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13 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 summerâone 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 damage in- creased with age of the plant.
14 In young barley plants, phosphorus-32 radiation damage was confined to cells in zones of active division (10). The lowest specific activity level at which damage was produced corresponded to 3. 2 me of phosphorus-32 per g of phosphorus, or about 170/ic of phosphorus-32 per g of dry plant tissue. A more complete treatment of this subject is found in the Proceedings of the First National Symposium on Radioecology (115).
