440 MUTATION AND PLANT BREEDING 



among the M 2 progenies. These criteria present a broad base for inter- 

 preting the effects of the radiation and the interaction between radi- 

 ation effects and secondary factors and for comparative analysis of the 

 action of chemical mutagens on plant cells. By measuring both chro- 

 mosomal and genetic damage induced by the same treatment, one 

 may obtain a greater understanding of the nature and control of the 

 induced mutation process. 



Increasing Induced Mutation Yield 



An increased total of induced mutations has been sought 

 through techniques that increase the radiation dose tolerance of tis- 

 sues and reduce the amount of chromosome damage without an associ- 

 ated reduction of induced mutation frequencies in the treated cells. 

 These techniques have involved the control and manipulation of 

 secondary factors in irradiated seeds, the transfer of plant-cell com- 

 pounds from radio-resistant to radio-sensitive species, and the use of 

 the chemical mutagens diethyl sulfate and ethyl methane sulfonate 

 which induce high mutation frequencies but relatively few chromo- 

 some aberrations. 



Increasing the radiation dose tolerance 



Under normal conditions, plant tissues have distinct levels of 

 tolerance to doses of ionizing radiation. Doses of radiation above these 

 levels lead to such low cell survival that most mutations are lost with- 

 in the plant. These levels, which vary from species to species, are 

 governed to a large extent by numerous secondary factors, only a few 

 of which are understood. Nevertheless, through proper manipula- 

 tion of a few of them in irradiated seeds, it has been possible to 

 increase the radiation dose tolerance, and, hence, mutation yield. 



Extensive studies have shown that after-effects, as they are related 

 to time, oxygen, moisture, and temperature, are a most important 

 influence on the degree of damage in X- and gamma-rayed seeds 

 (3, 5, 6, 9, 10, 21, 23, 29, 30, 31, 32). Through proper control and 

 manipulation of oxygen, moisture, and temperature before, during, 

 and after irradiation, the dose tolerance of the seed has been greatly 

 altered. Because of the several recent and extensive reviews on this 

 subject, only a brief summary will be presented here. 



Oxygen-effects and after-effects in irradiated barley seeds increase 

 radiation-induced damage, which in turn causes low cell and plant 



