Softening in Highbush Blueberry Fruits: Cell Wall Modification and β-Aminobutyric Acid Effect

Abstract

Blueberry fruit quality deteriorates significantly after harvest due to its rapid softening characteristics. Fruit quality changes and cell wall-associated softening mechanism were investigated during ripening of Bluecrop highbush blueberry fruits. The effects of preharvest β-aminobutyric acid (BABA) treatment on postharvest fruit quality and their softening were also evaluated during refrigerated storage. The fruit size greatly expanded along with a substantial accumulation of fruit weight between the reddish purple and dark purple stages. Fruit firmness declined significantly between the pale green and reddish purple stages, which corresponded to the increases in soluble solids content, pH, and total anthocyanin content. Respiration rate peaked at the reddish purple stage, coinciding with the peak of ethylene production which accounted for approximately 2 µL·kg-1·h-1. The softening of Bluecrop highbush blueberry fruits corresponded to extensively fractured microstructures of parenchyma tissues and alteration in cell wall ultrastructure, and significantly correlated with a reduction in fruit cell wall material contents. HCl-soluble pectins were converted to water-soluble pectins as ripening proceeded. Arabinose contents were substantially lost in HCl-soluble pectins, decreasing the ratio of arabinose plus galactose to rhamnose during ripening, and significantly correlated with fruit softening. Hemicellulose contents were significantly reduced with increasing ripening, showing a significant positive correlation with fruit firmness. Polygalacturonase, α-arabinofuranosidase, and β-galactosidase showed higher activities during early ripening, whereas the activities of α-mannosidase and endo-1,4-β-xylanase were higher during late ripening. Preharvest BABA treatment on fruit clusters at 20 mM significantly delayed color changes, enhanced individual soluble sugars and organic acids, and reduced decay incidence. BABA-treated fruits maintained higher skin firmness throughout the storage period. Cell wall material contents were significantly higher in BABA-treated fruits than in control. Moreover, preharvest BABA treatment significantly enhanced fruit pectin content at harvest and reduced activities of polygalacturonase and endo-1,4-β-xylanase, although hemicellulose content was not changed. These results indicate that cell wall-related softening events in Bluecrop highbush blueberry fruits extensively occur during early ripening and that preharvest BABA treatment improves the postharvest fruit quality including skin firmness during refrigerated storage. The results will be useful for managing softening-related fruit losses in highbush blueberries.