Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 172

Officially Released Mutant Varieties
Of a total of 1,847 accessions of the FAO/IAEA Mutant Varieties Database (http://
www-mvd.iaea.org), crop species are represented by 1,357 officially released mu-
tant cultivars and ornamental and decorative plant species by 490 mutant varieties.
Crop mutant cultivars were mainly developed in seed propagated plant species
(1,284 entries), whereas vegetatively propagated crops are represented by only 73
varieties. Among the cereals (869 mutant varieties), rice (333) ranks first, followed
by barley (261), bread wheat (147), maize (49), durum wheat (25), and others
(54). Most of the rice mutant varieties (67.6 %) were released as ' direct mutants ',
i.e. direct seed multiplication of selected mutants and their subsequent distribu-
tion to farmers. In addition, some mutants such as 'Reimei' (Japan) and 'Calrose
76' (United States) were successfully used in extensive crossbreeding programs.
Semi-dwarfness (129 varieties) and earliness (117 varieties) were the most often
selected characters from the treated populations. The list of improved characters
also contains traits desired for increasing sustainability in rice production, i.e. tol-
erance to cold (13) salinity (6), and photoperiod insensitivity (3). The vast majority
(201) of the directly released rice mutant varieties was induced with physical and
only 25 with chemical mutagens. Radiation was applied in 199 cases and laser
mutagenesis only in the development of two mutant varieties. Among the radiation
sources, gamma rays were used in 199 cases, including 37 varieties developed by
chronic gamma irradiation, followed by 14 with X-rays, 9 with neutrons, and 3
varieties with other sources of radiation. Methyl- and ethyl-nitroso-urea (12) as
well as ethyl methane sulphonate or EMS (9) was most commonly used as chemi-
cal mutagens to induce mutations for breeding new varieties. According to the
database, the mutant rice varieties were officially released in 26 countries. The
seven top countries are: China (117), Japan (46), India (31), United States (23) and
Vietnam (14). The economic impact of rice mutant varieties has been reviewed by
Rutger ( 1992 ).
Achievements Through Mutagenesis
Several achievements in crop improvement through mutation breeding: mutation
breeding efforts to date have resulted into two major outcomes—improved varieties
that are directly used as variety for commercial cultivation and new genetic stocks
with improved characters or with better combining ability. Although development
of new cultivars has been the primary objective of mutation breeding, the genetic
stocks developed can have numerous applications in plant breeding, from being
used as a donor parent in conventional breeding programme or as a parent in hybrid
breeding programme. Apart from these, mutation research itself has also a very dif-
ferent objective, i.e., mapping of genes. The technique of identification of a gene
by knockdown of the phenotypic expression through induced mutagenesis is a ma-