454 MUTATION AND PLANT BREEDING 



methane sulfonate, may be more efficient than radiation in certain 

 aspects of mutation induction. In barley seeds, these chemicals 

 induce very high mutation frequencies without an appreciable fre- 

 quency of gross chromosome aberrations, and apparently produce 

 a slightly different mutation spectra from X or gamma rays. 



It is obvious, however, that the required information for still 

 greater progress in increased efficiency of mutation induction in this 

 area is at present only fragmentary. Much more work should be 

 conducted in radiobiology, biochemistry, genetics, and cytology to 

 obtain more adequate information on induced mutations for plant 

 breeders of the future. 



In the final analysis, it is apparent that the greatest advances 

 toward increasing the efficiency of mutation induction in plant 

 tissues will come only when the basic mechanisms of radiation and 

 chemical mutagen effects in cells are completely understood. Studies 

 of the influence of secondary factors on radiation damage in seeds 

 have extended our concept of the physical and chemical pathways 

 of effects of ionizing radiation in cells. They have shown that much 

 of the radiation damage in the chromosomes or genes is due not to 

 the direct ionizing event but to intermediate agents (10, 32). Cer- 

 tain of these agents, free radicals, have been detected in seeds 

 following irradiation. Furthermore, they seem to be influenced by 

 the various secondary environmental factors to the same degree as 

 the radiobiological effects. The apparent association of these radia- 

 tion-induced radicals with biological damage was first demonstrated 

 with barley seeds and now has been confirmed and extended with 

 bacterial spores (34). Further studies on the nature of these inter- 

 mediate agents are necessary to increase our knowledge of mech- 

 anisms that produce radiation damage in seeds. 



It is unfortunate that much useful information obtained from 

 the seed radiobiological research has not been generally applied 

 by plant scientists, particularly plant breeders, in seed radiation 

 studies for practical or theoretical purposes. The techniques for 

 controlling secondary factors that have been described here and in 

 more detail in other publications have been ignored to a large 

 extent. And yet it should now be apparent to all who use seeds in 

 radiation experiments that the biological effects of a given dose of 

 radiation are meaningless unless the moisture content of the seed 



