Watermelon Crop Information
- by Todd C. Wehner
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
Centers of Origin
Watermelon is thought to have originated in southern Africa because it is found growing wild throughout the area, and reaches maximum diversity there. It has been cultivated in Africa for over 4,000 years. In 1857, David Livingstone reported watermelon growing profusely in the Kalahari Desert (Namibia and Botswana) after unusually heavy rainfall. The natives there knew of sweet as well as bitter forms growing throughout southern Africa. De Candolle, in 1882, considered the evidence sufficient to prove that watermelon was indigenous to tropical Africa, more specifically the southern parts of Africa.
Citrullus colocynthis is considered to be a wild ancestor of watermelon, and is now found native in north and west Africa. Fruit are small, with a maximum diameter of 3 inches. The flesh is bitter and the seeds are small and brown. Crosses of C. lanatus with C. colocynthis produced F1 hybrids with nearly regular meiosis. The pollen was 30-40% fertile, and 35% of the seeds were fertile.
Although Citrullus species grow wild in southern and central Africa, C. colocynthis also grows wild in India. India and China may be considered secondary centers of diversity for the genus. Cultivation of watermelon began in ancient Egypt and India, and is thought to have spread from those countries through the Mediterranean area, Near East, and Asia. The crop has been grown in the United States since 1629.
Germplasm is the foundation of breeding programs, so germplasm collection and evaluation are important aspects of breeding. Priorities for collection of Citrullus germplasm include India, especially the Indo-Gangetic plains and areas in the northwest parts of the country; Africa including the south and southwest (Kalahari Region); southern areas of the former USSR and Iran; and tropical Africa.
Recent work in germplasm collection and exchange has provided the USDA germplasm system with a total of 51 Citrullus accessions that were collected during a scientist exchange visit with the People’s Republic of China led by Wehner in 1993. Later, in 1996, a team of four researchers led by Wehner collected germplasm of Citrullus in the Republic of South Africa.
Centers of Diversity
The primary center of diversity for watermelon is southern Africa, with wild relatives also found in west Africa. The secondary center is China, and related species can be found in India. Areas of the middle east as well as countries near the Mediterranean Sea may also be good places to collect old land races and wild accessions of Citrullus.
T.W. Whitaker considered Citrullus colocynthoides to be the likely ancestor of watermelon. It is morphologically similar to C. lanatus, but with bitter fruit and small seeds. However, the bitter forms of C. lanatus were considered the probable ancestor of watermelon by others. That theory was supported based on the fact that they had the same number of chromosomes as C. lanatus, were freely intercrossable, and were found in the same areas of Africa and Asia. Citron was considered to be an intermediate stage between the primitive, bitter form of C. lanatus and the cultivated form of today.
Several germplasm collections, along with current varieties marketed by seed companies, represent the major sources of germplasm for watermelon breeders interested in the United States market. The USDA collection is stored at the Regional Plant Introduction Station, Griffin, Georgia with the backup collection at the National Seed Storage Laboratory (NSSL), Fort Collins, Colorado. There are 1644 accessions in the collection, with about 85% currently available to researchers, and the rest needing to be regenerated to increase seed quantity or germination percentage. The collection includes representatives of all Citrullus species and botanical varieties. In addition, approximately 300 heirloom varieties are kept at NSSL.
The Cucurbit Genetics Cooperative has curators who volunteer to collect and maintain seeds of gene mutants published for many of the cultivated cucurbit species. Some gene mutants are no longer available, but small amounts of seeds of some of the gene mutants can be obtained from the curator for that species, T. C. Wehner.
Additional collections are kept by seed savers and other groups interested in heirloom varieties, and by watermelon breeders around the United States. There are also watermelon germplasm collections in other countries that are being kept for national researchers in those countries.
Watermelon varieties have been described in the vegetable variety lists maintained by the American Society for Horticultural Science. Also, a complete set of descriptions for all vegetable crops from lists 1 through 25 has been collected into a book to be produced by ASHS Press. Seeds are available for many of the open pollinated and inbred varieties on the list, but there are a significant number of varieties that are no longer available. Watermelon breeders should obtain and evaluate a sample of the varieties available to become familiar with the diversity of germplasm. It is also useful to observe the improvement in horticultural traits that has been made in varieties developed over time.
A breeding program usually is started by intercrossing the best varieties currently available, or by crossing the best varieties with accessions having one or more useful traits missing from the elite varieties. Thus, in the beginning a watermelon breeder will need to obtain seeds of the best varieties, a set of varieties developed at different times in the past, a set of accessions from germplasm repositories, and lines with useful or interesting gene mutants.
A survey of popular varieties in the ten major watermelon-producing states in the United States by D.N. Maynard in 2000 indicated that popular varieties for commercial production were almost all hybrids, with few open-pollinated varieties being used commercially. Popular diploid (seeded) open-pollinated varieties (‘Allsweet’, ‘Black Diamond’, ‘Calsweet’, ‘Crimson Sweet’, ‘Jubilee II’, and ‘Legacy’) were grown mostly in one state each, suggesting regional adaptation or local demand. Hybrids generally were grown in several states, suggesting they have wider adaptation. The ‘Allsweet’ type, generally considered to be of high quality, was represented by more than half of the listed varieties (three of the open-pollinated and 11 of the hybrids). The most popular diploid (seeded) varieties were ‘Sangria’ and ‘Royal Sweet’ (seven states), ‘Fiesta’ (six states), and ‘Mardi Gras’ and ‘Regency’ (five states). For triploid (seedless) varieties, almost half of the varieties were ‘Tri-X-313’ type. The most popular triploid varieties were ‘Tri-X-313’ (ten states), ‘Summer Sweet 5244’ (nine states), ‘Millionaire’ (eight states), ‘Genesis’ (five states), and ‘Tri-X-Shadow’ (four states).
In order to develop improved varieties for an industry in a particular region of the world, the watermelon breeder will need to have seeds of varieties, breeding lines, populations, plant introduction accessions, and gene mutants that express the traits of interest at a high level. The breeder should identify a source that has the highest level of expression. That would be true whether the trait is quantitatively inherited (fruit yield, earliness, size, sweetness) or qualitatively inherited (dwarfness, anthracnose resistance, flesh color). If there is a choice of accession for a particular trait (for example, white flesh), it is better to use an adapted accession with the best genetic background. Thus, ‘Cream of Saskatchewan’ would be a better choice to use in the development of white flesh varieties for use in the United States, than a wild-type, white-fleshed citron having large vines, late maturity, hard flesh, bitter flavor, large green seeds, and seed dormancy.