SOURCES OF CONTAMINATION

Several aspects determine seed quality. The contaminants are chiefly responsible for poor quality seeds. The contaminants can be classified as genetical and physical contaminants.  Genetical contaminants consist of off-types, pollen shedders and shedding tassels.  Physical contaminants are inseparable other crop plants, objectionable weed plants and diseased plants.  Above mentioned sources of contaminants are discussed below.  

 

a. Off-types: Off-type is a plant, which differs, in morphological characters from the rest of the population of a crop variety.  Off-type does not confirm to the morphological characters as described by the breeder.  Off-type may belong to same species or different species.  Off-types are easily identifiable as it deviates from the given set of morphological characters such as bulb, tuber, root features, plant, plant type, branching, pigmentation, mottling, hairiness, stem features, leaf shape and arrangement, colour, shape and size of flower and flower parts and colour, size and shape of fruit and seed or characteristics such as duration to flower and maturity, tillering, male sterility, resistance to diseases etc. Plants of a different variety are also included under off-types.   

 

    

Volunteer plants and mutants are also off types. Male variants, female variants contaminantly sex expressing plants (i.e.) castor, Cucumber etc., are also treated as off types. 

 

To designate a plant as an off type, tracing to any variety is not necessary.  During field inspection, an offtype is counted irrespective of its growth stage.  If the crop plants and offtypes differ so widely in growth stages and contamination is not possible at any stage, in that case, the producer should be informed for their removal.

 

Selfed bolls / fruits: In hybrid seed production of cotton, tomato, brinjal, muskmelon etc. selfed bolls / fruits are treated as contaminants.  Selfed bolls / fruits are the bolls / fruits in the seed parent developed from the flowers, which escaped crossing and developed by self fertilization and improperly emasculated flowers.

 

b. Pollen shedders 

 

These are contaminant plants in the production of parental lines and hybrids. By Exploiting male sterility, (Genetic or Cytoplasmic) male sterile line (Line “A”), male fertile maintainer (Line “B”) and Restorer (Line “R”) are involved in parental and hybrid seed production. 

 

“A” line or male sterile line could not produce mature, reproductive pollen because of its male sterility.  “B” line or male fertile maintainer resembles “A” line in all morphological character, but produces mature viable healthy pollen grains.  When “A” line and “B” line are crossed the resultant progeny is male sterile “A:” line.  In hybrid seed production male sterile ‘A’ line and ‘R’ line are allowed to be crossed and the resulting hybrid is male fertile and produces, healthy pollen.  Hence the presence of “B” line in the “A” line (AxB) or (AxR) production is called pollen shedder.  Some times “A” line tends to exhibit symptoms of fertility on one or more side tillers or one side or part of an earhead.  These plants are also called as pollen shedders.  

 

c. Shedding Tassels. 

 

Tassel is the male or staminate inflorescence at the tip of a maize plant.  Shedding tassel means those plants which shed or shedding pollen in female parent rows.  In counting process sucker tassels, portions of tassels, and tassels on main plants should be counted as shedding tassel, only when 5 cm or more of the entire spike, the side branches or a combination of the two have the anthers exerted from the glumes and are shedding pollen.  

 

Tassels, which have exerted, but not yet shed, are not to be counted, even though these tassels are to be mentioned as “Not commenced shedding”.  Producer must be informed of these and asked to rogue these tassels before pollen shedding. 

 

d. Inseparable other crop plants. 

 

These plants are very similar and resemble closely to the seed crop plants in habit and growth characters. Separation of other crop seeds from seed crop seeds is also difficult even by mechanical means.  Since these other crop plants have similar habit and growth characters, counting is to be taken up along with other contaminants.  Presence of other crop plants in the seed field must be informed to the producer and asked to remove in time.

Crop designated as inseparable in various crops:

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Crop                          Designated inseparable crop Plants

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Wheat                     Barley, Oats, Gram and Triticale

Barley                     Oats, Gram, Wheat and Triticale.

Oats                       Barley, Wheat, Gram and Triticale

Triticale                  Wheat, Barley, Oats, Gram and Rye. 

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e. Objectionable weed plants 

 

Weeds whose seeds are difficult to be separated once mixed with crop seed or which are poisonous or injurious or having smothering effect on the main crop or are difficult to eradicate once established or are having high multiplication ratio thus making their spread quick.

 

OBJECTIONABLE WEED SPECIES IN CROP PLANTS

 

Sl.No.	Crop	Name of the weed species	Botanical name of the species
1 2   3 4 5   6 7 8 9         10 11     12 13   14       15 16     17 18 19	Paddy Wheat   Rape Taramira Sunflower   Safflower Berseem Lucerne Napier         Oats Bitter gourd     Cucumber Long melon   Snakegourd     Watermelon Bhendi     Amaranth Fenugreek Lettuce	Wild rice Wild morning glory, Gulli danda Mexican prickly Poppy Mexican prickly Poppy Wild sunflower Orabanche Wild safflower Chicory Dodder Canada thistle Dodder Jhonson grass Quack grass Wild morning glory Wild oats Balsam apple Bhat karela Jangle karela Cucumis Weed melon       Jangli Chachinda Wild watermelon Wild okra     Wild amaranth Senji Wild lettuce	Oryza sativa L Var.Fatua prain   Convolvulus arvensis L. Phalaris minor Retz.. Argemone mexicana L. Argemone mexicana L. Helianthus spp. Orabanche cumana Carthamus oxyacantha Chicorium intybus L. Cuscuta spp. Crisium arvense (L.) Scop. Cuscuta spp. Sorghum halepense Agrofyron repens Convolvulus arvesis Avena fatua L. Momordica balsamina   M.chochinchnensis      M.diocia Roxb. Cucumis hard wickil Royle Cucumis melo Cucumis prophetarum Trichosanthes palmata L. T. lobata T. cucumernia L. Citrulus colocythis L. A. ficulneus (L) A. manihot (L) A. moschatus (L) Amaranthus spinosus Melilotus spp. Lactuca scariola L.

        F. Diseases

Seeds are known carriers of harmful pathogens internally or externally or by both causing diseases which make a seed lot unfit for use.  Prophylactic control measures can be applied to prevent and control disease causing pathogens in seed lots.  In order to produce disease free seeds, some diseases are designated and standards have been fixed in some crops.  During field inspection symptoms of these designated diseases should be observed and counted during vegetative, flowering and maturity stages as indicated in IMSCS.  The list of designated diseases in various crops is given below.   

DESIGNATED DISEASES IN CROP PLANTS.