Monday, July 28, 2014

Meatless Monday: Seedless Watermelon…What’s Up With That?

Last week I promised to tell you the story of seedless watermelon in today’s Meatless Monday post.

The story is not a simple one to explain and I remember now why I started to blog about this last year and never finished the post.

Here is the puzzler: Seedless watermelons are all grown from seeds, although they themselves do not produce seeds. How is this possible?

The answer as brief as I can make it:
  • A seed-bearing watermelon contains two complete sets of chromosomes, making it a diploid (diploid for two) fruit.
  • When seeds from a diploid watermelon are treated with cholchicine, a chemical derived from the autumn crocus plant (used by humans to treat gout),they yield watermelons with four complete sets of chromosomes instead of two, making them tetraploids (tetra for four).
  • When the flowers from these tetraploid watermelon plants are cross-fertilized with flowers from diploid plants at the correct time, they produce fruits with triploid (tri for three) chromosomes; their seeds will grow healthy but sterile plants — seedless watermelons.
This process generates several questions:

Can a seedless watermelon be organic?
While it is possible to grow a seedless watermelon organically, there is no way to avoid the use of the chemical cholchicine to obtain seed for a seedless watermelon.

Is the seedless watermelon a GMO crop?
There is a lot of chatter on the internet about seedless watermelon being a GMO crop. This is not the case. Yes, a chemical is used to initiate a chromosomal change in the original seed, but the fruit is produced by hybridization. No genes from a different organism were introduced to modify the plant’s DNA, the definition of a genetically modified organism. Furthermore, the seedless watermelon plant is sterile, the end of the line [much like the sterile mule, the offspring of a horse and a donkey].

Are seedless watermelons are more difficult to grow than seeded varieties? Yes, on many counts:
  • First there is the cost of the seed that takes two generations to be produced.
  • Second, the seedless seed is more fragile. It requires germination at a higher temperature. For this reason, the seeds are grown as transplants to be moved to the field once a good root system is established, thus requiring the cost of the containers and the extra work of replanting. Seeds from seeded varieties can be sown directly in the field.
  • Finally, watermelons are monoecious, meaning that each plant bears both male and female flowers. The female flower on the triploid watermelon will set fruit, IF it is pollinated. Since the male flower on the triploid plant is sterile, the only solution until recently was to plant rows of diploid (seeded) watermelons in the midst of the triploid (seedless) melons. Since seeded watermelons are decreasing in market value, there is ongoing research to develop “in-row pollenizers,” some of which bear marketable fruits. 
There you have it – the story of the seedless watermelon in a nutshell. For more on the topic, check out this Texas Extension Service site for potential growers. Watermelon is a huge cash crop in our southern states.

Happy Monday. Have a great week!

On Mondays I often blog on food, food issues, or gardening in support of Meatless Monday, one of several programs developed in the Healthy Monday project, founded in 2003 in association with Columbia University’s Mailman School of Public Health, Johns Hopkins Bloomberg School of Public Health and Syracuse University’s Newhouse School of Communications. Meatless Monday’s goal is “to help reduce meat consumption 15% in order to improve personal health and the health of our planet.”

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