Agriculture Reference
In-Depth Information
pome fruits, even though other regulatory events precede and follow this rise
and there may be other aspects of regulation unrelated to ethylene (Knee,
). High ethylene concentrations can promote, but may not be required
for, many ripening processes. They can ensure that all ripening processes occur
synchronously under normal conditions. The low concentrations of ethylene
characteristic of pre-climacteric or non-climacteric apples and pears can have
major effects on ripening processes. A continuous treatment with ethylene at
approximately
ll
−
can elicit almost
.
µ
% of the maximum effect on
ethylene synthesis of 'Anjou' pear while
% response is attained at around
ll
−
. Softening of this pear is promoted by ethylene concentrations as low
µ
ll
−
(Knee,
as
.
µ
). Theoretically there is no lower limit at which
ethylene becomes inactive.
Knee (
) concluded that ethylene acts at a high affinity site to initiate
softening and chlorophyll degradation and that ethylene synthesis in mature
climacteric fruits is triggered by a high affinity response. It may act at a low
affinity site to stimulate respiration.
The higher the level of internal ethylene the lower the sensitivity to external
ethylene, but as the fruit advances towards autonomous ripening there is an
earlier response to any particular ethylene concentration.
The effects of ethylene on fruit ripening and fruit quality parameters
have been inferred from the pattern of natural changes in these but more
specifically demonstrated by the artificial supply of ethylene or ethylene-
producing chemicals and by the use of chemicals that inhibit ethylene syn-
thesis. The interpretation of any quantitative relationship between applied
ethylene and responses in mature climacteric fruit may be complicated
by the fact that endogenous ethylene production is stimulated by ethylene
supply.
Smith
et al.
(
) found that ethephon (
-chloroethyl phosphonic acid)
applied as an ethylene source
weeks before harvest increased internal
ethylene levels, fruit yellowing and starch loss and reduced fruit firmness and
titratable acidity. These effects were accounted for as consequent on advancing
the onset of ripening: there are no effects on the subsequent rates of change
but initial differences are maintained.
AVG (aminoethoxyvinylglycine) regulates ethylene biosynthesis by inhibit-
ingACCsynthase.Applicationintheorchardfourweeksbeforetheanticipated
harvest date delays fruit maturity on the tree as shown by its effect in delay-
ing the increases in internal ethylene concentration and soluble solids and the
changesinstarch,backgroundcolourandredcolour( Johnson,
-
).Thefruit
can consequently be picked later. Throughout several months of storage under
controlled atmosphere conditions, fruits from AVG-treated 'Cox' apple trees
produced only about a tenth as much ethylene as fruits from control trees
and were consistently firmer. Similar results of pre-harvest AVG sprays have
