Cucumber: Origin, Production and Varieties | India

After reading this article you will learn about:- 1. Origin of Cucumber 2. Production of Cucumber 3. Botany and Reproductive Biology 4. Genetics of Sex 5. Chemical Regulation of Sex Expression 6. Important Genes 7. Cultivar Groups 8. Plant Genetic Resources 9. Breeding Goals 10. Seed Production 11. Biotechnological Applications in Cucumber Breeding 12. Molecular Markers 13. Varieties.

Contents:

1. Origin of Cucumber
2. Production of Cucumber
3. Botany and Reproductive Biology of Cucumber
4. Genetics of Sex in Cucumber
5. Chemical Regulation of Sex Expression in Cucumber
6. Important Genes for Cucumber
7. Market Classes or Cultivar Groups for Cucumber
8. Plant Genetic Resources of Cucumber
9. Breeding Goals of Cucumber
10. Seed Production of Cucumber
11. Biotechnological Applications in Cucumber Breeding
12. Molecular Markers of Cucumber
13. Varieties of Cucumber

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1. Origin of Cucumber:

Cucumber probably originated in India from where it seems to have spread eastwards to China and westwards to Asia Minor, North Africa and Southern Europe and subsequently to entire Europe. It was introduced to the New World by Columbus who planted it in Haiti in 1494 and perhaps soon afterwards it was brought to the USA.

Although cucumber is known as only a cultivated plant, a Cucumis form Cucumis sativus var. hardwickii R. (Alex.), with 2n = 2x = 14 crosses readily with cultivated cucumber. Sir Joseph Hooker has collected specimens of var. hardwickii along the southern foot-hills of the Himalayas and found that its range of variability fell within that of C. sativus.

This has led to conclusion that var. hardwickii is either a feral or progenitor form of the cultivated cucumber i.e., C. sativus L. Isshiki (1992) examined the Indian wild cucumber (C. sativus. L. var. hardwickii Kitamura) and 81 accessions of cultivated cucumber for six isozymes, viz., malate dehydrogenase, 6-phosphogluconic dehydrogenase, phosphoglucomutase, phosphoglucoisomerase, shikimate dehydrogenase and isocitrate dehydrogenase and concluded that Indian wild cucumber is a distant relative of cultivated cucumber.

According to detailed accounts given by Staub (2008), cucumber was domesticated about 3000 years ago and is indigenous to India, (primary centre of diversity). C. sativus var hardwickii is a wild relative of C. sativus var sativus and grows in the foothills of the Himalayas and used by native peoples of northern India as a laxative.

This botanical variety is sympatric and cross-compatible with Cucumis sativus var sativus and has multiple fruiting and branching habit which is uncommon in cucumber. As against India which is the primary centre of diversity of cucumber, China is considered as secondary centre of diversity.

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2. Production of Cucumber:

Cucumber (Cucumis sativus L. 2n = 2x = 14) is one of the Asiatic species and member of the Cucurbitaceae which has 90 genera and 750 species. The African group has diploid chromosome number of 24. It is frost susceptible but grows well at temperature above 20°C. These are grown throughout the world to be consumed as fresh fruits, as slicing cucumber, and as pickles in immature stage.

After tomato and watermelon, cucumber and melon are cultivated more broadly than other vegetable species where 2.480 million hectares were harvested in 2005 producing 42.60 million tons under field and greenhouse culture.

Production of cucumber is second largest of all cucurbits where China (26.60 mt) Iran (1.40 mt), Russia (1.41 mt), Turkey (1.72 mt) and USA (0.98 mt) represent 75% of the world production as of 2005. In India, cucumber is estimated to be grown on 300000 ha and traded seed as 50 ton hybrid seed and 800 tons as OP seed with a market value of about Rs. 60 crores.

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3. Botany and Reproductive Biology of Cucumber:

Cucumber is an annual plant species and is found to be day neutral. Under greenhouse 3 generations/year can be grown. Basically, it is monoecious, trailing or climbing vine with angled, hirsute or rough stems. Leaves are triangular-ovate, somewhat three lobed with mostly acute curves. The staminate flowers are in clusters with short, slender pedicels. The pistillate flowers are usually solitary with stout, short pedicels.

The calyx and corolla of staminate, pistillate and even hermaphrodite flowers are five lobed. The staminate flowers have three stamens. Two stamens have two locules each and the third is unilocular. Filaments are free, but the stamens are more or less united by their anthers.

The pistillate flowers are epigynous and hermaphrodite flowers are perigynous. The pistil consists of from one to five (but usually three) carpels which in turn, produce ovaries with a corresponding number of locules. The pistillate flowers contain up to five stigmas. A typical leaf of cucumber is shown in Fig. 26.1.

The main stem of monoecious cucumber is usually characterized by three phases of sex expressions. Only staminate flowers are produced in the first phase fol­lowed by a phase of irregularly alternating female, male or mixed nodes and finally a phase of only pistillate flowers.

Lateral shoots of monoecious cultivars usually have stronger female tendencies. Fruits from perigynous flowers are more rounded as opposed to elongated ones from epigynous ones. The rounded fruits are horticulturally poor due to large seed cavity.

Sex expression is generally influenced by environment. Under long day and high light intensities staminate flowers predominate, whereas under short day and low light intensities female flowers predominate. For crossing purpose, pistillate flowers are closed with a rubber band/wrapped with cotton pad or covered by paper bag to protect against unwanted pollen 1 day prior to opening in the afternoon.

Unopened male flowers are also covered similarly at the same time. Anthesis takes place around 5.30 – 7.00 hr. Dehiscence occurs around 4.30-5.00 hr. Pollen fertility is up to 14 hr. Next day morning, pollination is carried out from pollen of protected staminate flower.

A thread is tied on selfed flowers and a label is tied on the crossed flowers. Selfed and crossed flowers are again covered by paper bag/wrapped in cotton pad. After about 4 weeks, the fruits are harvested and allowed to further ripening by about another week. For seed collection, the fruits are cut- open and seeds are collected in a glass jar with water and left as such for two days to remove gelatinous mass.

Then they are washed in a sieve and dried. Seeds should not be left in water for more than two days otherwise they may start germinating. Sixty days after pollination are usually preferred for optimum maturity of seeds.

The success of controlled pollination may be enhanced by removal of any previously set fruit as first fertilized flower inhibits the development of subsequent fruits. Therefore, controlled pollination should be done as soon as possible after flowering begins.

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4. Genetics of Sex in Cucumber:

Following main sex types are reported in cucumber:

(i) Monoecious plants: Staminate and pistillate flowers

(ii) Androecious plants: Only staminate flowers

(iii) Gynoecious plants: Only pistillate flowers

(iv) Hermaphrodite plants: Only hermaphrodite flowers

(v) Andromonoecious plants: Staminate and hermaphrodite flowers

The sex expression in cucumber is determined by three major genes, namely, F (also known as Acr), M and A. The F locus determines degree of femaleness (FF > Ff > ff). M locus determines whether flowers are unisexual (M-) or bisexual (mm).

The A locus conditions increased male tendency if the plant is homozygous recessive for aa and ff. Interaction between these three loci is responsible for producing basic sex types in cucumber.

Along with three major genes, there are several modifying genes and environmental factors influencing sex types in cucumber. The existence of sex modifying genes is supported by the observations that gynoecious plants differ in the level of gynoecy and their ability to confer femaleness in F₁ hybrids.

Monoecious plants also vary quantitatively in sex expression ranging from predominantly staminate to predominantly pistillate. There are at least five genes that modify the expression of gynoecy in cucumber. The hybrids between monoecious and gynoecious lines can manifest considerable variation in frequency of female flowers depending upon the level of gynoecy in the parents.

This variation in the level of gynoecy in the gynoecious x gynoecious and gynoecious x monoecious hybrids remain a potential deficiency in many commercial cultivars. Development of stable gynoecious lines is of commercial interest to produce F₁ hybrids of cucumber.

Roguing of monoecious segregates during increases of gynoecious lines is important in this context. Gynoecious lines are suggested to be grown at high plant density, high temperature and long days to promote male tendencies so that really stable gynoecious lines could be selected.

The hybrid populations for such selection can be created by:

Since Acr is only partially dominant in some backgrounds, the hybrid Acr Acr MM A- is supposed to result in more stable gynoecious performance. The gynoecious sex form in cucumber was spotted out as a chance segregate from a Korean gynomonoecious introduction Shogoin (PI 220 860) by Peterson and Anhder (1960) and this line has been used as a source to develop various gynoecious lines.

Researchers in Israel and Russia and Japan also proposed that hybrid cucumber seed could be produced by crossing gynoecious and monoecious lines. Other systems of producing gynoecious hybrid seed have subsequently been proposed, but gynoecious X monoecious hybrids are still the most widely grown.

Gynoecious hybrid cultivars produced by crossing gynoecious and monoecious lines have become important, particularly for mechanically harvested pickling cucumbers. Gynoecious hybrids are advantageous for mechanical harvesting, not only because of their hybrid vigor, but even more importantly because of their sex expression.

Partial dominance of the gynoecious gene usually confers a high degree of female sex expression to gynoecious hybrids, and the abundance of pistillate flowers results in uniform and concentrated fruiting. Consequently, high yield is often possible in a single mechanical harvest.

Many commercial seed-lots of gynoecious hybrid cucumber cultivars are a blend of gynoecious and monoecious types. Generally 10% of seed of a monoecious cultivar is blended with gynoecious hybrid seed. Seed of the monoecious parent of the hybrid can be used for this purpose.

The monoecious cultivar ‘Sumter’ has often been used as a blender for gynoecious or predominantly female pickling cucumber cultivars. Seed company breeders have developed monoecious cucumber lines specifically for use as blenders, selecting for early pollen production and for fruit type similar to the hybrid that the blender will pollinate.

Blending is done to improve pollination, but it has the disadvantage of compromising the uniformity that is an important attribute of hybrids. If the gynoecious hybrid is earlier than its monoecious pollinator, then the monoecious plants may not yet have fruit when the field is mechanically harvested.

Blending is not needed if the gynoecious hybrid is parthenocarpic and does not require pollination for fruit set. Genetic parthenocarpy has been bred into gynoecious hybrid cucumbers for greenhouse production.

A problem that has been encountered with many gynoecious X monoecious hybrid cucumber cultivars is that they are not dependably gynoecious. The occurrence of monoecious plants in plantings of gynoecious hybrid cultivars can have serious consequences on uniformity, earliness, and adaptation to mechanical harvesting.

Gene background is important for this to occur, and some cultivars heterozygous for the gynoecious gene are more reliably gynoecious or predominantly female than others. Environment is also quite important, and the proportion of gynoecious plants produced by the same seed lot of a hybrid may vary in different plantings.

(iii) Gynoecious X Gynoecious Hybrids:

Cucumber hybrids homozygous for F, the female sex expression gene, have the important advantages of being more stable for gynoecious sex expression than comparable hybrids heterozygous for that gene. They are more likely to produce entirely female plants, especially under environmental conditions or gene background that promote the development of male flowers.

Homozygosity for gynoecious sex expression is especially important for parthenocarpic greenhouse cucumbers. These cultivars are able to set seedless fruit when pollination insects are absent, but will set fruit with seed when pollinated.

The long fruit of these cultivars is often misshapen if pollinated, being enlarged at the blossom end where the seeds are located. Homozygous gynoecious cultivars reduce the likelihood of pollination and misshapen fruit, since they produce no pollen.

Homozygous gynoecious hybrid cucumber seed has been produced by crossing two gynoecious lines after one parent has been treated with a growth regulator to induce male flower formation. Hybrid seed produced in this way can be expensive, but the cost may be justified for greenhouse cucumbers since their value is high and the seed requirements are nominal.

Tropical true breeding gynoecious lines, viz. 87-304-6, 87-316, 87-319-12 and 87-338-15 have been developed in India by More and Sheshadari in late eighties. These have been found to be true breeding under high temperature and long photoperiodic conditions but are unstable and not in use.

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5. Chemical Regulation of Sex Expression in Cucumber:

Increasing Female Tendency:

(i) Auxin

(ii) Ethylene

(iii) Acetylene

(iv) 2-chloroethylphosphonic acid (ethephon)

Increasing Male Flower Promotion:

(i) GA₃

(ii) GA₄

(iii) GA₇

(iv) Silver nitrate (Ag NO₃)

(v) Silver thiosulfate [Ag (S₂O₃)₂]

(vi) Aminoethoxy-vinylglycine (AVG)

Growth Regulators to Maintain Gynoecious Cucumber Lines:

Robinson (1999) has discussed this aspect adequately quoting Anhder, Galun, Pike, Peterson, Shifriss, George, Tolla, Beyer, etc.

The information compiled by Robinson (1999) is as follows:

Since the maternal parent of gynoecious cucumber hybrid normally produces few or no male flowers, open- or hand-pollination cannot be relied on to increase its seed stocks. The commercial production of gynoecious hybrid cucumber seed was made possible only when it was discovered that gynoecious inbreds could self reproduce if a growth regulator is applied to induce male flower formation.

Gibberellic acid promotes male flower formation on gynoecious cucumber lines. A mixture of gibberellins 4 and 7 is more effective than GA for maintaining gynoecious lines. Different gynoecious inbreds may vary in response to GA application.

GA was initially used to maintain gynoecious lines used for producing hybrid seed. A problem often arose, however, in that not all of the treated gynoecious plants produced staminate flowers. Genetic variation in gynoecious lines results in plants with the strongest tendency towards maleness being more likely to respond to GA treatment by producing male flowers than plants with a higher degree of female sex expression.

Consequently, there could be a genetic shift in a gynoecious line after several generations of using GA to increase seed, and hybrids produced with an altered gynoecious line may produce an increased number of monoecious plants. Also, the number of male flowers induced by GA may not be sufficient for using a gynoecious line as a male parent of a homozygous gynoecious hybrid.

Because of these problems, many seedsmen now use a silver compounds rather than gibberellin to maintain gynoecious lines. Silver ions inhibit ethylene action and therefore promote male sex expression of gynoecious cucumbers. Silver nitrate induces significantly more staminate flowers.

Silver thiosulfate is now used by many seedsmen to increase seed of gynoecious cucumber lines. It effectively induces male flowering of gynoecious plants for an extended period, is less phytotoxic than silver nitrate, and does not cause the excessive stem elongation or malformed male flowers that is characteristic of gibberellin application.

The general recommendations to induce induction of male flowers on gynoecious lines are as follows:

1. Three applications of GA₃ at 1000 ppm, sprayed on at fortnightly intervals, commencing when the plants have two leaves.

2. As above, but using GA 4/7 at 50 ppm.

3. A single application of silver nitrate solution (600 mg 1^-1) before the first flowers open.

Hybrid seed is collected only from the female parent plants; the presence of any pistillate flowers on the male plants is not a problem but it is important that staminate flowers are completely suppressed on the female parent. This is normally achieved by two applications of ethrel (250 ppm), the first when the plants show their first true leaf and the second at the fifth true- leaf stage.

A visual check must also be made and any male flowers on the female parents removed by hand. The development of suitable male sterile lines or the application of a satisfactory gametocide would be a useful development in the production of hybrid cucumber seed.

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6. Important Genes for Cucumber:

Based on information provided by Robinson and Whitaker (1974), Robinson, (1976) and Tatlioglu (1993) the list of important qualitative genes in cucumber is summarized in Table 26.1.

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7. Market Classes or Cultivar Groups for Cucumber:

1. Eaten fresh or slicing market types

2. Processing or pickling types

European Cultivars:

European market types include glasshouse cucumbers, Mediterranean or Mini types for glasshouse or poly-tunnel production and processing cucumbers (gherkins). Mini cucumbers are 14-17 cm long and are marketed in Europe, North America, Greece, Spain and Turkey.

European glasshouse types are 32 to 40 cm long, gynoecious, parthenocarpic and resistant to powdery mildew, cucumber mosaic virus. They are result of Dutch breeding efforts. Some representative cultivars in Europe include Jessica, Optima, Flamingo, Toska 70, Averyl, Niagara, Ladner, and gynoecious mini cultivars are Jawell, Manar, Alamir, and Melita.

US Cultivars:

The initial successful cultivars of cucumber in US included introductions from Europe. These were Early Short Prickley, Long Green Turkey, Smyrna, Roman, and White Spined. Additional cultivars sold in US were China Long in 1962 and Chicago Pickling in 1888. Original development of cucumber varieties in USA started in 1880s. These historical developments are summarized in Table 26.2.

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8. Plant Genetic Resources of Cucumber:

International Plant Genetic Resources Institute (IPGRI) coordinates institutional germplasm holdings of cucumber in Europe. In USA, the U.S. National Plant Germplasm System (NPGS) maintains and evaluates cucumber germplasm.

The regional plant introduction station of NPGS, Ames, Iowa has about 1350 C. sativus accessions from all parts of world. In India, cucumber germplasm are conserved at NBPGR, New Delhi and IIVR, Varanasi and several SAUs. Cucumber plant introductions (Pls) have contributed enormously to cucumber improvement.

These important PIs used as useful donors in improvement of cucumber are as follows:

Several other prominent germplasm have been:

Riesenschaal – Germany

Zeppelin – Germany

Chinese Long – Japan

Tokyo Long Green – Japan

Spotvrije – The Netherlands

ILG 58049 – The Netherlands

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9. Breeding Goals of Cucumber:

1. Fruit Yield:

Quantitatively inherited trait, has low heritability, mainly influenced by genotype and environment and to a lesser extent by g x e interaction, hence selection for yield should occur during intermediate stages of selection, yield evaluation should be based on plot basis rather than on individual plants, yield may be effectively evaluated on small (one row, single replication and harvest), multi-locations (two-three) trials over seasons/years, several methods of yield measurement (fruit volume, mass, number, market value) compared and investigated.

1. The most efficient measurement of yield in plant breeding trials seems to be the total number of marketable and oversized fruits/plant, as it is highly heritable, more stable over time, is easier to measure.

2. Increase in number of harvests/plant, stem length, number of branches/plant, number of flowing nodes/plant, time to anthesis, percentage of pistillate flowers and percentage of fruit set.

3. Predominantly gynoecious types.

4. Earliness (days to first harvest).

5. More number of lateral branches

6. Desirable fruit size, shape and colour as per need of consumers/processing industry. In India there are two main segments from consumers point of view. There are long, green with light stripes particularly in north India and medium long, white with mild spines like Gypsy in western and south India.

7. Parthenocarpic cucumber, controlled by single recessive gene/several incompletely recessive genes/single dominant gene expressing incomplete dominance, desirable for glasshouse cultivation.

8. Better keeping quality of fruits with less shrinkage, non-bitter, crispy taste, free from carpel separation without hollow spots.

9. Resistance to disease:

i. Scab (Ccu)

ii. Downy mildew (dm)

iii. Bacterial wilt (Bw)

iv. Angular leaf spot (psl)

v. Anthracnose (Ar, cla)

vi. Target leaf spot (Cca)

vii. Fusarium wilt (Foc)

viii. Powdery mildew (pm-1, pm-2, pm-3)

ix. Cucumber mosaic virus (Cmv)

x. Green mosaic virus

xi. Gummy stem blight

xii. Phytophthora rot

10. Resistance breeding requires exact test protocols and extensive replicated testing at hot- spot locations in field and greenhouse, in multiple environments employing artificial and/ or natural inoculation.

11. Stress tolerance (temperature extremes, water deficiency).

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10. Seed Production of Cucumber:

In India, hybrid seed is produced on farmers’ field using trained manpower. One needs 75g seed of female parent and 25g seed of male parent per unit production plot of 1000 m². Unopened pistillate and staminate flowers are covered by paper pollination bag in afternoon.

The next morning, covered staminate flowers are collected and used in pollination of protected pistillate flowers. The crossed flowers are identified by thread around pedical. The seed multiplication ratio is about 100.

Hybrid seed production is usually done in either greenhouse or field environments (open field or cage isolation) by hand or insect pollination as elaborated by Lower and Edwards (1986). Typically, parental lines are multiplied in isolation or through controlled self-pollination. Open field isolation blocks need at least 1.5 km isolation distance.

Where the number of wild bees is in sufficient, beehives are introduced into isolation block or cages. In case of hybrid seed production on commercial scale, open field increases employ the strategic placement of rows such that cross- pollination can occur between lines of opposite sex types. Female: male rows ratio is usually 4:2.

Hybrids commonly result from:

gynoecious x gynoecious

gynoecious x monoecious

monoecious x monoecious

gynoecious x hermaphrodite

In case of gynoecious x gynoecious combinations, sex expression of one line is chemically altered by ethylene inhibitors such as silver nitrate, silver thiosulphate or aminoethoxy vinyl glycine. The seed of gynoecious lines is also produced using such compounds.

Likewise, staminate flower lines (monoecious, hermaphrodite, androecious) can be converted to pistillate lines by application of ethylene releasing compounds such as alpha-naphthalene acetic acid and ethephon i.e. 2-chloroethyl phosphonic acid. Chemicals are usually applied at least three times, beginning at first true-leaf stage, followed by 2 sprays at weekly intervals.

Isolation Distance:

1. Breeder/foundation seed – 800 m

2. Certified seed – 400 m

3. Cucumber not cross-compatible with C. melo, gourds, marrows, squash, pumpkins or watermelons

4. Ensure adequate number of pollinating insects for greenhouse production of cucumber

Cultivar Description:

There are open-pollinated and hybrid cultivars.

The following characteristics are based on UPOV (1993) and as described by George (1999):

1. Plant:

Growth type: determinate or indeterminate

Vigour: weak, medium or strong

Total length of first 15 internodes: short, medium or long

Length of internodes of side shoots: short, medium or long

2. Leaf:

Size of blade: small, medium or large

Intensity of green colour: light, medium or dark

Blistering: absent or very weak, weak, medium, strong or very strong

Undulation of margin: absent or very weak, weak, medium, strong or very strong

Length of terminal lobe: short, medium or long

Width of terminal lobe: narrow, medium or broad

Ratio length/width of terminal lobe: less than 1, equal to 1, or more than 1

3. Plant:

Sex expression: male and female flowers approximately equally present, mainly female flowers, or almost exclusively female

Number of female flowers per node: one to three, or more than three

4. Young fruit:

Type of vestiture: hairs only, prickles only or hairs and prickles

Density of vestiture: sparse, medium or dense

Colour of vestiture: white or black

Size of warts: absent or very small, small, medium, large or very large

5. Parthenocarpy: absent or present

6. Fruit:

Length: very short, short, medium, long or very long

Diameter: small, medium or large

Ratio length/diameter: small, medium or large

Core diameter in relation to diameter of fruit: small, medium or large

Predominant shape of stem end at market stage: necked, acute or obtuse

Length of neck: short, medium or long

Shape of calyx end at market stage: acute or obtuse

Ground colour of skin at market stage: white, yellow or green

Intensity of ground colour of skin: light, medium or dark

Ribs: absent or present

Prominence of ribs: weak, medium or strong

Colouration of ribs compared to ground colour: lighter, equal or darker

Vestiture: absent or very sparse, sparse, medium, dense or very dense

Warts: absent or present

Stripes (ribs excluded): absent or present

Length of stripes: short, medium or long

Mottling: absent or present

Predominant type of mottling: small and round, or large and irregular

Intensity of mottling: weak, medium or strong

Length of peduncle: short, medium or long

Thickness of peduncle: thin, medium or thick

Ground colour of skin at physiological ripening: white, yellow, green, orange or brown

7. Time of development of female flowers (80% of plants with at least one female flower): early, medium or late

8. Cotyledon bitterness: absent or present

9. Fruit bitterness: absent or present

10. Resistance to:

i. Scab: absent or present

ii. powdery mildew: absent or present

iii. downy mildew: absent or present

iv.- cucumis mosaic virus: absent or present

Cucumber Cultivar Types:

1. Field cucumbers, with prominent black or white spines.

2. The greenhouse or forcing type (often referred to as ‘English cucumber’). These have spineless fruit which can be produced parthenocarpically, there are also short-fruited cultivars in this group.

3. The Sikkim cultivars (originating from India) with reddish-orange fruits.

4. The small-fruited cultivars frequently used as ‘gherkins’ for pickling.

5. The ‘apple’ fruited cultivars which have ovoid-to spherical-shaped fruit, from diverse areas of the world including the USA and the Far East.

Harvesting:

Ideally the fruit should remain on the plant until it is fully mature. This is indicated externally by the development of the ripe rind colour characteristic of the cultivar; additionally the fruit stalk adjacent to the fruit withers when the seed is mature. In order to confirm the external signs that the seeds are actually mature, several fruits should be cut open longitudinally and the seeds examined. Mature seeds separate easily from the interior flesh.

The mature fruits are handpicked and put in a fruit crusher and seed extractor. Large-scale specialist producers use mechanized harvesting machines incorporating crushers and seed extrac­tors.. If the seeds are extracted by hand, the ripe fruits are cut in half longitudinally and the seeds are scraped into a container. The seed and juice mixture can be fermented for about a day before screening and washing the seed in suitable-sized sieves.

Seed Yield and 1000-Seed Weight:

The average seed yield under field conditions is 400 kg ha^-1, although yields of up to 700 kg ha^-1 are often reported. The yield from a single fruit depends on the cultivar and the amount of successful pollination, but estimates can be based on approximately 500 seeds per fruit.

The seed yield for F₁ hybrids produced in fields with a male: female population ratio of 1: 4 is 300-350 kg ha^-1. The 1000-seed weight of the smaller fruited cultivars is 25 g. The seeds of the longer fruited greenhouse cultivars have a 1000-seed weight of 33 g.

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11. Biotechnological Applications in Cucumber Breeding:

According to Tatlioglu (1993) the following areas hold promise in cucumber breeding:

(i) Propagation and maintenance of desirable genotypes

(ii) Induction of somaclonal variation and its application in breeding

(iii) In vitro mutagenesis on haploids and diploids

(iv) In vitro selection for disease resistance, cold resistance, herbicide tolerance

(v) In vitro fertilization

(vi) Protoplast fusion between un-crossable species and genera

(vii) Production of transgenic plants

(viii) Embryo rescue

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12. Molecular Markers of Cucumber:

Since the advent of the polymerase chain reaction (PCR), many techniques are now available to detect DNA polymorphism. These polymorphisms are useful in plant genetics and breeding.

DNA fragment and sequence analysis has been used for:

1. Genetic mapping experiments

2. Diversity analysis and

3. Marker assisted selection

Isozymes, RAPD and RFLP have been used in cucumber for genetic mapping and diversity analysis by several workers as reviewed by Fazio (2002). The use of these markers has been limited due to narrow genetic base (3-8% on a per band basis) of cucumber. However, a higher level of polymorphism has been documented in cucumber using simple sequence repeats (SSR).

Fazio (2002) have quoted research results, where in one case four of the seven SSRs used detected polymorphism among the 11 cucumber genotypes examined which allowed for calculation of gene diversity values ranging between 0.18 and 0.64.

In another experiment, 20 polymorphic SSRs were developed and tested in cucumber to more critically determine their potential value for genetic analysis. Polymorphisms were detected at 12 of 20 SSR loci (60%) having two to five alleles at each SSR locus examined.

Moderately large molecular marker database is available in cucumber for isozymes, amplified fragment length polymorphisms (AFLP) and sequence characterized amplified regions (SCAR) markers.

Using these markers, it has been possible to have diversity assessment which has allowed for the partitioning of commercial germplasm into market classes, and the grouping of accessions in the U.S. National Plant Germplasm System into geographic proximity and country of origin.

However, their limitations for use in legal applications is due to their paucity, and degree of consistency. Fazio (2002) designed experiments to develop additional SSR, SCAR and SNP (single nucleotide polymorphisms) markers and to optimize reaction conditions for PCR in cucumber.

They added 110 SSR, 4 SCAR and 2 SNP markers to the array of previously identified markers and concluded that these markers will enhance the use of genetic markers in breeding, diversity analysis, variety identification and protection of cucumber germplasm.

The application of genetic markers for MAS follows three major recurring cycles regardless of marker types. Markers are identified as potentially useful, and subsequently developed into efficient and effective genotyping systems.

Polymorphic markers are then placed on a genetic map and associated with QTL through progeny analysis for their subsequent use in MAS. A total of 39 new markers (SCAR and SNP) have been developed and seven of these have proven effective when multiplexed in MAS Nam (2006). A list of useful molecular markers as summarized by Staub (2008)) is given in Table 26.3.

Sex Expression:

Genetic analyses and QTL mapping studies have indicated that several loci are involved in sex expression. In a population fixed for the M and A genes (i.e., segregating only at the F locus), Serquen (1997a) estimated five effective factors involved in gynoecious sex expression in each of two locations.

Disease Resistance Markers:

RAPD markers linked to the downy mildew resistance gene (dm) have been recorded. F₃ family populations (WI 1983G x Straight 8 population and Zudm 1 x Straight 8 population) were evaluated over five locations in North America and Europe to identify RAPD markers linked to dm. Five markers were identified 15 to 33 cM away from dm, which was subsequently mapped (0.1 and 1.9 cM away) and co-segregated with ten other markers.

A scab resistance gene (Ccu) has also been mapped. Resistances to papaya ring-spot virus (PRV) and ZYMV have been found to be closely linked to each other (2.2 cM), and were also tightly linked (~5.2 cM) to three AFLP markers. Given their relatively closely linkage associations with resistance genes, markers from these studies will likely be exploitable in MAS.

Use of Molecular Markers in Breeding:

The pyramiding of simply inherited genes (e.g., disease resistance) during germplasm enhancement is common, and has proven useful in the improvement of many crop species.

In cucumber, the pyramiding of disease resistance genes resulted in important inbred lines and population e.g., WI 2757, WI 1983, WI 5207, M-17, ‘Lucia’, ‘Manteo’, and ‘Shelby, NCWBP, NCMBP, and NCEP1, NCWBS, NCMBS and NCES1 as reviewed by Staub (2008). Molecular markers provide a tool for the dissection of quantitative variation and thus are potentially important to cucumber improvement.

Cucumber possesses several characteristics that are favourable to MAS including a small genome size, low chromosome number, and rapid life cycle (three cycles per year). In addition, fairly saturated genetic linkage maps have been developed, and QTL analyses have identified several genomic locations involved with important traits.

Based on these associations, three experiments have been conducted to provide evidence for the potential benefits of MAS during population development and inbred line extraction in cucumber.

Major Breeding Achievements:

The early cucumber breeding achievements reviewed by Lower and Edwards (1986) include:

(1) Development and use of disease screening technologies to develop resistant cultivars;

(2) Identification of biochemical pathways which regulate sex expression,

(3) Development and implementation of controlled pollination procedures, and

(4) Characterization of genetics which stabilize gynoecious sex expression.

Early selection for disease resistance was primarily performed in the open-field conditions where the presence of economically important pathogens was unpredictable.

In the 1970-1980’s scientific collaborations between Drs. C.E. Peterson (cucumber breeder) and P.W. Williams (pathologist) at the University of Wisconsin resulted in the development of seedling screening methodologies that allowed for the highly controlled, high-throughput assessment of pathogen resistance in segregating progeny.

This led to the development of germplasm (i.e. lines, hybrids, and populations) with resistance to several important diseases, and the eventual transfer of this technology to the private sector by the late 1980’s. Biochemical and comparative analyses of sex morphotypes in early 1960’s led to the discovery of pathways that regulate sex expression in cucumber.

This allowed for a better understanding of the biochemistry and physiology underlying sex expression that led to the ability to convert sex types for genetic manipulation. Chemical sex conversion allowed for more rapid cultivar improvement since plant types could be more predictably recovered from selection using more sophisticated selection techniques (e.g., reciprocal recurrent selection, tandem selection).

The combination of the ability to manipulate sex expression, methodologies for accurate prediction of disease resistance, and sophisticated selection techniques allowed for the development of sex stable gynoecious lines that could be crossed to produce hybrids with distinctly improved attributes.

Among those that provided such improvements were Drs. C. Barnes (Clemson University), B.Kubicki (Warsaw Agricultural University), H. Munger (Cornell University), C.E. Peterson (Michigan State University/USDA, ARS at the University of Wisconsin) and R.L. Lower (North Carolina State University/the University of Wisconsin).

Beginning in the early 1980s improved techniques for germplasm evaluation (e.g., improved field plot techniques) were documented and instituted for the systematic application of complex breeding systems resulting in improved germs-plasm (e.g., incorporation of exotic genes).

These techniques and publicly released germplasm (gynoecy, disease resistance) have been used widely by the seed industry. It is likely that genes for parthenocarpy will be increasingly used to increase yield and fruit quality in the next decade.

More recently, the creation of sophisticated computer algorithms and the development of molecular marker technologies has allowed for an in-depth quantification of some economically important metric traits, the development of unique genetic stocks, and an improved understanding of the cucumber genome.

Much of the U.S. research on molecular marker development, map construction and QTL analysis between 1980 and 2000 was partially funded by the seed industry. The use of new technologies (e.g., molecular markers) and genetic stocks [e.g., RIL and near- isogenic lines (NIL)] will likely increase in the future as they augment conventional breeding.

Their wide-scale use will result from the availability of precise phenotypic data (i.e., cost and time), the development of a highly saturated map with attending marker-QTL association (i.e. the identification of trait-linked SNP markers), and the ability to detect and characterize epistatic interactions as summarized by staub (2008).

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13. Varieties of Cucumber:

Straight Eight:

This is a variety of cucumber having cylindrical, very symmetrical fruits introduced by Ferry- Morse Seed Co., Detroit, Michigan, USA, in 1935. It was introduced to India and released by IARI, Regional Station, Katrain, Kullu Valley. It is early, suited for hills, and has dark green leaves, 20-25 cm long thick straight fruits with round ends. The fruits are light green. It is a heavy yielder.

Japanese Long Green:

This is a temperate variety specifically suited to hills and lower hills. It is extra-early with 45 days maturity. Fruits are 30-40 cm long, with white skin, white spines and light green crisp flesh. It has been released by IARI, Regional Station, Katrain.

Poinsette:

This is an American variety initially introduced and multiplied by National Seeds Corporation of India. Fruits are 20-30 cm long, dull dark green in colour. It is resistant to downy mildew, powdery mildew, anthracnose and angular leaf spot. It has been originally bred at Charleston, South Carolina, USA.

Pusa Sanyog:

This is an F₁ hybrid between a Japanese gynoecious line and Green Long Naples. It matures in 50 days. Fruits are 28-30 cm long, cylindrical and have dark green skin with yellow spines. It has been released by IARI, Regional Station, Katrain. This hybrid could never reach to market in India as seed could not be produced and marketed.

Pant Khira 1 (PCUC 28):

This is a selection from inbreds of indigenous germplasm at Pantnagar, released in 2001. The fruits are 20 cm long, cylindrical with light, white stripes. Yield is approximately 150 q/ha.

Pant Sankar Khira 1:

This hybrid (PCUC 28 X PCUC 8) developed at Pantnagar was released in 1999. Fruits are about 20 cm long. Yield potential is about 200 q/ha. Predominant hybrids in market by seed industry in India are Gypsy (white fruits), Malini (green fruits) and Harshini (green fruits).