CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 393 



influence the expression of a given character. Furthermore, apparent- 

 ly these genes are often in the homozygous dominant condition on 

 more than one pair of the homeologous chromosomes. Now, if in 

 diploids a heterotic effect is obtained when differences exist between 

 alleles on a pair of homologs, it seems possible that in a polyploid 

 differences between genes which govern the expression of the same 

 character, but are on homeologous chromosomes, may also confer 

 increased vigor to plants in which the condition exists. If such is the 

 case, the self-fertilizing polyploid would have an advantage over the 

 diploid in that the "heterozygous" state (between genes on homeolo- 

 gous chromosomes) could be perpetuated by selfrng and thus provide 

 permanent heterosis by merely maintaining the alleles on one pair 

 of homologous chromosomes in the homozygous dominant condition. 



In regard to the possible usefulness of this approach in plant 

 breeding, it seems relevant to mention a promising study that is in 

 progress, using recurrent irradiation on hexaploid oats. In this pro- 

 gram the following procedure has been used. Seed from a single plant 

 from three different oat varieties, Park, Missouri 0-205, and Clint- 

 land, was increased and irradiated. In the X a , 1,000 fertile panicles 

 were chosen from different plants in each of the three varieties. The 

 seed from these panicles was re-irradiated and planted in panicle 

 rows. From these panicle rows one fertile panicle was chosen from 

 one agronomically suitable-looking plant to form the seed source for 

 the next cycle of radiation. This procedure has been continued 

 throughout the entire program, which has now run through six cycles 

 and is presently in the seventh. 



Some interesting general observations on this material are worth 

 reporting, although at this time there are no grounds for assuming 

 that a state of permanent heterosis exists in any of the material. First, 

 and most significant, is the fact that variability in the material for 

 height, panicle type, and maturity date was not in evidence until 

 after the second cycle of radiation. Subsequent to this there has been 

 a continued increase in variability for these characters. At the end 

 of the fifth cycle of radiation the variability was so extreme in all of 

 the material, with a considerable number of types that evidenced 

 characteristics of agronomic interest, that a number of plant selections 

 were made for testing with the parental type. These selections were 

 increased in 1959 and the promising ones were turned over to experi- 



