28 L. H. GRAY 



The term in D^ is more important than that in D at closes above 

 6 krad. 



The rather close parallel between the influence of small amounts of 

 water on the sensitivity of seed, and on the changes induced by radia- 

 tion in non-living systems, as illustrated, for example, by the experi- 

 ments of Clegg (1957) with cellulose fibres, supports the view that we 

 are here concerned with a profound modification in the reaction chains 

 brought about by the presence of water. 



When seeds of still higher water content ( ~ 15 per cent water in the 

 embryo) are irradiated in the presence of nitric oxide, the radiation 

 damage is enhanced to about the same extent as when the same seeds 

 are irradiated in the presence of oxygen, i.e. the seeds now respond in 

 a manner typical of vegetative bacteria and the cells of higher plants 

 and animals. 



The influence of progressively increasing water content on the radio- 

 sensitivity of Agrostis seeds, irradiated in the presence of nitrogen, 

 oxygen and nitric oxide, is nicely illustrated by Fig. 1 taken from the 

 paper by Sparrman et al. (1959). 



BIOLOGICAL SYSTEMS OF NORMAL (HIGH) WATER CONTENT 



Lifetimes of some of the radicals produced by irradiation of seeds 

 having low and intermediate water content are long enough for study 

 by means of conventional ESR spectroscopy. When we attempt to 

 obtain information about the lifetimes of radicals produced in dilute 

 aqueous solutions, or in cells of high water content, a number of 

 difficulties arise, both in experimentation and in interpretation. 



Difficulties of experimentation 



Major difficulties arise on account of the short lifetime of the inter- 

 mediates. We know from the observations of Howard-Flanders and 

 Moore (1958) that essentially all the intermediates involved in the 

 aerobic chains leading to loss of reproductive integrity in Shigella 

 flexneri have lifetimes less than '20 msec, and estimates based on the 

 way in which the sensitivity of these cells varies with the concentration 

 of oxygen in the surrounding medium suggest that the lifetimes of the 

 intermediates which react with oxygen may be in the 100 /^sec range 

 (Howard-Flanders, 1958). 



In our laboratory we are attempting to investigate these lifetimes 

 directly, both in chemical systems and in living cells, by means of a 

 pulsed radiation source and pulsed analytical techniques. The source 

 (Boag and Miller, 1959) delivers 1*5 MeV electrons in jmlses of 2/xsec 

 duration. The dose, which can be delivered in a single pulse, depends on 



