CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 389 



nomic significance. By and large these studies have involved attempts 

 to induce so-called point mutations. The organisms most commonly 

 studied have been the small-grained cereals, peanuts, and corn. Data 

 obtained from studies with these species have been discussed in other 

 sections of this Symposium. The intent of the present report is to 

 mention only theoretical considerations that may be of relevance to 

 the plant breeder and which have received very little attention by 

 most workers. 



In using ionizing radiations in plant improvement, one must 

 take into consideration the degree of ploidy and the breeding 

 behavior or mode of reproduction of the species with which he 

 intends to work. In addition, one must be cognizant of the fact that 

 ionizing radiations not only induce point mutations but also all 

 manner of structural chromosomal anomalies, such as reciprocal 

 translocations, inversions, duplications, and deletions. Furthermore, 

 he must be aware that the frequency with which these types of events 

 are produced, and their ease of detection, will be dependent upon 

 the species with which he is working and the ontogenetic stage of the 

 plant to which the irradiation is given. 



To reduce this complex of variables in the present report, unless 

 otherwise stated, we shall consider only what may possibly be achieved 

 through seed irradiation of the naturallv self-fertilizing cereals which 

 occur in nature as diploids, tetraploids and hexaploids. 



Mutations From a Single Dose of Radiation 



On a priori grounds, in the X 2 from populations receiving one 

 dose of radiation it would be expected that plants expressing mutant 

 phenotypes for qualitative characters would occur commonly in the 

 diploid species, with a lower frequency in tetraploid species, and only 

 rarely in hexaploid species. The reason for this is simply that most 

 radiation-induced mutations are recessives and the simultaneous 

 mutation of genes which influence the expression of the same charac- 

 ter, but are located on homeologous chromosomes, would be a rela- 

 tively rare event, even under optimal conditions of treatment. The 

 essential validity of this consideration has been borne out by chloro- 

 phyll mutation studies on tetraploid and hexaploid oats and wheat. 



In addition to the above consideration, there is another important 

 factor which makes it difficult to recover a large number of mutant 



