Commercial production of seedless watermelon began in the s. Early seedless varieties did not have the sugar and flavor levels of seeded types, but plant breeders have improved these traits and new varieties no longer have these problems. However, one problem that does continue is seed germination. Initially, seed germination of seedless watermelon was quite low. Each time the seeds are watered it lowers their temperature. On a small scale, warm temperatures can be maintained by watering transplant flats, covering them and letting them heat up in the sun in the greenhouse for a day or more.
Then, plant the seed and cover them again until seedlings emerge. Either process will take four to five days. After emergence, seedlings are then finished off in the greenhouse for three weeks and then transplanted to the field late May or early June. Following these steps generally produces a more than 90 percent germination rate. High germination rate is important since seed of seedless types is quite expensive compared to seeded varieties. The standard number of chromosomes in watermelon is This is called the diploid number di meaning two, as in dissect — cut in two.
With this even number, cell division is highly regular and produces pollen and egg cells with 11 chromosomes that recombine to produce seed with the usual 22 chromosomes. Through a chemical process, the chromosome number can be doubled from 22 to 44 tetraploid, tetra meaning four. Cell division in plants with 44 chromosomes is, again, highly regular and will produce pollen and egg cells with 22 chromosomes that recombine to produce seed having 44 chromosomes.
However, if pollen from a plant with 22 chromosomes is placed on a female flower of a plant with 44 chromosomes, the resulting seed will have 33 chromosomes triploid — three sets of the base number of 11 chromosomes. This odd number does not produce or rarely produces viable pollen and eggs in the resulting seedlings. 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 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. Watermelon plants are usually diploid, like us, meaning they have two sets of chromosomes, the packages of DNA with instructions for life. 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. 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. 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.
0コメント