Dear Dr. Ramkishore Singh Rathore Sir,

Please follow the following link and the attachment for answer to your query

http://agriquest.info/index.php/causes-for-varietal-deterioration-maintenance-of-genetic-purity-and-principles-of-seed-production

Regards,

Dr. Arvind Singh

Generally, a small number of plants are maintained by the plant breeder or a germplasm bank. Care is taken to prevent contamination, including cleaning harvesting equipment, growing in isolation from other varieties (if outcrossing), and manual removal of phenotypic off-types. Periodically, "breeder's seed" from these plants is increased for use in production.

BY growing progeny rows in isolation  and selecting the good plants, one can maintain a good crop variety  for a longer period, Growing progeny rows and selection of good plant purifies the phenotype whereas growing in isolation allows little bit crossing between plants to maintain some vigour. Alternate years growing decreases the rate of inbreeding depression of a genotype. Thus purity and vigour both are maintained symultaneously.

Ramkishore, the response to your query will depend on whether the crop in question is open pollinated or self-pollinated. Based on my previous experience in plant breeding, in both cases of open and self-pollinated crops, they can be isolated by time, place and space in efforts to maintain its purity; however in the case of open pollinated, in addition to isolation in time, place and space, some specific plant breeding techniques are required. Furthermore, some manual selection based on the variety phenotype can be made in both cases in efforts to further maintain varietal purity.

I hope this response supplement and/or complement  the ones posted previously by other colleagues.

Further to the above responses, in the traditional agriculture, farmers usually harvest selected panicles from rice, sorghum, etc., and cobs from maize for seeds just  prior to harvesting the crop for the purpose of  maintaining varietal purity. For other seed producing species of crops - i.e. vegetables producing pods, the same is practised. Although some crossing goes on, this has been a long used technique for maintaining varietal purity at the best out of farmers' situations. On the top of that the above responses ensure the genetic purity, as mentioned. 

Estimado Dr., primero verifique que tipo de semillas son: Recalcitrantes u Ortodoxas. Las semillas ortodoxas pueden ser almacenadas a bajas temperaturas de 0 a 4°C generalmente y con bajos contenidos de humedad en las semillas de 5 al 10%. Las semillas recalcitrantes no toleran la deshidratación ni el almacenamiento a bajas temperaturas, son semillas de vida corta a diferencia de las ortodoxas. Saludos