222 MUTATION AND PLANT BREEDING 



raised from 26 to 41 after 10 selection cycles. In contrast, the con- 

 trol strains (Cp and Dp) did not undergo any appreciable change. 



Mutagenic Agents as a Tool for Special Purposes 



Physical and chemical mutagens may be exploited in plant 

 breeding in various other ways than those discussed above. A num- 

 ber of other possibilities are outlined in Table 2 and will be brief- 

 ly discussed here. The use of translocations in transferring a valu- 

 able character from one species or genus to another has recently 

 been reviewed (28), including the typical and fully analysed 

 example of Sears (98). In the same review (28), the problem of 

 diploidization of artificially produced tetraploids is outlined. 



Use of duplications to produce "directed mutations" has been 

 suggested by Hagberg (50, see there also older references). For this 

 project, it is necessary to have a very large set of translocations in the- 

 genotype in which the breeding work shall be done and to have 

 these translocations localized. If a greater gene content of the chro- 

 mosomes is also mapped, then it should be fairly easy to produce 

 definitely localized duplications (and deletions) of short segments 

 carrying desired genes through crosses of adequate translocation 

 types. 



Irradiation may also be useful in apomictic species to produce 

 transitory sexuality for one or a few generations, as has been shown 

 by Julen (62) for Poa pratensis. This enables the breeder to make 

 crosses and to select in later generations new apomictic forms with 

 improved characters. 



That irradiation can be exploited for the production of hap- 

 loids has been known for a long time and is outside the scope of 

 the present paper. Attempts have also been made to overcome the 

 interspecific incompatibility by exposing male or female gametes 

 or tissues to gamma radiation prior to crossing. In one instance, 

 that of the cross Brassica oleracea X B. nigra, the technique appeared 

 to be successful and many interspecific hybrids were produced 

 (16). 



Methods of Induction and Selection of Mutants 



The significance of mutations in plant breeding will largely 

 depend on the progress of their production methods. It is impor- 

 tant to obtain a greater yield of total mutations and particularly of 



