Seed type and characterstics

Seed type and characterstics

1. TYPE AND CHARACTERSTICS OF SEED Seed are the protectors and propagators of their kind. Thus farmers entire crop depends on the quality of seed they use for sowing or planting. Seed may be defined as a fertilized ovule consisting of intact embryo, stored food and seed coat which of viable and has the capacity to germinate. It may also be called as a “unit of reproduction of flowering plants and may be described as a plant embryo, in a dormant state, surrounded by a food supply and protective outer skin or seed coat.” Seed is the living link between parent and its progeny. Biologically, seed is the ripe fertilized ovule and a unit of propagation / reproduction of flowering plants. Agronomically , a seed material or propagule is the living organ of crop in rudimentary form use for propagation. It can be any part of the crop from which a new crop will grow.

2. CLASSES OF SEED After harvest the seed is subjected to analysis and germination test. The standards of certification of each crop differ, but in all cases they result in the sale of quality seeds to the farmers. Four classes of pure seeds are recognized by the International Crop Improvement Association. Breeder or Nucleus seed : It is directly controlled by the organizing or sponsoring plant breeder or institution. It provides for initial or recurring increase of the foundation seed. This is the seed that is produced directly under supervision of plant breeder. Foundation seed: This includes elite seed, would be seed stock (seeds, tubers, bulbs, plants etc.) that are so handled as to maintain specific genetic identity and purity that may be distributed by representatives of an experimental station. Foundation seeds are the source of all other certified seed classes, either directly or through registered seed agencies. It as also known as mother seed.

3. Registered seed: This is the progeny of the foundation seed that is so handled as to maintain satisfactory genetic identity and purity and has been approved by the certifying agency. This class of seed is of a quality suitable for production of certified seed. It can be produced by the farmer and other growers under special contract with the certifying agency. Certified seed: This is the progeny of foundation, registered or certified seed that is so handled as to maintain satisfactory genetic identity and purity that is approved by certifying agency. This is the seed designed for the use by the farmers for crop production. Two classes of certified seeds are produced i.e. F 1 and F 2 . Hybrid seed: It is the seed produced by crossing two or more homozygous inbred lines to obtain a desirable type with high yield potential. Only F 1 generation of hybrids is recommended for use as seed for commercial production .

4. Characteristic of good seed The seeds in its real meaning must possess the following characters: It must be true to its type. The seeds must be healthy, pure and free from all the inert materials and weed seeds. The seeds must be viable, the germination capacity is up to the standard and it has been tested recently. The seeds must be uniform in texture and structure. The seeds must be truthfully labeled and produced under due care and strict supervision so that it does not degenerate quickly. The seed must not be affected by any seed born disease.

5. According to their nature and precaution which seed are produced they are classified into: TYPE OF SEED COLOUR OF TAG Breeder seeds or nucleus seeds Gold/ Yellow Foundation seeds White Certified seeds Blue Registered seed Purple Truthfully labeled seeds - QUALITY OF SEED: Viability and vigour are two important characters of seed quality. Viability can be expressed by the germination percentage which indicate the number of seedlings produced by a given number of seeds. Vigour of seed and seedlings is difficult to measure, Low germination percentage, low germination rate and low vigour are often associated. Seeds with low vigour may not be able to withstand unfavorable condition in the seedbed. The seedling may lack the strength to emerge if the seeds are planted too deep or if the soil surface is crusted.

6. Germination is measured with two parameters: The germination percentage and germination rate. Vigour is indicated by the higher germination percentage, high germination rate and quicker seedling growth. Germination percentage is the number of seeds germinated to no. of seeds planted and its expressed in two ways: The number of days require to produce a given germination percentage. The average number of days required for radicle or plumule to emerge. SEED PRODUCTION Purity of seed is important and it has to be maintained during seed production. Impurities of seed occur due to cross pollination, from a mechanical mixture etc. Purity of seed is maintained by isolation and roguing .

7. ISOLATION Contamination by cross pollination with a different but related cultivar has to be prevented. It is primarily achieved through distance, but it can also be attained by enclosing plant or growing of plants in cases, enclosing individual flowers or removing male flower parts and then employing artificial pollination. Isolation distance of different field and vegetable crops vary according to the crop, according to type of seeds and different category of seeds to be produced like foundation, certified. Separation of different cultivars is needed to production of self- pollinated plants to prevent mechanical mixing of seeds during harvest. The main distance usually specified between plots is 3 meters. ROUGHING Off type plants are to be removed before flowering so as to avoid contamination of off type plants which may arise because recessive genes are present in heterozygous conditions even in highly homozygous cultivars. Volunteer plants arising from accidentally planted seeds or from seeds produced by earlier crops is another source of contamination.

8. CULTIVAR It is a contraction of cultivated variety. Cultivar is synonymous with variety. A variety is a subdivision of a species with some special characteristics. There are several varieties in rice ( Oryza sativa) such as Saket - 4, Samba mahsuri , Sarju-52 and each variety has different characteristics regarding tillering , plant height, duration, grain quality etc. A cultivar is also a unique population of plants but it is artificially maintain by human efforts and is named. Many cultivars cannot continue to exist without human effort. A cultivar should have proper name so that it can be identified. HIBRIDIZATION A hybrid line is the F 1 generation of two inbred lines. Hybridization may be made between two inbred lines (single cross) or two single crosses (double cross) and inbred line and an open pollinated cultivar (top cross) or between a single cross and inbred line (three way cross). Inbred lines are true breeding cultivars that result from enforced self- pollination of selected parent plants followed by continued selection to a desired type in succeeding generation. Once are desired has been selected it is maintained by growing plants in isolation and allowing them to cross pollinate or self- pollinate naturally.

9. SEED ANALYSIS Optimum crop stand establishment depends upon quality of seed used and soil environment in which it is sown. SEED TESTING It is the procedure for obtaining reliable information on five aspects of seed quality, viability, purity, vigour , seed health and presence of noxious weed seed. VIABILITY TESTING S eed viability is the capacity of non-dormant seed to germinate under favorable conditions. Petri dish/ Rolled towel test is done to test the viability of seed.

10. TETRAZOLIUM TEST It is the calorimetric test in which a bio chemical reaction causes the test solution to change colour under certain conditions. Tetrezolium (2,3,5- triphneyl tetrazolium chloride) solution is colourless but changes to red insoluble compound called formagan upon being reduced by hydrogen ion. Viable seeds will change colour to red and dead or non- respiring seeds remain colourless . This test is quick and reliable. SEED PURITY It is the percentage of pure seed (seed without contaminants) in the sample tested. Contaminants include seeds of other crops, weed seed and inert matter. PURE LINE SEED Pure line seed (PLS) is the percentage of desired cultivar that will germinate. It is the function of both percent purity and percent germination it is calculated in percent. Percent (PLS) = (% Germination x % Purity) / 100

11. SEED VIGOUR It indicates the properties of seed that determine its potential for rapid, uniform emergence and development of normal seedlings under a wide range of field conditions. It is influenced by genetic factors and external environmental condition during seed development and maturity. An environment of high temperature and humidity adversely affects seed vigour . SEED HEALTH It evaluate the presence of pathogens and insect pest on the seed. Seed health may be evaluated visually (change in testa , colour , presence of spores etc.) after incubating on an appropriate medium for disease development. It can also be determined by biochemical test such as enzyme linked immunosorbent assay (ELISA).

12. SEED DORMANCY In induced or secondary dormancy, some seeds may achieve dormancy due to interaction of seed with environment (temperature). Such dormancy may continue even after the removal of seed from such environment. When non dormant imbibed seeds are buried deep in soil, inadequate light, temperature, excess carbondioxide or a combination of these may induce the dormancy. Such dormancy lasts until the seed experiences such conditions necessary to break it. ENFORCED DORMANCY Some seeds have dormancy thrust upon them due to conditions of inadequate oxygen, excess carbondioxide , cold temperature, ethylene etc. induced by human or nature is called enforced or environmental dormancy. Non dormant seeds fail to germinate under such forced unfavorable condition. Weed seeds buried deep in the soil are put under enforced dormancy.

13. RELATIVE DORMANCY Certain varieties of lettuce germinate well at 20 o C or below. However, they are dormant at high temperature (thermal dormancy) and require light for germination. EPICOTYL DORMANCY Some species of plants show epicotyl dormancy, combination of root and epicotyl dormancy or double dormancy as in the case of Sanguinaria sps . This can be overcome by moist storage at 1.0 to 10 o C for two to three months after the seed has germinated to form a root. MULTIPLE DORMANCY Some seed required passage of one cold period for the after ripening of radicle and embryos. During the next summer, the radicle emerges and become established. However, the epicotyl remains dormant until the passage of another cold period.

14. ENVIRONMENTAL CONTROLS OF SEED DORMANCY The environmental controls of seed dormancy are: Temperature: Some seeds required low temperature to break dormancy. Temperature as low as 10 o C is sufficient for breaking dormancy. Low temperature treatment can depress the inhibitor content of dormant seeds and high temperatures. Generally increase the dormancy of seeds rather than improving germination. Light: Light requirement of seeds may serve to bring about germination when the seeds reaches soil surface or surrounding vegetation has been disturbed. It is a common requirement for germination of many weed seeds. Water: The seed coats may contain large amounts of some osmotic materials which can restrict the amount of water entering the embryo through osmotic means. Water itself can relieve dormancy in many types of seed specially from seed coat. Drying can relieve dormancy in some cases as in tomato seeds.

15. VIABILITY OF SEEDS Viability is the capacity of seed to germinate to maintain its normal life process, capacity of growing or maintaining its living state. A viable seed can germinate under favourable conditions, provided dormancy is removed. A viable seed may be dormant or non-dormant. Under optimum conditions, a viable weed seed germinates with in ten days while crop germinates within 7 days. LONGEVITY OF SEEDS Longevity or duration of seed viability depends on storage environment and the genetic constitution of the seed. A given storage environment may lengthen the life span of one species and shorten that of another. Even when the seeds is intact with parent plant, the seeds may deteriote with time depending on the weather and other biotic factors. Relatively dry weather is favourable for good seeds. The maturity stage at harvest is the major factor responsible for part of the variation in viability and seed side. Seed may be non- viable due to various reasons, like mechanical injury, insect, disease damage and aging.

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