Fruit softening correlates with enzymatic and compositional changes in fruit cell wall during ripening in 'Bluecrop' highbush blueberries

Blueberry fruits undergo rapid softening, which has important consequences for postharvest shelf life. However, the mechanism of softening is not well understood. Here, we examined fruit quality characteristics and cell wall associated fruit softening in 'Bluecrop' highbush blueberry by investigating cell wall composition, cell micro-structure, and cell wall modifying enzyme activities at different fruit ripening stages. Fruit firmness significantly declined during ripening, with the greatest reduction between the pale green and reddish purple stages. The decrease in firmness corresponded to an extensively fractured microstructure in the parenchyma tissues and positively correlated with reductions in the contents of fruit cell wall materials. The increased amount of water-soluble pectins correlated with a decline in HCl-soluble pectins during ripening. A substantial loss of arabinose and a decreased ratio of arabinose and galactose to rhamnose in HCl-soluble pectins significantly correlated with fruit softening. Hemicellulose was significantly reduced, mainly between the pale green and reddish purple stages, showing a significant positive correlation with fruit firmness. Polygalacturonase, alpha-arabinofuranosidase, and beta-galactosidase showed higher activities during early ripening, whereas the activities of alpha-mannosidase and endo-1,4-beta-xylanase were higher during late ripening. These results indicate that cell wall-related processes are a key feature of early ripening in 'Bluecrop' highbush blueberry fruits. Our results set the stage for efforts to improve fruit shelf life by managing softening-related fruit losses in highbush blueberries.