Glossary Plant Breeding

principally in three different ways: (a) direct use by increase in masse (e.g., Taichung
Native 1ofriceandSonoro 64 of wheat), (b) selections made from introductions
(Sonalika from segregating materials of bread wheat), and (c) use in hybridisation.
However, the present role of plant introductions, owing to emphasis for area-specific
breeding programmes, is limited to providing a reservoir of germplasm at the disposal
of plant breeders.
Plasticity. The degree to which the expression of an individual’s genotype can be
modified by environmental factors, irrespective of the adaptive value of that
modification. The plasticity may be specific to a particular character, particular
environment, or specific in direction. It may be under genetic control, and can be
fundamentally changed by selection.
Plastid. A cytoplasmic organelle in plant cells, primarily concerned with the formation
and storage of soluble and insoluble carbohydrates. Plastids include chloroplast
(having chlorophyll pigments), chromoplast (containing carotenoids commonly found
in fruits and flowers), and leucoplast (with no visible pigments, such as amyloplast).
Plastogene. Any one of the hereditary determinants located in the plastids of plant cells.
Plastom. The sum total of plastogenes present in the plastid DNA.
Pleiotropy. A phenomenon of having more than one seemingly unrelated phenotypic
manifestation of one gene; such a gene is called pleiotropic gene. Mutation in such a
gene is called pleiotropic mutation. For instance, the recessive gene for white-eye
colour in Drosophila melanogaster also affects the testicular membrane, the shape of
spermatheca, longevity, and general viability. The well-known - Dee-geo-woo-gen -
has not only shortened the plant height but has also changed angle of inclination of
leaves, lodging tendency, fertilizer responsiveness, etc. in rice. The above examples
indicate that each “major” gene could have one or more side effects; and if side
effects were favourable, selection for major effect would be accompanied by
correlated response for all other side effects. However, unfavourable side effects
would preclude exploitation of pleiotropy in crop improvement. It should be noted
that most genes, if not all, affect more than one characteristic.
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