Agriculture Reference
In-Depth Information
September. The firmness at maturity, i.e. at the optimum harvest date for
immediate consumption or that for long term storage, varies from cultivar to
cultivar, e.g. from
newtons for 'McIntosh' to
N for 'Delicious' in the
same season (Lau,
), and also varies from season to season so is not a
reliable sole indicator of the best date for harvesting. The decline in firmness
continues after harvest in store, slowing this decline being a key objective
of storage technology. Other things being equal, the firmness of fruits after
storage is a linear function of their firmness at harvest ( Johnson and Ridout,
).
Respiration rate
The rate of respiration per unit fresh weight is high early in the season, during
the cell division phase of fruit growth and then declines to a very low level
(Bepete and Lakso,
). Then, when the fruits reach physiological maturity
and ripening processes are initiated, apples and European pears typically show
a marked increase in respiratory activity resulting in increased evolution of
CO
(Kidd and West,
). This increase, referred to as the
respiration climacteric, precedes visible symptoms of ripening but once it has
occurredtheripeningprocessisirreversible.Subsequenttothisclimactericrise,
which can occur in apples left on the tree and in harvested fruits, respiration
rates decline once more (Figure
; Rhodes,
a
).
Japanese pears can show a respiration climacteric but some cultivars do not
do so (Downs
et al.
,
.
).
Ethylene evolution
Soon after pollination ethylene production is high and gradually declines dur-
ing cell division and expansion. In apples and pears showing a respiration
climacteric ethylene production increases rapidly just prior to obvious signs
of ripening. In 'climacteric' apples and pears the change in ethylene produc-
tion may coincide with that in respiration but it may be significantly later
(Reid,
). The time of the peak rate of ethylene production in pears co-
incides with that of respiration but in apples it comes later (Rhodes,
).
The change in ethylene production is much greater than that in respiration.
Preclimacteric apples produce
lkg
−
h
−
of ethylene, climacteric apples
.
µ
lkg
−
h
−
(Knee,
µ
).TherateofCO
evolution increases by only
about
-
% (Rhodes,
). Typical curves for apple ethylene evolution
are shown in Figure
b
. The rapid rise in ethylene production involves an
autocatalytic effect. Ethylene is always present in the fruit tissues at a very
low concentration but young fruitlets are not capable of responding to this
endogenous, or to exogenous, ethylene by ripening. Once a critical point of
.
