Agriculture Reference
In-Depth Information
adjacent to aerobic regions within the fruit. This is a very
different situation from that imposed by 'anoxic shock'
treatments where the change in O 2 supply is not only
sudden but also presumably throughout the whole fruit.
'stay-green' phenomenon seen in some plant species. In
the 'stay-green' types, there is an inhibition of pheophor-
bide a oxygenase, but this was not the case for banana.
Drury et al . (1999) evaluated ripening related gene expres-
sion in the peel at 20°C and 35°C. At the high temperature,
the majority of clones was unaffected. This supported the
view that the effect of high temperature on peel de-greening
was not a reflection of the inhibition of the ripening
process. They thought the important change was the down
regulation of ATP-citrate lyase (ATP-CL) at 35°C compared
with 20°C. They proposed that in senescence, ATP-CL
may be involved in regulating the energy supply within
cells and it may have a role in the conversion of chloroplast
to chromoplast.
Plainsirichai et al . (2003) used auto-fluorescence to
examine the changes in the chlorophyll of the peel of the
dessert banana cv 'Chinese Cavendish' (AAA, Cavendish
subgroup) and the cooking banana cv 'Bluggoe' (ABB
group) ripening at 20°C or 30°C. They found that chloro-
phyll auto-fluorescence disappeared from the peel of both
cultivars ripened at 20°C but was retained in cv 'Chinese
Cavendish' at 30°C but only in the cells adjacent to the
surface of the peel. In the remainder of the peel, chloro-
phyll auto-fluorescence disappeared as the fruit ripened.
These observations are consistent with those of Drury et al
(1999) and earlier studies, that chlorophyll is degraded
at  high temperature in bananas albeit at a slower rate
than  at 20°C. The retention of chlorophyll in the cells
adjacent to the surface of the peel has implications for
studies where the whole peel is macerated to extract
biochemical components in an attempt to discover what is
happening to the chlorophyll.
Interactions between the peel and the pulp
The banana and plantain fruit have two major morphologi-
cal components, the peel and the pulp. The peel is the wall
of the inferior ovary and has completed much of its growth
in length just after the inflorescence emerges from the top
of the pseudostem (anthesis). The pulp develops, after
anthesis, from the outer edge of the loculus, that is, the
internal walls of the peel. As the pulp grows, the fruit
expands radially and the weight of the peel decreases as a
proportion of the weight of the whole fruit. Bananas are
growing rapidly when they are harvested and banana growers
use the lateral dimensions of the fruit as a measure of maturity
for harvest. Thinner fruit is sent to more distant markets
because it is less mature and has a longer green-life than
fruit that is more 'full'. Consumers judge the ripeness of
the pulp by the colour of the peel and so it is important to
match the stage of ripeness of the peel with that of the pulp.
That the two can be separated is of interest in commerce
and biologically. For the Cavendish cultivars, the tem-
perature of ripening is a simple way to change the rate of
ripening of the pulp, independently of the peel. At high
temperatures, the pulp ripens in advance of the peel and the
reverse is true at cool temperatures (Peacock 1980).
Differences between the peel and pulp have been of interest
to physiologists for many decades (Gore 1914).
De-greening of the peel
During ripening the chlorophyll in the peel breaks down,
revealing yellow pigments. At temperatures above 24°C,
chlorophyll breakdown is slowed down and the pulp may
become over-ripe while the peel is still green (Seymour et al .
1987). Plantains do not show this behaviour (Blackbourn et
al . 1990). Drury et al . (1999) followed the catabolism of
chlorophyll in the peel of bananas ripened at 20°C or 35°C.
Chlorophyll was degraded at both temperatures, but the
rate of loss was reduced at 35°C compared with that
at  20°C. They observed that all the steps involved in
chlorophyll catabolism that they examined functioned at
35°C as well as at 20°C. Therefore, they suggested that the
slower rate of chlorophyll catabolism at 35°C may be
related to the release of the chlorophyll from the thylakoid
membranes, preventing its movement to the chloroplast
envelope where disassembly begins. Drury et al . (1999)
found that the mechanism of retention of green colour in
bananas ripening at high temperature differed from the
The pulp and the peel during ripening
The main issues are where does ripening start, pulp or peel,
and what mechanisms control C 2 H 4 biosynthesis and action in
the peel and the pulp, given the central importance of C 2 H 4
in the ripening of banana? In exploring these issues differ-
ences in methodology and the use of 1-methylcyclopropene
(1-MCP) to block ethylene action are highlighted as we
work towards uncovering the control mechanisms.
The pulp and the peel undergo ripening, and most of the
genes that are up- or down-regulated in the pulp as it ripens
(Medina-Suárez et al . 1997) also change in the peel (Drury
et al . 1999). The degradation of chlorophyll during ripening is
unique to the peel. The banana fruit ripens in response to
exogenous C 2 H 4 and so in commerce the peel is exposed
to  C 2 H 4 before the pulp. Does exogenous C 2 H 4 stimulate
ripening in banana in the same way as endogenous C 2 H 4 ?
Dominguez and Vendrell (1993) investigated the capacity
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