344 MUTATION AND PLANT BREEDING 



A chronic or recurrent treatment will act similar to a delayed 

 application, but offers, in addition, chance for a continuous selection 

 against very harmful mutations and a successive accumulation of 

 more vital changes. Pursuant to this reasoning, Mikaelsen (77) found 

 chronic radiation of barley definitely more effective than the com- 

 parative, acute dose already given to the seed. By applying a recurrent 

 treatment, gross deleterious effects may be eliminated before new 

 valuable mutations are added step by step. In vegetatively propagated 

 plants, where a shift in generation over a sexual phase may break 

 down a complex but well-balanced, heterozygous genotype, such a 

 recurrent treatment may have great possibilities even to inactivate a 

 dominant gene in homozygous position. As will be discussed by 

 Caldecott (11) in this symposium, recurrent treatment of successive 

 generations of seed-propagated plants has special merit, since a muta- 

 tion can here be removed from competition with the parental geno- 

 type before new mutations are induced. 



In certain situations, mutation rate can be enhanced by proper 

 selection of M] individuals. Selection can directly be made for domi- 

 nant mutations or recessive changes in heterozygous position. Induced 

 mottling or streaking of primary leaves or other signs of injury are 

 generally correlated with mutagenic effect (55, 68, 117) and can 

 preferably be used as a basis for selection when treatment is not 

 uniform among individuals. Thus, Blixt, et <d. (6) were able greatly 

 to increase mutation rate by selecting mottled pea seedlings treated 

 with ethyleneimine. A similar improvement is reported by Gregory 

 (33) after selection for X x seedling injury in peanuts. Since there does 

 not appear to exist any correlation between chromosome and point 

 mutations if competition between shoot initials is eliminated (27, 29), 

 a shift to either category can be obtained by selecting semifertile or 

 fertile Mi plants, respectively. 



Even with the best choice of parental material, the highest 

 efficiency in mutagenic application, and the most appropriate han- 

 dling of the treated generation, the vast majority of scorable muta- 

 tions will be harmful or practically uninteresting. The economical 

 success of mutation breeding will, therefore, depend greatly on the 

 efficiency of selection in the segregating generations. It is also symp- 

 tomatic that the most rapid and elegant completions in mutation 

 breeding are generally directly associated with a specific screening 



