Agriculture Reference
In-Depth Information
used. The GDD can be defined as the sum of the mean daily
temperatures above a lower base - and below a maximum
threshold value that will allow the plant to complete its total
life cycle, or to reach a phenological stage (Souza 1990).
This calculation is used to predict and plan harvesting times
(Montenegro 1980, Ometto 1981, Warrington & Kanemasu
1983, Roberto
et al
. 2005). Citrus fruit gradually becomes
edible on the tree and remains so beyond harvesting.
Maturity therefore reflects the accumulation of sugars and
the loss of acidity and a range of other biochemical changes.
Temperatures also affect maturation of fruit. The lower the
fruit temperature, the earlier the chlorophyll degradation
and carotenoid accumulation will occur (Ladaniya 2008).
Flesh maturity also differs between fruit harvested with the
same rind colour. Changes in polygalacturonase, cellulase
and pectinmethyl esterase activity have been observed dur-
ing fruit maturation (Ladaniya 2008).
several varieties of citrus fruit at different temperatures,
which showed the correlation of increased respiration rates
with higher temperatures. The respiratory rates of citrus
fruit are further influenced by humidity. Murata and
Yamawaki (1989) showed that the respiratory rates of
navel orange, Iyo tangor, Seminol tagelo, Natsudaidai,
Hassaku and Anbokan are lower in fruit conditioned at low
humidity (64% RH, 20°C) than fruit kept at high humidity
(92%, 20°C), during and after conditioning. Citrus does
not show any respiratory rise accompanied by major
changes in flavour and biochemical composition after
harvest in relation to ripening (Ladaniya 2008).
PHYSIOLOGICAL DISORDERS
Creasing
Rind mineral levels, rootstock type and irrigation manage-
ment are all factors contributing to creasing or albedo
breakdown. Fewer symptoms were observed in navels sub-
jected to reduced water volumes, whether delivered as part
of a partial root zone drying strategy or otherwise (Treeby
et al
. 2000). This work also highlighted the impact that
reduced water volumes have on final fruit size. A particularly
interesting outcome of this work was the recognition that
low water volumes affected trees less on some rootstocks.
Maturity indices
Sweet oranges, mandarins, grapefruits and pomelos are
considered mature when juice content and the total soluble
solids:acidity ratio have attained certain minimum limits
for palatability. Total soluble solids constitute about 80%
sugars, 10% acids and 10% nitrogenous compounds. In cit-
rus fruit that are used for table purposes (such as fresh
fruit) and that are processed into juices, maturity is deter-
mined mainly on the basis of the ratio of total soluble
solids (TSS) to titratable acidity (TA) (Ladaniya 2008).
Citrus fruit are marketable when a minimum TSS:TA ratio
is reached. This minimum ratio varies with location and
local standards, but generally ranges from 7-9:1 for
oranges and mandarins; to 5-7:1 for grapefruit. The
TSS:TA ratio is not of importance to lemon and lime pro-
ducers, since the fruit are harvested based on minimum
size and juice content and for processing, on acid and peel
oil content (Davies & Albrigo 1994).
Rind staining
Rind staining is a condition that can occur while the fruit
are still on the tree, and symptoms may be expressed during
storage (Agusti
et al
. 2001). The Spanish have serious rind
staining problems with 'Navelate' fruit during maturation
on the tree, but 'Navelina', 'Washington navel' and 'Lane
late' fruit can also develop this disorder during post-harvest
storage. These symptoms are seen on fruit harvested from
inland Australia, where climatic conditions similar to the
Mediterranean-type conditions in Spain prevail. Affected
fruit are characterised by collapsed areas in the rind, which
become reddish-brown over time. The symptoms appear
on fruit at nonchilling temperatures, and fruit blemish
increases with maturity. Water stress is thought to play an
important role in its development. Delivery of bins to pack-
ing sheds should be prompt. Storing fruit at low tempera-
tures (2°C) is considered useful to reduce the symptoms.
De-greening with ethylene may also delay its expression.
Respiration and transpiration
During maturation and senescence, citrus fruit exhibit rela-
tively low respiration rates and levels of ethylene produc-
tion (Kader 1992). The respiratory rate of the rind is nearly
ten times as high as that of vesicles (Murata 1972); the
rind, therefore, has an important physiological role in
the qualitative changes that take place during storage of the
fruit. Respiratory rates of citrus fruit are affected by several
factors, including temperature, humidity, air movement,
atmospheric compositions (O
2
, CO
2
, C
2
H
4
and other ole-
fins), dropping, bruising and microbial infection. These
factors all affect the fruit quality. Haller
et al
. (1945)
measured the respiratory rates and heat of respiration of
Oleocellosis
Oleocellosis (oil spotting) is cell collapse and blemish
caused by the release of rind oils after impact or abrasion.
Rupture of the oil glands result in necrosis of the adjacent
epidermis, inducing the formation of irregularly shaped
