Growing seedless watermelon is a sophisticated blend of genetics and specialized horticulture, distinct from traditional farming methods. Developed in the mid-20th century, the fruit appeals to consumers because it lacks the hard, black seeds of its conventional counterpart. A seedless watermelon is a triploid variety, meaning its genetic makeup prevents the formation of mature seeds. This sterility necessitates specific breeding methods to produce the initial seed and unique cultivation techniques to grow the fruit successfully.
The Genetic Foundation of Sterility
Seedless watermelon production relies on polyploidy, the state of having more than two complete sets of chromosomes. Standard, seeded watermelons are diploid, possessing two sets of chromosomes, which allows for normal reproduction and fertile seeds. The seedless variety is a triploid, carrying three sets of chromosomes (33 total), which causes its sterility. During meiosis, the three sets cannot divide evenly, resulting in non-functional gametes. The fruit still develops ovules, but they remain soft, white, and underdeveloped, making the melon appear seedless. This genetic manipulation is achieved through traditional breeding methods, resulting in a female sterile triploid hybrid.
Producing the Triploid Seed
The sterile triploid seed is manufactured by specialized producers through a controlled cross-breeding process. This process begins by chemically treating a standard diploid watermelon plant to double its chromosomes, creating a tetraploid plant (four sets). Breeders typically use a chemical like colchicine, which disrupts cell division. These tetraploid plants are maintained to establish a stable breeding line.
The final step involves crossing this tetraploid female plant with a standard diploid male plant. The tetraploid contributes two sets of chromosomes, and the diploid contributes one, resulting in an embryo with three sets. The seeds harvested from this cross are the sterile triploid seeds. Due to the difficulty in maintaining parental lines and executing the hybrid cross, these triploid seeds are significantly more costly than traditional seeds, sometimes costing $150 per thousand.
Field Cultivation and Pollination Requirements
Seed Starting
The triploid seed is typically grown into a transplant rather than direct-seeded into the field. This is necessary due to its thicker seed coat and weaker germination rate. The high cost of the seed also makes it impractical to risk poor germination in the field.
Pollination Strategy
When planted, the sterile triploid plants cannot produce fruit on their own because their pollen is non-viable. To overcome this sterility, farmers must interplant a standard, seeded diploid watermelon variety alongside the triploid plants. This pollinator provides the viable pollen needed to stimulate fruit development on the sterile triploid vines. A common planting configuration involves a ratio of one row of the seeded pollinator for every two to three rows of the seedless variety.
Role of Bees
Although the triploid flower receives viable pollen, the genetic makeup of the triploid plant ensures that the resulting fruit remains seedless. The presence of active honeybees is highly important. Bees must visit the sterile female flower multiple times, sometimes 16 to 24 visits, to ensure adequate fertilization and fruit set.