W. D. JACKSON 



was 2 • 5 kr. When fresh seed was irradiated in an atmosphere of" nitrogen, 

 following 14 days' storage in that gas, much less damage resulted. It was 

 estimated graphically that a dose of about 7-0 kr would be required to 

 produce the same damage as the dose of 2-5 kr in air or the four years' 

 storage in air. 



It appears likely that oxygen is involved in the ageing process. It has long 

 been known as one of the principal factors in seed storage^-' ^^. It has also 

 been shown by Conger and Fairchild^- that oxygen causes breakage in 

 dormant pollen grains. The results of storage of fresh seed for four weeks in 

 atmospheres of nitrogen, 20 per cent oxygen in nitrogen, and in pure oxygen, 

 are shown in Figure 2. The damage is scored as number of acentric fragments 

 per cell. It is seen that pure oxygen causes appreciable damage in four 



Equivalent dose 

 of X-rays in N2 

 3 kR 



100 



%02 — 28 days, 20''C, ^0% , r.h. 



Figure 2. Frequency of chromosome breaks per cell in seed stored for four 



weeks in oxygen-nitrogen mixtures, showing the exponential increase in 



damage with oxygen concentration 



weeks. The estimated equivalent dose of X-rays given in nitrogen is approxi- 

 mately 3 kr so that one month's storage in pure oxygen is roughly equivalent 

 to two years' storage in air. 



The influence of oxygen combined with a small dose of X irradiation on 

 fresh seed is shown in Figure 3. A dose of 1 • 2 kr given to seed in a nitrogen 

 atmosphere after 14 days' storage in that gas causes the average of one 

 fragment in 10 cells. The damage is approximately eight-fold in pure 

 oxygen, and about three-fold in air. Utilizing the result obtained in the 

 control series we may deduct a small part from the total effect at high oxygen 

 tensions, due to the direct effect of oxygen, and we may regard the damage 

 caused in pure nitrogen as the direct effect of irradiation. This partition of 

 the damage into its relative parts is shown in Figure 4. The shaded area 

 represents the contribution to the total damage by the interaction of oxygen 

 with the irradiation. This contribution rises more rapidly at low oxygen 

 concentrations, although it increases throughout the range of oxygen 

 concentration. 



When the seed is held in the phials for a further 14 days of post-storage 



195 



