104 



MUTATION AND PLANT BREEDING 



Table 8. — Physiological Factors which Affect Sensitivity. * 



•After Guntkel and Sparrow (62). Also see footnote to Table 6. 



certain physiological or biochemical steps involved in protein and 

 nucleic acid synthesis (2, 66). Various factors listed above under 

 chemical factors could perhaps have also been included in this section. 

 Also, please refer to Table 7. 



Summary and Conclusions 



This paper attempts to review certain aspects of the physical 

 nature of ionizing radiations and their effects upon nuclei, chromo- 

 somes, and genes. The major physical characteristics of a number of 

 the ionizing radiations most important to radiobiologists are pre- 

 sented and some methods of treatment and facilities commonly used 

 are described. The major biological effects, including nuclear and 

 chromosomal changes known to be induced by ionizing radiation in 

 plants or plant cells, are listed and explained briefly. Special attention 

 is given to the problem of chromosome breakage and aberration. The 

 kinetics of the dosage response curves obtained under different con- 

 ditions for various types of aberrations and mutations are explained 

 and discussed. The possible relationship between chromosome aber- 

 ration and mutation, and the factors which influence radiosensitivity 

 are considered in some detail. 



The review is mainly concerned with fundamental aspects of 

 radiation cytogenetics and does not attempt to relate this knowledge 

 to practical problems of plant breeding. It is concluded that further 

 comparative studies of the effects of different physical and chemical 



