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Peach fruit softening appears to be associated with changes in cell wall polymers, particularly pectins and hemicelluloses. To determine changes of cell wall polymers associated with peach fruit softening, we conducted sequential extractions of pectin and hemicellulose from softening fruit. A more tightly bound hemicellulose fraction contained considerable amounts of pectin associated sugars. This fraction was separated into charged and neutral fractions, using anion exchange chromatography, and then fractionated into two apparent molecular weight classes by size exclusion chromatography. Virtually all of the charged fraction eluted in the higher apparent molecular weight fraction. The neutral sugar fraction segregated into both apparent molecular weight size classes, with a redistribution from the large to the small size class during softening. This redistribution was accompanied by changes in neutral sugar composition. A possible relationship between changes in this fraction and fruit softening will be discussed. Supported by USDA grant 92-34150-7190 and the Oklahoma Agricultural Experiment Station.
The mechanism of softening was studied in a rapidly softening peach cultivar `Belle of Georgia' by assessing changes in pectins and hemicellulose from enzymically inactive cell walls. Cell wall preparations were sequentially extracted with imidazole and sodium carbonate (pectin extracts), and potassium hydroxide (hemicellulose extracts). The pectin extracts were particularly enriched in galacturonic acid, arabiiose and rhamnose, and contained only small amounts of hemicellulose associated sugars. Hemicellulose extracts were enriched in xylose, glucose, mannose, and fucose. More tightly bound hemicellulose fractions contained considerable amounts of pectin associated sugars. The proportion of pectin associated sugars in hemicellulose extracts was greater for cell wall extracts of softened fruit. Some possible relationships between pectin/hemicellulose solubility and fruit softening will be presented. Work was supported by USDA grant 90-34150-5022 and the Oklahoma Agricultural Experiment Station.
Softening to a normal melting flesh texture in peaches involves a combined participation between polymers located in the middle lamella and primary cell wall. Pectins located in the primary cell wall polysaccharide matrix which cosolubilize when hemicellulose is extracted with KOH have received less attention than the chelator or sodium carbonate soluble pectin likely to be associated with the middle lamella. We conducted a series of extractions for cell walls prepared from softening peach fruit (47, 30, and 15 N firmness) using 0.5 m imidazole, sodium carbonate and a graded series of KOH. Hemicellulose-associated pectin was a substantial proportion of most KOH extracts (30 to 50 mole percent) and fractionated on size exclusion chromatography as a high apparent molecular weight peak which became more prominent as fruit softened and could be separated from two lower apparent molecular weight peaks by anion exchange chromatography. The nature of a hemicellulose-pectin interaction in peach was apparently by physical entrapment, versus covalent cross-linking. Softening related changes in hemicellulose-associated pectin will be addressed.
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.
Pectin and hemicellulose were solubilized from cell walls of peach [Prunus persica (L.) Batsch] fruit differing in firmness by extraction with imidazole and sodium carbonate (pectin extracts), followed by a graded series of potassium hydroxide (hemicellulose extracts). The extracts were subjected to size exclusion chromatography. In imidazole extracts, as fruit softened, there was an increase in proportion of a large apparent molecular mass peak, with a galacturonosyl to rhamnosyl residue ratio resembling a rhamnogalacturonan-like polymer. A smaller apparent molecular mass peak was enriched in galacturonic acid and probably represented a broad polydisperse peak derived from more homogalacturonan-like polymers. In sodium carbonate extracts, a homogalacturonan-like polymer appeared to elute primarily as a higher apparent molecular mass constituent, which increased in quantity relative to other constituents as fruit softened. In cold 1 mol·L-1 KOH extracts three peaks predominated. A xyloglucan-like polymer appeared to elute predominantly in the second peak and fucose was strongly associated with it. In 4 mol·L-1 KOH extracts (tightly bound hemicellulose) the higher apparent molecular mass peak was predominantly acidic and presumably of pectic origin. The smaller apparent molecular mass peaks were composed primarily of neutral sugars, the second peak became smaller and the third peak larger as fruit softened. The ability to separate pectin and xyloglucan-like polymer as two separate fractions based on charge suggests that the nature of any pectin-hemicellulose interaction in this fraction is probably one of physical entrapment of pectin fractions by hemicellulose and not principally by covalent crosslinking between the two polysaccharide classes in peach. Flesh firmness serves as an important determinant of quality in peaches. Our results indicate that apparent molecular mass of both pectins and hemicelluloses changed as peaches softened, resulting in alteration of cell wall structure and associated with decreased tissue cohesion.