Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 49

that the cell wall is fl exible, as is the foil
balloon, but it does not stretch easily
unless modifi ed. If the cell loses some of its
water through evaporation, it is softer and
more fl exible, like the foil balloon would
be if it lost some of its air. To take this
analogy a bit further, the foil balloon can
be made to rupture at lower or higher
pressures depending on the thickness and
strength of the material used. Similarly, the
pressure required to rupture a fruit cell
depends on the strength and composition
of the cell wall. None the less, a fruit is a
bit more complicated than the single
balloon because the fruit is not just one cell
but many cells that can be made to slide
past one another, and slippage can also
contribute towards softening.
Imagine a bag full of marbles. If the
marbles are glued tight to each other, they
act as one big incompressible rigid block,
but loosen the glue between the marbles,
and the marbles readily move about inside
the bag. The middle lamella between plant
cells acts like a glue that adheres adjacent
cells. Completely dissolving the middle
lamella causes the fruit to become softer.
However, if softening were simply
dissolution of the middle lamella to allow
cells to slide past each other, eating a fruit
might be more like eating a bag of sand.
The cells must also rupture to allow the
ripened contents to escape.
Thus it can be seen that fruit softening
is a combination of changes in turgor
pressure, primary cell-wall construction
that affects the pressure needed to rupture
the cell, and cell-to-cell adhesion that
affects how readily the cells slide past one
another. However, there are limits to how
much these parameters can change whilst
still maintaining a living organ and
palatable fruit. At this point, it is worth
mentioning a basic principle necessary to
maintain a living cell deep inside a fairly
large organ like a fruit. All plant cells,
including fruit cells, require effi cient gas
exchange (Ho et al. , 2011). Oxygen is taken
up for respiration, and carbon dioxide is
given off. If the environment surrounding
the cell is anaerobic (i.e. oxygen depleted)
for too long, the cell will die. Animals

facilitate gas exchange by using a
circulatory system, but plants typically
rely on passive diffusion of oxygen to and
from the cell surface. The primary cell
wall is fi lled with liquid, but some portion
of the intercellular space between cells
must be gaseous (Ho et al. , 2011). There
must be a contiguous air channel from the
surface of the fruit to its centre to facilitate
gas diffusion. If the primary cell-wall
structure and middle lamella were to be
degraded to the point that the intercellular
gas space was lost, either from collapse of
the fruit under its own weight or swelling
of cells to the point that gas exchange was
greatly compromised, the fruit would spoil
and become rotten. We all know what a
rotten fruit looks like. Rotten fruit can be
caused by pathogens, mechanical damage,
or over-ripening and collapse of the fruit
structure.

4.1.1 Turgor: waxing and packaging

Water relationships are not the focus of this
chapter, and factors that affect turgor
pressure and water loss are complex. None
the less, because water relationships are so
important to softening and maintaining
fruit quality, we will briefl y discuss what
has been done to understand and regulate
this important parameter. You might be
surprised how many fruits and vegetables
are waxed: apples, pears, tomatoes, cucum-
bers and many more (Hardenburg, 1967).
The commercial practice of waxing fruits
and vegetables to reduce water loss and
extend shelf-life has been used for many
years (Hardenburg, 1967). In addition to
wax, the coating often incorporates fungi-
cides to reduce postharvest infections
(Hall, 1989). In some cases, the wax may
reduce not only water loss but also gas
exchange enough that it may limit
oxidative respiration without the fruit
becoming anaerobic (Jeong et al. , 2003).
Limiting oxidative respiration goes hand in
hand with reducing metabolic rate and
slowing the ripening process, which
includes softening (Tucker and Laties,
1985; Asif et al. , 2006; Zabalza et al. ,