Bananas - Crop Post-Harvest: Science and Technology - page 36

these actions would reduce C 2 H 4 synthesis and Inaba et al .
(2007) described it as a negative feedback regulatory
mechanism. Golding et al . (1998) concluded that most
changes in fruit biochemistry related to ripening depend on
the functioning of C 2 H 4 receptors during the first 24 hours
after initiation of ripening by propylene and that sensitivity
to C 2 H 4 increases after ripening is initiated.
Since C 2 H 4 is known to suppress C 2 H 4 evolution in
banana (Vendrell and McGlasson 1971), Inaba et al . (2007)
investigated the feedback mechanisms of C 2 H 4 in banana
peel and pulp. They used cv 'Grand Nain' (AAA, Cavendish
subgroup) grown in the Philippines. They applied 1-MCP
before and after ripening had been initiated by propylene
and measured C 2 H 4 evolution as well as the activities of
ACC synthase and oxidase, the genes associated with these
enzymes and the content of ACC in the tissues. In the pulp,
propylene stimulated C 2 H 4 production within 24 hours and
then, adding 1-MCP doubled the production of C 2 H 4 . By
contrast, in the peel tissue, 1-MCP reduced C 2 H 4 production
to very low levels, compared with the effect of propylene
alone. In addition, the peel began to produce C 2 H 4 one day
later, after ripening began, than the pulp. Inaba et al . (2007)
suggest that the peel and the pulp have different feedback
regulatory mechanisms for the biosynthesis of C 2 H 4 . For
the pulp, an increase in C 2 H 4 reduces ACC synthase activity
and there is also an effect of the supply of co-substrate and
co-factors to ACC oxidase which produces C 2 H 4 from
ACC. If co-substrate or co-factors become limiting then
C 2 H 4 production is reduced.
In the peel, 1-MCP suppressed activity of ACC synthase
and ACC oxidase, implying that C 2 H 4 has a positive
feedback mechanism in this tissue (Inaba et al . 2007).
Ethylene promoted the activity of ACC oxidase and ACC
synthase and we would expect that this would stimulate
C 2 H 4 evolution. This is not the case because C 2 H 4 promotes
the activity of malonyl ACC transferase which deprives
ACC synthase of substrate. While the peel displays a
positive feedback mechanism for C 2 H 4 biosynthesis, the
amount of C 2 H 4 produced is governed by the amount of
ACC channeled to malonyl ACC, which is not available for
synthesis of C 2 H 4 (Dominguez and Vendrell 1993).
Inaba et al . (2007) proposed that in banana exogenous
C 2 H 4 induced ripening which in turn induced endogenous
production of C 2 H 4 . In the pulp, C 2 H 4 biosynthesis is
under negative feedback control and in the peel, while
under positive feedback control, C 2 H 4 evolution is limited
by the availability of free ACC. The combined effect of
the two tissues is observed in the whole fruit, an
increase in C 2 H 4 evolution early in ripening followed by a
decrease. However, this pattern can be modified by other

environmental factors and so the contribution of pulp and
peel to these changes needs to be separated. A heightened
awareness of the differences between physiology of the
peel and the pulp in banana may lead to deeper knowledge
of the control mechanisms in each tissue and their
interaction. It would be especially interesting to explore
these effects in different cultivars, such as the cv 'Sucrier'
(AA) with its peel spotting problem and a cultivar of the
AAA, Cavendish subgroup where the peel and pulp
respond differently to high and low temperature.

Ethanol and acetaldehyde
There are two aspects of the metabolism of ethanol and
acetaldehyde that are important for bananas. One is the
endogenous production of ethanol by ripening fruits, even
when in air and the other is the use of exogenous ethanol
or acetaldehyde to modify ripening behaviour, reduce
astringency or enhance flavours. Pesis (2005) reviewed
experiments on the use of ethanol and acetaldehyde to
modify the post-harvest behaviour of fruits.

Ethanol metabolism
Ethanol is produced in anoxic tissue, in situ , when pyruvate
decarboxylase (PDC) converts pyruvate to acetaldehyde,
which in turn is converted to ethanol by alcohol dehydroge-
nase (ADH). The last reaction is reversible. Ethanolic
fermentation is the main metabolic pathway that produces
energy in anaerobic tissues in plants since O 2 deficiency
inhibits pyruvate dehydrogenase, cutting off the tricarbox-
ylic acid (TCA) cycle. However, the ethanolic fermentation
pathway produces only one tenth of the energy, per unit of
hexose metabolized, compared with the TCA cycle (Gibbs
and Greenway 2003). Thus, O 2 deficiency creates an energy
crisis in plant cells. Bulky tissues, such as fruits and storage
roots, produce ethanol even when they are situated in air.
The ethanol may be produced in regions of these organs
that are deficient in oxygen, as suggested by Gustafson
(1930, 1934) for ripening tomatoes, or it may be activated
by other mechanisms such as a deficiency of energy (Gibbs
and Greenway 2003) or by acidosis, an acidification of the
cytoplasm (Longhurst et al . 1990). In fruit tissues under
anoxia, ethanol accumulates during continued exposure,
while acetaldehyde soon reaches a maximum level. Since
plant tissue is permeable to ethanol it can readily diffuse
from anoxic to more aerated regions of these organs and
enter general metabolism. There it is oxidized to acetalde-
hyde, which in turn can be converted by aldehyde dehydro-
genase to acetyl coenzyme A, a precursor of acetate esters.
Acyl-CoA is the precursor for longer esters, compounds
that are responsible for the aroma of ripened fruits.