Causes for varietal deterioration, Maintenance of genetic purity and principles of seed production
GENERAL PRINCIPLES OF SEED PRODUCTION
Production of good quality seed is an exacting task requires high technical skills and heavy financial investments. During seed production strict attention must be given to maintain genetic purity and exploit its potentiality in next generation. In other words, seed production must be carried out under standardized and well organized condition.
Causes for Deterioration of Genetic Purity
The genetic purity of a variety or trueness to its type deteriorates due to several factors during the production cycles.
Kadam (1942) listed the following important factors responsible for deterioration of varieties.
1. Developmental variations
2. Mechanical mixtures
4. Natural crossing
5. Minor genetic variations
6. Selected influence of pest and diseases
7. The technique of the plant breeder
1. Developmental Variations
When seed crops are grown under environments with differing soil, fertility, climate photoperiods, or at different elevations for several consecutive generation's developmental variations may set in as differential growth responses. It is therefor, preferred to grow the varieties of crops in the areas of their natural adaptation to minimize developmental shifts.
2. Mechanical Mixtures
Mechanical mixtures, the most important reason for varietal deterioration, often take place at the time of sowing if more than one variety is sown with the same seed drill, through volunteer plants of the same crop in the seed field, or through different varieties grown in adjacent fields. Two varieties growing next to each other field is usually mixed during harvesting and threshing operations. The threshing equipment is often contaminated with seeds of other varieties. Similarly, the gunny bags, seed bins and elevators are also often contaminate, adding to the mechanical mixtures of varieties. Roguing the seed fields critically and using utmost care during seed production and processing are necessary to avoid such mechanical contamination.
Mutations do not seriously deteriorate varieties. It is often difficult to identify or detect minor mutations occurring naturally. Mutants such as 'fatuoids' in oats or 'rabbit ear' in peas may be removed by roguing from seed plots to purify the seeds.
4. Natural Crossing
Natural crossing can be an important source of varietal deterioration in sexually propagated crops. The extent of contamination depends upon the magnitude of natural cross-pollination. The deterioration sets in due to natural crossing with undesirable types, diseased plants, or off types. In self-pollinated crops, natural crossing is not a serious source of contamination unless variety is male sterile and is grown in close proximity with other varieties. The natural crossing, however, can be major source of contamination due to natural crossing.
Extent of genetic contamination in seed field due to natural crossing depends up on
1. The breeding system of the species
2. Isolation distance
3. Varietal mass
4. Pollinating agent.
The isolation of seed crops is the most important factor in avoiding contamination of the cross-pollinated crops. The direction of prevailing winds, the number of insects present and their activity, and mass of varieties are also important considerations is contamination by natural crossing.
5. Minor Genetic Variations
Minor genetic variations can occur even in varieties appearing phenotypically uniform and homogenous when released. The variations may lost during later production cycles owing to selective elimination by the nature. The yield trials of lines propagated from plants of breeder's seed to maintain the purity of self-pollinated crop varieties can overcome these minor variations. Due care during the maintenance of nucleus and breeder's seed of cross-pollinated varieties of crop is necessary.
6. Selected Influence of Pest and Diseases
New crop varieties often are susceptible to newer races of pests and diseases caused by obligate parasites and thus selectively influence deterioration. The vegetatively propagated stock also can deteriorate quickly if infected by virus, fungi or bacteria. Seed production under strict disease free conditions is therefore essential.
7. The Techniques of the Plant Breeder
Serious instabilities may occur in varieties owing to cytogenetic irregularities in the form of improper assessments in the release of new varieties. Premature release of varieties, still segregating for resistance and susceptibility to diseases or other factors can cause significant deterioration of varieties. This failure can be attributed to the variety-testing programme.
In addition to these factors, other heritable variations due to recombination's and polyploidization may also take place in varieties during seed production, which can be avoided by periodical selection during maintenance of the seed stock.
GENETIC PURITY MAINTENANCE
Hartmann and Kester (1968) & Agarwal (1980) described steps to maintain the genetic purity of variety during seed production.
The following measures have been suggested to safeguard the genetic purity
1. Adoption of Crop: Growing crops only in areas of their adaptation to avoid genetic shifts.
2. Approved Class of Seeds: Use of only approved class of seed in seed multiplication and adopt generation system.
3. Preceding Crop Requirement: Inspection and approval of seed plots prior to planting
3. Isolation: Isolation of seed crops from various sources of contamination by natural crossing or mechanical mixtures.
4. Roguing: Roguing of off types differing in characteristics from those of the seed variety.
5. Field Inspection: Qualified and experienced personnel of seed certification agency should inspect seed crops at all appropriate stages of growth and verify seed lots or purity and quality.
6. GOT: Periodic testing of varieties for genetic purity.
Standardized seed production, besides genetic principles, involves the application of the following agronomic principles to preserve good seed quality and abundant seed yields.
1. Selection of suitable Agro-climatic region
Ø The seed crops have to be grown only in areas well adapted to the photoperiodic and temperature conditions prevailing.
Ø Region of moderate rainfall and humidity are much more suited to seed production than regions of high rainfall and humidity.
Ø In general, regions with extreme summer heat and very cold winters should also be avoided for seed production unless particular crops are especially adapted to grow and product under these conditions.
2. Selection of seed plot
Ø Seed production plot should have good texture and fertility.
Ø Should be free from volunteer plants weeds and other crop plants.
Ø Soil should be free from soil borne diseases and insect pests.
Ø The previous crops should not be same crop.
Ø The plot should get adequate isolation distance.
3. Isolation of seed crops
Ø The seed crop must be isolated from other nearby fields from the same crop
or any contaminating crop as per certification standards.
Ø Time isolation could also be used in some crops. This is a must to meet the
standards for genetic purity of seeds.
Ø Isolation of seed production of different varieties is also necessary to avoid
4. Preparation of land:
Ø The land must be prepared well.
Ø Good land preparation helps in improved & uniform germination resulted in good stand establishment.
5. Selection of variety:
Ø Variety should be adopted to agro-climatic condition
Ø Variety should be high yielder
Ø Variety should posses other desirable characters like disease resistance, earliness, grain quality etc
6. Selection of Seed:
Ø Seed purity: The seed used for raising a seed crop should be of known purity, appropriate class and invariably obtained from authorized official agency.
7. Seed Treatment:
Ø If the seed is not treated already, it should be treated with appropriate fungicides/insecticide prior to sowing.
8. Time of sowing:
Ø Should be sown at their normal planting time
9. Seed rate:
Ø Lower seed rate than usual to facilitate rogunig operation/seed inspection.
10. Method/depth of sowing:
Ø Invariably sown in rows at right depth.
Ø To rouge plants (off types, pollen shedders, diseased plants, etc.,) at the earliest possible but before flowering
Ø The number of rouging varies with the crop, purity of the source seed and the stage of the multiplication of the seed crops.
Ø Rouging in most of the field crops may be done at - Vegetative, Pre-flowering, Flowering and Maturity stages
12. Supplementary pollination
Ø In cross-pollinated crops supplementing the natural pollination is very much essential to increase the seed yield.
Ø Provision of honeybees, hand pollination, floral mechanism, and rope pulling are some of the techniques used in crosspollinated crops to increase the seed set and there by seed yield.
13. Weed Control:
Ø Compete with seed crop and reduces seed yield and quality
Ø Presence of weed seeds at the time of harvest leads to mixing of weed seeds with crops seeds and difficult to separate during seed processing
Ø Harbors pests and diseases
14. Disease and Insect management:
Ø Reduces seed yield and quality
Ø Infection of seeds with spores
Ø Important role for proper development of plant and seeds
Ø N-Good healthy growth P-Fruiting and seed development K- flowering and seed development
Ø Proper nutrition to achieve synchronization
Ø Required for translocation of all the nutrients
Ø Seed production areas should be dry regions with assured irrigation
17. Harvesting of seed crops
Ø Time and method of harvesting are important.
Ø Harvest when the seed is fully matured (Moisture content less than 20%)
Ø Early or late harvesting affect seed quality.
Ø Every effort should be made to avoid chance of mechanical mixing, mechanical injury to seeds during harvesting, threshing, cleaning drying and packing.
18. Drying of seeds
Ø In order to preserve seed viability and vigour, it necessary to dry seeds to safe moisture content levels as early as possible without heaping wet seed \ pods.
19. Storage of Raw Seeds
After sun drying, the seed should be filled in clean bags.
Ø The bags should be dipped in 2 % Melathion, dried and cleaned before they are filled.
Ø Mark the bag with name of variety and other details.
Ø The bags should be stacked on wood pallets but not directly on the floors.
Ø The height of the stack should not be more than 3 to 4 m for cereals, 2.5 to 3 m for other crops.
Ø The godowns should be dry, cool and clean and spray with Melathion and later fumigate as and when necessary.
SEED MULTIPLICATION RATIO
It is nothing but the number of seeds to be produced from a single seed when it is sown and harvested. According to expert group on seeds (1989), the seed multiplication ratio for different crops are as follows.