NILAN AND KONZAKI MUTATION INDUCTION EFFICIENCY 439 



is because the seed has several unique properties not possessed by the 

 more actively metabolizing tissues, such as the vegetative bud or fresh 

 pollen. One important characteristic of the seed in these radiobiologi- 

 cal studies is its adaptability. It can be irradiated under a range of 

 conditions that greatly alter the cellular environment. For instance, 

 the seed can be desiccated, soaked, or frozen. It can be maintained 

 under a vacuum, almost free of oxygen, or under high pressures of 

 oxygen or other gases for extended periods. When dry it is resting, 

 almost biologically inert, and the severe environmental treatments 

 apparently cause little or no biological damage. After such treatments 

 and controlled rehydration, the irradiated seed can be measured for 

 damage using several biological criteria. 



Rigid controls of environmental conditions before, during, and 

 after exposure to radiation provide a means for learning about the 

 specific modifying factors that influence the degree of radiation 

 damage. Controls are also necessary for revealing the basic physical 

 and chemical processes responsible for the cell damage incited by 

 radiation (23, 32). 



For other reasons, such as ease of handling during irradiation, 

 chemical treatment, and culture, the seed has been the most widely 

 used plant organ for the induction of mutations in plant breeding. 

 Thus, the fundamental studies of the induced mutation process in the 

 seed can provide pertinent information which may have an immedi- 

 ate and direct application for increasing the efficiency of mutation 

 induction in crop plants. 



Among the seeds used for radiobiological and chemical mutagen 

 studies, the barley seed (Hordeum vulgare or Hordeum distichum) 

 has been by far the most useful (29). The chief advantage in using this 

 seed in these studies is that the effects caused by radiation and by 

 chemical mutagens can be measured in terms of several criteria. These 

 include the linear rate of Mi seedling growth over a finite period; 

 survival of Mi plants following treatment and frequencies of seedling 

 leaf flecking and cholorophyll-deficient chimeras in the M T plants; the 

 number of spikes per Mi plant and number of seedlings produced 

 in the Mi plant and spike; frequencies of chromosome bridges and 

 fragments in the shoot-tips of mutagen-treated seeds; chromosome 

 translocation and inversions at meiosis in the M x plants; and the fre- 

 quency and proportion of chlorophyll-deficient seedling mutations 



