WHERE DO SEEDLESS WATERMELONS COME FROM?

 WHERE DO SEEDLESS WATERMELONS COME FROM?

 

How do seedless watermelons get that way?

My daughter loves watermelon and I was thinking she was getting big enough to have a seed spitting contest. But watermelon seeds seem to be vanishing. I'm used to seedless grapes but only recently became aware of this watermelon situation. Seedless watermelons have been growing in popularity since 1990. From the standpoint of a plant, the whole point of fruit is produce seeds, so I wondered what kind of hanky panky was going on to produce seedless watermelons. Turns out that they are like mules, self-sterile hybrids and involve a lot of work.
Diploid
Watermelon plants are usually diploid, like us, meaning they have two sets of chromosomes, the packages of DNA with instructions for life.
Triploid
Seedless watermelons are triploid. They have three sets of chromosomes. This odd number results in them being sterile and not producing seeds. The way they become triploid is by mating a diploid male with a tetraploid female. Tetraploids have four sets of chromosomes.
Tetraploid
The way you get tetraploids is by applying a chemical called colchicine which messes with cells as they are dividing. You add it to diploid seedlings and then some cells become tetraploid. You have to cultivate these over several generations to get enough that produce enough viable seeds with suitable traits.
Mixing It Up
Watermelon plants have male flowers and female flowers. The female flowers have a little pea-sized melon behind it. You remove the male flowers on the tetraploid plants because the female tetraploid flowers produce triploid fruit. It doesn't work with a male tetraploid and female diploid. Pollination can be done by hand or using bees.
Go Time
Seeds from the triploid fruit grow into triploid plants. They don't produce much pollen, so you plant some diploid plants. This pollen stimulates the triploid female flowers to produce fruit. Because the number of chromosomes is not compatible, they don't have seeds.
The seedless watermelons are smaller and rounder. They are supposed to be sweeter and last longer. But do you think it's really worth all the extra work?

GROWING SEEDLESS WATERMELONS

 Seed Production. The obvious question asked about growing seedless watermelons is: "How does one obtain seed of a seedless watermelon?" Obviously, you cannot save seed from a seedless watermelon. So, where do the seeds come from? Simply stated, the number of chromosomes (the threadlike bodies within cells that contain the inheritance units called genes) in a normal watermelon plant is doubled by the use of the chemical colchicine. Doubling a normal (diploid) watermelon results in a tetraploid plant (one having four sets of chromosomes). When the tetraploid plant is bred back, or pollinated, by a diploid or normal plant, the resulting seed produces a triploid plant that is basically a "mule" of the plant kingdom, and it produces seedless watermelons. Seed of seedless varieties are available from most major seed companies.

General Climatic Requirements. Seedless watermelons are a warm-season crop, preferring relatively high temperatures for optimum growth. Daytime temperatures of 80 to 95 degrees F. and night temperatures of 60 to 70 degrees F. are best. When temperatures are lower, plant growth is slowed considerably. With favorable weather, seeded fields can produce ripe fruit in 85 to 100 days.
Planting. Poor seed germination is the main problem with growing seedless watermelons. When direct seeding, the soil temperature should be minimum of 70 degrees F. at a depth of 4 inches. Soil temperatures below 70 degree F. will reduce germination and emergence. When growing transplants, use 12- to 2-inch seedling cells or peat pellets. Soak the planting medium thoroughly, and let drain 4 to 6 hours before sowing. Plant 1 or 2 seeds per cell or pellet. The greenhouse temperature should be 75 to 85 degrees F. during the germination period. Do not allow the growing medium to become dry, but do not over water during initial germination. Begin watering, as needed, after 10 to 15 percent of the seedlings have emerged. Plants should be ready for transplanting in 3 to 4 weeks. Transplants should have not more than 3 true leaves when set in the field. Use of older, larger transplants can cause slow, stunted growth and poor yields. In-row and between-row spacing is generally 48 X 80 inches.
Pollination. The male and female flowers are born separately on the watermelon plant. Female flowers must be pollinated for fruit to set. Also, cross pollination must occur between a seedless and a regular type watermelon for seedless fruit to be produced. This is best accomplished by planting a standard watermelon variety in the garden. Approximately one-third of the plants in the garden should be of the standard or 'pollinator' variety. Honey bees are the principal insects that pollinate watermelons. Pollination is a must, and poor or partial pollination may result in misshapen fruit and no seedless melons.
Harvesting. The lower side or 'ground spot' of the fruit should be cream-colored or yellowish. Thump fruit to check for ripeness. The result will vary. Generally, a solid sound indicates ripeness, while a sharp echoing sound indicates a greener fruit. The tendril or 'tail' which occurs in the axils of leaves (where the leaf attaches to the vine) along the stem can be used as an indicator of ripeness. Experienced harvesters say that if the 2 tendrils nearest the fruit are dry, the seedless watermelon is ripe. It is important to note that the first few mature melons in the garden may frequently contain small seeds. This condition is most prevalent under stressed conditions, such as low soil moisture, insufficient fertilizer, temperature extremes, or disease pressure, which affect normal plant development.

Each planting of seedless watermelons actually produces 3 different types of watermelons -- the regular seeded watermelons (from pollinator plants), the true seedless melons, and a light-green tetraploid melon that produces a very limited number of seeds, from which next year's planting can be made.