Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 48

4

Cell-wall Metabolism and Softening
during Ripening

Mark L. Tucker*
US Department of Agriculture, Beltsville, MD, USA

pressure. Although many factors can affect
turgor pressure, understanding the con-
sequences of water loss is not too
complicated. A partially dehydrated leaf
will be more fl exible. Similarly, a de-
hydrated fruit will be softer than a fully
hydrated fruit. It also seems fairly obvious
that if you loosen up the cell wall to make
it more fl exible and extensible, the fruit
will be softer. However, the cell wall is a
complex structure and how specifi c
changes in the cell wall affect softening is
not always obvious.
Let us continue with some additional
basic concepts that should help us under-
stand the role of the cell wall in a large
multicellular organ like a fruit. Let us
begin by building a mental picture of the
basic function of the cell wall by com-
paring it to two types of balloons, a latex
balloon and a foil balloon. The latex
balloon starts out small and continues to
expand as you fi ll it with gas until at some
point it explodes. The foil balloon is fl at,
fl imsy and fl exible until it is completely
full of gas (turgid) and does not expand
much more once completely fi lled but
rather explodes if too high a pressure is
reached. The fruit cell walls act more like
the foil balloon. Once the cell is turgid, it
does not readily expand. The key here is

4.1 Introduction

The fi nal stage of fruit development is
ripening, which typically includes trans-
formation from an inedible hard organ into
a palatable softer version. Fruit softening is
a combination of changes in fi rmness and
texture. Firmness can be defi ned as com-
pressibility or the force required to deform
the surface of the fruit (Brookfi eld et al. ,
2011). Texture is defi ned as a sensory
attribute and is more diffi cult to measure
with instrumentation (Mohamed et al. ,
1982; Garcia-Ramos et al. , 2005; Brookfi eld
et al. , 2011). Texture includes crispness,
viscosity and juiciness (Brookfi eld et al. ,
2011). Texture is often best measured by
human tasters (Brookfi eld et al. , 2011).
Before moving on to the specifi c details of
what happens during ripening to con-
tribute to softening, let us identify a few
basic principles to help visualize what
softening really is. The edible parts of fruit
are not woody (lignifi ed), not even before
they ripen. In other words, the cell walls in
the edible parts of fruit are not rigid. They
can fl ex. The fl exibility of the cell wall is
more obvious in thinner structures like
leaves. Leaves are not as 'stiff as a board'.
The primary force that maintains structure
in a leaf and fi rmness in fruit is turgor