Scaling-up and Production:
The commercial seed grower obtains a license or seeds from the plant breeder and begins the process of generating sufficient seed for the farmer to grow the fruit for marketing. The true-breeding parent line is placed in an isolation field, typically one to five acres of only the parent line and no other watermelons plant varieties. In the isolation field the parent line open-pollinates with the assistance of bees and may yield hundreds of pounds of stock seed. Bees are economical and efficient at pollination because they will visit the same flower as many as eight or nine times. One disadvantage of allowing bees to do the pollinating is that bees may carry pollen from different varieties of watermelon plants if those plants are available. Ideally an isolation field is three miles from other sources of pollen or the watermelon fields are placed between fields containing other crops such as corn.
Stock seed of the tetraploid(female) and diploid(male) are planted in the seed production field. Hand pollination or other techniques are used to transfer pollen from the male diploid flower transferred to the female tetraploid flower. The triploid seed is present in the resulting fruit, whose cells are tetraploid, and that seed is sold to growers for planting in the next season. Flowers from the triploid plants require a diploid pollinator to stimulate the production of the seedless fruit. The triploid plants are sterile because of an imbalance in chromosome number. The ovules have a tetraploid or 4N number of chromosomes while the pollen has only a diploid or 2N number. Commercial triploid growers who market triploid fruit need to grow some pattern of diploid pollinator plants in their fields along with the triploid plants to induce the watermelon fruit to develop. Typically a triploid field will be 25-33% diploid pollinator plants.
Since triploid plants produce seedless fruit, the farmer needs to buy new seed each year from the seed suppliers. This provides an incentive to the plant breeder to develop new varieties with different characteristics and the seed grower to continue to generate triploid seeds.
An alternative to the commercial seed production might be to use tissue culture techniques to produce the triploid plants. While theoretically sound, in practice the process of getting sufficient numbers of triploid plantlets coming from greenhouses as uniform transplants has not been as effective or economical as traditional seed production.