438 MUTATION AND PLANT BREEDING 



The results already obtained from these investigations concerned 

 with increasing the efficiency of mutation induction as well as indica- 

 tions of necessary future research will be the subject of this paper. 



Investigations Related to the Induced Mutation Process 



The manipulation of the induced mutation process appears to 

 be a promising way for increasing the efficiency of mutation induction 

 in plant improvement programs. Here the prime objective is to 

 increase total mutation yield. Eventually we may obtain sufficient 

 control and direction over the induced mutation process so that 

 specific mutations may be produced. 



Some progress toward both objectives has been achieved through 

 the control and manipulation of secondary factors that influence the 

 response of plant tissues to ionizing radiation. Progress has been 

 achieved also through the use of certain chemical mutagens, such as 

 diethyl sulfate and ethyl methane sulfonate, which induce a spectrum 

 of effects different from radiation. 



Plant radiobiological studies have shown that there are many 

 factors that alter the response of tissues to the sparsely ionizing radi- 

 ations such as X and gamma rays. Of these, the factors that have been 

 more intensively investigated (21, 29) 2 are genotype, age, stage of 

 cellular development, chromosome number and size, nuclear volume, 

 moisture, temperature, atmosphere (oxygen, nitrogen, carbon di- 

 oxide, hydrogen sulfide, etc.), infra red radiation, and chemicals 

 (colchicine, cysteamine, etc.) (21, 29). 



Most modifying factors, however, do not seem to change the 

 response of plant tissues to neutrons and other densely ionizing types 

 of radiation (21, 29). Therefore, in mutation breeding, neutrons give 

 repeated and predictable results regardless of the physiological condi- 

 tion and environment of the plant tissue. In spite of this advantage, 

 neutron treatments cannot be experimentally modified for increasing 

 total mutation yield and altering the induced mutation process. In 

 this section, therefore, reference will be made only to the investiga- 

 tions involving X and gamma radiation. 



In the investigations of the influence of secondary factors in irrad- 

 iated tissues, the seed has been the chief experimental material. This 



-Sec References, page 455. 



