102 MUTATION AND PLANT BREEDING 



effect on the yield (per r of exposure) of breaks or on any of the 

 known genetic changes derived from breakage. 



c. DNA content per nucleus. — The DNA content is related to 

 average nuclear volume, and it appears that the larger the amount of 

 DNA per complement, the higher the radiosensitivity (164, 168). 



d. Amount of hetero chromatin. — The amount and distribution 

 of heterochromatin has been shown to affect the radiosensitivity of 

 nuclei in various ways (39, 157). 



e. Number of nucleolar organizing regions. — The number of 

 such regions is often related to the amount of heterochromatin and 

 may be related to the average amount of nucleolar material per 

 nucleus. Both of these factors are known to affect the radiosensitivity 

 of cells (41, 57). 



f. Number of nuclei. — It has been shown, for example in fungi, 

 that the number of nuclei can affect the radiosensitivity (81). It 

 would also be expected that multinucleate cells would be more resist- 

 ant than the normal type of mononucleate cell. 



g. Stage of the nuclear cycle. — The stage of the nuclear cycle has 

 been known for many years to affect the amount of chromosome 

 breakage (14, 125, 138, 157) and the amount of mutation produced 

 per roentgen of exposure. In some cases the extent of the differences 

 is quite large. For instance, in Trillium erectum there is about a 

 60-fold difference in amount of fragmentation between the least sensi- 

 tive and most sensitive stages of meiosis (39, 157). In Zea mays muta- 

 tion rates also vary widely during microsporogenesis (148). Changes in 

 lensrth of chromonemata at different stages of meiosis in Trillium 

 also has been correlated with sensitivity (169). 



h. Average length of nuclear cycle or of intermitotic time. — It 

 has been shown, especially for chronically irradiated material, that 

 the longer the average nuclear cycle, the more radiation damage is 

 accumulated under a given dose rate (121, 163). 



i. Position of a gene in tJie chromosome. — It has been clearly 

 shown that the position of a gene with reference to the centromere 

 has a significant effect on the frequency of deletion (42, 163). 



j. Position of the centromere and length of chromosome arm. — 

 Both of these factors are known to affect radiation response, although 

 more work is required to establish clearly the degree of the effect 

 (41, 42). 



