times higher than in tissues incubated in low (≤0.03 mol·L-1) or high (≥0.27 mol·L-1) concentrations of CaCl2. At osmotically equivalent concentrations, turgor potential was up to 40% higher in Ca-than in sorbitol-treated tissue. The results suggest that postharvest treatment with 0.14 to 0.20 mol·L-1 solutions of CaCl2 are best for maintaining fruit water relations and storage life of `Golden Delicious' apples while minimizing the risk of salt-related injuries to the fruit. While higher concentrations of CaCl2 may better maintain firmness, these treatments adversely affect fruit water relations and increase the risk of fruit injury.

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Abstract

Red tart cherries treated with succinic acid 2,2-dimethyl hydrazide (Alar) 5 were studied for changes in anthocyanin pigments and also histochemical changes. Anthocyanin biosynthesis was altered as a result of the treatment. The significant effect of Alar was to change markedly most of the individual pigment ratios. Of note is the fact that the concentration of the pigment peonidin 3-rutinoside was unaltered. The treated fruit showed an increase in total anthocyanin pigment.

Alar treatment also caused both quantitative and qualitative changes in cell wall carbohydrate composition. Treated fruit had more total wall material and showed callose formation. Furthermore, the sprayed fruit contained more pectin and hemicellulose. The amount of cellulose and lignin was not affected. The relation between changes in cell wall polysaccharides and increased firmness of sour cherries as a result of the treatment is discussed.

Open Access

Mature green and pink tomato (Lycopersicon esculentum Mill.) fruit were subjected to ionizing irradiation in the range of 0.7 to 2.2 kGy from gamma-or X-ray sources. Firmness of whole fruit and pericarp tissue, pericarp electrolyte leakage, and pericarp cell wall hydrolase activities were measured following irradiation and during postirradiation ripening at 20 °C. Irradiation-induced softening was evident in mature-green and pink fruit within hours following irradiation, and differences between irradiated and control fruit persisted throughout postirradiation storage. Trends of firmness loss were much more consistent and showed much greater dose dependency in pericarp tissue than whole fruit. Irradiation enhanced electrolyte efflux in fruit of both maturity classes. Fruit irradiated at the mature-green stage softened during postirradiation storage but exhibited an apparently irreversible suppression in polygalacturonase activity, with levels remaining <10% of those of nonirradiated fruit. Polygalacturonase activity was less strongly affected in irradiated pink fruit than in mature-green fruit, but activity remained reduced relative to the controls. Pectinmethylesterase and β-galactosidase activities were significantly enhanced in irradiated fruit of both ripening stages in the early period following irradiation, but reductions were noted after prolonged storage.

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Three onion (Allium cepa L.) cultivars, `Southport White Globe', `Grano', and `Pukekohe Longkeeper' were grown at low to high S (at 0.5, 1.8, 3.0 or 4.0 meq·L-1) in hydroponic culture. Differential solvent extractions of bulbs were used to isolate quantitatively cell contents, cell wall proteins, and cell wall residue. The weight of the cell fractions, their S content, and the S content of intact bulbs were determined. Bulb characteristics of fresh weight (FW), firmness, soluble solids concentration (SSC), and soluble sugars were also determined. For all three cultivars, bulb FW increased with S from 0.5 to 4.0 meq·L-1. Sulfur had a significant effect on bulb firmness. Onion bulbs grown with S at 0.5 meq·L-1, the lowest S concentration, were significantly softer than onion bulbs grown at the highest concentration of 4.0 meq·L-1. Varying the S supply had a major effect on dry weight (DW) allocation to the cell wall residue. Bulbs of all three cultivars grown at the lowest S had significantly less DW in the cell walls compared to S at 3.0 or 4.0 meq·L-1. In contrast to the effect of S supply on DW allocation, varying S supply had no effect on total bulb S, free SO4 -2, and on the S content of the cell contents and the cell wall residue and only a minor effect on cell wall proteins. There was no significant effect of S supply on either SSC or soluble sugars. At low S nutrition, which is limiting to the growth of onion bulbs, cell wall deposition is reduced, with a consequent decrease in bulb firmness. The S composition of the cellular components is maintained at the expense of bulb growth.

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Changes in cell wall polysaccharides associated with peach fruit softening were characterized over two harvest seasons. Enzymically inactive cell walls were prepared from mesocarp tissues of peach fruit harvested at three stages of softening. Pectin-associated and hemicellulose-associated polysaccharides were extracted from the cell walls sequentially, and glycosyl residue compositions were determined by GLC. Pectin extracts from both years were richest in galacturonosyl, arabinosyl, and rhamnosyl residues. Hemicellulose extracted with 1 m potassium hydroxide contained a high mole percentage of xylosyl, glucosyl, and fucosyl residues. Hemicellulose extracted with 4 m potassium hydroxide contained a substantial amount of pectin-associated sugar residues in addition to hemicellulose-associated sugar residues. During softening in both years, sugar compositions for cell walls, aqueous phenol-soluble polysaccharides, and imidazole extracts reflected a decrease in galacturonosyl residues and a concomitant increase in arabinosyl residues on a mole percent basis. The degree of change for galacturonosyl residues in these fractions depended on season, with greater variation exhibited from fruit at earlier stages of softening. With the notable exception of the seasonal variation exhibited for galacturonosyl residues in cell walls, the relative stability of other glycosyl compositional changes over seasons indicates conserved changes for pectins and hemicelluloses occur during peach fruit softening.

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Abstract

Respiration, ethylene production, firmness, polygalacturonase activity, cell wall composition, and soluble uronide content were measured during ripening of two tomato (Lycopersicon esculentum Mill.) genotypes, ‘Manapal’ and dark green (dg). Respiration rates and cell wall uronide contents of the two genotypes were similar. Climacteric ethylene production rates of dg fruit were about half that of ‘Manapal’ fruit. Firmness and polygalacturonase activity of dg tomatoes were similar to that of ‘Manapal’ fruit until 55 days postpollination, when dg fruit were twice as firm as ‘Manapal’ fruit and exhibited greater polygalacturonase activity. Soluble uronide content did not differ between the two genotypes, except at 50 days postpollination, when that of dg fruit was 60% that of ‘Manapal’ fruit. Cell wall uronide content of dg fruit was 1.5 times greater than ‘Manapal’ fruit at 55 days postpollination. Although dg fruit contained larger, absolute amounts of cell wall noncellulosic neutral sugars than ‘Manapal’ fruit, net changes in sugar composition were similar throughout ripening. Also, ratios of cell wall arabinosyl or galactosyl residues to cell wall galacturonic acid were similar in both genotypes. These data suggest that firmness differences between dg and ‘Manapal’ fruit are not due to differing activities of polygalacturonase or changes in cell wall composition during ripening, but to other factors that may affect solubilization of cell wall uronides.

Open Access