From the Archives
of the Rare Fruit Council of Australia
by A. J. Joubert
Fruit Crack in
Lychees
Scientific
name: Litchi
chinensis
Family:
Sapindaceae
One
of the most common problems in litchi production is the large
percentage of fruit that crack during the ripening process. The causes
of fruit crack cannot be attributed to a single factor, and the
phenomenon can differ from orchard to orchard or from one region to the
next. Fruit crack occurs during the third stage of fruit development.
This is after cell division has ceased in the pericarp (skin) and cell
stretching takes place.
It has been established that cell
division in the skin ceases about 70 days after fertilisation. From
this stage until the end of the fruit growth period, i. e. after
another 35 days (Stage III), cell stretching takes place in the skin.
During this period, the edible portion (aril) that grows between the
seed and the skin, enlarges very rapidly, causing tremendous inner
tension against the skin which then has to stretch very quickly.
Skin
strength, i. e. resistance to cracking, is determined by the number of
cells formed during the first 10 weeks of fruit development. Since the
litchi skin has this weak characteristic, it is essential that optimum
conditions for cell division be created during fruit growth Stages I
and II. Any lack, for example, of water or nutrients at this stage,
will hamper cell division and too few cells will be formed. The tension
in the cells during Stage III will then be very high, resulting in skin
crack. It is notable that the skin cracks after long, wet spells when
much water is taken up. The skin cracks according to an easily
identifiable pattern across the length (tip) of the fruit.
A
second cause of fruit crack is skin damage during Stage III when only
cell stretch takes place. Damaged cells cannot recover during this
period and since the cells are dead at that time, they will also not be
able to stretch and the skin will tear as the tension (pressure) of the
enlarging aril increases inside the skin. This tension causes the skin
to crack at damaged or weak spots. The most common factors that play a
role in this regard are insect, hail and sunburn damage when relative
humidity is low.
Browning is a symptom of cell necrosis which
starts in the internal cell layers (mesocarp) of the skin once cell
division has stopped and the cell begin to stretch. In an advanced
stage, the browned cells die and dry out. It has been established that
browning is not caused by insect damage or fungal diseases. However, as
yet, the primary cause of browning has not been established, but the
following factors could be directly involved:
Early ageing and dying of the cells due to an imbalance in nutrients.
The tremendous cell tension in the pericarp that could cause cell
damage.
High temperatures together with low humidity during the fruit growth
period.
Secretion of volatile, harmful substances by the actively-developing
aril that could adversely affect the ageing pericarp cells.
To
summarise, browning is a condition of early cell deterioration and
necrosis. To prevent fruit crack, it is recommended that litchis be
grown in such a way that all the deleterious factors can be restricted
or eliminated.
(Reprinted from Farming in South Africa 1983). Extract from Sunshine
Coast Sub Tropical Fruits Association Newsletter May, 1986.
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