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IX. SUMMARY 



Radioisotopes in faillout enter plants by three principal pathways: (1) direct 

 absorption by the aboveground parts; (2) absorption by the stems and roots from the 

 root mat of grass; and (3) absorption by the roots from the soil. Contaminated soil 

 adhering to the aboveground parts of the plants may contribute to the observed up- 

 take of fission products. 



Foliar deposition and absorption depend on the surface area of the aboveground 

 portion of the plant and the characteristics of the surface. The greater the surface 

 area, the greater the interception per plant. Pubescence increases the retention of 

 the fallout dust against washing and therefore the period of absorption. Many ele- 

 ments seem to be absorbed, some to a greater extent by this method than through 

 the roots. Fallout particles can be washed from the leaf surface, and even small 

 amounts of absorbed elements can be leached from the leaf. 



Plant-base absorption is a relatively recent concept and its general contribu- 

 tion has not been adequately evaluated. 



No single crop of plants has been reported to absorb from the soil as much as 

 10 per cent of the applied dose of fission products. There are two main reasons: 

 (1) the soil has an affinity for the fallout nuclides because most of them are cations; 

 and (2) the plant itself discriminates against them to a certain extent. The uptake 

 of short-lived isotopes, such as barium- 140 and iodine- 131, through the roots is 

 relatively unimportant as most of the isotope decays during the period required for 

 it to reach the roots. 



The uptake of cations by roots is probably by a carrier mechanism. Stron- 

 tium and calcium compete for the same binding sites on this carrier, whereas 

 cesium and potassium compete for another common site. 



With the exception of strontium, and possibly cesium, the longer-lived fission 

 products are taken up in relatively small amounts and therefore are not as important 

 as strontium and cesium with respect to uptake from soils. 



Because strontium is chemically similar to calcium, the strontium content of 

 plants is often reported as a strontium-to-calcium ratio as well as an absolute 

 amount of strontium. Both values have some importance in assessing hazards in 

 the subsequent links of the food chain. The usual maximum uptake of strontium ap- 

 pears to be about one per cent of the applied dose per crop. The average DF for 

 strontium to calcium between the soil and plant tops appears to be close to unity. 

 This factor varies ajnong plants and even among different parts of the same plant 

 and by different soil extractants, but the range of variation is considered to be of 

 little practical importance. Variations in the root zone and differences in the 

 vertical distribution of faillout strontium-90 and csdcium in the field can have greater 

 effects. 



