seed production and seed multiplication (T.Y.B.Sc)

Principles of seed production and stages of seed multiplication

 Seed quality

            A good should have the following quality characters.

            1.Improved variety : It should be superior to the existing variety i.e. the yield should be higher by 20-25% than the existing variety or it should have some desirable attributes like disease resistance , drought resistance ,salt tolerance etc., with good yield potential.

            2. Genetic purity: The seed should be true to type .The seed should possess all the genetic qualities /characters , which the breeder has placed in the variety, genetic purity has direct effect on the yields .If there is deterioration ,there would be proportionate decrease in the yield of performance.

            3.Physical purity: Physical purity of a seed lot refers to the physical composition of the seed lots. A seed lot is composed of  pure seed , inert matter , broken seeds , undesired seeds, Soil and dust particles, weed seeds etc. Higher the content of pure seed better would be the seed quality . Pure seed together with germination gives the planting value of seed lot.

Table 4.1 Minimum limits of pure seed and germination percentage

Crop	Pure seed (%) (Min.)	Germination (%) (Min.)
Paddy ,Jowar, Groundnut , Cowpea	97	70
Wheat ,Maize	97	80
Bajra, soyabean, Pea	96	70
Cotton	95	55
Barley , Chickpea	97	75
Castor	98	70

            4.Seed germination and vigour : Seed germination refers to the ability of a seed when planted under normal sowing conditions to give rise to normal seedling. Seed vigour refers to the sum total of all seed attributes that give effective plant stand in the field. Higher germination percentage and vigour gives adequate plant population and uniform growth, which have profound effect on yield and determine the planting value of seed.

           5.Freedom from weeds and other crop seeds : There are certain weed species, which are very harmful to the crop and once established they are difficult to eradicate. An absolute freedom from seed of such species is highly desirable and is one of the important criteria for determining the planting quality of the seeds.

            6. Seed health : Seed health refers to the presence or absence of disease organisms or insect pests on the seed. The quality of a seed depends on its health, hence the seed should be free from seed born disease and insect pests.

          7.Seed moisture: The seed moisture is the most important factor in determining the seed germination and viability during storage. At high seed moisture content there is high incidence of pest attack and at moisture content above 16% seed get heated and the viability is lost. Hence the seed should be stored at safe moisture levels of 11-13% .

          8. Seed size, weight and specific gravity: Seed size, weight and specific gravity has been found to have positive correlation with seed germination and vigour in many crops . Therefore the seed should be hold with specific gravity.

          9. Seed colour : The colour of the seed often reflects the condition during seed maturation . The farmers from ancient times have regarded good normal shine as invariable quality guides. The colour and shine deteriorates only when the weather conditions are adverse during maturation or when insects infest the crop or when it is handled badly . The seed lots having high genetic purity , high germination and with a minimum amount of inert matter , weed  seeds and other crop seeds and are free from diseases is said to be high quality and if it is lacking of these it is said to be of low quality.

principles of quality seed production

          Seed production or maintenance of a genetic constitution of the seed is quite specialized and scientific procedure and is not similar to general food crop production .  It is important that seed of a new and superior variety should be multiplied and made available in quantities as soon as possible so as to benefit the farmers. Also the seed of released varieties must be maintained in such a way that stocks of pure propagating seed are constantly moving into commercial channels. Seed production is carried out under standardized and well organized conditions. During seed production following principles should take in to consideration to obtain quality seed.

A)  Genetic principle

a)Maintenance of genetic purity of a variety

 Deterioration of varieties:

        Genetic purity a variety can deteriorate due to several factor during production cycles. The important factors of apparent and real deterioration of varieties are as follows :

1.     Development variation : When a seed crop is grown in different environmental conditions such as different soil and fertility conditions, under saline or alkaline conditions or under different photo-periods or different elevations or different stress conditions for several consecutive generations the developmental variations may arise as differential growth response . To avoid or minimize such developmental variations the variety should always be grown in adaptabale area or in the area for which it has been released . If due to some reasons (for lack of isolation or to avoid soil born diseases) it is grown in nonadaptable areas it should be restricted to one or two seasons and the basic seed i.e. nucleus and breeder seed should be multiplied in adaptable areas.

2.     Mechanical mixtures : This is the major source of contamination of the variety during seed production. Mechanical mixtures may take place right from sowing to harvesting and processing in different way such as;

a)      Contamination through field- self sown seed or volunteer plants.

b)      Seed drill – if same seed drill is used for sowing two or three varieties

c)      Carrying two different varieties adjacent to each other .

d)      Growing two different varieties adjacent to each other.

e)      Threshing floor

f)       Combine harvesters or threshers

g)      Bag or seed bins

h)      During seed processing

             To avoid sort of mechanical contamination it would be necessary to rouge the seed fields at different stages or crop growth and to take utmost care during seed production ,harvesting ,threshing , processing etc.

3.     Mutation: It is not of much importance as the occurrence of spontaneous mutations is very low. If any visible mutations are observed they should be removed by roughing. In case of vegetatively propagated crops periodic increase of true to type stock would eliminate the mutants.

4.     Natural crossing  : It is an important source of contamination in sexually propagated crops due to intogression of genes from unrelated stocks/genotypes. The extent of contamination depends upon the amount of natural cross-fertilization, which is due to natural crossing with undesirable types, offtypes, and diseased plants. On the other hand natural crossing is main source of contamination in cross- fertilized or often cross-fertilized crops. The extent of genetic contamination in seed fields is due to natural crossing depends on breeding system of the species, isolation distance, varietal mass and pollinating agent. To overcome the problem of natural crossing isolation distance decreases the extent of contamination. The extent of contamination depends on the direction of the winfd flow, number of insects presents and their activity.

5.     Genetic drift : When seed is multiplied in large areas only small quantities of seed is taken and preserved for the next years sowing. Because of sub-sampling all genotypes will not be represented in the next generation and leads to change in genetic composition. This is called as genetic drift.

6.     Minor genetic variations : It is not of much importance, however some minor genetic changes may occur during production cycles due to difference in envinronment. Due to these changes the yields may be affected. To avoid such minor genetic variations periodic testing of the varieties must be done from breeder seed and nucleus seed in self-pollinated crops . Minor genetic variations is a

common feature in often cross-pollinated species; therefore care should be taken during maintenance of nucleus and breeder seed. 

7.     Selective influence of disease : Proper plant protection measures must be taken against major pests and diseases other wise the plant as well as seed get infected.

a)      In case of foliar disease the size of the seed gets affected due to poor supply of carbohydrates from infected photosynthetic tissue.

b)      In case of seed and soil borne diseases like downy mildew and ergot of jowar, smut of bajra and bunt of wheat, it is dangerous to use seeds for commercial purpose once the crop gets infected.

c)      New crop varieties may often become susceptible to new races of disease.

8.     Techniques of the breeder : Instability may occur in a variety due to genetic irregularities if it is not properly assessed at the time of release. Premature release of variety, which has been breed for particular disease, leads to the production of resistant and susceptible plants which may be an important cause of deterioration. When Sonalika and Kalyansona wheat varieties were released in India for commercial cultivation the genetic variability in both the varieties was still in flowering stage and several secondary were made by breeders.

9.     Breakdown of male sterility : Generally in hybrid seed production if there is any breakdown of male sterility it may lead to a mixture of F₁ hybrids and selfers.

10.                     Improper seed certification : It is not a factor that deteriorates the crops varieties, but if there is any lacuna in any of the above factors and if it has not been checked it may lead to deterioration of crop varieties.

Maintenance of genetic purity during seed production

            Horne (1953) had suggested the following methods for maintenance of genetic purity;

1.      Use of approved seed in seed multiplication

2.      Inspection of seed fields prior to planting

3.      Fields inspection and approval of the crop at critical stages for verification of genetic purity, detection of mixtures, weeds and seed borne diseases.

4.      Sampling and sealing of cleaned lots

5.      Growing of samples with authentic stocks or Grow-out test

Various steps suggested by Hartman and Kestar (1968) for maintaining genetic purity as follows;

1.      Providing isolation to prevent cross-fertilization or mechanical mixtures

2.      Roughing of seed fields prior to planting

3.      Periodic testing of varieties for genetic purity

4.      Grow in adapted areas only to avoid genetic shifts in the variety

5.      Certification of seed crops to maintain genetic purity and quality

6.      Adopting generation system

Safeguards for maintenance of genetic purity

              The important safeguard for maintaining genetic purity during seed production are;

1.     Control of seed source : This seed used should be of appropriate class from the approved source for raising a seed crop. There are four classes of seed source viz. Nucleus seed , breeder seed, Foundation seed and certified seed,

2.     Preceding crop requirement: This has been fixed to avoid contamination through volunteer plants and also the soil borne diseases.

3.     Isolation : Isolation is required to avoid natural crossing with other undesirable types, offtypes in the fields and mechanical mixture at the time of sowing, threshing, processing and contamination due to seed borne disease from nearby fields. Protection from these sources of contamination is necessary for maintaining genetic purity and good quality of seed.

4.     Roughing of see fields : The existence of off type plant is another source of genetic contamination. Offtype plants differing in their characteristics from that of the seed crop are called as off types. Removal of off types is referred to as roughing.

The main source of off types are

a.       Segregation of plants for certain characters or mutations

b.      Volunteer plants from previous crops or

c.       Accidentally planted seeds of other variety

d.      Diseased plants

                        Off type plant should be roughed out from the seed plots before they shed pollen and        

                         Pollination occurs. To accomplish this regular supervision of trained personnel is required.

5.     Seed certification : Genetic purity in seed productions maintained through a system of seed certification. The main objective of seed certification is to make available seeds of good quality to farmers. To achieve this qualified and trained personnel from Seed Certification Agency (SCA) carry out field inspections at appropriate stages of crop growth . They also make seed inspection by drawing samples from seed lots after processing. The SCA verified for both field and seed standards and the seed lot must confirm to get approval as certified seed.

6.     Grow – Out Test (GOT) : varieties that are grown for seed production should be periodically tested for genetic purity by conduction GOT to make sure that they are being maintained in true form. GOT test is compulsory for hybrids produced by manual emasculation and pollination and for testing the purity of parental lines used in hybrid seed production.

                                                                                                             

  Stage of seed multiplication

         The benefits of an improved variety are not released unless enough true seed has been produced for its commercial spread. The initial amount of pure seed which is limited in quantity is multiplied under various stages or classes or seed these are :

1.      Nucleus seed

2.      Breeder seed

3.      Foundation seed

4.      Certified seed

5.      Truthful seed

Nucleus seed :

          It is the initial amount of pure seed of an improved variety available with plant breeder who has developed it. The nucleus seed is sent percent pure genetically as well as physically and is very limited in quantity. It does not having any seed certification tag.

Breeder’s seed :

          It is the seed obtained from the progeny of nucleus seed. It is directly supervised by a breeder concern with the crop. Its genetical  and physical purity to be 100 per cent. The yellow tag is attached to breeder seed bag.

Foundation seed : 

         It is seed obtained from nucleus or breeder’s seed. It is produced on seed multiplication farm of a state government or agricultural universities. Foundation seed plots are jointly inspected by the SCA (seed certification agency) , it is not as pure as the nucleus and breeder’s seed are . The bags are sealed with white coloured label.

Certified seed :

       It is progeny of foundation seed, and its production is so handled as to maintain specific genetic identities and purity standards as prescribed for the crops being certified. The bags are sealed with blue coloured label.                     

Truthful seed :

         It is seed obtained from certified seed. It is produced on seed multiplication farm of a state government or agricultural universities and at the farm of progressive farmers. Truthful seed plots are jointly inspected by the SCA (seed certification agency) . The minimum standard of purity and germination percentage is maintained. The bags are sealed with green coloured label.