The catalytic capacity of tomato polygalacturonase (PG) toward soluble pectic polymers is in excess of activity expressed in vivo; however, in vitro assays of PG have traditionally been performed under conditions (pH 4.0 to 4.5, 150 mM NaCl) that likely do not reflect the apoplastic environment of ripening tomato fruit. In this study, hydrolysis of pectin by purified tomato PG (isozyme 2) was examined in response to K+ (the predominate apoplastic cation) and over the pH range from 3.0 to 6.0. In the presence of K+, PG activity toward polygalacturonic acid measured reductometrically increased nearly 3.5-fold from pH 4.0 to pH 5.5. In the presence of Na+, activity decreased 90% over the same pH range.
PG-mediated degradation of cell wall from mature-green fruit showed divergent hydrolytic patterns in response to pH and K+. At pH 4.5 in the presence of K+ (as KCl), catalysis resulted in both solubilization and extensive depolymerization of cell wall pectin, with oligomers accounting for a significant portion of the hydrolysis products. At pH 5.5, the total quantity of wall pectin released in response to PG2 was similar to that at pH 4.5; however, oligomer production was strongly suppressed at the higher pH. At pH values favoring extensive depolymerization, low mol mass products were produced at 5 mM K+ and increased to a maximum at 100 mM K+. At higher pH, hydrolysis patterns were not affected by [K+]. pH and ionic effects may contribute to the distinctive patterns of pectin hydrolysis observed for different fruits.
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.
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).
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.
Abstract
Red tart cherries treated with succinic acid 2,2-dimethyl hydrazide (Alar) 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.
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.
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.
shelf-life conditions. Sometimes, softening produced by CI is accompanied by gelification ( Arnal and Del Río, 2004a ). Various studies have related CI in persimmon to changes in cellular structure; a greater and more accelerated cell wall solubilization
mango fruit is also closely associated with rapid fruit softening caused by cell wall depolymerization ( Yashoda et al., 2006 ). Immature to mature green mangoes are more sensitive to CI compared with ripe fruit and fail to ripen adequately ( Sivakumar
sclerenchyma cells formed by the secondary thickening of cell walls, followed by the deposition of lignin on the primary walls of parenchyma cells. In pears, stone cells in the fruit flesh are an important determinant of fruit texture ( Tao et al., 2009 ) and