A research paper on onion. After reading this you will learn about:- 1. Origin of Onion 2. Production of Onion 3. General Botany 4. Breeding Objectives 5. Qualitative Genetics 6. Cytology 7. Morphological, Biochemical and Molecular Markers 8. Day-Length Response 9. Breeding Methods 10. Selection Techniques 11. Biotechnology 2. Seed Production in India 13. Seed Production of Multiplier Onion and Others.
Contents:
- Research Paper on the Origin of Onion
- Research Paper on the Production of Onion
- Research Paper on the General Botany of Onion
- Research Paper on the Breeding Objectives of Onion
- Research Paper on the Qualitative Genetics of Onion
- Research Paper on the Cytology of Onion
- Research Paper on the Morphological, Biochemical and Molecular Markers of Onion
- Research Paper on the Day-Length Response of Onion
- Research Paper on the Breeding Methods of Onion
- Research Paper on the Selection Techniques of Onion
- Research Paper on the Biotechnology of Onion
- Research Paper on the Seed Production of Onion in India
- Research Paper on the Seed Production of Multiplier Onion
- Research Paper on the Varieties of Onion
- Research Paper on the Methods Used for Seed Production of Onion
- Research Paper on the Production of Hybrid Seed of Onion
Research Paper # 1. Origin of Onion:
The origin of onion still remains unresolved. According to Vavilov (1951) onion originated in the area of Pakistan. Jones and Mann (1963) have suggested Pakistan, Iran, and the mountainous areas to the north. Two types of onions are commercially grown in India.
(i) Common onion (Allium cepa var. cepa):
This is most important in commercial trade. Bulbs are large, normally single and plants reproduce through seeds.
(ii) Multiplier onion (Allium cepa var. aggregatum):
This produces bulbs of smaller size and they are several in number to form an aggregated cluster. Propagation is usually vegetative via daughter bulbs.
Research Paper # 2. Production of Onion:
Onion (Allium cepa, 2n = 2x = 16) is an important vegetable crop grown throughout the world. A global review of major vegetables shows that onion ranks second after tomato in area. Approximately 58 million tons of onions are produced on 3.2 million hectares globally.
India grows onion in approximately 7,56,000 hectares with production of about 12.16 million tons. The leading onion growing countries are China, India, USA, USSR, Japan, Spain, Turkey, Brazil, Italy, Egypt, Pakistan, Iran and Brazil. As per FAOSTAT (2006), the worldwide production of onion is given in Table 25.1.
Research Paper # 3. General Botany of Onion:
The flowering stalk of onion is an apical extension of the stem but without nodes and internodes. The growth of flower stalk ceases when umbels start to flower. The length of scape (flower stalk/seed stem) is controlled by genetic factors, long stalks Dwl, being dominant over dwarf, dwl stalks.
Spherical umbel which terminates the stalk may have 50 to 2000 flowers, common range being 200-600. Inbreds produce significantly less flowers per head than heterozygous plants. Each flower is attached to a slender pedicel.
Secondary umbels (those arising from branches that have already flowered) have approximately 30-50% of the number of flowers in the primary heads. The flower stalk of onion reaches to a final length of 1-2 m. The growth of flowering stalk is stopped when umbels start flowering.
The flowering structure is called an umbel which is an aggregate of many small inflorescences (cymes) of 5 to 10 flowers, each of which opens in a definite order causing flowering to be irregular and to last for 2 or more weeks. Each individual flower contains 6 stamens, 3 carpels united into one pistil and 6 perianth segments. The pistil contains 3 locules each of which has 2 ovules (Fig. 25.1).
The flowers also contain nectaries which secrete nectar to attract insects for cross- pollination. The flowers are protandrous and anthers shed pollen over a period of 3-4 days prior to the time when full length of style is attained. Thus, stigma becomes receptive 3-4 days after shedding of pollen grains and protandry leading to cross-pollination is favoured.
Selfing:
Selfing in onion is done only on a limited scale as it becomes difficult to maintain the inbred lines beyond S2 generation due to drastic inbreeding depression. Selfing is done by putting individual cages over the plants.
Flies are generally used to ensure pollination within cages. Sometimes it is convenient to enclose 2-3 umbels of the same plant in a muslin cloth bag before anthesis. After anthesis, the umbels are rubbed against each other daily for a few days to ensure self-pollination.
Crossing:
As soon as few buds in an umbel open, the whole umbel of the female parent is bagged in a muslin cloth bag. The flowers are removed daily for a few days until the peak flowering has reached after which buds are emasculated as they open and when sufficient buds have been emasculated the remaining young flower buds are removed.
The umbel of pollen parent covered by a muslin cloth bag is cut off and its stalk placed in a glass bottle filled with water. This bottle is fastened to a bamboo/wooden stake and fixed in soil close to the female parent. Female parent umbel (emasculated one) and the pollen parent umbels are now enclosed in the same common bag.
For a few days in the morning, the male umbel is gently rubbed over the emasculated umbel to ensure pollen shedding and cross pollination. A few common houseflies can also be introduced into the bag for pollen transfer.
However, it must be understood that cross-pollination through hand emasculation is extremely difficult. Therefore the second choice is to make what is called a fertile x fertile cross where 2 parental lines are caged together and then pollinated by hand or with common houseflies, blow flies or bees.
This method is effective and feasible only when F1 can be differentiated from both the parental lines in the bulb stage. In that eventuality seeds from both the parental lines should be planted in separate progeny rows and hybrid bulbs are identified from the bulbs resulting from selfs of the parental lines.
The F1 bulbs are harvested and caged together to give rise to an F2 progeny. During breeding programme, individual cages are used for selfing of individual plants, small cages for massing of progeny row (2×2 feet, made of nylon screen fitted on frame) and large cages (12 X 100 feet) for seed increase of elite lines for commercial testing.
Bees are used as pollinators in large cages. Small field isolation should be at an isolation distance of about 1600 m from other flowering onions to ensure production of non-contaminated seeds.
Research Paper # 4. Breeding Objectives of Onion:
1. High Yield
2. Longer bulb storage life
3. Resistance to diseases (purple blotch, basal rot, stemphyllium blight, bacterial storage rot), downy mildew
4. Resistance to insect pests (thrips)
5. Resistance to abiotic stresses (moisture stress, high temperature, salinity, alkalinity)
6. Bulb quality (size, shape, colour, pungency, firmness, dormancy, amount of soluble solids). Dormancy is important because onions are normally stored for longer time. High TSS (14%) is important for dehydration industry producing onion chips and powder. The amounts of s-alkyl cysteine sulfoxide precursors and the enzyme alliance contribute to the yield of sulphur compounds that constitute the pungency of the onion bulb.
7. Single centred bulbs specially for fresh market
Research Paper # 5. Qualitative Genetics of Onion:
As per information compiled by Pike (1986) the qualitative genes are listed in Table 25.2.
Khar and coworkers (2008) concluded that bulb colour in onion is controlled by at least five major loci (I, C, G, L and R). However, their segregation data also showed that red bulb colour is controlled by at least three loci.
Inheritance of Male Sterility:
The hybrid onion programme originated in 1925 with the discovery by Dr. H.A. Jones of a bulb (13-53) in material of ‘Italian Red’. Dr. Henery A. Jones, a former Professor of Vegetable Crops at the University of California, Davis, and Research Horticulturist with the US Department of Agriculture and later on with the Dessert Seed Company, El Centro, California is truly ‘Mr. Onion’ having spent a life time of productive research with onion as quoted by Whitaker (1979).
Bulb 13-53 was probably the single, most important onion bulb ever grown, because of its impact on onion breeding and plant breeding in general. It was a great pleasure that 13-53 could be propagated vegetatively as it produced bulbils in the umbels in place of seeds.
Also male sterility was found to be stable under a wide range of environmental conditions. The bulbils were saved and replanted to generate new plants which were crossed with other onion cultivars and the progenies were grown to understand the inheritance of this male sterile character.
The result proved that male sterility was determined by an interaction between a recessive nuclear gene and a cytoplasmic factor. The cytoplasmic factor was designated N for normal fertile cytoplasm and S for the sterile cytoplasm.
The nuclear genetic condition was designated as Ms/- for the normal fertile condition and ms/ms for the sterile condition.
Various combinations of genetic factors and cytoplasmic conditions are shown in Table 25.3:
The above table shows that male sterility can be maintained only when S msms (A line) is crossed as female parent with N msms as the pollen parent. The line N msms is also known as maintainer or B line.
If any male sterile line (S msms) is available and that is called as A line, the B line (N msms) can be identified by crossing A line with several other lines and if the progeny turns out to be completely male sterile in a particular case, that particular pollen parent will be considered as N msms type as shown here.
The above results show that line No. 2 is of N msms type. From this a combination of A and B lines can be developed by back-crossing as shown in Fig. 25.2 in which A line shall become identical to that of B line after 5-6 generations of back-crossing.
The results show that by BC5 the A line originally used has become almost identical to the recurrent parent B line (line No. 2) for all the characters except fertility.
Research Paper # 6. Cytology of Onion:
The Allium cepa species are diploid with basic chromosome number of x = 8 (2n = 16). Occasional tetraploids have also been reported. Of eight pairs of chromosomes, seven pairs are metacentric or sub-metacentric and one is satellited sub-telocentric chromosome with nucleolus organizer region.
Onion chromosomes are relatively large and spread easily during slide preparation for cytogenetical studies. Heterochromatic DNA is distinguishable from euchromatic DNA by Geimsa C banding, G banding, Q banding and fluorescence. The heterochromatic segments lie near the ends of chromosomes and show low quinacrine fluorescence and appear as dark bands with Geimsa staining. The satellites are also deeply stained.
There are morphological and cytological similarities between the species of section Cepa, but still strong crossing barriers exist between them. This prevents gene flow between the two even where sympatric distribution of two species occurs. Introgression of genetic material from wild to cultivated species is also difficult. Low success has been recorded in several interspecific hybridizations.
Research Paper # 7. Morphological, Biochemical and Molecular Markers of Onion:
Cramer and Havey (1999) have presented an extensive review on this subject.
Based on this the morphological markers of onion are given in Table 25.4:
Isozymes were identified as biochemical markers for onion during the 1980s. 24 different isozymes have been characterized in onion seeds and roots (Table 25.5).
The RAPD and RFLP markers are listed in Table 25.6.
8. Day-Length Response of Onion:
Onions are day length dependent, and therefore cultivars adapted to a certain latitude cannot be grown commercially in other latitudes. Therefore, different onion types depending on the latitude they grow can be distinguished into 4 classes, namely, long, long day (LLD), long day (LD), intermediate day (ID), and short day (SD) onion cultivars.
LLD cultivars are grown in Europe, north of Paris onwards, North America, South Canada, North East USA. LD cultivars can be found in Europe between Paris and Barcelona and in USA from Wisconsin in east to Montana in the west.
ID cultivars are grown in Europe between Barcelona and Morocco and Idaho in USA. The SD growing areas include North Africa from Morocco to equator, State of Taxas, Mexico and more to south and Indian subcontinent.
Research Paper # 9. Breeding Methods of Onion:
Breeding of Open Pollinated Varieties:
Open pollinated varieties are defined as genetically variable populations which are maintained and multiplied by mass pollination in isolation. The simplest population improvement method applicable to onion is mass selection in which 1-5% of mother bulbs of the desired characteristics are selected from a chosen basic source population at harvest or after storage. Mass selection is based on phenotypic selection.
The next generation is built-up by mixing the seeds of selected individuals of the current generation and subsequently a new selection cycle can start. This procedure can be repeated 3-5 cycles before commercial release of the improved selection.
Mass selection can be started on all kinds of basic materials, for example, commercial OP cultivars, fertile F1 hybrids, native OP cultivars, introduced OP cultivars or even elite partial inbred lines. Later on these selected bulbs are replanted in isolation to produce mass pollinated seed. These steps could be repeated till substantially improved new population is produced.
The success of this method depends upon:
(i) Genetic variability in the initial population, and
(ii) High heritability of the character under selection
A modification of this mass selection is stratified mass selection in which the field plot of the bulb crop is subdivided into equal sized area and mass selection is carried out into each area to make allowance for the effect of variable growing conditions within the field.
More refined open-pollinated breeding schemes are based on family selection under those situations where heritabilities are particularly low. In these schemes, selection is based on family mean performance rather than the individual bulb as the unit of selection. The families could be half-sib families or S1/S2 families. A generalized scheme is shown in Fig. 25.3.
A year-wise schematic breeding plan to breed open-pollinated onion cultivars as outlined by Pike (1986) is shown in Table 25.7.
Breeding of Hybrids:
The first commercial F1 hybrids were commercialized in early 1950s in USA. Since then F1 hybrids have increased their share more particularly in temperate regions as compared to the tropical one. Now a days, the global temperate onion seed market (long day types) includes over 90% of F1 hybrids, however, for the tropical regions, OP cultivars still predominate.
The reported advantages of F1 hybrids are as follows:
1. Exploitation of heterosis
2. Increase in uniformity
3. Relative ease to combine desirable traits specially those governed by dominant genes
4. Built-in variety protection
The latter two advantages are quite obvious and of considerable significance. However, advantages in terms of heterosis and ensuring uniformity are less evident. Breeding lines developed in long-day onion cultivars are mostly I2/I3 and are subsequently maintained by mass selection and therefore these parental lines do have considerable variation within themselves and hence the F1 hybrid produced using such parental lines do manifest some heterogeneity and variation.
In case of short-day onion cultivars, this problem is even worse as hardly any inbreeding is applied in the parental lines. Therefore, one can understand that uniformity in F1 hybrids is not necessarily higher as compared to OP varieties which have been developed though strong positive mass selection.
Presence of significant heterosis in onion is a matter of debate. However, hybrid onion breeding in India has got momentum with increased focus on this issue by IIHR, Bangalore and Directorate of Onion and Garlic Research (DOGR), Rajgurunagar, Pune. IIHR, Bangalore has released two hybrids Arka Kirthiman and Arka Lalima based on A/B line MS 48 (dark red, oval globe, medium in size) ar.d MS 65 (dark red, flat-globe, big in size).
These A/B lines and corresponding C lines are available to interested users on signing of certain MOU. DOGR – Pune is using these two male sterile lines and pursuing its own onion hybrid breeding programme. In nut shell, the system is based on CMS system based on A, B and C or R lines. The development of an F1 in onion requires the development of a male sterile line (A line), an inbred maintainer for A line (B line) and a pollinator male parent (C line).
These lines have the following genetic constitution:
A line = S msms
B line = N msms
C line = N MsMs
The procedure of developing A and B lines by backcrossing has been outlined in Fig. 25.2. It may be possible to find male sterile individuals within local adapted populations, and if so, appropriate pairs of male sterile/maintainer lines can be developed from these.
Alternatively, the breeder may start with known male sterile lines, usually from un-adapted cultivars made available to him by other onion breeders or genetic conservationists. These are converted to male sterile lines by crossing and backcrossing with selected maintainer lines.
Disregarding the original source of male sterile line, the objective is same i.e. the production of isogenic pairs of male sterile/maintainer lines of agronomic value.
This involves initial test-crossing and subsequent back-crossing. The initial test crosses are grown to flowering and only those test crosses are used in further back-crossing which are found to be male sterile (see Fig. 25.2).
After 4-5 backcrosses, the lines A (S msms) and B (N msms) become almost genetically identical except for sterility in line A and the other line remains fertile. A line is maintained by crossing with B line and B line is maintained through selfing.
Development of pollinator line i.e. C line (N MsMs) proceeds concurrently with that of male sterile lines. Usually open-pollinated selection/promising crosses are used as base population for this purpose. Generally 3 generations of selfing with field selection are carried out.
The outline of a basic onion hybrid breeding scheme is shown in Fig. 25.4. A schematic plan for breeding improved onion hybrids as outlined by Pike (1986) is given in Table 25.8.
Maintenance and Multiplication of Lines A, B and C:
The first increase of seeds of the lines A, B and C is done in insect proof cages followed by an increase in the open but not more than twice to obtain a high degree of purity in stock seeds of the parental lines. The seeds of lines A and B can be increased in same cage while the line C must be grown separately in another cage.
A nylon or wire net cage of 20 x 20 or 24 x 24 mesh measuring 6m x 3m X 2m with a small door at one corner for entrance and exit is suitable for this purpose. This size of cage can accommodate 4 rows of bulbs spaced 60 cm apart to produce about 1-2.5 kg of seeds. In the cage, bulbs of lines A and B should be planted in alternate rows with equal number of bulbs of each. Before flowering the cage is put over the plants.
The umbels should not touch the cage as it would allow the stigma to protrude and get contaminated with foreign pollen. About 3 days before the commencement of flowering a bee hive of medium size colony with a queen is placed inside the cage.
10% sugar is used as feeding medium. It is necessary to rogue out the pollen bearing plants from A line and pollen sterile plants if any from B line every morning before the dehiscence of anthers. The C line can be multiplied in the same way in a separate cage.
For planting large areas of commercial seed crop, it will be necessary to increase the seed of A, B and C lines in the open in isolation with an isolation distance of 3.2 km. The plot size is about 0.4 ha.
Research Paper # 10. Selection Techniques of Onion:
(i) Yield evaluation at uniform density
or
Yield estimation using simple covariance model when the range of density is small
(ii) Maturity – time of foliage collapse
(iii) Bolting – Good assessment by early sowing
(iv) Quality assessment –
Visual
Colour
Skin retention
Thickness
Shape
Uniformity
Single centre bulbs
Widely recognized phenotypic correlations between large size, softness, low pungency and poor storage ability should also be taken into account.
(v) Storage capacity – by storing bulbs in environmental conditions which closely follow those under which a commercial crop would normally be stored and subsequently recording the bulbs for:
marketability
losses due to rotting
sprouting
(vi) TSS – high TSS (16%) of importance in varieties suitable for dehydration
(vii) Disease resistance:
Pink root (Pyrenochaeta terrestris)
Fusarium basal rot (Fusarium oxysporum f. sp. cepae)
Leaf blight (Botrytis squamosa)
Neck rot (Botrytis allii)
White rot (Sclerotium cepivorum)
Smut (Urocystis magica)
Downy mildew (Peronospora destructor)
Puple blotch (Alternaria porri)
Black mould (Aspergillus niger)
Stemphyllium blight (Stemphyllium vesicarium)
Smudge (Colletolrichum circinas)
Bacterial rots (Erwinia carotovora)
(viii) Insect-pests:
Thrips (Thrips tabaci)
Maggots (Delia antiqua)
Important sources of resistance to major diseases and insect-pests are summarized in Table 25.9.
Research Paper # 11. Biotechnology of Onion:
In onion the first molecular map was developed by Havey and coworkers. The low density map was based on an intraspecific cross between cultivar Brigham Yellow Globe and Ailsa Craig. There were F2 derived 58 F3 lines and 116 markers were mapped in 12 linkage groups covering 1060 cM.
Van Heusden (2000) developed a low density AFLP map based on a selfed progeny of 65 plants based on an interspecific cross. In this case around 500 markers were mapped on both parental maps. QTLs have been added to the molecular maps. These QTLs are involved in the organo-sulphur and carbohydrate metabolism. Both these traits are of importance as they determine pungency and bulb hardiness respectively.
Fernandes Santos (2010) developed marker-assisted selection of maintainer lines in onion. Traditional onion hybrid production requires the development of maintainer and male sterile lines and also a pollinator line with good combining ability.
These authors reported the identification of maintainer and male sterile lines within Brazilian ‘Baia Periforme’ derived population ‘Alfa Sao Francisco’ associating random field pairing of male fertile plants with selected male sterile plants and PCR-based marker system monitoring S, T and N-cytoplasm’s.
Male sterile plants produced flowers with light green anthers which were easily detected in the field. A frequency of 2.0% male sterile plants was estimated in the Alfa Sao Francisco sampled population.
Male sterile plants produced the 5′ cob-marker 180-bp and the orf A 501-marker 473-bp fragments suggesting T cytoplasm type while the maintainer line produced only the 5′ cob-marker 180 bp. These identified lines will be important to develop tropical onion hybrids, well-adapted to Brazilian low latitudes and to future comparative studies with other onion cytoplasmic genie male sterility systems.
Double haploids technology provides a solution to the problem of considerable phenotypic variation in present day onion hybrids, which are based on the crossing of two partially inbred lines. Several scientists have shown that haploia induction in onion based on gynogenic embryo induction is possible as reviewed by Shigyo and Kik (2008).
However, induction of double haploids through microspore culture is considered more efficient. There are reports of limited success of DH production using microspore in onion. However, current limiting factors in the application of DH technology in onion breeding are the genotypic dependency of induction frequency and severe inbreeding depression.
Onion in general has proved difficult crop for genetic transformation/production of transgenics. Klein (1987) were the first to prove that onion could be transformed using a high velocity micro-projectile bombardment method. Eady (2000) demonstrated development of genetically stable transformants of onion mediated by Agrobacterium tumefaciens.
Transgenic onions have been produced carrying herbicide resistance. Herbicide resistance is based on the Basta gene making onions resistant to herbicide Round-Up. However, transgenic onion has not been commercialized as yet.
Research Paper # 12. Seed Production of Onion in India:
Good quality seed is very much essential to have increased production of good quality onion. Seed production of onion has long been and will continue to be a highly specialized industry. Years of experience are required to become conversant with many of the details such as best cultural practices, cultivar characteristics, roguing, plant protection measures, pollination, harvesting, threshing, seed cleaning, packing and storage.
Present requirement of onion seed in India is about 6500 tons. Major onion seed growing areas are in Maharashtra. Gujarat and Karnataka totaling 12000 – 14000 ha. Seed yield is about 500 kg/ha. Govt. departments supply about 500 tons, private companies 2500 tons and the rest is farmers own produce.
Onion is said to be biennial so far seed production is concerned. The seed production of all varieties which are produced in India is possible in plains. Following are the details which one must know for good quality seed production.
Research Paper # 13. Seed Production of Multiplier Onion:
Since this crop is mainly propagated vegetatively, well grown, uniform bulbs of a particular variety are selected and bulblets are separated. Healthy and uniform bulblets are selected and used for planting. The planting details including cultural practices followed for production of planting materials are same as that for production of bulbs for consumption. The seed of multiplier type is also produced as in common big onion.
Seed Certification:
The purpose of seed certification does not differ from other crops. It is taken up to maintain and make available, to the public, through certification high quality seed propagating material of superior varieties so grown and distributed as to ensure genetic identity and genetic purity.
The different steps/requirements of certification are mentioned as under:
Minimum Seed Certification Standards for Common Onion:
I. Application and Amplification of General Seed Certification Standards:
The general seed certification standards are basic and together with following specific standards constitute for certification of onion seed.
II. Land Requirement:
Land to be used for seed production of onion shall be free of volunteer plants.
III. Field Inspection:
A. Mother Bulbs Production Stage:
A minimum of two inspection shall be made as follows:
(a) The first inspection shall be made after transplanting of seedlings in order to determine isolation, volunteer plants, off types including bolters and other relevant factors.
(b) The second inspection shall be made after the bulbs have been lifted to verify the true characteristics of bulbs.
B. Seed Production Stage:
A Minimum of four inspections shall be made:
(a) The first inspection should be made before flowering in order to determine isolation, volunteer plants, off types including bolters and relevant factors.
(b) The second and third inspection shall be made during flowering to check isolation, off types and other relevant factors.
(c) The fourth inspection shall be made at maturity to verify the true nature of the plant and other relevant factors.
IV. Field Standards:
(A) General Requirements:
Isolation:
Onion seed fields shall be isolated from the contaminants shown in column 1 of the table below by the distance specified in columns 2, 3, 4 and 5.
(B) Specific Requirements:
* Maximum permitted at second inspection at mother bulb production stage.
** Maximum permitted at and after flowering at seed production stage.
V. Seed Standards:
Minimum Seed Certification Standards for Onion Hybrids:
I. Application and Amplification of General Seed Certification Standards:
(a) The general certification standards are basic and, together with the following specific standards constitute the standards for certification of hybrid onion seed.
(b) The general standards are amplified as follows to apply specifically to the hybrid seeds of onion.
II. Eligibility Requirement for Certification:
(a) An inbred line to be eligible for certification shall be from such a source that its identity may be assured and approved by the certification agency.
(b) Hybrid seed to be eligible for certification shall be the progency of two approved, inbred lines, one of which shall be male sterile.
III. Classes and Sources of Seed:
(a) An inbred line shall be relatively true breeding strain resulting from self-pollination with selection.
(b) The foundation class seed shall consist of an approved male sterile line to be used as a female parent and an approved inbred line to be used as a male parent for the purpose of producing hybrid seed.
(c) A male sterile line shall be a strain (A) carrying cytoplatmic genetic male sterility, which sheds no viable pollen and is maintained by the normal sister strain (B) which is used as pollinator.
(d) The certified class seed shall be the hybrid seed to be planted for any use except seed production.
IV. Land Requirements:
Land to be used for seed production of hybrid onion shall be free from volunteer plants.
V. Field Inspection:
A. Mother Bulb Production Stage:
A minimum of two inspection shall be made as follows:
a) The first inspection shall be made after transplanting of seedling in order to determine isolation, volunteer plants, off types including bolter and other relevant factors.
b) The second inspection shall be made after the bulbs have been lifted to verify the true characteristics of bulbs.
B. Seed Production Stage:
A minimum of four inspections shall be made as follows:
(a) The first inspection shall be made before flowering in order to determine isolation, volunteer plants, outcrosses, planting ratio, errors in planting and other relevant factors.
(b) The second and third inspections shall be made during flowering to check isolation, pollen shedding umbels, off-types and other relevant factors.
(c) The fourth inspection shall be made at maturity to verify the true nature of umbels and other relevant factors.
VI. Field Standards:
A. General Requirements:
Isolation:
Onion seed fields offered for certification shall be isolated from the contaminants shown in column 1 of the table below by distances specified in columns 2, 3, 4 and 5 of the said table.
(B) Specific Requirements:
VII. Seed Standards:
Same as in case of common onion given above except for genetic purity which is 95% for hybrids.
Research Paper # 14. Varieties of Onion:
A number of onion varieties have been evolved in India by different Universities and Institutes for cultivation in different regions. There is, however, enough scope to develop white varieties with high TSS suitable for dehydration, short day yellow, varieties for export and varieties resistant to diseases and insect pests.
Varieties developed by different Universities/Institutes have been classified in two groups i.e. common onion and multiplier onion. The common onion has been grouped in three sub-groups based on colour i.e. red, yellow and white.
Big Common Onion:
Red Coloured Bulbs:
Punjab Selection:
Developed at the PAU, Ludhiana and identified in 1975 for zones IV, VII and VIII, plant height 55-65 cm, 6-9 leaves per plant, bulbs quite firm with good keeping quality, TSS about 14%, average yield 200 q/ha, recommended for rabi season.
Pusa Ratnar:
Developed at the NBPGR, New Delhi and identified in 1975 for zones IV and VII, plant height 30 cm, leaves dark green with waxy bloom, bulb more exposed above ground at maturity, colour dark red, obvate to flat, globular shape, less pungent and neck drooping, storage quality average, plants mature in 125 days after transplanting, TSS about 11-12%, average yield 300 q/ha, recommended for rabi season.
Pusa Red:
A very popular short day variety developed at IARI, New Delhi and identified in 1975 for zones IV, VII and VIII, plant height 30 cm, bulb medium in size, average weight 70-90 g, bronze red in colour, flat to globular shaped and less pungent, keeping quality good, plants mature in 140-145 days after transplanting, TSS about 12-13%, average yield 250 q/ha, can be grown both in late kharif and rabi season in Maharashtra.
N2-4-1:
This variety developed during early sixties for rabi season by the Department of Agriculture, Maharashtra and identified in 1985 for zones IV, and VII, average bulb diameter 4-6 cm, brick red colour, shape globe, pungent in taste, keeping quality good, plants mature in 140-145 days after transplanting, TSS about 12-13%, average yield 300-350 quintal per hectare, recommended mainly for rabi season but can be grown both in late kharif and rabi seasons in Maharashtra.
Pusa Madhavi:
Selection made at the IARI, New Delhi from a local collection of Muzaffarnagar, identified in 1987 for the Indo-gangetic plains, bulbs medium to large in size, light red in colour and flattish round in shape, keeping quality good, plants mature in 130- 145 days after transplanting, average yield potential 300 q/ha, recommended for cultivation during rabi season.
Arka Niketan:
Developed through mass selection from a local collection (IIHR-153) at the IIHR, Bangalore, identified in 1987 for zones VII and VIII, bulbs globular with thin neck, attractive light red colour, 4-6 cm in size, TSS 12-14%, pungency and dry matter also high, keeping quality good, matures in 145 days after transplanting, average yield 340 q/ha, though recommended for rabi season mainly, can be grown in late kharif also in Maharashtra.
Agrifound Dark Red:
Selection made by National Horticultural Research and Development Foundation formerly known as Associated Agricultural Development, Foundation Nashik, from a local stock of kharif onion grown at Nashik. Identified in 1987 for the plains of Sutlej- Ganga, bulbs dark red, globular in shape, 4-6 cm in size with tight skin, moderately pungent, TSS 12-13%, matures in 95-110 days after transplanting, average yield 300 q/ha, keeping quality average, is recommended for kharif season.
Kalyanpur Red Round:
Developed at Chandra Sekhar Azad University of Agriculture and Technology, Kanpur, is a popular variety of Uttar Pradesh. Bulbs are bronze red in colour, globular in shape and moderately pungent, TSS 13-14%, matures in 150-160 days from the date of sowing, keeping quality good, average yield 250-300 q/ha.
Agrifound Light Red:
Developed at National Horticulture Research and Development Foundation Nashik, by mass selection from a local stock of rabi onion grown in Dindori area of Nashik, the bulbs of globular shape with tight skin, light red colour and 4-6 cm in size, TSS 13%, has good keeping quality, maturity 160-165 days after sowing, average yield 300-325 q/ha, notified in 1996, recommended for growing during rabi all over the country, can be grown in late kharif season also in Nashik district of Maharashtra.
Hisar 2:
The variety developed by Haryana Agricultural University, Hisar, the bulbs bronze red in colour and globular in shape with tight skin and sweet to pungent taste, TSS 11.5-13%, maturity in 165 days after sowing, bulbs yield 200-250 q/ha, suitable for cultivation during rabi in Haryana and Punjab.
Punjab Red Round:
Developed at PAU, Ludhiana, the plant is medium tall with green leaves, bulbs shining red in colour, medium to large in size and round in shape with thin neck, is early maturing and high yielding variety, average yield about 300 q/ha.
Arka Pragati:
Developed at IIHR, Bangalore, is improvement over the local collection IIHR-149 from Nashik and developed by mass selection method, variety has attractive pink colour, globe shaped bulbs of uniform size with thin neck, high pungency and early maturity, matures in 140-145 days after sowing, its yield is about 200 q/ha.
N-53:
The variety developed by the Department of Agriculture (Maharashtra) during 1960 and released at the national level in 1984, is very popular for growing during kharif season all over the country. Bulbs flatish round in shape, red in colour, medium to large and mildly pungent, TSS 11-12%, keeping quality poor, ready for harvest in 90-100 days after transplanting, average yield 250 q/ha.
Baswant-780:
Developed at MPAU, Rahuri (M.S.) by selection and improvement over N-53 for kharif season, released in 1986, bulbs crimson red in colour, globe in shape and mildly pungent, less bolting with less twins, TSS 12%, keeping quality average, ready for harvest in 100-110 days, average yield 250 q/ha, suitable for kharif season in Maharashtra.
Udaipur-101:
Developed at RAU, Bikaner Campus, Udaipur, bulbs dark red, flattish globular in shape, sweet but slightly more pungent, TSS 12-14%, matures in 150-160 days after sowing, average yield 200-300 q/ha, recommended for Rajasthan and other adjoining states during the rabi season.
Arka Kalyan:
Developed at 11 HR, Bangalore through selection in local cultivar of Kadwan region of Maharashtra, dark red, globular bulbs, pungent, 80-100 days maturity, 240-300 q/ha, suitable for Kharif season.
Bhima Super:
Developed by Directorate of Onion and Garlic Research, Puna through selection in Basawant 780, 95% single centre bulbs, suitable for Kharif and late Kharif in MS, Karnataka, Gujarat, globe shaped, TSS 10-11, 300-400 q/ha, 110-115-days maturity.
Bhima Raj:
Suitable for Kharif and late Kharif, dark red, globular bulbs, 250-300 q/ha, good yielder in Rabi in north India, 80-90% single centre bulb.
Bhima Red:
Selection from Basawant 780, suitable for Kharif and late Kharif, 200-250 q/ha in Kharif and 450-500 q/ha in late Kharif, dark red bulbs, 80-90% single centre bulbs.
Upaipur-103:
Developed at RAU, Bikaner Campus, Udaipur, bulbs red but lighter than Udaipur 101, oblate globular shape, sweet but slightly more pungent, TSS 10.5-13%, matures in 150-165 days after sowing, average yield 250-300 q/ha, also recommended for Rajasthan and other adjoining states during rabi season.
Line-28:
The onion selection made by NHRDF from Patiala area of Punjab, leaves dark green, erect with little waxy coating, bulbs dark red coloured, globular round in shape and have reddish thick inner scales, fleshy thick, scales with red colour, bulbs have 2-3 outer scales, moderately pungent in taste with TSS of 13-14%, can be grown in Northern India as well as Central and Western India, suitable for rabi season.
Phule Samarth:
The selection made by MPKV, Rahuri for cultivation during kharif and early rabi (rangda) seasons, recently released by the state government, bulbs are dark red in colour, globular round shape and thin neck and early maturity, bolting and double bulb percentage less compared to other varieties.
Yellow Coloured Bulbs:
Early Grano:
Introduced by IARI, New Delhi from USA, produces globular bulbs of yellow colour with very mild pungency, size 7-8 cm, crop matures in 95-110 days, TSS 6-7%, poor in storage, average yield 500-600 q/ha, suitable for cultivation in plains during kharif and rabi seasons for salad purpose.
Spanish Brown:
Long day variety developed by IARI, Regional Research Station, Katrain, Kullu, bulbs attractive brown in colour with mild pungency, TSS 13-14%, crop maturity in 160-180 days, good in storage at hills, yield 280-300 q/ha.
Phule Suvarna:
Developed by MPKV, Rahuri (M.S.) and recommended in 1997, year round plantation possible, yellow coloured variety suitable for export to Europe, Australia and America, bulb size medium to big, less pungency, TSS 11.5%, excellent keeping quality (4-6 months), suitable for rangda and rabi season, yield 240 q/ha.
White Coloured Bulbs:
Pusa White Round:
Developed from line 106 at IARI, New Delhi, identified in 1975 for zone IV, VI, and VIII, bulbs white in colour and roundish flattish in shape, TSS 12-13% drying ratio 8:1, crop matures in 130-135 days after transplanting, yield 300-325 q/ha, good in storage.
Pusa White Flat:
Developed by selection form local material at the IARI, New Delhi, identified in 1975 for zone IV, VI, and VII, bulbs attractive white, flattish round with 12-14% TSS and drying ratio 9:1, maturity in 130-135 days after transplanting, good in storage.
Punjab-48:
A variety bred at the PAU, Ludhiana and identified in 1975 for the sub-humid plains of Sutlej-Ganga, bulb flattish round and white in colour, has very good texture and flavour, suitable for dehydration, good in storage, average yield is about 300 q/ha.
Udhaipur-102:
The developed at RAU Bikaner Campus, Udaipur, bulb are white in colour, round to flat in shape with low percentage of small bulbs, size 4.5-6.5 cm, matures in 120 days after transplanting, TSS about 12%, average yield is 300-350 q/ha.
N-257-9-1:
A variety developed by Department of Agriculture, Maharashtra by selection from local material and maintained at the MPAU, Rahuri and identified in 1984 for Zone VI, bulbs globular shaped and white in colour, high yield potential with good keeping quality, suitable as a rabi crop in onion growing regions of Maharashtra, average yield 250 q/ha.
Phule Safed:
Developed by selection from local material from Kagal at MPKV, Rahuri and released in 1994, bulb globular in shape with white colour, TSS 13%, suitable for dehydration, average yield 250-300 q/ha.
Agrifound White:
Selection made by National Horticultural Research and Development Foundation, Nashik from local stock of white onion grown during rabi season in Named area of M.P., bulbs globular in shape with tight skin, silvery attractive white colour and 4-6 cm diameter, TSS 14-15%, good keeping quality, matures in 160-165 days after sowing, yield 200-250 q/ha, suitable for rabi season, good variety for dehydration.
Agrifound Rose:
Developed at National Horticultural Research and Development Foundation. Chickballpur, formerly known as Associated Agricultural Development Foundation, pickling type, grown in Kolar and Bangalore districts of Karnataka and Kuddapah district of Andhra Pradesh exclusively for export, bulbs flattish round in shape, deep scarlet red in colour and 2.5-3.5 cm size, matures in 95-110 days from sowing, TSS 15-16%, average yield 190-200 quintals per hectare, suitable for growing in kharif season in Kuddapah district and in all the three seasons in Karnataka.
Akra Bindu:
Developed for export markets from a local collection from Chickballapur (Karnataka) through mass selection at IIHR, Bangalore, bulbs deep pink colour, small size (2.5-3.5 cm) of flattish globe shape, bulbs free from premature bolters and splits and have high pungency and TSS 14-16%, matures in 100 days from sowing, average yield 250 q/ha.
Among local types, Sukhsagar (red bulb) and Pillipati Local (red bulb) have been accepted in West Bengal and Gujarat respectively. From Pvt. Seed Companies, Cyclone (Enza Zaden), Lucifer (Bejo Sheetal), Colina (Nunhems), Reforma (Bejo Sheetal) and Mercedes (Seminis) have been marketed as introduced types.
Multiplier Onion:
Co-1:
Developed at TNAU, Coimbatore, medium plant height with light green leaves, medium size bulblet of red colour, 7-8 bulblets per plant, average weight of bulblets 55-60 g per clump, yield 100 q/ha, adapted throughout Tamilnadu, maturity in 90 days, fairly pungent with medium TSS.
Co-2:
A variety bred at TNAU, Coimbatore, medium plant height with light green cylindrical foliage, moderately bigger size bulblets of crimson colour, 7-9 bulblets per plant, medium bulb weight.
Research Paper # 15. Methods Used for Seed Production of Onion:
1. Seed to Seed Method:
This is also called “in situ” method. The bulb is left and allows to bolt and flower in the same field where the transplanting was earlier done. The seeds are sown in the nursery from June to August and transplanting is done from August to October. Bolting starts in January- February and seeds are ready for harvest by mid-May.
Advantages:
1. Low cost of the production
2. Early maturity
3. No need to store the bulb
Disadvantages:
1. Not possible to select the bulb and thus high quality seed production not possible
2. Low seed yield
2. Bulb to Seed Method:
When bulbs are lifted after proper selection they are replanted in the field, it is known as bulb to seed method.
There are two methods in this:
(a) Annual Method:
The seed are sown in June-July, transplanting in August-September. Bulbs are ready by November- December. The bulbs are lifted and cured. Selected bulbs are replanted in another field after about fifteen days that is from end of November to mid of December.
The bolting in this case starts in February and seed are ready by May. Since the method takes one year, it is called annual method. Seed for N-53, Agrifound Dark Red, Baswant-780 and Arka Kalyan as well as Phule Safed varieties are produced by this method.
(b) Biennial Method:
For the main season crop the nursery transplanted in December end or in the first week of January. The bulbs are ready by the end of April-mid of May. The selected bulbs are stored up to mid-October and are again selected and planted in the well prepared fields. Since this takes about one and a half year, it is called as biennial method. Seeds of Pusa Red, N-2-4-1, Agrifound Light Red and other Rabi varieties are produced by this method.
Advantages:
1. Selection of true type bulbs possible and hence quality seed produced by this method
2. Higher yield obtained
Disadvantages:
1. It takes more time
2. More cost is involved
Since bulbs to seed methods is the only method which gives pure and high quality seed, details of this method are discussed in the following paragraphs:
Cultural Operations for Production of Mother Bulbs:
These are the same as required for bulb production for commercial use.
Storage of Mother Bulbs:
In case of one year method, selected bulbs are planted in well prepared beds after curing for 10-15 days, but under biennial methods bulbs are stored from June to September in well ventilated rooms called chawls in Maharashtra and kupe in the North that is Haryana. In Maharashtra, the sides of chawls are prepared with the help of bamboo spaced at the distance of 2-3 cm. These are about 1.20 cm wide and 1.5 m high. These are, however, extended to any length.
Thatched roof is prepared on top to protect the bulbs from rain/sunlight. The roofs are also made up of tiles or asbestos sheets. The flooring is done with the help of the soil mixed with the sand and small stone pieces.
After this bamboo pieces or sticks made up of any other wooden material are placed on the floor and onions are loaded on this. This types of flooring saves onion from rotting, as it maintains low temperature and also dampness is avoided. It is, however, better to provide ventilation from bottom to avoid loss due to decay.
The kupe are almost similar type except that at the bottom also floor is prepared with the help of bamboos and kept about 30 cm height to have easy air circulation from the bottom. The best method of storing the onion without much damage is to store in cold storages which are erected especially for the onion (Temp 0-2° C and RH 65-70%).
It has been observed that if the onions which were stored at 0-2° C temperature in the cold storage and are planted directly in the field, the sprouting is very slow. Sometimes the sprouts are also less. For proper sprouting, the temperature in the cold storage is raised to 15-20° C about 10-15 days before the onions are removed.
It has also been found that the plants developed from the bulbs which have been stored at the optimum temperature of about 15° C temperature usually flower earlier and produce more seed yield than plants developed from the bulbs that have been stored at higher or lower temperature.
Cultural Operations for Seed Productions from the Bulbs:
Temperature throughout the year and day length during the season set broad limits to the areas that are suitable for seed production. When the seeds are produced from the bulbs that have been raised during the preceding year, the length of the growing season that is required for the crop is relatively short. The long days early in the season which are characteristic of high latitude favour rapid bulbing rather than flowering.
A fairly cool temperature over the considerable time usually while bulbs are in storage or overwintering in the field, conditions the plant for seed stalk formation. Temperature of 4.5 to 14°C are favourable for this conditioning. Longer these conditions, more flower stems will each plant produce and more flowers will be there in each umbel.
High temperature early in the season or at the time of the year when the young scapes are flowering causes many plants to produce bulbs instead of the flower stems and reduces greatly the number of the flowers or umbels. The best seed producing areas are those where there is low humidity.
Long rainy periods or heavy dew and fog, favour the development of downy mildew, stemphylium blight and purple blotch. While onions are in flower, clear and bright days are necessary to ensure insects activity for pollination.
It is equally important to have hot and dry weather during the harvesting, curing and threshing of the seed. Onion seed crop can be grown on a wide range of soils but very light sandy soils are avoided. Heavier soils should be preferred because they are cooler, have a high water retaining capacity and are more productive.
Selection of Bulb, Land Preparation and Planting:
Medium size bulbs (4.5-6.5 cm for big onion and 2.5-3 cm for small onion) are selected for planting for getting maximum yield. Last week of October to first fortnight of November is recommended for planting of bulbs in rabi onion varieties. Kharif onion bulb is available in November end or December beginning.
The bulbs could be planted by 15th December from the same crop and this way seed production can be taken within a year. Maharashtra, Madhya Pradesh, Karnataka, Gujarat, and Rajasthan states are considered to be better in respect of seed production without much damage from diseases and also yield is better.
Seed should never be produced in plains of Tamilnadu because bulbs do not get conditioning thereby number of sprouts are 2-3 as also whatever seeds are formed, these are not of good quality. Big size bulbs increase in number of sprouts/scapes and thereby produce more seed yield per hectare. It is, however, better to select medium size bulbs for economic seed production.
The main objection in small size is that they do not express the true shape of the varieties, making it difficult to rogue off types. The best planting time is October end to November end. If the planting is done early, the crop gets damaged from rain which comes in March/April. If late planting is done, the vegetative growth is less and due to this there are lesser number of flowers and also the seeds in umbel.
Attack of the thrips is also more. Selected bulbs of 4.5 cm-6.5 cm size are planted in well prepared field at the distance of 45 cm x 30 cm in normal loam to clay loam soil. If the spacing is kept closer because of the less ventilation, drying of the field is slow after irrigation thus heavy dew and the rains favour the disease. In sandy soils spacing may, however, be reduced to even 30 x 30 cm.
The depth is kept 7.5 cm. Irrigation is given immediately after transplanting. 25 to 30 quintals bulbs for big and 15-18 quintals for small onions are enough to plant one hectare area. Seed crop also needs to be fertilized for optimum yield. 80-100 kg of N, 60 kg of P2O5 and 50 kg of K2O per hectare is recommended in general.
If soil is low in nitrogen 120 kg N per hectare should be applied. 50% N and full dose of P and K should be applied as basal dressing and rest 50% of N should be applied as top dressing after 45 days of planting. Weedicides like Pendimethalin @2.5-3.5 lit/ha or Oxyflurfen @ 0.15-0.25 lit/ha with 1 hand weeding at 45 days give good control of weeds.
The cutting of 1/3 top of bulb and dipping for 5 minutes with 1% carbendazim + 1% potassium nitrate or 1% potassium orthophosphate is found to give increased seed yield in kharif onion variety Agrifound Dark Red. Further by cutting 1/3 of top portion of each bulb before planting one can select for single centre bulbs.
Irrigation and After Care:
Irrigation is given at an interval of 7-10 days depending upon the types of soils and season. Earthing is done after two months of the transplanting to avoid lodging. Weeding and hoeing are done as per requirement. Specific precautions are taken in irrigating the crop more frequently during flowering and seed setting.
In variety Agrifound Light Red irrigation at 10 days interval gives higher yield. Saturating soil surface and keeping it wet, however, should be avoided as this facilitates the development of neck rot and other diseases which cause seed stalks to die prematurely. The use of drip irrigation and mulching with wheat straw or paddy straw gives higher seed yields.
Isolation:
Onion is highly cross pollinated, wherever there is going to be seed production of different varieties, an isolation distance of 500 meters is kept between two varieties. This is necessary in case of the pedigree or certified seed production. For stock or foundation seed production, the isolation distance should be 1000 m.
Roguing:
Diseased and off type plants are rogued out before flowering for quality seed production. Because onion is highly cross pollinated that too by bees, it is necessary to have good number of bees for good pollination and seed setting.
Following points should be taken care of for improving the pollination:
1. Bee colonies are kept in field.
2. Irrigation should be given more frequently at the time of flowering and seed setting.
3. Only safer insecticides are sprayed during flowering.
4. In case of more wind, sometimes bees do not sit on the flowers. To avoid this, plant wind breaks all around the field.
Harvesting and Curing:
All seed heads do not mature simultaneously, therefore, harvesting is done in instalments. When seed inside capsules become black and 20 to 25% black seeds are exposed the umbels are cut with 10 to 15 cm stem attached. The umbels are spread on ground or canvas to avoid attack of mould and to obtain uniform dying.
Threshing and Cleaning:
Seed is threshed when the capsules are brittle and break readily. Threshing is done by flairing method. Cleaning is done by air screen machine by using 1/14 x 1/2 as grading screen.
Drying, Packing and Storage:
For safe storage seeds are dried to a level of about 6% moisture and then packed in aluminium foil or tins which are considered to be moisture proof. Seeds are then stored in air-conditioned and dehumidified store having temperature of about 16-20° C and relative humidity of 30-40%.
In hot and humid climate the viability of onion seed is lost within a year. It has been reported that onion seed if dried to 6.0% moisture level and stored in sealed containers, the life of seed may be prolonged to 3-4 years without loss in germination.
Seed yield:
6 to 8 quintals per hectare is the normal yield. Some time when weather is favourable as high as 10-12 q yield can be obtained from one hectare area.
Research Paper # 16. Production of Hybrid Seed of Onion:
The hybrid seed is produced in the open in an isolated field. The bulbs of A and C lines are planted alternately in a ratio of 4 rows of the A line to 1 row of the C line. The conventional ratio is 8: 2. However, a higher planting ratio gives better seed yield. In UK 9: 1 ratio has been found good. The flowering of A and C lines must synchronize.
If needed synchrony can be achieved by:
(i) Adjustment of storage temperature (warmer storage temperature within the range of 0-12°C causes earlier flowering)
(ii) Adjustment of planting dates (early planting causes earlier flowering)
In large fields 3-4 beehives/0.40 ha are placed to ensure large population of honey bees for cross pollination. Daily roguing of pollen shedding plants and other off type plants in A line in the morning before dehiscence of anthers is essential.
Frequently the seed from the pollinator row is discarded. The male plants are destroyed before the female is harvested to avoid contamination of the female seed with accidentally harvested seed from the male parent.
If both the male and female parents are to be harvested the male should be harvested before the female and earlier than the optimal harvest time so that the more valuable hybrid seed can be harvested at its optimal time and free from contamination by accidentally harvested seed from the male.
Hybrid seed production often produces lower seed yields per unit of land occupied by the female line than the yield of open-pollinated cultivars. The process of stabilizing a male sterile line can reduce its vigour and fruitfulness due to inbreeding depression.
Studies have attributed this to:
(i) Reduced number and size of umbels
(ii) Reduced period of receptivity in individual flowers
(iii) Ovule abortion
This tendency may be overcome by using single cross females without losing too much uniformity in the hybrid.
Seed Production Method for Hybrids:
This is based on Pathak (1999). Hybrid onion seed for commercial use is produced either using the bulb-to-seed method or by the seed to seed method. The bulb-to-seed method is more commonly used because of its high seed yield, and ease in roguing of unwanted plants before producing seeds in the next generation. The seed-to-seed method requires larger quantities of basic seed, and relatively pure basic seed stock, because roguing is difficult.
Seed Increase of Parental Lines:
The initial seed increase of A, B, and C lines is carried out in isolation, usually in screen cages. The A and B lines are generally planted in the same cage, while the C line is planted separately in another cage. The size of the cage depends on the amount of seed to be increased. Equal numbers of bulbs of the A and B lines are usually planted in separate rows in the cage.
At anthesis a net cage cover is put over each group of plants. It is essential to stake and tie up the seed stems on the outside rows of the cages to prevent them from touching the net and becoming contaminated with foreign pollen carried by insects.
Once anthesis starts, each plant is checked carefully for fertile pollen production. It is essential to rogue out any pollen-fertile plants in the A line and pollen-sterile plants from the B line. This will ensure the purity of seed.
Once all of the plants are checked, a beehive is placed inside the cage to help ensure pollination. Seed harvesting is performed very carefully in lines A and B to avoid mechanical mixing. Line C is also maintained in isolation by use of a net cage.
For large-scale seed increase, the parental lines are planted in the open field, under proper isolation. The isolation distance between two fertile onion lines should be greater than 3 km. Stock seed of the A line is produced by planting alternate rows of A and B lines. The field size is adjusted based on the seed requirement.
Roguing of off-type plants is carried out in the same way as in the cages. Seeds of the C line are increased in a similar manner, taking care to isolate the site. It is advisable to keep bee hives around the seed production plot to insure pollination.
Production of F1 Hybrid Seed:
Identification of a suitable location is the key to commercial seed production. The seed production area should have low humidity, mild, cool temperatures during initial crop growth, followed by increasing temperatures at later stages of growth. Long, rainy periods or heavy dew and fog increase the risk of diseases like downy mildew, purple blotch and stemphyllium leaf blight.
While the crop is in flower, clear, bright days are necessary to insure high insect activity for pollination. It is equally important to have hot, dry weather during the harvesting, curing, and threshing of the seed.
In Europe, major onion seed production areas are located in Italy, southern France, and Spain, because of warm, dry summers which help in the production of disease-free seed crops. In India, the major onion seed production area is in the state of Maharashtra which is also the major onion producing state.
Mother Bulb Production:
Mother bulb production practices are generally the same as commercial production practices, except in some cases a higher seed rate is used to prevent the production of very large bulbs, which generally do not store well. Special care should be taken to control major diseases affecting the crop.
Seedlings of lines A, B, and C are generally planted in the same field but well separated from each other and with proper labeling to avoid mechanical mixing. Bulb size generally plays an important role in seed production. The larger the mother bulb, the higher the seed yield per plant but they do not store as well.
Smaller size mother bulbs, when planted at higher plant densities in the field, produce increased seed yield per hectare, despite a reduction in per plant yield. The desired size of mother bulb also depends on the cultivar or inbred parent; however, bulbs of 4 to 6 cm in diameter are preferred.
Mother bulbs are harvested when the tops have fallen. Curing the bulbs for approximately one week will increase their storage life. Tops are pruned after the neck is dried. It is very important to label each inbred line properly before storage. Utmost care must be taken during handling and storage to keep the inbred lines separate. Bulbs are stored until they are planted for seed production.
Storage of Mother Bulbs:
Mother bulbs are stored under similar conditions as market crops. Ambient temperature storage is preferred among overwintering cultivars, as these are vernalized by cold ambient conditions after the planting of mother bulbs.
Several tropical areas generally lack enough cold weather to induce vernalization, and the cultivars grown in these areas do not generally require very cold temperatures for flower induction. These bulbs are stored at ambient high temperatures before planting in the cool season.
Generally, the ideal storage temperature to prevent sprouting and rotting is 0°-3°C, however, this is not suitable for inducing flowering. Storage conditions of the mother bulbs, especially temperature and duration, generally affect flowering date, number of umbels, and finally seed yield.
Three months storage of bulbs at 10°C is sufficient to induce flowering in most cultivars. Mother bulbs for seed production should be planted when the temperature is low to avoid the inflorescence suppressing effects of high temperature.
In the tropics, onions are grown mainly as a winter crop, and mother bulbs are harvested in the spring, stored over the summer, and planted in the autumn. No vernalization treatment is required for tropically adapted lines.
Planting of Mother Bulbs:
In warm climates, onion bulbs for seed production are usually planted in the autumn, while in cold temperate climates, certain over-wintering cultivars are planted in autumn and others in the spring. Bulbs are generally planted in rows 50 to 100 cm apart to facilitate cultural operations.
In the case of inbred lines, it is advisable to keep higher plant density to increase seed yield, as per-plant yield will be generally low. High plant density, however, has disadvantages- ventilation around the plant is impeded, drying after irrigation is slow, and heavy dew and moisture make the plants more prone to diseases.
Ideally, space the plants sufficiently far apart between rows to provide good ventilation, and maintain a high population by planting the bulbs close together within the row. Carefully selected bulbs of male sterile (A line) and pollen parent (C line) are planted alternately in the field.
Sufficient information is available on the required proportion of male and female plants and their planting arrangement in the production field to optimize seed yields. In the conventional method, eight rows of male-sterile parent are alternated with two rows of the pollinator parent. A ratio of 12 male-sterile: 2 pollinator rows has also been suggested.
Isolation:
Commercial F1 hybrid seeds are produced in isolation during the optimum season for seed production. Onion is an insect-pollinated, outcrossing species and requires isolation from other onion seed production fields, and even from chance bolters from commercial onion fields.
The location of onion seed production fields should be planned well in advance, so that adequate isolation is obtained. Complete isolation in a field-grown crop is practically impossible, as pollination is entirely by insects which are able to carry pollen from field to field over long distances.
The greater the distance between onion fields, the lesser will be the amount of outcrossing. The isolation distance from other onion fields should be at least two km; however, three km or more isolation distance is preferable to avoid contamination from wild insects. The best isolation distance for onion hybrid seed production is five km.
However, for seed production of open-pollinated cultivars, recommended isolation distance in India is as follows:
1. Breeder/Foundation seed – 1000 m
2. Certified seed – 500 m
Roguing:
Onion, being a highly cross pollinated crop is always at risk of unwanted pollen contamination by insect pollinators, even when utmost care is taken. Roguing is carried out at the bulb production stage as well as at the time of flowering.
Bulb color, shape, foliage type, and seed stalk height are some of the parameters used in identifying off-type plants. In hybrid seed production fields, the fertile plants in the female line are immediately rogued out to avoid contamination.
Pollination:
To harvest the maximum amount of high quality seed, the flowering period of the A and C lines must be synchronized. If this does not occur naturally, it can often be achieved by adjusting planting dates. Number of days to anthesis of a line is known to be influenced by storage temperature and by planting date of bulbs.
Honey bees are the most important among several insect species that pollinate onion flowers. To ensure proper pollination it is essential to place beehives in the seed production field during the flowering period. Generally, 8 to 10 hives per hectare are enough.
It is recommended not to place all the hives at one time. A few hives should be placed when 50% of the umbels have open flowers. This is mainly to ensure adequate nectar and pollen to forage so that the bees do not move to other fields in search of food.
The rest of the hives are introduced periodically during the entire flowering period of the crop. Insecticide sprays to control thrips or other insects should be managed carefully. Insecticides used to control these pests may be harmful.
Seed Harvest:
Seed maturity takes approximately 30-50 days after anthesis, depending on the cultivar. Seed stalks of A and C lines are harvested separately when the seeds are mature. Care should be taken to avoid mixing seeds between A and C lines.
The C line can be harvested before the A line, earlier than the optimal harvest time (when about 25% of the umbels show few open fruits), so that the valuable hybrid seed can be harvested at its optimal time and free from contamination.
Frequently, the seed from the C line is discarded. In this case, male plants are destroyed as soon as the pollination is completed. Seeds collected from the female parent (A line) are commercially marketed as F1 hybrid seeds.
Onion seed may be harvested either by hand or by machine. Seed is hand harvested when about 25% of the umbels show few open fruits and the black seeds are visible. The umbels are cut with about 15 cm of scape attached. Mechanized harvesting is recommended when ripe seeds are visible in 1-3% of the umbels.
After harvest, the umbels can be dried in several ways. Sun drying on canvas or plastic sheets is commonly practiced when the weather is clear and sunny. The umbels are spread loosely in a layer about 20 cm thick. To achieve uniform drying and to avoid rotting, the umbels are turned regularly. Umbels can also be dried on racks in sheds, or in bins with forced warm air.
To avoid damage, the temperature of the warm air should not exceed 32°C until the seed moisture content is less than 18%; 38°C until less than 10%, and 43°C when below 10%. The threshed seed is dried to 12% moisture.
Cultivar Descriptions of Onion:
There are open-pollinated and F1 hybrid cultivars. The following outline is based on UPOV (1976) and as described by George (1999).
1. Foliage:
Length: short, medium or long
Attitude: erect or semi-erect
2. Leaf:
Curvature: absent, weak, medium or strong
Maximum diameter: small, medium or large
Waxiness: absent or present
Colour: light green, medium green, dark green or bluish-green
Degree of waxiness: weak, medium or strong
3. Bulb:
Size: small, medium or large
Height: small, medium or large
Diameter: small, medium or large
Shape: elliptic, ovate, broad elliptic, circular, broad ovate, rhombic, transverse elliptic or transverse narrow elliptic
Width of neck: thin, medium or thick
Thickness of skin: thin, medium or thick
Basic colour of the skin: white, yellow, red or brown
Colour of the skin: white, straw-yellow, yellow, pale brownish-yellow, dark yellow, greenish-yellow, bronze coloured, brown ochre, orange-brown, pale orange-red, pale red, reddish-brown or dark red
4. Skin adherence: loose, medium or tight
Thickness of rings: thin, medium or thick
Firmness of flesh: soft, medium or firm
Colour of flesh: white, reddish or purplish
Position of root disc: deep, flat or outstanding
Diameter of root disc: small, medium or large
Predominant number of axes (to be established in autumn on mature material before storage): one, two, three or more than three
Colour of epidermis of rings: absent, greenish or reddish
Cross-section: asymmetrical or symmetrical
Dry matter content: low, medium or high
Time of harvest maturity: early, medium or late
5. Male sterility (percentage of plants): < 10% or > 50%
Seed Yield:
1. Open-pollinated cultivars – 500 – 1000 kg/ha
2. Hybrid – 100 – 500 kg/ha
3. 1000 seed weight – Approximately 3.6 g
4. Seed multiplication ratio (SMR) – 100