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Important Factors Responsible for Deterioration of Genetic Purity of Crop Variety

1) Developmental Variation:

When varieties are multiplied outside area of normal cultivation. There is danger of developmental variation and genetic change or shift may appear in the varieties.
Sometimes it becomes necessary it raise the seed crop outside their area of adaptation to maintain steady supply of good quality seed. E.g Seed production of flowers there should not be rains during harvesting period; hence seed production needs to be taken outside the normal area of cultivation, where there are no rains in post flowering period.

Similarly seed production of rainfed cotton varieties can be taken under irrigated conditions which reduces land requirement significantly as productivity of material as irrigated crop is high. Disease free potato seed production is always taken at hilly region of simala though potato is extensively cultivated in plain regions. The extent of developmental variation increases with
1) Number of generation out the area of adaptation.
2) Condition of adaptation – The varieties bred for extreme condition i.e disease resistance, drought resistance against cold, show greater deterioration.
3) Mode of pollination –the cross pollinated crops varieties deteriorate faster than self pollinated.
4) Stability of genotype-Unstable genotype deteriorates fastly.

2) Mechanical Mixture:

This is dangerous source of variety deterioration. It occurs commonly when
1) More than one variety is sown in same piece of land.
2) When same drill used for sowing number of varieties.
3) It occurs when numbers of varieties are threshed on same yard.
4) It also occurs when same threshing machine is used for threshing number of varieties.
5) It also occur when gunny bags and storage bins are reused for storage of seed.
It happens when proper care is not taken during different operations. To avoid mechanical mixture to keep sufficient isolation is always desirable.

3) Mutation:

This is not serious factor for varietal deterioration. Minor mutations are difficult to identity. To avoid deterioration due to mutation minute observations and timely roughing is essential.

4) Natural Crossing:

It depends upon natural cross fertilization it may be due to
1) Natural cross with undesirable types
2) Natural crossing with diseased plants.
3) Natural crossing with off types.

Once the natural cross fertilization taken place the deterioration take place at cellular level and such deterioration is can not be rectified. In self pollinated crops the natural crossing is not serious factor of source of contamination and deterioration but it is serious in cross pollinated /Crops. The extent of cross pollination depends on varies factors

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