Agriculture Reference
In-Depth Information
can make the apple feel 'greasy', and if severe, the fruit can
become unmarketable.
the  activity of the enzymes responsible for ethylene
biosynthesis, cell wall degradation, and aroma volatiles
accumulation.
Although ethylene is the dominant trigger for ripening
in climacteric fruit, it has been suggested that both ethylene
dependent and ethylene independent gene regulation
pathways coexist and coordinate the ripening process
in  climacteric fruit. For example, sugars and acid
accumulation has been shown to be ethylene independent,
whereas flesh softening has been shown to have both
ethylene dependent and ethylene independent components.
This is because softening is regulated by a series of
differentially regulated genes.
APPLE PHYSIOLOGY, MATURITY
AND RIPENING
Apples are a climacteric fruit that show a burst of ethylene
production and an increase in respiration during ripening.
The manipulation of ethylene production and action has
become very important in efforts to improve quality and
extend storage life of apples. Given its importance, the
following section focuses on the physiological role of
ethylene (further information can be found in Volume 1,
Chapter 2).
Ethylene
Ethylene is a plant hormone that acts along with other
plant  hormones (auxins, gibberellins, kinins and abscisic
acid) to exercise control over ripening processes ( Pech et al .
2002; Wills et al . 2007). In addition to its recognition as a
'ripening hormone', ethylene is involved in a range of other
developmental processes including flowering, abscission
and senescence of various organs in responses to environ-
mental stress. As a ripening hormone, ethylene is a promoter
of aging and senescence that causes cells to shift from a
growth program to that of senescence (Fluhr &  Mattoo
1996). The regulation of ethylene production in  higher
plants such as apples is regulated by endogenous and
exogenous biotic and abiotic factors (Fluhr & Mattoo 1996).
Ethylene synthesis in higher plants takes the
following sequence: methionine → S-adenosylmethionine
(S-AdoMet) → 1-aminocyclopropane-1-carboxylic acid
(ACC) → C 2 H 4 (ethylene) (Kende 1993; Fluhr & Mattoo
1996; Alexander & Grierson 2002). Generally, the rate
limiting steps in the ethylene biosynthesis pathway are
catalysed by the enzymes 1-aminocyclopropane-1-
carboxylic acid synthase (ACS) and 1-aminocyclopropane-
1-carboxylic acid oxidase (ACO) (Fluhr & Mattoo 1996),
and the formation of ACC is the major rate limiting step
in  the biosynthetic pathway. Positive feedback regulation
of ethylene biosynthesis is also a characteristic feature of
ripening where exposure to exogenous ethylene results in a
large increase in ethylene production due to the induction
of ACS and ACO (Alexander & Grierson 2002). Both of
these enzymes are encoded by multigene families and
their  expression is differentially regulated by various
developmental, environmental and hormonal signals.
It has been shown that ethylene affects the transcription
and translation of many ripening related genes (Alexander
& Grierson 2002). The resulting biochemical changes that
precipitate ripening in apple fruit include increases in
Fruit softening
Fruit softening is a key aspect of apple ripening and it is
often used as a predictor of fruit quality. The softening of
apples is the consequence of cell wall degradation by
enzymes including polygalacturonase enzymes, pectin
methyl esterase and glycosidases (Kays 1991). The
softening process is regulated by the presence of ethylene.
It is important to note that there are marked differences
between apple varieties in terms of their rate of softening
(Johnston et al . 2002). Work has shown that the different
rates of softening between varieties can in part be
explained  by different expressions of polygalacturonase
genes (Wakasa et al . 2006).
As well as softening, other changes that occur during
ripening are the changes in background colour from green
to yellow, loss of acidity, the conversion of starch to sugar
and the synthesis of aromatic compounds. These all lead to
a product that is edible and enjoyable to eat (Watkins
2003).
Determination of commercial apple maturity
For the longest storage life with optimum eating quality,
apples should be harvested just before the rise in the
climacteric (Little & Holmes 2000). However, if the fruit
are harvested too early, they will not ripen properly, have
reduced flavour and will be susceptible to storage disorders
such as superficial scald and bitter pit. Conversely, if the
fruit are harvested when the fruit are well into their
climacteric (i.e. over-mature), the fruit do not store well,
soften rapidly, develop off-flavours and are susceptible to
disease. The determination of optimal apple maturity is
difficult, but it is critical to optimise fruit storage and
quality (Faragher et al . 1984). In addition, a number of
factors can significantly influence fruit maturity between
and within orchards and among fruit within a single tree.
For example rootstock, the nutrient status of the fruit and
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