IN-VIVO AND IN-VITRO SWELLING OF CELL-WALLS DURING FRUIT RIPENING

Swelling properties of the cell walls of nine temperate fruit species, selected for their different ripening anti textural characteristics, were studied during ripening. Cell wall swelling was examined in intac t fruit using microscopy techniques and in vitro, using cell wall mate rial isolated from fruit tissue. In fruit which ripened to a soft melt ing texture (persimmon, avocado, blackberry, strawberry, plum), wall s welling was pronounced, particularly in vitro. In-vivo swelling was ma rked only in avocado and blackberry. Fruit which ripened to a crisp, f racturable texture [apple (two cultivars). nashi pear, watermelon] did not show either in-vivo or in-vitro swelling of the cell wall. There was a correlation between swelling and the degree of pectin solubilisa tion, suggesting that wall swelling occurred as a result of changes to the viscoelastic properties of the cell wall during pectin solubilisa tion. Chemical and enzymatic removal of pectin from kiwifruit cell wal l material supported the idea that swelling is associated with movemen t of water into voids left in the cellulose-hemicellulose network by t he solubilised pectin. However. the results also suggested that swelli ng in vivo was more complex than this, and that the physicochemical ch anges which led to swelling included other elements of cell wall modif ication involving the site and mechanism of pectin solubilisation and- or the cellulose-xyloglucan complex.