Form of Germplasm
1) Land Races:
“Primitive cultivars which were selected and cultivated by farmers for many generations.
Land races even respond- to –selection for high yield, but to certain extent. Since land races posses’ valuable alleles, their conservation is essential. The main drawbacks of land races are that they are less uniform and low yielders. Land races were first collected and studied by N.I Vavilov in rice. Land races have been collected in maize, sorghum, pearl millet and many other crops especially in South Asia.
2) Obsolete Cultivars:
Improved varieties of recent past are known as obsolete cultivars. These are the varieties which were popular earlier and now have been replaced by new varieties. These varieties have several desirable characters and constitute an important part of gene pool. For example, Wheat varieties K-68, K-65, and Pb-591 were most popular traditional tall varieties before introduction of high yielding dwarf Mexican wheat varieties. These varieties are well known for their attractive grain colour and chapatti making quality. Now these varieties are no more cultivated. They are good genetic resources and have been widely used in wheat breeding programmes especially in India for improvement of grain quality. Now such old varieties are found in the gene pool only.
3. Modern Cultivars:
The currently cultivated yielding varieties are referred to as modern cultivars. Modern cultivars are also known as improved cultivars or advanced cultivars. These varieties have high yield potential and uniformly as compared to obsolete varieties and land races. Modern cultivars constitute a major part of working collections and are extensively used as parents in the breeding programmes for further genetic improvement in various characters. Hence these cultivars are in great demand. These varieties are the outcome of scientific plant breeding and have been developed for modern intensive agriculture. However, modern cultivars have narrow genetic base and low adaptability as compared to land races.
4. Advanced Breeding Lines:
Pre-released plants which have been developed by plant breeders for use in modern scientific plant breeding are known as advanced lines, cultures and stocks. They include advanced cultures which are not yet ready for release to farmers. Sometimes advanced breeding lines and stock are not very much productive, but constitute valuable part of gene pool for various economic characters.
5. Wild Forms of Cultivated Species:
Wild forms of cultivated species are available in many crop plants. Such plants have generally high degree of resistance to biotic and abiotic stresses and are utilized in breeding programmes for genetic improvement of resistance to biotic and abiotic stresses. They can easily cross with cultivated species. However, worms of many crop species are extinct. Moreover, entire range of diversity of available wild forms is rarely tapped. They constitute small part of gene pool.
6. Wild Relatives:
Those naturally occurring plant species which have common ancestry with crops and can cross with crop species are referred as wild relatives or wild species. Wild relatives are important sources of resistance to biotic (disease and insects) and abiotic (drought, cold , frost, salinity, etc) stresses, however, wild relatives are used as last resort in crop improvement programmes, because their use in crossing leads to: 1) Hybrid sterility, 2) Hybrid and 3) Transfer of several undesirable genes to the cultivated species along with desirable alleles. This group constitute a minor part of gene pool. Interspecific derivatives are added to the gene pool.
Mutation breeding is used as when the derived character is not found in the genetic stocks of cultivated species and their wild relatives. Mutations do occur in natures as well as can be induced through the use of physical and chemical mutagens. The extra variability which is created through induced mutations constitutes important component of gene pool. Mutants for various characters sometimes may not be released as a variety, but they are added in the gene pool. For example, mutant gene pool Dee-Geo-Woo- Gene in rice and Norin-10 in wheat proved to be valuable genetic resources in the development of high yielding and semi dwarf varieties in the respective crop species. In seed propagated crops, 410 varieties have been released through the use of the mutants in the crosses. (IAEA, 1991).