NILAN AND KONZAK: MUTATION INDUCTION EFFICIENCY 443 



radiation dosages. For instance, air-dried barley seeds, about 9 per 

 cent moisture, given no proper pre- or post-treatment to control the 

 oxygen- and after-effects can not survive 20 Kr of X-rays (even less if 

 stored), whereas seeds given proper treatments to control after-effects 

 can survive dosages of 80 to 100 Kr (23, 24). Mutation yields from 

 these latter dosages are the highest yet recorded for irradiated 

 barley seeds. Furthermore, with these treatments, more predict- 

 able and reproducible results have been obtained in seed irradiation 

 experiments. 



Another important means to alter the dose tolerance of plant tis- 

 sues is the application of certain compounds that affect plant response 

 to radiation. Only recently has this possibility been investigated 

 (1, 2). Soaking pine seeds in extracts of mustard seeds increased their 

 LD 50 , where soaking mustard seeds in extracts of pine seeds reduced 

 their LD.- )0 . Protective effects were obtained after treatment with both 

 acidic and basic fractions from mustard seeds. Furthermore, the phen- 

 olic and neutral fractions of the mustard seed extracts contained ger- 

 mination inhibitors. A study of the chemical fractions of pine seed 

 extract revealed that the sensitizing action was centered in the 

 organic acid fraction and was possibly a result of the presence of oleic 

 and linoleic acids. It was suggested (1) that peroxides produced by 

 irradiation of the unsaturated fatty acids may be the primary cause of 

 the sensitization. 



To date, the LD 50 has been measured only in terms of seed ger- 

 mination. The influence of these radio-resistant or radio-sensitive 

 fractions on the induced-mutation process has not been investigated. 

 It appears, however, that this line of research may open up a whole 

 new approach to the manipulation and possible control of the radi- 

 ation tolerance of plants for increased mutation induction. 



Increasing the ratio of induced mututations to chromosome 

 aberrations 



Possibly one of the chief causes of cell death in tissues irradiated 

 at high dosages are chromosome aberration. Caldecott (3) has shown 

 that chromosome aberrations in barley increase exponentially, where- 

 as the mutations increase linearly with dose. This means that at high 

 doses, chromosome aberrations concurrently induced limit the 

 mutation yield. Therefore, for radiation to become more efficient 

 for inducing mutations in plant breeding, the frequency of chromo- 



