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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 - 25c for the first three days after sowing. However, good results have also been observed at 30c (day) and 20c (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 27c and 19c, respectively.

Night temperatures below 13c 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.

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 - 20c). Seed packaging opened too long before sowing, and frequent and rough handling will lower watermelon seed viability.

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