The Development of Different Textures in Two Apple Cultivars: A Cell Wall Study

Abstract

What makes the ripe 'Scifresh' apple firm, and the 'Royal Gala' soft? The cell wall is important for providing structural integrity to fruit tissues during all stages of development. In the literature, cell wall changes during apple fruit ripening in storage have been extensively studied, but there is paucity of information regarding these changes during fruit growth. This study is the first to examine two cultivars of apple with different textures, during both fruit growth and ripening in storage, using an integrated approach of cell wall chemical composition analysis, enzyme activity assays, immunolabelling, microscopy and solid-state 13C NMR studies. The aim was to identify cell wall differences which could explain the difference in textures observed. In spite of comparable ethylene production, 'Scifresh' was able to remain firm and crisp during ripening in storage, whereas 'Royal Gala' softened more rapidly. Cell wall differences between the two cultivars were established early in growth (fruitlet), and majority of these differences occurred in the pectic domain. 'Scifresh' exhibited less pectin solubilisation, higher amount of galactose, higher degree of pectin methyl esterification, more polysaccharides strongly associated with cellulose and higher arabinan mobility. Enzymes β-galactosidase, α- arabinofuranosidase and pectin methylesterase were more active in 'Royal Gala' fruitlet than 'Scifresh', while activity of polygalacturonase was only detected in ripe 'Royal Gala' fruit but not in 'Scifresh'. Structurally, mature fruits of 'Scifresh' had larger cell sizes that appeared to be more angular shaped and more efficiently packed, which could contribute to increased intercellular adhesion. Knowledge of the cell wall components, structural properties and the growth stage of which is responsible for the development of different apple textures can be used as targets for breeding apple varieties with preferred textural attributes and longer storage life that fetch higher commercial value.