Cell wall enzymes and cell wall changes in 'Flavortop' nectarines: mRNA abundance, enzyme activity, and changes in pectic and neutral polymers during ripening and in woolly fruit

Most 'Flavortop' nectarines [Prunus persica (L,) Batsch (Nectarine Group)] that were placed directly into 0 degrees C storage developed chilling injury after removal, while preconditioning fruit for 2 days at 20 degrees C (delayed storage) reduced chilling injury substantially, Chilling injury was expressed as the development of a dry, woolly flesh texture during ripening. Delayed-storage fruit were as firm as control fruit when placed in storage, but softened more during storage. Analysis of cell wall components showed that in woolly fruit a higher percentage of pectin was retained in the sodium carbonate fraction, although during ripening polymers in this fraction decreased in molecular mass (Mr). In the guanidine thiocyanate hemicellulose fraction of woolly fruit, the associated pectin and hemicellulose remained as large polymers, while in delayed-storage fruit they decreased in Mr during ripening. Endo-polygalacturonase (PG), pectin esterase (PE), and endo-glucanase (EGase) activities of delayed-storage fruit were the same as control fruit at the beginning of storage, although exo-PG was higher. However, differences were observed at the end of storage. Endo-PG activity was lower in control than delayed-storage fruit at the end of storage while PE activity was higher, and exo-PG and EGase activities were similar. These differences in activity were not reflected in the mRNA abundance of the respective enzymes, Endo-PG and PE message was similar in all fruit at the end of storage and increased during ripening, while EGase message was low at all times except in control fruit after storage and development of woolliness, Prevention of chilling injury by delayed storage appears to be due to the ability of the fruit to continue a progressive, slow cell wall degradation in storage which allows normal ripening to proceed when the fruit are rewarmed, Regulation of the softening process did not appear to be by enzyme synthesis, since mRNA. levels of the enzymes did not correspond with enzyme activity.