452 MUTATION AND PLANT BREEDING 



for the mutations rather than a sectorial chimera as is the case with 

 irradiated seeds. Moreover, better comparisons would be possible 

 with chemical and ultraviolet treatments of pollen. Before these 

 studies can be practical, however, certain major difficulties must be 

 surmounted. These include maintaining pollen viability in some 

 species and adequate control over modifying factors. 



As early as 1930, Stadler (37) compared the probable advantages 

 of inducing mutations in zygotes or pro-embryos over those of seeds. 

 However, because of several difficulties inherent in these materials, 

 including the low dose tolerance, no further studies were conducted 

 to explore the merit of zygotes or even immature embryos in higher 

 plants as organs for the induction of mutations by use of radiation. 



In the past two years, the use of these organs for mutation 

 induction in barley has been investigated with some promising 

 results (26, 27). The irradiation of pro-embryos has resulted in a sig- 

 nificant increase in the size of the M 2 mutant population and in 

 the frequencies of tillers of a single plant with the same mutation 

 compared with that obtained from treated seed. These advantages 

 indicate that irradiated zygotes or pro-embryos may have definite 

 practical significance for the induction of mutations in certain self- 

 fertilizing crops. However, the efficiency of this technique will need 

 considerable improvement to approach the results now possible 

 with seed irradiation. 



Diploidization of Polyploids 

 Another means of facilitating the detection and selection of 

 mutations is by the diploidization of loci in polyploids through 

 mutagen treatment. Many of our crop species are polyploids; thus 

 the duplication, triplication, etc., of loci, restrict the segregation 

 and appearance of induced mutations. It may be possible to alter 

 these loci through mutagen treatment so that they behave as in a 

 diploid. Their alleles would then segregate in normal Mendelian 

 ratios, and induced changes at these loci would be easier to detect. 

 Some success with this technique has already been achieved and 

 reported at this symposium (6) and elsewhere (4). 



Alteration of Reproductive Mechanisms 



The reproductive system in cross-fertilized species influences 

 and limits the segregation and recovery of induced mutations. For 



