Aspects of plant breeding
Present day crop plants are from wild species reared by careful
domestication, cultivation and management. We have several wild varieties
existing in nature just as in the case of salinity tolerant wild rice.
Through gene manipulation, the gene for salinity tolerance could be cloned in a
rice variety. In such of those areas where shortage of fresh water exists,
rice can be cultivated using seawater and can even be grown in extreme
Similarly, we need many more such wild plants showing increased capacity to extremes of climatic conditions and disease resistance for plant breeding programmes. In order to safeguard the biodiversity and certain important valuable crops from going into extinction, scientists are protecting these crops by establishing gene or germplasm banks by preserving their seeds.
It is one of the oldest procedures in which individual plants or group
of plants are sorted out from mixed population.
Thus eliminating undesirable ones. Selection methods are of two types – mass selection and pure line selection.
In this method, plants are selected based on their desirable morphological characters (phenotype). Their seeds are composite or mixed and the progenies are grown in masses. They are not individually tested.
After repeated selection for about five to six years, selected seeds are multiplied and distributed to farmers. The only disadvantage of mass selection is that it is difficult to distinguish hereditary variation from environmental variation.
Pure line selection
A pure line is a collection of plants obtained as a result of repeated
self-pollination from a single homozygous individual. Hence, a variety
formed by this method shows more homozygosity with respect to all genes.
One disadvantage is that new genotypes are never created by this method.
Genetic variability is essential for adaptations in different environmental
and seasonal conditions.
Crops like sugarcane, potato, tea, banana and certain species of grasses
are asexually propagated and produce very poor seeds. Based on their
phenotypic appearance, the method of clonal selection is employed to
select improved variety from a mixed population (clones).
Selected plants are multiplied through vegetative propagation to give rise to a clone. The genotype of a clone remains unchanged for a long period of time.