226 MUTATION AND PLANT BREEDING 



the procedure in recurrent irradiation programs, and various other 

 topics. Previously it was suggested to use a dose which leads to about 

 50 per cent surviving Mi plants (23, 43). In contrast to these earlier 

 findings we have recommended applying higher doses which result 

 in 10 to 20 per cent (or even less) survival. At 90 per cent lethality of 

 Mi plants the mutation rate may be nearly doubled as compared 

 with 50 per cent lethality. The discrepancy with the older results 

 is mainly a consequence of the fact that the mutation frequency 

 was not measured correctly (27, 32). 



There are large differences in the radiosensitivity between 

 various species, necessitating quite different doses. In an extensive 

 study, Sparrow, et al. (103) considered nuclear volume (of mer- 

 istematic cells), polyploidy, and chromosome number as the major 

 factors determining radiosensitivity. A table of doses leading to 

 approximately 50 per cent survival in various agricultural plants 

 can be found in Gustafsson and v. Wettstein (48). 



Diplontic Selection 



Mutagenic treatment of seeds or other parts of the plant results 

 in the formation of chimeras. The efficiency of mutant selection 

 depends, therefore, on the problem as to which parts of the plant the 

 progenies should be grown and whether the elimination of mutations 

 within the plant (diplontic selection) can be inhibited. Unfortunate- 

 ly, little is known in this field. After treatment of barley seeds, we were 

 able to show that about the first five tillers per plant possess a 

 considerably higher mutation frequency than tillers formed later 

 (27, 30, 33). It has therefore been recommended to space the Mi seeds 

 extremely close in order to obtain high mutation rates (34). We also 

 have demonstrated that the size of chimeras varies with the dose. With 

 very low X-ray dose the average size of a mutated sector may comprise 

 about one quarter of the generative tissue of a single spike. With 

 increasing dose this size increases correspondingly and finally all the 

 florets of a spike contain the same mutation. With greater tillering, 

 the mutated sector may then even include several spikes of a plant 

 (30, 33). The decrease of mutation frequencies in later formed tillers 

 has recently also been demonstrated in rice by Bekendam (4, 5). In 

 addition, the same author was able to show that in rice, as in barley, 

 after irradiation of seeds the generative tissue of a single head may 

 derive from one to four embryo cells. The chimeric structure has also 



