Watermelon
Crop Information
Germplasm Resources
- 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.
Germplasm Repositories
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.
Important Varieties
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.
