One of the major limiting steps in breeding seedless watermelons is the small number of tetraploid inbreds available. Stage 1 involves choice of diploid lines to use in tetraploid production. The grower will want to discard the gray fruit so they are not marketed as seedless watermelons by mistake. Stage 2 is the production of tetraploid plants. Many methods have been used effectively in other crops to produce polyploids, including tissue culture regeneration, temperature shock, and X-rays.
Colchicine inhibits spindle formation, and prevents separation of chromosomes at anaphase.
Of all the methods of colchicine application, shoot apex treatment at the seedling stage was found most effective. When the cotyledons first emerge from the soil, the growing point is treated with colchicine to stop chromosome division and result in a shoot with four sets of chromosomes rather than two. The treatment produces plants that are diploid, tetraploid, or aneuploid, so it is necessary to identify and select the tetraploids in later stages. Some diploid cultivars and breeding lines produce a higher percentage of tetraploids than others. Tetraploids can be detected by the direct method of counting chromosomes of cells under the microscope, or by comparing stem, leaf, flower, and pollen size with diploid controls.