Cell wall metabolism during the development of chilling injury in cold-stored peach fruit: association of mealiness with arrested disassembly of cell wall pectins.

Abstract

Partially tree-ripened ripe fruit of peach (Prunus persica L.) were stored for 1-4 weeks at 5 degrees C and then ripened at 20 degrees C for 3 d to induce chilling injury. With increasing cold storage the incidence and severity of mealiness symptoms increased progressively, manifested as reduced quantities of free juice and internal flesh browning. Relative to juicy fruit, tissue of mealy fruit showed altered intercellular adhesion when examined by microscopy and, upon crushing, a higher proportion of cells remained intact and did not release cellular contents. Substantial alterations in the metabolism of cell wall polysaccharides were observed. Chelator-soluble polyuronides from mealy fruit were partially depolymerized during cold storage in a manner dissimilar to that in unripe or ripe juicy fruit, and were not depolymerized further during the ripening period. The solubility of these high molecular weight pectins remained low, and did not show the increase characteristic of juicy fruit. Furthermore, in mealy fruit the dramatic decline in the polymeric Ara content of base-soluble, matrix glycan-enriched fractions occurring during normal ripening was absent, indicating diminished disassembly of an arabinan-rich polysaccharide firmly attached to cellulose. A corresponding rise in the polymeric Ara content of the most soluble pectin fraction was also absent, as was a decline in the Gal content of this extract. The depolymerization of matrix glycans showed only minor differences between juicy and mealy fruit. After cold storage and ripening, the activities of endo-1,4-beta-glucanase (EC 3.2.1.4), endo-1,4-beta-mannanase (EC 3.2.1.78), beta-galactosidase (EC 3.2.1.23), alpha-arabinosidase (EC 3.2.1.55), and particularly endo-polygalacturonase (EC 3.2.1.15) were lower in mealy fruit than in juicy fruit, whereas pectin methylesterase activity (EC 3.1.1.11) was lower in slightly mealy and higher in very mealy fruit. The data suggest that cold storage affects the activities of numerous cell wall-modifying enzymes, with important consequences for pectin metabolism. These changes alter the properties of the primary wall and middle lamella, resulting in tissue breakage along enlarged air spaces, rather than across cells, which reduces the amount and availability of free juice upon tissue fragmentation.