After reading this article you will learn about:- 1. Origin of Watermelon 2. Botany of Watermelon 3. Production 4. Gene List 5. Breeding Goals 6. Breeding Methods 7. Genetic Resources 8. Use of Genetic Male Sterility that are Identified in Watermelon 9. Diseases Resistance 10. Integration of New Biotechnologies into Breeding in Watermelon 11. Seed Production 12. Isolation Distance 13. Varieties.
- Origin of Watermelon
- Botany of Watermelon
- Production of Watermelon
- Gene List of Watermelon
- Breeding Goals of Watermelon
- Breeding Methods of Watermelon
- Genetic Resources of Watermelon
- Use of Genetic Male Sterility that are Identified in Watermelon
- Diseases Resistance of Watermelon
- Integration of New Biotechnologies into Breeding in Watermelon
- Seed Production of watermelon
- Isolation Distance of Watermelon
- Varieties of Watermelon
1. Origin of Watermelon:
The primary centre of diversity lies in Southern Africa where wild forms are still found. Domestication is likely to have occurred in Egypt and India. Primary centre of diversity for watermelon is Southern Africa. The secondary centre is China and related species can be found in India.
2. Botany of Watermelon:
Plants are vinyl type, monoecious, with angular stem. Leaves are pinnately divided into 3 or 4 pairs of lobes (Fig. 28.1). Watermelon is the only economically important cucurbit with pinnately divided lobed leaves. All other species have whole (non-lobed) leaves. The leaves are pinnately divided into 3-4 pairs of lobes, except non-lobed (sinuate) gene mutant controlled by the nl gene.
In a typical vine, lateral branches arise quite late and usually there is dominance of single runner stem. However, in some of the new dwarf types (dw1 and dw2), multiple branching occurs simultaneously from the crown of the plant providing more potential bearing areas. Corolla is rotate, five lobed, sulphur-yellow or chrome-yellow.
The plant is classified as naturally cross-pollinated crop, but there is considerable amount of selfing and sibbing also. Fruit size varies from 1.5 kg to 50 kg but 10-12 kg fruit size is most common. Fruit shape varies from long cylindrical to spherical with various intermediate shapes. Rind colour varies from white to shades of green with conspicuous stripes and mottled rind. Rind is hard but not durable.
Fruit flesh colour may be white, yellow, orange, pink, red and texture may vary from firm to fibrous. The greater part of fruit flesh is mostly derived from placentae, (endocarp) in contrast to melon where edible part of fruit is mesocarp. Flowers are less showy, mostly located as singly as separate pistillate and staminate flowers on the same plant (monoecious cultivars).
The pistillate or hermaphrodite flowers normally occur in every seventh leaf axil while the intervening axils are occupied by the staminate flowers. Three stamens are at the base of the corolla. Typical ratio for staminate to pistillate flomers is 7: 1. Watermelon flowering and fruit development are promoted by high light intensity and high temperature.
In andromonoecious cultivars also, harmaphrodite flowers do not ensure self-pollination. They too need insect visit to effect pollination. Flowers open shortly after sun rise and remain open only one day. Usually, anthers have dehisced before the flower anthesis. Pollen is visually evident in sticky masses adhering to anther.
The stigma is receptive throughout the day. Female bud to be self-pollinated/cross-pollinated is protected by putting small screen cage over it or by wrapping it in unopened condition by cotton-rubber band. The staminate flowers are prepared by bending back the petals until they break followed by brushing the mass of sticky pollen against the stigmatic surface of the protected pistillate flower.
For increasing fruit setting under controlled pollination, open-pollinated fruit should be removed as there is inhibitory effect produced by fruits already set. Unopened buds are covered in afternoon followed by pollinations next morning.
3. Production of Watermelon:
Watermelon (Citrullus lanatus (Thunb.) Matsam. & Nakai, 2n-2x = 22) is most commonly grown in the Middle East, the United States of America (Florida, Georgia, California, Texas), Africa, India, Japan and Europe. Plant growth is favoured by high temperature and adequate sunlight.
Watermelon fruits are produced in different sizes (ice-box, small, medium, large, giant), different shapes (round, oval, blocky, elongate), different rind patterns (gray, narrow stripes, medium stripes, wide stripes, light solid, dark solid) different flesh colours (white, yellow, orange, red) and different types (seeded or seedless).
Commercially most popular seeded cultivars are red flesh, blocky shape, 8-11 kg (e.g. Allsweet). For seedless watermelon, the popular cultivars are red flesh, oval shape, 5-8 kg. (e.g. Tri-X-313). Watermelon hybrids from private seed companies have become very popular in India and 70 tons of hybrid seed is traded annually.
4. Gene List of Watermelon:
As per compilation by Robinson (1976) and some modifications, the qualitative genes of watermelon are listed in Table 28.1.
5. Breeding Goals of Watermelon:
2. Pistillate flowers at lower node number
3. Tough skinned fruits for long distance transportation
4. Dark red flesh
5. Firm and non-fibrous flesh texture
6. Black seed
7. Proper sugar to acid ratio
8. TSS content not less than 10%
9. Fruits with smaller and fewer seeds with attractive deep red flesh
10. Firm flesh
11. Intermediate fruit shape between typical long and round ones as most elongated cultivars have a tendency to produce so called gourd neck fruit, whereas round-fruited cultivars tend to be susceptible to ‘hollow heart’. The intermediate fruit shape is the advantage of F1 hybrids between long and round-fruited parental lines.
12. High yield
13. Resistance to diseases, viz., Virus
14. Fusarium wilt (race 0, 1,2) Anthracnose, gummy stem blight Powdery mildew
15. Resistance to insects (cucumber aphid, fruitfly, cucumber beetle, red pumpkin beetle)
6. Breeding Methods of Watermelon:
Major objectives for watermelon breeding include proper fruit type, early maturity, high fruit yield, high sugar content, tough flexible rind, and proper seed type. For example, seed type changes significantly for different market classes.
Parental lines for seedless hybrids should have small seeds, whereas confectionery seed types should have large seeds. For commercial cultivars, black seeds are preferred because of their contrast with red, yellow, or orange flesh.
After determining the breeding objectives, methods for measurement of the traits of interest should be developed, selection methods should be determined (specifying the operations to be carried out for each generation), and parents with high expression of the traits of interest should be chosen.
Vine type should be long for commercial production and dwarf (bush) for home garden. It may also be possible to use the dwarf plant type for once-over harvest in commercial production. Sex expression should be monoecious, with a ratio of 7 staminate: 1 pistillate flowers, or better (preferably 4:1). Andromonoecious sex expression and ratios of 15:1 are more typical of older cultivars.
For production in most areas, watermelon must have resistance to Fusarium wilt. Races 0 and 1 are common, and race 2 is becoming important, especially in Texas and Oklahoma where plastic mulch culture and fumigation are less common.
Production areas in the southern United States usually have anthracnose race 1 and may also have problems with race 2. Gummy stem blight is a disease for which resistance is needed. Powdery mildew is becoming a problem.
Main breeding methods are as follows:
1. Pedigree Method:
This is the most common method of watermelon breeding to develop cultivars or the parental lines for use in hybrid breeding programme. The parental lines are chosen in such a manner that both are well adapted to the location and complement each other for some of the traits.
The selected parents are crossed and F1 is produced. F1 is self-pollinated or even sib-pollinated to produce F2, the first segregating generation where selection for highly heritable traits like earliness, fruit shape and fruit quality can be applied. Selected F2 plants are advanced to F3 generation where plant to progeny i.e. F3 families are grown.
Here selection focuses on superior plants in superior families. In F4 generation, selection should emphasize on family row performance for quantitative traits. Plants within better rows, should be selected on the basis of qualitative traits.
By F6, the lines become uniform and homogeneous and should be treated as new inbred to develop new hybrids or should be further evaluated in trials as new OP cultivars. The population size in segregating generation could stick to 54 F2 plants to 36 F3 families to 24 F4 families to 18 F5 families.
Single-seed-descent is a modification of pedigree breeding in which inbred lines are developed rapidly by self-pollination in greenhouses and winter nurseries, and selection is not practiced until later generations, such as F3 to F6.
This method requires less record keeping and works better where the main objective is to improve quantitative traits such as yield and earliness, rather than qualitative traits such as flesh colour and disease resistance.
However, traditional pedigree breeding is probably the more useful method for watermelon since there are many qualitative traits that can be selected in early generations. In that way, plants or families having unsuitable traits that are simply inherited (such as poor fruit flesh colour) can be eliminated in early generations.
2. Backcross Breeding:
Backcross breeding is used to transfer a highly-heritable trait into an otherwise superior cultivar. The superior parent is referred to as the recurrent parent. Often, six generations of selection and backcrossing to the recurrent parent are required to recover the genotype of the recurrent parent (except for the addition of the new trait) without the other undesirable traits from the non-recurrent (donor) parent.
For the transfer of a trait controlled by a recessive gene, the recurrent parent is crossed with the donor parent, and the F1 backcrossed to the recurrent parent. In one scheme, the F1 is self- pollinated to produce the F2, which will segregate for the trait of interest. Individuals having the trait can then be backcrossed to the recurrent parent to produce the BC1.
The BC1 generation is then tested for the trait, and individuals having it are self-pollinated once again to produce a segregating generation for selection and backcrossing to the recurrent parent. The process is repeated until the BC6 generation when the best individuals are self-pollinated and selected for the trait to produce the improved cultivar which need not be evaluated again.
For the transfer of a trait controlled by a dominant gene, the recurrent parent is crossed with the donor parent, and the F1 backcrossed to the recurrent parent. The BC1 generation is then tested for the trait, and individuals having it are again backcrossed to the recurrent parent.
The process is repeated until the BC6 generation when the best individuals are self-pollinated and selected for homozygous expression of the trait using progeny testing.
3. Hybrid Breeding:
Hybrids are usually made between two monoecious inbreds. For triploid hybrid production, the seed parent should have a distinctive rind pattern that has recessive inheritance. For hybrid production with less labour input, the seed parent could be male sterile.
The seed increase of the male sterile inbred would be accomplished by pollinating male sterile plants with the heterozygote (Ms ms) as the pollen parent. For seedless hybrid production, the seed parent would be a tetraploid inbred. Once good parental lines are available, they are crossed in pairs keeping complementarity of parents into account.
Although there is not much advantage of hybrids over open-pollinated cultivars for most traits, it is thought that the former are more uniform. Thus, it may be possible to get the same yield in fewer harvests because of more uniform growth and a more concentrated fruit set. Hybrids offer several advantages over open-pollinated cultivars.
A major advantage is the production of seedless triploids, which are produced by crossing a tetraploid female inbred with a diploid male inbred. Hybrids also can express heterosis, with the hybrid performing slightly better than the best parent in some cases.
The amount of heterosis in watermelon is around 10%. Another advantage is the ability to get an intermediate fruit shape by crossing an elongate-fruited inbred with a round-fruited one.
Watermelon hybrids may not offer distinct yield advantage, however they have uniformity. They are seedless in case of triploids. It is easy to manipulate fruit shape through hybrids in watermelon as one can get intermediate (blocky) fruit shape by crossing elongate-fruited parent with a round fruited parent.
Inbreds can be used to combine dominant genes for resistance from each parent into a hybrid that has more dominant genes expressed than either parent. A hybrid that has large seeds for the grower to plant and small seeds in the fruit sold to the consumer can be produced by crossing a large-seeded female inbred with a small-seeded male inbred.
Finally, hybrids provide a way for the seed company to protect their proprietary inbreds. The disadvantages of hybrids are that they add an extra step to the breeding process, and increase the cost of seeds since they are produced by hand pollination rather than by bee pollination.
7. Genetic Resources of Watermelon:
USA has been leader in watermelon breeding. Several germplasm collections, along with current cultivars marketed by seed companies, represent the major sources of germplasm for watermelon breeders. The USDA watermelon collection is stored at the Regional Plant Introduction Station, Griffin, Georgia with the backup collection at the National Seed Storage Laboratory, Fort Collins, Colorado.
There are 1644 accessions in the collection, with most currently available to researchers. The collection includes representatives of all Citrullus species and botanical varieties. In addition, approximately 300 heirloom cultivars are kept at the National Seed Storage Laboratory.
The Cucurbit Genetics Cooperative, USA 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 can be obtained from the curators for that species T.C. Wehner and S.R. King. In India, watermelon germplasm are conserved at IIVR, Varanasi, NBPGR, New Delhi and a few SAUs and IIHR, Bangalore.
8. Use of Genetic Male Sterility that are Identified in Watermelon:
Either controlled pollinations or the elimination of pollen-producing flowers can allow the transfer of pollen from only one male parent line to a female parent line. One way to eliminate pollen is to use male sterility. Several male sterile genes have been identified in watermelon. One has particular merit. It is denoted simply as male sterile.
This recessive gene primarily affects the development of the male flower and has little effect on the rest of the plant. No linked seedling marker for this gene has been identified.
This gene eliminates ail pollen production and effectively produces an ideal female parent. A 1: 1 male fertile: male sterile population can be planted, and the male fertile plants rogued prior to pollination when the male fertile plants with normal plump male buds can be distinguished from male sterile plants with poorly developed male buds.
This use of this gene is further enhanced with a seedling marker. This system exploits a recessive seedling marker which is not linked with the male sterile gene. The seedling marker is incorporated into the female line and used to identify inbred seedlings that may show up in the planting of the F1 hybrid seedlings. This system is similar to that which is in use to produce hybrid seeds of muskrtielon Punjab Hybrid at PAU, Ludhiana.
Currently, very limited production of hybrid watermelon seed employs male sterility. Most of the diploid hybrid seeds are produced by hand pollination. Thus, an important consideration for choosing a hybrid seed production site is the availability of skilled labour for the 3-4 week pollination period. At least six skilled workers are needed for pollinating one acre of a hybrid watermelon seed production field.
Female and male parents of the hybrid are planted in a ratio of 10 female to 1 male. The male parent is planted 7-10 days earlier than the female parent to ensure adequate pollen at the pollination time. The male parent plants are removed from the field after pollination is complete. Therefore, the male parent is usually planted in outside rows or in the corner of the field.
In commercial hybrid seed production, the pollinated flowers are marked with coloured plastic bands or string. A different colour is used every 5 days so harvest can be made according to the time of pollination. Fruits for hybrid seed production are not harvested until over-ripe, usually 7 weeks after pollination.
Only marked fruits are harvested for seed. It is highly recommended that a well-trained individual carefully inspects the planting for disease. Discarding diseased fruit, even the entire field if necessary, is the best way to prevent seed-borne disease. In addition, 24 hour fermentation of diploid seed before washing almost entirely eliminates the bacterial pathogen for fruit blotch.
9. Disease Resistance of Watermelon:
Fusarium wilt (Fusarium oxysporum f. sp nivenum):
Soil inhabiting fungus, invades root of plants, resulting in wilting and eventual death of plant. W.A. Orton (1911) in a classical investigation found that resistance to Fusarium wilt is determined genetically. His preliminary experimental work indicated that the semi-wild, non-edible stock melon or citron was resistant to Fusarium wilt. The resistant stock citron was crossed with the cultivar Eden, susceptible to diseases.
The F1from this cross was exceptionally vigorous and resistant to Fusarium. F2 progenies were grown and tested for resistance over a period of several years. Finally a selection was chosen that combined a high level of resistance with what was hoped were satisfactory horticultural characters.
The result was the cultivar “Conqueror’. Unfortunately, ‘Conqueror’ never justified the high hopes of its sponsor, chiefly because of poor consumer quality. Orton’s work, however, did trigger a vast amount of research aimed at developing wilt resistant cultivars.
Conqueror, Summit, Charleston Gray, Dixielee
Anthracnose (Colletotrichum lagenarium):
It attacks both plants and fruit and may cause serious damage to fruit during post-harvest handling.
Congo, Fairfax, Charleston Gray, PI 189 225, PI 271 775, PI 271 778
10. Integration of New Biotechnologies into Breeding in Watermelon:
In watermelon, biotechnology is being used to propagate plants in tissue culture, to study genes at the molecular level, to develop molecular markers for selecting useful genes, to isolate the DNA of useful genes, and to incorporate useful genes into cultivars using genetic transformation.
Tissue culture involves the production of cells or plants from plant parts. It offers a method for propagation of valuable plants such as tetraploid parental inbreds, or triploid seedless hybrids.
With the increased demand for seedless cultivars, breeders are interested in producing tetraploids in quantities large enough for a hybrid seed production block. Since tetraploid watermelons have slow growth and low seed production, tissue culture can be useful for multiplying new tetraploid lines. Protocols have been developed for propagation of tetraploid plants.
Methods have been developed for the production of tetraploid plants by the regeneration of cotyledons of seedlings cultured in vitro. It has been possible to produce tetraploid plants from different watermelon cultivars, as an efficient alternative to the standard method of using colchicines to double the chromosome number of diploid plants.
Marker Assisted Selection:
Molecular markers can be used as reference points in mapping genes on a chromosome. The information is useful in the selection of plants that carry a marked gene of interest. Plant breeders can discard plants from a segregating population that are missing the gene of interest.
In that way, field testing can be done using only the plants having a particular set of traits. Markers can also be used to identify cultivars (DNA fingerprinting), and to estimate the genetic relatedness of a set of cultivars or individuals in a population.
Over 40 genes have been described in watermelon. The genes are involved in disease resistance, flower type, fruit shape, and fruit quality. If molecular markers can be identified that are closely linked to those genes, then selection might be performed more rapidly, or earlier. For example, selection might be carried out in a seedling test instead of waiting for the plants to produce fruit.
Genetic transformation provides methods for inserting single genes into plants while overcoming barriers to interspecific crossing. The soil-borne bacterium Agrobacterium tumefaciens is often used as the vehicle for transferring genes into plants.
Plant cells can also be transformed using micro-particle bombardment, where DNA-coated particles are shot into plant cells. Several studies have reported transformation of watermelon using Agrobacterium or micro-particle bombardment. Transformed cells must by regenerated from sterile culture to produce plants containing the new gene to be used in a breeding program.
Transformation of watermelon plants has been used to confer virus resistance. Several virus species cause disease in watermelon. These include squash mosaic virus (SQMV), cucumber mosaic virus (CMV), papaya ring-spot virus-watermelon strain (PRSV-W), zucchini yellow mosaic virus (ZYMV), and watermelon mosaic virus (WMV).
Transfer of virus coat protein genes into watermelon plants may confer resistance to the virus disease, and provide plant breeders with new resistance genes.
11. Seed Production of Watermelon:
According to George (1999), it is as follows:
2. Season of cultivation: including suitability for intensive and/or extensive protected cropping
3. Plant vigour
4. Foliage leaf protection against sun-scald
6. External shape round, oval or long
7. Relative size and weight at market maturity
8. Rind colour, bi-colour, colour pattern, striped
9. Internal thickness of rind, flesh colour, intensity of flesh colour (especially towards the centre of the fruit), absence of central cavity
10. Seed external colour when mature, striped or single colour, relative size
11. Resistance to specific pests, pathogens or other disorders, e.g. Fusarium sp., Didymella bryoniae (gummy stem blight) and Colletotrichum sp. (anthracnose or sun-scald).
Watermelon flowers are insect pollinated, mainly by honeybees. The plants are self-compatible, but because flowers are unisexual a high percentage of cross-pollination occurs. It is essential to place colonies of bees hive on the perimeter of watermelon fields.
This is done to increase seed yield and it is also claimed that by supplying a high population of pollinating insects adjacent to, or within the seed production plot or field, the incidence of cross-pollination with other fields of watermelon which may be a different seed category or cultivar is minimized.
12. Isolation Distance of Watermelon:
a. Breeder/foundation seed – 800 m
b. Certified seed – 400 m
13. Varieties of Watermelon:
This is a variety originally introduced by M Hardin, Geary, Oklahoma, USA in the year 1956. It is early and resistant to drought. From USA, it was introduced to India by 1ARI. The vines are medium long. Fruits are smaller (3-5 kg), round in shape. The rind is bluish black. Flesh is sweet (11-13% TSS) with deep pink colour. Seeds are brown and small. Yield potential is 150 q/ha.
It is a mid-season Japanese introduction by IARI. Fruits are medium weighing 6-8 kg. The fruits are round oblong, non-striped with light green rind. Flesh is deep pink with 11-13% TSS. Fruits ripen in about 95 days. Yield is 225 q/ha.
It is a seedless triploid variety of watermelon developed at IARI, from a cross of Tetra-2 (4X) x Pusa Rasaal (2X). The fruits have dark green skin with faint stripes. Fruits are somewhat triangular in shape with tough rind, red flesh and white remnants of false seed.
TSS is 12-13%. Average fruit weight is 5-6 kg. The number of fruits per vine varies from 3 to 6. It takes 115-120 days for first fruit harvest. It could not become popular due to irregular fruit shape and high cost of seed. Now it is not available.
This has been bred at IIHR, Bangalore from a cross of IIHR 21 x Crimson Sweet. Fruits are round to oval with green rind, dark green stripes and weight is about 6 kg. The flesh is deep crimson, with granular texture. It is very sweet with 12-15% TSS. It has multiple resistance to powdery mildew, downy mildew and anthracnose. Yield potential is 500 q/ha.
It is a mid-season F1 hybrid cultivar evolved at IIHR, Bangalore by crossing a local watermelon of Rajasthan (IIHR 20) with Crimson Sweet. Fruit are oval and deep blue angular stripes. Average fruit weight is 5-6 kg. Flesh is bright crimson, sweet with 11-13% TSS. Yield potential is 600 q/ha.
It is a late maturing cultivar (125 days). Fruit is round and light-green. Rind is thick with good keeping quality. Flesh is sweet with TSS around 11% and dark-red colour. Average fruit weight is 6-8 kg. Seeds have black tip and margin. Yield is 40-45 tonnes/ha. It is released by Agricultural Research Station, Durgapura, Rajasthan.
It is a late cultivar. Fruit weight is 4-5 kg. Skin is green with stripes. Flesh is yellow in colour, and moderately sweet. Seeds are large. It is released by Agricultural Research Station, Durgapura, Rajasthan.
There are several cultivars, locally grown which are named after the region in which they are grown, such as Farrukhabadi, Moradabadi of Uttar Pradesh. Most of them have fruits with dark- green colour or pale-green with black stripes, oblong to round shape weighing 8-10 kg with thick rind.
The flesh colour could be variable from pale-pink to pink with big flat black to brown seeds. In Rajasthan, Mateera cultivar is grown in rainy season around Bikaner. There is local cultivar of Jamuna river-bed called Katagolan whose flesh is not sweet but it keeps well for over 2-3 months at ambient temperature during July-September. The flesh becomes dry and fibrous instead of fermentation and rotting.
Prominent hybrids by private seed companies in India are NS 295, Tambola, Madhuri (all Jubilee segment, oval, 6-10 kg fruits, striped rind, red flesh), Black Magic, Black Sugar, Augusta (spherical, 6-8 kg, black rind, Sugar Baby type) and Kiran and Rasraj (3-4 kg, oblong, green skin/striped skin, ice box type).
All these hybrids are sweeet, juicy (12-13% TSS) and have high shelf- life and transportability. With the introduction of these hybrids, Indian consumers are enjoying delicious watermelons round the year. This is the impact of hybrid technology for anybody to see.