Lower cell wall pectin solubilisation and galactose loss during early fruit development in apple (Malus x domestica) cultivar 'Scifresh' are associated with slower softening rate.

Substantial differences in softening behaviour can exist between fruit even within the same species. Apple cultivars 'Royal Gala' and 'Scifresh' soften at different rates despite having a similar genetic background and producing similar amounts of ethylene during ripening. An examination of cell wall metabolism from the fruitlet to the ripe stages showed that in both cultivars pectin solubilisation increased during cell expansion, declined at the mature stage and then increased again during ripening. This process was much less pronounced in the slower softening 'Scifresh' than in 'Royal Gala' at every developmental stage examined, consistent with less cell separation and softening in this cultivar. Both cultivars also exhibited a progressive loss of pectic galactan and arabinan side chains during development. The cell wall content of arabinose residues was similar in both cultivars, but the galactose residue content in 'Scifresh' remained higher than that of 'Royal Gala' at every developmental stage. The higher content of cell wall galactose residue in 'Scifresh' cell walls correlated with a lower ß-galactosidase activity and more intense immunolabelling of RG-I galactan side chains in both microscopy sections and glycan microarrays. A high cell wall galactan content has been associated with reduced cell wall porosity, which may restrict access of cell wall-modifying enzymes and thus maintain better structural integrity later in development. The data suggest that the composition and structure of the cell wall at very early development stages may influence subsequent cell wall loosening, and may even predispose the wall's ensuing properties.

Solid-state 13C NMR study of the mobility of polysaccharides in the cell walls of two apple cultivars of different firmness.

Solid-state (13)C nuclear magnetic resonance (NMR) was used to compare differences in mobility of the cell wall polysaccharides of 'Scifresh' and 'Royal Gala' apples after 20 weeks of storage. The texture of 'Scifresh' apples was markedly firmer than that of 'Royal Gala' at the end of storage. In a novel approach Two Pulse Phase Modulation (TPPM) decoupling was combined with cross polarisation (CP) and single pulse excitation (SPE) experiments. The resulting high resolution solid-state SPE spectra, unprecedented for apple cell walls, allowed a detailed insight into the physical and chemical properties of very mobile polysaccharides such as the arabinan and galactan side chains of the pectic polysaccharide rhamnogalacturonan I (RG-I). NMR showed that the cellulose rigidity was the same in the two cultivars, while arabinans were more mobile than galactans in both. Unexpectedly, arabinans in 'Scifresh' cell walls were more mobile than those in 'Royal Gala' which was unforeseen considering the greater firmness of the 'Scifresh' cultivar.

Cell wall structures leading to cultivar differences in softening rates develop early during apple (Malus x domestica) fruit growth.

BACKGROUND: There is a paucity of information regarding development of fruit tissue microstructure and changes in the cell walls during fruit growth, and how these developmental processes differ between cultivars with contrasting softening behaviour. In this study we compare two apple cultivars that show different softening rates during fruit development and ripening. We investigate whether these different softening behaviours manifest themselves late during ethylene-induced softening in the ripening phase, or early during fruit expansion and maturation. RESULTS: 'Scifresh' (slow softening) and 'Royal Gala' (rapid softening) apples show differences in cortical microstructure and cell adhesion as early as the cell expansion phase. 'Scifresh' apples showed reduced loss of firmness and greater dry matter accumulation compared with 'Royal Gala' during early fruit development, suggesting differences in resource allocation that influence tissue structural properties. Tricellular junctions in 'Scifresh' were rich in highly-esterified pectin, contributing to stronger cell adhesion and an increased resistance to the development of large airspaces during cell expansion. Consequently, mature fruit of 'Scifresh' showed larger, more angular shaped cells than 'Royal Gala', with less airspaces and denser tissue. Stronger cell adhesion in ripe 'Scifresh' resulted in tissue fracture by cell rupture rather than by cell-to-cell-separation as seen in 'Royal Gala'. CDTA-soluble pectin differed in both cultivars during development, implicating its involvement in cell adhesion. Low pectin methylesterase activity during early stages of fruit development coupled with the lack of immuno-detectable PG was associated with increased cell adhesion in 'Scifresh'. CONCLUSIONS: Our results indicate that cell wall structures leading to differences in softening rates of apple fruit develop early during fruit growth and well before the induction of the ripening process.