1.2 Deterioration causes of crop Varieties and their control:
Genetic principles :The genetic purity of a variety can deteriorate due to several factors during production cycles. Kadam (1942) listed some important factors which deteriorate the genetic purity of seeds:
1. Developmental variations
When the seed crop is grown under different soil and fertility conditions or
under various agroclimatic conditions or under different photoperiods or at different
elevations for several consecutive generations, the developmental variations may
arise due to differential growth response and there will be a genetic shift to the
changed agro-climatic conditions. This change in genetic shift causes deterioration
of variety. To minimize such types of variations it is advisable to grow seed crops in their areas of adaptation and growing seasons.
2. Mechanical mixtures
This is an important source of variety deterioration during seed production. The mechanical mixture may occur at the time of sowing if more than one variety is
sown with the same seed drill, through volunteer plants of the same crop, or through different varieties grown side by side in the same field get mixed at the time of harvesting or threshing. Sometimes the seed produce of all the varieties is kept in the same thresher for the processing which may result in the deterioration of variety.
Sometimes gunny bags and seed bins used for packing the seeds may contaminate the seeds. To avoid such types of mechanical mixtures it is necessary to rogue the seed fields. Care should be taken at every stage of seed production, harvesting, threshing, processing, and packing.
3. Mutations:
Natural mutation is a continuous process in nature and it is difficult to
identify or detect minor mutations, however, such mutant plant is observed in the
seed field then it should be rogued out to avoid contamination hence, constant
inspection of the seed plot is to be made rigorously.
4. Natural crossing:
Most of the field crops sexually propagate, hence natural out-crossing is another important source of contamination and varietal deterioration, due to the introgression of genes from unrelated stocks through the natural crossing.
A. The breeding system of species (nature of pollination, self or cross)
B. Isolation distance: It is the minimum separation required between two or more varieties of the same crop species for the purpose of keeping seeds genetically pure.
C. Varietal mass i.e. no. of varieties grown at a time in the field.
D. Pollinating agents – Insects, predators, wind and wind direction.
E. Climatic factors – Humidity, temperature, wind velocity at the time of anthesis, and pollination.
5. Residual or Minor genetic variability :
Sometimes, minor genetic variation may exist even in the varieties appearing uniform and homogenous at the time of release. During the later cycle of seed production, these variations may affect the yield and quality of the variety and deteriorate the variety. Therefore, it is essential to take precautions during the maintenance of the nucleus and breeder seed.
6. Disease infestation :
Sometimes, an improved variety deteriorates because it is infested by a disease to which it was considered resistant at the time of its release. This may result either due to the absence of such disease in the area where the variety was tested or due to the formation of a new pathogenic race of a disease.
7. Techniques of plant breeder :
Cytological irregularities may cause instability in a variety if it is not properly assessed by the breeder at the time of their release. Premature release of variety may segregate for undesirable characters, which may deteriorate the variety. Therefore, proper care is required in testing the variety before its release.
Other factors, such as the breakdown of male sterility and heritable variations arise due to environmental conditions and considerably deteriorate the genetic purity of variety.
1.3 Maintenance of genetic purity during seed production
Different workers have made suggestions to maintain the genetic purity of seeds duringseed production. Notable among them are Horne (1953) and Hartmann and Kester (1968)
Suggestions made by Horne (1953)
1. Seeds approved by the competent authority and inspected by the competent agency before planting.
2. The seed field should be approved and inspected by the competent agency before planting.
3. Inspection of seed field and approval of growing crops at a critical stage for
verification of genetic purity, and detection of mixtures, weeds, and seed-borne diseases should be made.
4. Proper care should be taken at the time of sampling and sealing of cleaned seed lots.
5. A comparison of potentially approved stock should be made with authentic stock by growing the samples.
Suggestions made by Hartmann and Kester (1968)
1. Adequate isolation should be provided to the seed field to prevent contamination by natural crossing or mechanical mixtures.
2. Roguing of seed field should be carried out prior to the stage at which they could
contaminate the seed crop.
3. The variety should be tested periodically for its genetic purity.
4. The seed crop should be grown only in the areas of its adaptation to avoid genetic drift.
5. The seed crop should be certified by the agency to maintain genetic purity and
quality of seeds.
6. The seed production should be restricted to three generations only i.e. starting from breeder seeds, it should only be multiplied up to the foundation and certified class.
7. Variety grown for seed production should be tested for genetic purity by growing out
test to ensure it is being maintained in its true breeder form.
Agronomic principles of seed production :
In the seed production program in addition to genetic principles, certain agronomic principles are to be followed to preserve good quality and abundant seed yields which are as under.1. Selection of a suitable agro-climatic region: Regions or areas with moderate rainfall and humidity are more suitable for seed production. Most of the field crops require a dry sunny period and moderate temperatures for flowering and pollination. The crops pollinated through wind require bright sunny weather with gentle wind velocity which results in good seed setting. Thus, regions with extreme summer heat and very cold winters with excessive rainfall should be avoided for seed production.
2. Selection of seed plot: The plot selected for seed crop must have the following
characteristics
I. Soil texture and fertility of the plot should be according to the requirement of
the seed crop.
II. The soil of the plot should be free from weeds, volunteer plants, insects, pests
and soil-borne diseases.
III. The plot selected for seed production purposes should have not been used to
grow the same crop in the previous season.
IV. The plot should be well-leveled and it should be feasible to isolate it from
another side as per the requirement of seed certification.
3. Isolation of seed crop: The seed crop must be isolated from other nearby fields of the same crop and other contaminating crops. Enough isolation distance should be provided as per seed certification standards. For the nucleus and breeder, seed production isolation may be provided by enclosing plants or groups of plants in a cage or by removing male flower parts and then artificial pollination.
4. Preparation of land: Good land preparation helps in improved germination, good
stand establishment, and eradication of weed plants, it also helps in water
management and uniform irrigation. So land preparation is an important aspect.
5. Selection of variety: The variety selected should be adapted to the agro-climatic
conditions. It should be a higher yielder and possess desirable attributes like resistance to disease, insects, and pests, earliness, and good grain quality.
6. Seed: The seed to be used for raising the seed crops should be obtained from an officially authorized agency having known genetic purity and appropriate class. The tag and seal of the seed bag purchased should be intact and the validity period of the seed should not be expired.
7. Seed treatment: Appropriate seed treatment must be given to the seeds before sowing. For seed-borne diseases and pests, the seeds should be treated with fungicides, insecticides, and pesticides. In the case of legume crops the seeds must be inoculated by the bacterial culture for higher modulation, water soaking treatment for softening the hard seed coat, and mechanical scarification or acid treatment for breaking the dormancy of the hard seed.
8. Time of planting: The seed crop should be sown as per the recommended time of sowing. In a hybrid seed production program, staggered planting may be adjusted in order to have synchronized flowering (flowering of male and female plants at the same time). There should be sufficient moisture at the time of sowing to have good germination.
9. Seed rate: Recommended seed rate for different crops should be followed in order to carry out roguing and inspection of the seed crops.
10. Method of sowing: In order to take effective plant protection measures, roguing operations, and field inspection, line (row) sowing is the appropriate method to be followed in the seed production program. In the case of hybrid seed production, female and male parents should be planted in the definite proportions of 4:2 or 6:2 rows.
11. Depth of sowing: The seed crop having small seeds should be planted at shallow depths while those having bigger seed sizes may be planted at a deeper level.
12. Roguing: Roguing means the removal of off-type, diseased plants, weed plants, volunteer plants, or plants differing in characteristics from the seed crop variety. Timely and adequate roguing is important in order to maintain the genetic purity of seed crops.
13. Supplementary pollination: If the seed crop is pollinated by insects then provision of the honey bee and other beneficial insects may be made nearby the seed crop field to ensure good pollination and setting.
14. Weed control: Efficient weed control is a basic requirement of seed crops in producing good quality seeds and obtaining higher yields. Therefore, the seed field should be free from weed plants, or either it should be at a minimum level. In no case, the weed plants should be allowed to flower or seed set.
15. Plant protection measures: The quality of seeds may be poor if damaged by insects, pests or disease and there will be a reduction in yield, so proper plant protection measures should be taken up at the appropriate times as per recommendations.
16. Fertilizer application: For proper growth and development of seed crops application of fertilizers at the proper time and as per recommended doses is important. Hence, it is necessary to have knowledge of the nutritional requirements of the seed crops.
17. Irrigation: Irrigation is important to ensure good and uniform germination at
planting time so it is advisable to irrigate the seed crop as per its requirements.
Irrigations should be stopped two to three weeks before seed maturity to allow the
soil and plants to dry.
18. Harvesting of seed crops: Early or late harvesting will affect the yield and quality of seed hence, the seed crop should be harvested at the optimum time to allow maximum yield. Moisture content below 20 percent is the optimum time to harvest the seed crop.
19. Drying of the seeds: In order to preserve seed viability and vigour, it is necessary to dry seeds to safe moisture content. Seeds of the field crops are dried to room temperature.
20. Storage of raw seeds: The best method of storing seeds for a short period is gunny bags. After sun drying the seed should be filled in a neat and clean bag. The stacks of bags should not be made directly on the floor but arranged on wooden platforms in dry, cool, clean, and rat-proof storehouses or godowns. The storehouse should be fumigated from time to time to protect seeds from stored grain pests.
Maintenance of Nucleus and Breeder seed
The nucleus or breeder seed should be produced in such a manner that it strictly
satisfy the genetic purity and identify the quality of seeds and subsequently maintain the quality of foundation and certified seed produced/multiplied from it.
Maintenance of Nucleus and Breeder in self-pollinated crops
The variety of self-pollinated crops should be completely homogenous (uniform).
However, in practice, some amount of variation may occur during the seed production cycle due to natural crossing. Therefore, purification of such variety during the maintenance of the nucleus/breeder seed is necessary. The methods of maintaining the nucleus/breeder seed can be divided into two groups.
(1) Maintenance of a newly released variety
(2) Maintenance of established variety.
Harrington (1952) outlined the procedure for maintenance of the nucleus seed of a newly released variety.
(A)Sampling of the variety to obtain nucleus seed :
The selected plants or lines which are highly promising in yield trials or breeding nurseries should be sampled for seed purification. A maximum of fifteen such promising lines at a research station should be sampled in one year. Approximately 200 plants from the central rows should be selected. To avoid the shattering of the seeds, these plants should be pulled 4 to 5 days before the grains are fully mature. To prevent breakage or loss, these plants should be tied in a bundle and wrapped in a cloth or paper. Store properly these bundles till the final yield results are available. Discard any of these bundles found inferior in yield and quality based on the results.
(B) Table examination of samples :
The two hundred plants of each sample should be threshed separately, clean the
seeds and examine in piles on the table. Discard the pile found off type, diseased or not uniform. The seed of these selected two hundred plants is called a nucleus seed, which is to be sown in a variety purification nursery.
(C) Locating and seeding of nucleus seed:
Each nucleus seed should be grown on clean fertile land at the research farm in
the region or area for which it is to be released. The land to be used for sowing the
nucleus seed must not have the same crop sown in the previous year. The 200
progenies of a nucleus should be sown in 50 double rows plots in four series. To
facilitate the examination of rows during different growth stages the plot-to-plot distance
should be at least 45 cm. It should be isolated properly to prevent contamination by
natural crossing.
(D)Inspection of two rows plot and removal of off types :
From the seedling stage up to maturity, the plot should be examined critically.
Differences in early plant growth, rate of growth, time of heading, plant height, head
characters, and disease reaction should be observed if any plot differs from the
average in such traits, it should be removed. From flowering to maturity, roguing
should be done based on ear-head characters.
(E) Harvesting and threshing of the nucleus :
Each plot of the nucleus should be harvested separately, and tied in a bundle with
proper labeling. Thresh the bundle individually, and clean the seeds taking care that it
should not get mixed with the seeds of another plot. The seeds should be treated with
fungicide and insecticide, bagged and labeled properly, and stored as “Breeder’s
stock seed”.
(II) Maintenance of Breeder seed of newly released variety :
1. Breeder’s stock seed obtained from the nucleus should be sown on clean fertile land. The land to be used for sowing should not have a crop of the same kind grown in the previous year. The land requirement varies with the crop. e.g. In the case of wheat about 1.2 ha. while in the case of transplanted paddy the land requirement is 3 hectares.
2. The seed should be properly isolated as per the seed certification standard.
3. The field should be produced at the research farm of the university and in the area or region for which the variety has been bred.
4. The sowing should be done in rows keeping the sufficient spacing between rows so as to permit examination of plants for mixtures or off type.
5. Roguing should be critically done before flowering i.e. during the growth period and
after flowering i.e. during the reproductive stage.
6. Harvesting, threshing, drying, cleaning, bagging, and labeling should be carried out with the most care to avoid contamination.
7. A portion of breeder seed should be retained by the plant breeder to multiply the
stock with 100 % genetic purity.
Maintenance of Breeder seed of established variety :
The breeder seed of established variety can be maintained by two ways.1. By raising the crop in isolation: The breeder seed of established variety can be
maintained by growing it in isolated plots and by rigorous roguing during
various stages of crop growth by observing plant characters.
2. By Bulk selection: In this method, 2000 to 2500 plants representing typical
plant characters of the variety are selected, harvested, and threshed separately.
The seeds from each selected plant are examined by table examination making
piles of the individual plant and if seeds of any pile are found off-type or dissimilar
then it is discarded The remaining piles are bulked to constitute the breeder seed.
How long a particular method should be used depends on the rate of
deterioration in a variety either through natural crossing or mutation or mechanical
mixture. If enough care is taken while production, we can maintain genetic purity
for several generations.
Carry-over seed: A portion of breeder seed retained by the plant breeder for the continuation of a variety is called carry-over seed. The breeder must retain enough quantity of seeds to safeguard against the loss of variety if there is a complete failure during the foundation seed multiplication phase.
Methods of maintaining nucleus and breeder seed in cross-pollinated crop :
The maintenance of a variety of cross-pollinated crops is complicated because it
involves the maintenance of parental material and the method of breeding the variety.
Maintenance of nucleus and breeder seed of newly released variety :
It involves self-pollination, sib pollination, or a combination of both procedures.
Generally, sibbing (mating between siblings) is preferred because it does not reduce vigor excessively. However, if a change in breeding behavior is observed then selfing should be used as a means of stabilizing the inbred lines. Alternate selfing and sibbing are generally practiced to maintain the parent material.
Seed multiplication is carried out in isolation. Roguing is carried out at different
growth stages. Harvesting is done at physiological maturity. Generally, the ear-to-row line (Progeny rows) is harvested, separately. Threshing or shelling is carried out in bulk or individually and then composited by examining the seeds.
Maintenance of Breeder seed of inbred :
The breeder’s stock obtained from the nucleus seed is planted in an isolated field.
Attention is paid to land, isolation, roguing, harvesting, shelling, and drying so as to
maintain the maximum possible genetic purity.
Maintenance of breeder seed of established variety :
1. By raising the crop in isolation and roguing the off types thoroughly at various
stages of crop growth right from sowing to maturity.
2. By mass selection: The crop is grown under isolation and roguing is carried out at different growth stages. At maturity 2000 to 2500 true-to-type plants are selected.
The selected plants are harvested separately and after careful examination, they are bulked to constitute the breeder seed.
1.4 Seed quality - definition and importance of seed quality
This
quality character could be obtained with seed lots by proper cleaning and
grading of seed (processing) after collection and before sowing/storage.
2. Genetic purity:
It is the true-to-type nature of the seed. i.e., the seedling/plant/tree from the seed should resemble its mother in all aspects. This quality character is important for achieving the desired goal of raising the crop either yield or for resistance or for desired quality factors.
3. Physiological Quality:
It is the actual
expression of seed in further generation/multiplication. The physiological quality characteristics of the seed
comprise seed germination and seed vigour.
The liveliness of the seed
is known as viability. The extent of liveliness for the production of good seedlings
or the ability of the seed to the production of seedlings with normal roots and shoot
under favorable conditions is known as germinability. Seed vigour is the energy or stamina of the
seed in producing elite seedlings. It is
the sum total of all seed attributes that enables its regeneration under any
given conditions. Seed vigour determines
the level of performance of seed or seed lot during germination and seedling
emergence.
The seed which performs well
at sowing are termed quality seed and based on the degree of performance in the production of elite seedling it is classified as high, medium, and low vigour
seed. The difference in seed vigour is
the differential manifestation of the deteriorative process occurring in the
seed before the ultimate loss of ability to germinate. The difference in seed vigour will be expressed
in the rate of emergence, uniformity of emergence, and loss of seed germination.
Hence it is understood
that all viable seeds need not be germinable but all germinable seeds will be
viable. Similarly, all vigourous seeds
will be germinable but all germinable seed need not be vigourous. The physiological quality of the seed could be
achieved through the proper selection of seed (matured seed) used for sowing and by
caring for quality characters during extraction, drying, and storage.
Seed with good vigour is
preferable for raising a good plantation as the fruits, and the economic come out
are to be realized after several years.
Hence, seed selection based on seed vigour is important for raising a perfect finalized plantation.
4. Seed Health
The health status of the seed is
nothing but the absence of insect infestation and fungal infection, in or on
the seed. Seed should not be infected
with fungi or infested with insect pests as these will reduce the physiological
quality of the seed and also the physical quality of the seed in long-term
storage. The health status of the seed also
includes the deterioration status of the seed which is also expressed through the low
vigour status of the seed. The health status of the seed influences the seed quality
characters directly and warrants their soundness in seed for the production of
elite seedlings at the nursery/field.
Hence the quality seed should have
- High genetic purity
- High pure seed percentage ( physical purity)
- High germinability
- High vigour
- Higher field establishment
- Free from pests and disease
- Good shape, size, color etc., according to the specification of varied
- High longevity/shelf life.
- Optimum moisture content for storage
- High market value
Characteristics of good quality seed
Ø Higher genetically purity:
Breeder
/Nucleus - 100%
Foundation
seed - 99.5%
Certified
seed - 99.0%
Ø Higher physical purity for certification
|
Maize , Bhendi |
99% |
|
All crops ( most) |
98% |
|
Carrot |
95% |
|
Sesame, soybean &jute |
97 % |
|
Ground nut |
96 % |
Ø Possession of good shape,size , color, etc., according to
specifications of variety
Ø Higher physical
soundness and weight
Ø Higher germination ( 90 to 35 % depending on the crop)
Ø Higher physiological vigour
and stamina
Ø Higher storage capacity
Ø Free from other crop seeds ( Expressed in number /kg)
|
Crop |
Designated
inseparable other crop seeds |
|
Barley |
Wheat ,oats &gram |
|
Oats |
Wheat ,gram & barley |
|
Wheat |
Oats, gram &barley |
Other crop seeds are the
plants of cultivated crops found in the seed field and whose seeds are so
similar to crop seeds that is difficult to separate them economically by
mechanical means. Cause physical
admixture with the crop seed only when this crop matures approximately at the
same time when the seed crop matures.
Ø It should be free from
objectionable weed seeds
These are plants of weed species that are harmful in one or more of the following ways.
- The size and shape of their seeds are so similar to that of the crop seed that is difficult to remove their seed economically by mechanical means.
- Their growth habit is detrimental to the growing seed crop due to competing effect.
- Their plant parts are poisonous or injurious to human and animal beings
- They serve as alternate hosts for crop pests and diseases.
Objectionable Weeds of Seed Crop Plants
|
Crop |
objectionable weeds |
|
Berseem |
Chicory(Chicorum intybus) |
|
Cucurbits |
Wild Cucurbita sp. |
|
Kasuri methi |
Melilous sp. |
|
Lettuce |
Wild lettuce(Lactuca sativa) |
|
Bhendi |
Wild Abelmoschus sp |
|
Rape &Mustard |
Argemone
mexicona |
|
Wheat |
Convolvulus arvensis(Hiran kuri) |
|
Paddy |
Wild paddy (Oryza sativa var. Fatua) |
Ø It should be free from
designated diseases
It refers to the diseases specified
for the certification of seeds and for which certification standards are to be
met with these diseases would cause
contamination, when they are present in the seed field or within the specified
isolation distance ( eg. loose smut of wheat). For this, the certification
distance has been prescribed as 180 meters.
|
Crop |
Designated disease |
Causal organism |
|
Wheat |
Loose smut |
Ustilago tritici |
|
Sorghum |
Grain smut or Kernel smut |
Sphacelotheca sorghii |
|
Mustard |
Alternaria blight |
Alternaria sp |
|
Pearl millet |
Grain smut Green ear Ergot |
Tolyposporium penicillariae Sclerospora graminicola Claviceps microcephala |
|
Sesame |
Leaf spot |
Alternaria sp |
|
Brinjal |
Little leaf |
Datura virus 2 |
|
Chilies |
Anthracnose leaf blight Leaf blight |
Gloesporium piperatum Alternaria solani |
|
Cucurbits |
Mosaic |
Cucumis virus |
|
Cowpea |
Anthracnose |
Colletotricum sp |
|
Bhendi |
Yellow vein mosaic |
Hibiscus virus 1 |
|
Potato |
Brown rot Root knot nematode |
Pseudomonas solanacearum Meloidogyne incognita |
|
Tomato |
Early blight Leaf spot |
Alternaria solani Xanthomonas vesicatoria |
Ø It should have optimum
moisture content for storage
Long-term storage - 6 - 8 %
Short-term storage - 10-13%
Ø It should have a high market
value
Role of good quality seed
Therefore most care must be given to the use of quality seed and thus certification
guarantees quality and ensures high and assured yield under environmental
stress conditions. This emphasizes the
need for increasing the area under quality seed production. So one has to take
efforts to produce quality seeds and boost the yield by seed to seed seedling
concept.
Significance of quality seed
- Ensures genetic and physical purity of the crops
- Gives desired plant population
- Capacity to withstand the adverse conditions
- Seedlings produced will be more vigorous, fast-growing, and can resist pest and disease incidence to a certain extent
- Ensures uniform growth and maturity
- Development of the root system will be more efficient that aids the absorption of nutrients efficiently and results in higher yield.
- It will respond well to added fertilizer and other inputs.
- Good quality seeds of improved varieties ensure a higher yield at least 10 – 12 %
1.5 Classes of seeds :
The seed of a released and popular variety produced by the scientific method is referred to as an improved seed or quality seed. Variety refers to a genotype, which has been released for commercial cultivation either by the state variety release committee or the central variety release committee.
Improved seed results in (1) better production (2) vigorous seedling growth (3) higher crop stand (4) better quality of products and (5) higher crop yield. Thus the production of improved seeds is an important aspect of seed technology.
There are four classes of improved seed viz.,
1. Nucleus seed: It is the initial seed of an improved variety which is always limited in quantity. It is produced by the original plant breeder of a variety. It is produced at the research farm of the concerned crop under the supervision of the original plant breeder. It is genetically and physically 100 % pure. There is no need for seed certification for the nucleus seed. Nucleus seed is used for the production of the breeder seed.
2. Breeder seed: It is the progeny of nucleus seed produced under the strict supervision of the original plant breeder at the research farm of the concerned crop research station. It is produced in isolation from other varieties. The isolation distance differs from species to species. It is also genetically and physically100 % pure. Genetic purity is maintained by proper roguing. Seed certification is not necessary for breeder seed. However, the seed plot is inspected by a monitoring team led by the original plant breeder and a representative from NSC, and the state seed certification agency. Breeder seed is used for the production of foundation seed.
3. Foundation seed: It is the progeny of the breeder seed. It is produced by the NSC or State Seed Corporation under the strict supervision of a research scientist. Its production is taken up at the seed multiplication farms of Govt, the research farm of ICAR institute or University farm, or on progressive farmer’s fields. Proper isolation distance is the pre-requisite for the production of foundation seed, which varies from crop to crop. Genetically foundation seed is 100 % pure while permissible physical purity is 98 %. In the case of foundation seed, certification is undertaken by State Seed Certification Agency. Foundation seed is used for the production of certified seed.
4. Certified seed: It is the progeny of the foundation seed. It is produced in the field of progressive farmers under the strict supervision of the State Seed Certification Agency. Proper isolation is adopted for its production which varies from crop to crop. It has 100 % genetic purity and 98 % physical purity. The other crops' seeds and weed seeds should not be more than the prescribed standards which vary from species to species. Seed certification from the State Seed Certification Agency is required for certified seed to meet the requirements of purity and germination of certified seed.