442 



.MUTATION AND PLANT BREEDING 



survival. Typical oxygen- and after-effect data are presented in Figure 

 1. Seeds at 4 per cent moisture were gamma-rayed from 10 Kr to 60 Kr 

 and then either planted immediately or stored in oxygen or nitrogen 

 for 1 week before planting. Severe after-effects occurred in the oxygen- 

 stored seeds, while considerably fewer after-effects occurred in the 

 nitrogen-stored seeds. It is also interesting to note that a typical dose 

 response was exhibited by the seeds planted immediately after radi- 

 ation, but no dose response occurred in the irradiated, stored seeds. 



These oxygen- and after-effects now can be controlled (32). They 

 occur only slightly in seeds with over 12 per cent moisture, and are 

 even more reduced if these moist seeds are partially evacuated of 

 oxygen before irradiation and then irradiated at dry ice (—78° C) 

 temperature. Controlling both the oxygen- and after-effects also 

 depends on the rehydration of the irradiated seeds in oxygen-free 

 water. The length of time necessary for rehydration depends upon 

 temperature of the water. Usually 1 1/ 2 to 2 hours at 30° C is adequate. 

 Longer rehydration may be required for very dry seeds. Typical 

 results of these environmental controls on irradiated barley seeds are 

 shown in Figure 2. 



By manipulating the oxygen and moisture content and control- 

 ling the after-effects, seeds can be made to tolerate and survive high 



CO 







— **.> 



3% EMBRYO MOISTURE 

 OXYGEN BUBBLED 



""^'^.•••A-.../ 13% EMBRYO MOISTURE 



4% EMBRYO MOISTURE "Ut-^-.. HELIUM BUBBLED 



HELIUM BUBBLED ""' 1 ** , -^k. 



4% EMBRYO MOISTURE 

 OXYGEN BUBBLED 



10 20 30 40 50 60 70 80 90 100 



3 



DOSE OF GAMMA RAYS IN IO Q r 

 Figure 2. — The interrelation of moisture content and oxygen effect 

 when seeds are frozen in dry ice (—80°C) prior to and during irradia- 

 tion. 



