Cultivar-Specific Flesh Mealiness in Apple Fruit Associated with Divergent Cell Wall Metabolism and Accelerated Senescence

Flesh mealiness, a textural disorder in apples, reduces storage quality and consumer acceptance. The ‘Delicious’ and ‘Fuji’, prominent apple cultivars in China, exhibit contrasting susceptibility to mealiness, though the underlying mechanisms remain unclear. This study compared cytological, physiological and cell wall metabolic changes between mealy ‘Oregon Spur II Delicious’ and non-mealy ‘Miyazaki Spur Fuji’ during ambient storage. Toluidine blue staining and scanning electron microscopy revealed that ‘Delicious’ exhibited larger intercellular spaces and cell separation in contrast to ‘Fuji’. This observation aligns with the earlier onset of mealiness in ‘Delicious’: its mealiness degree increased from 3.06% at harvest to 19.62% after 28 d of storage (a 6.4-fold rise), whereas that of ‘Fuji’ only increased from 2.13% to 3.90% (1.8-fold). This pronounced increase in ‘Delicious’ was accompanied by a significant increase in air space volume and a reduction in expressible juice. Furthermore, the occurrence of mealiness in ‘Delicious’ involved a sharp increase in respiration rate and ethylene production, alongside rapid declines in firmness and starch content. Notably, there was a substantial accumulation of water-soluble pectin (WSP) and chelator-soluble pectin (CSP) in ‘Delicious’, whereas the content of Na2CO3-soluble pectin (NSP) remained consistently lower. Monosaccharide composition analysis confirmed significantly reduced arabinose and galactose levels across pectin fractions (WSP, CSP, and NSP) in ‘Delicious’. Correspondingly, immunofluorescence labeling showed a pronounced degradation of arabinan and galactan within the side chains of rhamnogalacturonan-I (RG-I). In addition, the activities of pectin methylesterase, α-L-Arabinofuranosidase, and β-D-Galactosidase remained significantly elevated in ‘Delicious’. Collectively, these findings demonstrate that cultivar differences in flesh mealiness are attributable to divergent physiological senescence and cell wall disassembly processes.