NILAN AND KONZAK: MUTATION INDUCTION EFFICIENCY 453 



this reason, very little research has been conducted to determine 

 the possibilities of mutation induction in these species. In the 

 self-incompatible species, this limitation might be overcome by 

 producing self-compatible or self-fertile strains of these plants through 

 physical or chemical mutagen-induced changes of the self-incom- 

 patible (S) alleles. A precedent for this approach may be found in 

 a paper by Lewis (25). 



Recurrent Mutagen Treatments 



Another means to produce higher mutation yields is mutagen 

 treatment of successive seed generations prior to selection of muta- 

 tions. In a self-fertile crop, such as barley, the procedure would be 

 to treat the seeds from fertile inflorescences at each generation. As 

 shown by Gaul (14), the frequency of point mutations or muta- 

 tions not associated with gross aberrations is independent of fer- 

 tility. Thus, through successive mutagen treatments of seeds from 

 only fertile inflorescences, it should be possible to accumulate high 

 frequencies of mutations in a line. 



This approach, as outlined by Caldecott (4) in more detail for 

 irradiation treatments, is essentially a mechanical means for remov- 

 ing induced chromosome aberrations while increasing the total 

 accumulated radiation dose that can be applied. It is probably far 

 less efficient than the controlled seed irradiation procedures described 

 earlier, but since it can be applied independently from other con- 

 trols or treatments, it has special merit. 



Summary and Conclusions 



Considerable progress has been made in recent years in increas- 

 ing the efficiency of mutation induction by radiation in seeds. 

 Greater precision, repeatability, and prediction in radiation experi- 

 ments have been achieved and higher frequencies of induced muta- 

 tions have been obtained through rigid control of after-effects, 

 moisture, oxygen, and temperature. Furthermore, through con- 

 trol and manipulation of these secondary factors, certain deleterious 

 effects of the radiation, such as low survival and high chromosome 

 aberration frequencies can be reduced; and some control and 

 direction of the induced mutation process may be realized. 



Certain chemical mutagens, such as diethyl sulfate and ethyl 



