|
Germinating
watermelon
seed
Lefroy Valley Magazine
June 2003
Germination is often thought of as the
whole process from sowing until the full
emergence of the two cotyledon
leaves.
There are, in fact, two vastly different
phases during this period, namely (i)
germination and (ii) growth of the radicle
and hypocotyl until emergence. If
both of these phases are better understood
and dealt with separately, less
problems would occur during these initial
stages of plant production. In this article,
seed dormancy, germination, and
seedling propagation of watermelon seed
will briefly be addressed.
Seed Germination
Germination is a complex physiological
process with photo-dependency,
initiated by water absorption during a
period of optimum temperatures.
When these three criteria are met,
seed dormancy is lifted and the
radicle emerges, signalling the end of
the germination process.
Sowing Depth in the Field
The size of a seed is a good indication
of its optimum sowing
depth. When seed is sown too
deep, most of the reserves in the
endosperm will be used for
germination and radicle growth, leaving
very little for further development. A
good rule of thumb is to sow at a depth
of four times the seed width.
Seeded Watermelon(Diploid)
The seed absorbs water for its metabolic
processes, embryo expansion, and radicle
and hypocotyl growth. Because of its
size, watermelon seed requires more
water during the initial stages of germination
than smaller seed varieties do. If
germination takes place at relative
humidity of 90%, no additional water is
required during the first three days
after sowing.
Over-watering, especially during cooler
periods, will result in a lowered
respiration rate and less energy
production. Together with seedlings
stretching in low light and unhygienic
sowing conditions, over-watering
also increases the susceptibility of seed
and seedlings to water-borne pathogens.
Using the water disinfectant Sporekill
and incorporating the bio-inoculant
Trichoderma into the seedling growth medium limits the damage caused by these
pathogens. The optimal temperature range for successful and uniform
germination is a constant 23 - 25°c for the first three days after
sowing. However, good results have also been observed at 30°c (day)
and 20°c (night).
Seedless Watermelon
Water uptake through the seed coat is
slower in seedless watermelons,
compared to normal watermelon, which
means that the seed can easily be
suffocated. When there is excess soil
moisture, different procedures should be
observed.
Procedure for germinating seedless melons in
transplant trays:
Prior to sowing, pre-wet a light
seedling mix with approx 500ml of
water for every 1000gm of mix and
pre-heat in a germination room
overnight at 30oc.
Sow seed into indentations using warm
seedling mix at 1 cm depth and
cover lightly with same warm mix or
Vermiculite. DO NOT WATER.
Seed coat adherence to cotyledons can
be virtually eliminated by placing the
seed in the transplant trays pointy
end up at 45-90o angle (may not be a
practical option for Nursery
production).
Place in germination room at 30oc at
high humidity and remove when most
radicles have emerged.
Move trays to greenhouse and hold
at 27oc until emergence is complete.
The high temperature assists uniform
emergence.
Water only as necessary irrigate
sparingly for the first week.
DO NOT OVERWATER. Plants
should be hardened off by reducing
irrigation and transplanted at third
true leaf stage.
|
Seedling Propagation
The greenhouse structure should be
sterilized and all sources of possible
contamination should be removed before
the arrival of the seedling trays. A
preventative program against insect
virus vectors should also be followed.
The optimum air and root-zone
temperatures in the structure are 27°c
and 19°c, respectively.
Night temperatures below 13°c result in
slow growth and cold stress,
increasing susceptibility to
disease.
A relative humidity of 70% is
preferable, but good ventilation
throughout the day is even more
important. Ventilation is necessary
for optimal CO2 levels within the
structure and for removing any
free-water from the leaf surfaces.
High light intensity within the
structure ensures sturdy compact
seedlings.
Transplanting
Watermelons are frost-sensitive and should only be transplanted in
warm, well watered soil. Transplant shock can be minimized by
transplanting in the afternoon or by using the anti-transpirant
Vapor Guard.
Apply Kic Start during cool transplant
periods. Seedling growth is boosted, as it
promotes early root growth, which is the
first priority after transplanting.
Seed storage
Watermelon seed will last 3 - 4 years
when kept dry (10 - 15% seed moisture
content), and at cool, constant
temperatures (14 - 20°c). Seed packaging
opened too long before sowing, and
frequent and rough handling will lower
watermelon seed viability.
|
