338 MUTATION AND PLANT BREEDING 



Table 1. — Factors Influencing the Genotypic Response to Progressive Mutation 



in Cultivated Plants. 



Nutrimental and industrial or ornamental use 



Age from standpoint of evolution, cultivation, regional adaptation, and scientific breeding 



Extent and availability of natural variation, including possibilities for interspecific gene 



transfers 

 Anatomy and differentiation of meristematic tissue 

 Annual and biannual or perennial habit of growth 

 Autogamy, allogamy, or clonal propagation 



Genie duplication and tolerance to chromosomal rearrangements 

 Inherent mutagenic resistance and environmental possibilities to its modifications 

 Degree of heterozygosity 



Phenotypic buffering in relation to characteristics 

 Presence of desirable precursor genes 

 Combining ability with genotypic background 

 Goal of the specific breeding program and availability of effective selection techniques 



acute and chronic radiation with chromosome size and level of ploidy 

 as the most decisive characteristics (39, 52, 72, 73, 80, 100). It is inter- 

 esting, though by no means conclusive, that practical results in muta- 

 tion breeding were early reported for rape and mustard, two extremes 

 in high seed radio-resistance among agricultural crops (2, 3). 



Clear differences in primary radio-resistance are also found 

 between closely related genotypes. The degree, and thus the chance 

 to obtain maximum yield of mutation, may be controlled by inherent 

 differences in simple metabolic activities (51). It may depend on indi- 

 vidual chromosomes (114) or even genes (30, 61, 62, 99). It has been 

 proved to vary witth degree of genotypic stability (8), can easily be 

 recombined, and is dependent on type of radiation (34, 35). From 

 Drosophila experiments, Ave can expect varietal differences in the 

 response to chromosome breaks (16, 18). Gregory (34, 35) has even 

 shown that it is possible to induce mutants with improved resistance 

 to primary radiation injury. 



An example on induced decrease in radio-resistance is given in 

 Table 2. Such a phenomenon has implication also in connection with 

 recurrent radiation experiments in the same or successive generations. 

 In the latter case, Shestakov, et al. (cited by Hoffmann, 48) found an 

 increased sensitivity to radio-phosphorus in the second generation of 

 treatment, and the same trend was found by Hoffmann (48) in acute 

 X-irradiation of successive generations of wheat. 



With respect to the degree of heterozygosity, it appears that not 



