Agriculture Reference
In-Depth Information
of fresh avocados increased from 0.18 kg in 1970 to
0.68-1.0 kg in the late 1980s, which is equivalent to
nectarines and comparable to pineapples (0.77 kg), but
considerably lower than bananas (11 kg), apples (8.7 kg)
and oranges (6.6 kg). Avocado is consumed as a fresh fruit,
besides its use in the oil, cosmetic, soap, and shampoo
industries. Unlike many fruits that typically have a sweet or
acidic taste, avocados have a smooth, buttery consistency
and a rich flavour. A popular use is as a salad fruit, but
avocados are also processed into guacamole and can be
used in sandwich spreads. Avocado paste with flavour
extracts and skimmed milk can also be used to make ice
cream. Oil extracted from avocados can be used for cooking
and preparation of salads, sauces and marinades. Avocado
oil also can be used for skin care products such as sunscreen
lotions, cleansing creams and moisturizers, or for hair
conditioners and makeup bases. Several more uses have
been added around the world. For example, in Mexico and
Brazil, it is added to ice creams and sorbets; in Japan it is
eaten in sushi rolls; in Cuba the pulp is mixed with capers,
green olives, lemon juice and olive oil to make a sauce
which is served with steamed fish; and in Nicaragua it is
stuffed with cheese, fried and baked. In other countries
such as Taiwan, it is eaten with milk and sugar; in Korea it
is mixed with milk and used as a facial cream and body
lotion; in Indonesia it is mixed with coffee, rum and milk to
make a refreshing beverage; in the Caribbean it is mixed
with salt, garlic, and coconut and served as an entrée; in the
Philippines the avocado purée is mixed with sugar and milk
to make a beverage which is served as dessert (Yahia 2003).
Hormonal control of fruit development
Two important endogenous factors affecting the physiology
of fruit growth include hormones and nutrients (Bower &
Cutting 1988; Cutting et al . 1986b, 1986c). At all stages of
fruit development, it was observed that the mesocarp
contained lower levels of the auxin, indole acetic acid
(IAA) than the seed and testa (Cutting et al., 1985). Auxin
increased the skin strength of the fruit and regulated
endosperm development. There is strong evidence that
ethylene is involved in the abscission of young avocado
fruitlets. Davenport and Manners (1982) observed that an
increase in ethylene did not occur in fruit that failed to
abscise. The prevention of the ethylene peak and associated
fruitlet abscission could, therefore, be under the control of
other plant growth substances such as cytokinins, which
showed a strong peak during this period. Blumenfeld
and  Gazit (1970) found high levels of cytokinins in both
the cotyledons and testa of avocado, which decreased
with development. The high levels of cytokinin activity in
the young seed served to increase the sink strength of
the  fruit for nutrients and other metabolites. The high
cytokinin levels detected in young fruitlets may actively
assist in increasing the sink strength of the fruit, and
therefore promote fruit growth (Bower & Cutting 1988).
Stress-induced increases in abscisic acid (ABA) caused an
irreversible loss in fruit growth, and the physiological
mechanism associated with ABA-induced retardation of
'Hass' avocado fruit growth appeared to be inextricably
linked to a decline in cytokinin content and included:
diminution of mesocarp and seed coat plasmodesmatal
branching, gating of mesocarp and seed coat plasmodes-
matal by deposition of electron dense material in the neck
region, abolishment of the electrochemical gradient
between mesocarp and seed coat parenchyma, and arrest of
cell-to-cell chemical communication (Moore-Gordon et al .
1998). Adato and Gazit (1976) were unable to elicit
initiation of fruit ripening, except with a very high dose of
IAA. IAA decreases to low levels in all parts of the fruit
with the onset of maturity (Wolstenholme et al . 1985).
A  direct role of auxins in the initiation and development
of the ripening response is not clear. Gazit and Blumenfeld
(1970a) showed low levels of cytokinin activity in the
mesocarp by the time the fruit was mature. Cutting et al .
(1986a) were unable to confirm the role of cytokinins in
avocado fruit ripening, although there were indications that
the interaction with ABA may occur, as suggested by
Letham and Palni (1983). Lieberman et al . (1977) found
that exogenous addition of gibberellins had little effect on
avocado ripening. They showed an increase in ethylene
and  ripening following application of ABA before the
FRUIT DEVELOPMENT
The pericarp, which is the fruit tissue proper excluding the
seed, comprises the rind (exocarp), the fleshy edible
portion (mesocarp) and a thin layer next to the seed coat
(endocarp) (Biale & Young 1971). The large seed of the
avocado consists of two fleshy cotyledons, plumule,
hypocotyl, radicle and two seed coats adhering to each
other. The endosperm disappears in the course of
development. The cotyledons consist of parenchyma tissue
interspersed with idioblasts and contain starch as the main
storage material (Biale & Young 1971). The avocado fruit
is unusual in that cell division in the mesocarp is not
restricted to the initial period of growth but also continues
during fruit development and even occurs in the mature
fruit attached to the tree (Van Den Dool & Wolstenholme
1983; Lewis 1978). In some cases, cell enlargement stops
when the fruit reaches about 50% of its size at full maturity,
while cell division accounts for the continued growth
(Cummings & Schroeder 1942).
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