How Are Seedless Watermelons Grown?

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Seedless watermelons are grown by crossing watermelons with 2 sets of chromosomes and 4 sets of chromosomes. This produces a watermelon with 3 sets of chromosomes, which is, ultimately, sterile and therefore, seedless.

Watermelons are known for two things: the sweet juice of their crushed-ice-textured flesh and seed spitting. However, since the 1990s, people seem to play the sport less and less; in fact, some watermelons don’t have any seeds now.

Seedless watermelon half
Seedless watermelon. (Photo Credit : Pixano)

A seedless fruit is an oxymoron. From experience, and intuition, we know fruits should have seeds. We learn it in school too, “Flowers become fruit which have seeds. Seeds germinate to make a new plant.” And thus plant life survives. A fruit without a seed would mean that the plant that bore it is sterile, it is the final heir of that plant.

But, plant geneticists (or scientists that study and sometimes influence the genetics of a plant) discovered ways to create seedless fruits.

At the heart of our inquiry lies the question: how do we grow a plant when its fruits contain no seeds? The inventors therefore had two problems to solve: growing a plant that bears seedless fruits and somehow ensuring that it continues to procreate, despite its sterility. So, how did they resolve these issues?

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Cross Breeding

To nature, sterility is an atrocity, a heinous crime. Nature encourages the survival of its species (precisely, its genes) at all costs. Such a crime can only be committed by tinkering with a plant’s cellular structure either, naturally or artificially, through deliberate cross-breeding, for example.

A watermelon embedded with seeds possesses a pair of 11 chromosomes, thread-like entities that carry the genes. 22 chromosomes in total. Such a species is called a diploid.

However, the chromosomes can be doubled by treating the plant with the chemical colchicine. The new plant will have 4 sets of 11 chromosomes, 44 chromosomes. It is a tetraploid plant.

Now, when the tetraploid is pollinated by a diploid, or when the diploid’s male reproductive cell fuses with the tetraploid’s female reproductive cell, a seed possessing 33 chromosomes is formed (3 sets of 11 chromosomes), a triploid.

This seed grows into a watermelon plant that bears seedless watermelons.

Mule horse
The seedless watermelon plant is often called the “mule” of the plant kingdom, for a mule is the sterile offspring of a male donkey and a female horse. (Photo Credit: Pixabay)

It is imperative to know that by “seedless”, we mean that the watermelon isn’t replete with the seeds that we normally refer to as seeds — the brown, minuscule pebbles — but rather, it possesses the white, translucent and softer ones that are less hated. These are immature seeds whose coatings haven’t completely developed.

The first problem is now solved; we discovered how to create a seedless variety of watermelons, but how do we ensure that it procreates?

In a vegetation of seedless watermelon plants, only two-thirds are triploids, while the rest are diploids. A watermelon plant sprouts both male and female flowers, but its sterility renders its male flower impotent. To ensure the continuation of its seedless species, the female flower must be pollinated.

The pollens are provided by the diploids planted around it. This is primarily achieved by agents, such as a honey bee, which acts as a wing-man and carries pollen from the neighboring diploids and drops them onto the triploid’s females. The flower matures and transmutes into a delightful fruit, except that it is seedless.

Seedless watermelon
(Photo Credit: Pxhere)

Seedless fruits aren’t necessarily grown by cross-breeding. Vines that bear seedless grapes are grown by cloning. They are descendants of the very first vine that bore seedless grapes. For more details, click here.

Also Read: How Are Seedless Fruits And Vegetables Grown?

The Costs

Of course, the convenience of eating a watermelon bereft of seeds does come at a cost. Its beauty seems to be starkly overshadowed by its blemishes. Seedless watermelons are warm-season crops; thus, for propitious germination, they must grow in day temperatures of 80 to 95ᵒF and night temperatures of 60 to 70ᵒF. The growth will not be optimum if these temperatures aren’t maintained; to ensure that, they are grown in greenhouses with facilities that come at very high costs.

Triploid seeds are produced by maintaining the two parental lines: the tetraploid and the diploid. Small scale seed production can be done by hand pollination. Large scale seed production is done by growing the two parents in cages or isolation blocks and introducing bees when they are flowering. All the male flowers from the female parent are removed to make sure that the female parent does not get pollinated by its own pollen. A complicated and labor intensive process! That is why seedless watermelon is so expensive. Every year growers buy this seed and plant their seedless watermelon crop.

Watermelon farm
Seedless watermelon farm. (Photo Credit: Pixabay)

Despite its limitations, seedless watermelons are grown widely. Vendors earn a fortune since the market is huge. In the few years following its invention, the fruit resembled its seeded cousin, but didn’t taste as sweet as one would expect. However, improved breeding techniques have instilled the uniquely sweet taste as well. Could it get any more convenient?

Also Read: How Are Seedless Grapes Grown?

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References (click to expand)
  1. Seedless watermelon – how do they do that? - MSU Extension. Michigan State University
  2. Where Do Seedless Watermelons Come From?. Texas A&M University
  3. Where does seedless watermelon come from?.
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About the Author

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.