Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 163
8.2 Fruit softening
Ripening-associated fruit softening is usually represented by a decrease in the firmness of
the tissues involving modifications to the polysaccharide components of the primary cell
wall and middle lamella that cause a weakening of the structure. This interplay between
primary cell wall and middle lamella components is a very complex process involving
many families of cell wall-modifying enzymes. Additionally, structural proteins such as
expansin also play role (Brummell, 2006; Vicente et al., 2007). Developmental regulation
of expression of many of the genes encoding the cell wall-modifying enzymes underlying
this process has been demonstrated. Many of these proteins are expressed at the onset
of fruit ripening and secreted into the extracellular spaces from the symplast. In addition
to developmentally regulated fruit softening, other mechanisms such as water relations
involving turgor pressure and free radicals may also contribute to fruit softening (Fry et al.,
2001; Dumville and fry, 2003).
Softness and textural characteristics of ripe fruit have been suggested to be determined
by the ratio between the declining firmness of primary cell wall and the declining strength of
the intercellular adhesion (Harker et al., 1997). It has been suggested that both the cell wall
and the middle lamella must weaken for fruit to change from hard unripe to soft/crisp and
yet juicy (Brummell, 2006). Relatively robust intracellular connections with the weakening
of the primary cell walls would keep fruit firm and crisp. On biting, cells in such fruit will
split open resulting in release of cellular contents and making fruit juicy when chewed. Cell
separation due to breaking of the intracellular connection would result in fruit that is both
soft and juicy. In case the primary cell walls remain strong and the intracellular adhesion
is too weak, the fruit tissue will be soft with an unpleasant dry texture as observed in apple
and peach injured by chilling (Harker and Hallett, 1992; Brummell et al., 2004b; Brummell,
2006). Overripe fruit exhibit loss of both primary cell walls and intracellular connections.
However, detailed characterization of the structural and compositional changes is needed
to strengthen this hypothesis.
8.3 Structure and composition of primary cell walls in fruits
The primary wall is important for structural and mechanical support of the plant body.
It maintains and determines cell shape and form, resists internal turgor pressure of cell,
controls rate and direction of growth, regulates diffusion of material through the apoplast,
carbohydrate storage, and provides protection against pathogens, dehydration, and other
environmental factors. Besides polysaccharides, a range of structural and enzymatic proteins,
hydrophobic compounds, and inorganic molecules also exist in cell wall. The primary
cell wall is highly hydrated, and the aqueous components contain various dissolved
solids, ions, and soluble proteins including enzymes. Several models for plant
cell walls have been proposed. These models are based on the same common fundamental
components (Table 8.1) but differ in terms of how these components interact
with each other. Interactions proposed among various components include cellulose microfibrils
cross-linked with hemicellulose with pectin acting as cement, Ca 2+ bridges connecting
uronic acid carboxyl function and borate diesters of two rhamnogalacturonan II
monomers, covalent cross-linkage among different classes of pectin to form a single heterogeneous
network, covalent bonding between pectin and xyloglucan, as well as pectin and