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VI. RADIATION EFFECTS 



The severity of associated heat and blast effects from nuclear test detonations 

 have tended to obscure radiation effects on plants. However, radiation effects may 

 be a significant force in modifying the ecological systems after a nuclear attack. 



At present, the effects of ionizing radiation have been observed for only a few 

 hundred of the more than a million and a half different kinds of organisms. Most of 

 these data were obtained under experimental conditions of minimum environmental 



stress. 



A. External Radiation 



The median lethal dose for flowering plants ranges from about 1, 000 to 

 150,000 roentgen units, and the sensitivity of a particular plant may vary widely 

 according to the particular stage in its life cycle (90). The variation in sensitivity 

 between plants has been correlated with characteristics of the cell nucleus. Plants 

 with low chromosome number and high nuclear volumes are the most sensitive (122). 



Pine trees appear to be relatively more sensitive than other trees. At an un- 

 shielded reactor site, pines died after receiving 2,000 or more rads in an initial 

 burst, but pines at greater distances died after accumulating about 8,000 rads; 

 hardwood trees in the area showed little effect (90). With gamma radiation from 

 cobalt-60, pines showed detectable effects from two roentgens per day for an average 

 of 240 days per year over a period of nine years (122). 



Several other observations have been made on irradiated trees (90). The winter 

 dormancy is prolonged by an amount proportional to the dose received during the 

 preceding summer — one to two weeks' delay for several hundred rads. The terminal 

 buds are more sensitive than the lateral buds and, of the lateral buds, those farthest 

 from the trunk are most sensitive. 



Two years after a nuclear explosion at the Marshall Islands, the number of 

 different plant species showing pathological effects and abnormalities increased 

 with an increase in fallout (23). However, differences in edaphic factors such as 

 soil fertility may confound these observations (39). 



B. Internal Radiation 



The radiation emitted by the absorbed radionuclides may also cause damage. 



In greenhouse experiments, at concentrations of 5 mc of strontium-90 or 13 mc 

 of cesium- 137 per g of wheat leaves, the protein levels decreased and the carbohy- 

 drate levels increased (34). A 30- to 50-per cent decrease in yield of grain was ob- 

 served at those concentrations of radioactivity. Resistance to radiation daunage in- 

 creased with age of the plant. 



