Which came first, the watermelon or the seed?

Every so often, someone asks me something like "Gee, if seedless watermelons don’t have seeds, where do they get the seeds from to plant them?" That is a good question. It also shows that they are aware of the basic cycle of crop agriculture… you plant a seed, which grows into a plant, which produces a flower, which produces a fruit, which produces more seeds, which you take and plant for the next year’s crop. But here you have a fruit with no seeds in it. Where do the seeds that produce that melon come from?

To understand the process, let’s review some basic genetics. Normally, plants have two sets of chromosomes. That’s abbreviated by plant breeders and geneticists as 2N, or diploid. There actually are quite a few exceptions to this rule in the plant kingdom, but for now, let’s keep the matter simple.

To produce a new fruit, a flower must be fertilized. The ovum and pollen (gametes) only have one set of chromosomes (1N or haploid). When these combine, you end up with an embryo plant inside a seed that will go on to produce fruit and seeds at the 2N level… 1N + 1N = 2N.

As you might have guessed, something quite different is going on genetically with seedless watermelons. The process starts with a chemical called colchicine. The plant breeder selects a breeding line with the traits he desires, and treats the growing apex of the plant with colchicine. This chemical is known for its ability to double the chromosome level of a plant. Thus, when that watermelon plant produces flowers, fruits and seeds, there will be four sets of chromosomes…. 4N or tetraploid.

Once a tetraploid watermelon line has been established, it will be fertile and can be used to cross with regular diploid watermelons. The gametes from a teteraploid plant will be 2N (half of 4N). When you make that diploid-tetraploid cross, you will combine the gametes from each parent…. 1N (from the diploid parent) and 2N (from the tetraploid parent). These two gametes combine to produce a triploid embryo plant inside the seed (1N + 2N = 3N). The triploid embryo from this cross is viable, and will grow a triploid plant. However, the fruit produced by that plant is sterile, and will not produce seeds. Thus, a seedless watermelon is produced.

But the story isn’t over yet. To get this seed to produce a seedless watermelon successfully, there are several cultural considerations that must be observed. I’ll cover those in my next column.