Genetic purity refers to true to type or percentage or ratio of contamination of seed caused by undesired genetic varieties or species. The production of genetically pure seed requires high technical skills and comparatively heavy financial investment. One of the major objective to maintain genetic purity is to produce hybrid seed and quality seed. Quality seeds are the basic inputs of the crop production . It improves productivity of crops by about 25% and reduces the amount of seed requirement by 25%. There is less chances of weed , insect and diseases and decrease cost of production.
The genetic purity of variety or trueness to its type deteriorates due to several factors during the production cycle. Developmental variations ,mechanical mixtures ,mutation , Natural crossing ,minor genetic variations ,selected influence of disease and pest, technique of plant breeders are the main cause of varietal deterioration.
1. Developmental variations : several consecutive generation’s developmental variations may set in different growth responses when seed crops are grown under environments with differing soil, fertility ,climate photoperiod , or at different elevations.so the varieties should be grown in their areas of their natural adaptation .
2. Mechanical mixtures : mechanical contamination at the time of sowing, harvesting ,threshing, processing ,and handling of seed is the major the major cause of varietal deterioration. So rouging the seed field at different stages of crops is necessary.
3. Mutation : It is often difficult to detect minor mutations occurring naturally in crop variety. So detectable mutants can be removed by rouging.
4. Natural crossing : natural crossing is an important source of varietal deterioration in cross pollinated crops. The deterioration sets due to natural crossing with undesirable types, diseased plants or off types. The varietal deterioration due to natural crossing in field depends on the breeding system of species, isolation distance , varietal mass and pollinating agent . Maintaining the isolation distance between the seed crops is the major way to avoid natural crossing.
5. Selected influence of disease and pest: In case of foliar disease ,seed size is affected due to poor supply of carbohydrates from infected photosynthetic tissue. In case of soil and seed borne disease once crop is infected use of their commercial purpose is dangerous. New crop varieties become more susceptible to newer races of pest and diseases. Hence these varieties can out from seed production programme.
6. Minor genetic variation: It can occur even in varieties appearing phenotypically uniform and homogenous when released. The variation may lost during later production cycles owing to the selective elimination by the nature . The use of pure breeder seed in self pollinated crops and due care during maintenance of nucleus and breeder seeds of often fertilized crop varieties is necessary to avoid minor genetic variation.
Therefore , during seed production strict attention and especial genetic principles should followed to maintain genetic purity. Major ways to maintain genetic purity during seed production are:
1. Control of seed source : seed of the appropriate class must be selected from the appropriate source seed production .There are five classes of seed ;nucleus seed , breeder seed ,foundation seed ,certified seed and improved seed. Nucleus seeds and breeder seeds is used for genetically pure seed production.
2. Isolation : crop field must isolated from other field to avoid natural crossing and mechanical mixtures with other undesirable type off types in the field .
3. Adaptation of crops: crops should be grown only in the areas where they can adopt to avoid genetic shift or developmental variation.
4. Field inspection : qualified , experienced and trained personnel should be employed to inspect crop at different stages of growth and to verify seed lots or purity and quality.
5. Rouging: It means removal of off types plants .Rouging of off types differing in their characters of those of seed variety is very important. The existence of off type plants i.e. the plants with differ in their characteristics from those of the seed variety is another potent source of genetic contamination.
Similarly , periodic testing of varieties for genetic purity, preceding crop requirement : inspection and approval of seed plots prior to planting, adopting generation system are the ways to maintain genetic purity during seed production.
