gaul: induced mutants in seed-propagated species 225 



tion of seeds. Certainly seed treatment offers a great number of 

 advantages (77), but this does not necessarily mean that it is the 

 best method for the induction of mutations in plant breeding. In 

 Table 6 some other stages of plant development are shown which 



Table 6. — Plant Stages for Mutagenic Treatment. 



1. Seeds, dormant, presoaked. germinating 



2. Pollen 



3. Flowers, prior to, during, and after meiosis 



4. Zygotes, immediately after fertilization 



5. Buds in various stages 



6. Any other stage 



can be used. Particularly, flowers which have passed meiosis and 

 also zygotes deserve attention. In the last case, the whole organism 

 is represented by one cell only; consequently, no chimera forma- 

 tion will take place and high mutation rates might be expected 

 because there is no intercellular competition. After treatment of 

 seeds or buds diplontic selection results in a great loss of muta- 

 tions (34). With gamma-radiation sources, like cobalt 60 and caesium 

 137, acute irradiation of various plant-stages can easily be done, and 

 there are also useful methods for the application of chemical 

 mutagens (88). 



The experience with chronic irradiation has shown that this 

 treatment apparently offers no advantage as compared with acute 

 irradiation. The "maximal" mutation rate seems to be rather low- 

 er with chronic irradiation which may be easily explained by the 

 phenomenon of diplontic selection (27, 28, 30, 33). However, these 

 results are based mainly on chlorophyll mutations and it is unknown 

 if the same holds true for vital mutations. Theoretically, diplontic 

 selection could eventually be utilized as a screening procedure 

 for progressive mutations, but experimental evidence is lacking for 

 this speculation. 



In relation to radiation of seeds, we have recently presented a 

 number of detailed suggestions on how a breeder may best proceed 

 (34). These suggestions were based mainly on our experience with 

 barley, and they include the description of an early test to find the 

 most efficient dose on the base of the seedling length (31). Further- 

 more, suggestions are given as to the most suitable sowing technique, 



