Cell Wall Ultrastructure and Xylan Distribution in 'Bluecrop' Highbush Blueberry (Vaccinium corymbosum) Fruits during Ripening

Fruit firmness is largely determined by cell wall, cell shape, cell-to-cell adhesion, and tissue anatomy. This study was conducted to investigate cell wall ultrastructural changes and xylan distribution during ripening and softening of 'Bluecrop' highbush blueberry (Vaccinium corymbosum) fruits. Fruits at pale-green, reddish purple, and dark-purple stages, which represented large green, turning point, and ripe stages, respectively, were used in this study. Cell walls and middle lamella were degraded, while plasma membranes were separated from primary cell walls at the reddish purple and dark-purple stages. The immunofluorescence labelling of xylan was abundant at the pale-green stage once the fruits were firm. However, the labelling became weaker at the reddish purple and dark-purple stages, and it was not well confined to cell wall areas, implying cell wall alteration at late ripening stages. These results demonstrated that the cell wall, middle lamella, and xylans were extensively degraded during the ripening and softening of 'Bluecrop' highbush blueberry fruits.