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20 IX. SUMMARY Radioisotopes in fallout 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 among 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 fallout strontium-90 and calcium in the field can have greater effects.
21 The average maximum uptake of cesium appears to be about one tenth of one per cent of the applied dose. The cesium-to-potassium DF is smallâabout 0. 2 for uptake in nutrient solutions and 0. 02 for additions to the soil. In general, it appears that grasses accumulate less strontium than legumes. The fruit and seeds contain less strontium than the leaves or stems because stron- tium tends to accumulate in the vascular tissues of the plants. In contrast with strontium, which only moves readily upward, cesium is easily translocated through- out the plant, with perhaps slightly higher accumulation in young leaves and flowers. Strontium and cesium are retained in the soil partly by ion-exchange bonds on clay minerals and organic colloids. A part of the strontium may be synthesized into organic compounds by the microbial population. A third means of retention in the soil can involve fixation processes. A large fraction of cesium-137 appears to be fixed irreversibly. Exchangeable strontium is leached through soils at the rate of about one inch per 100 inches of leaching water. The downward movement of stron- tium, and probably cesium, is essentially an exchange reaction and proceeds by successive desorption-adsorption sequences. The soil that will provide minimum uptake of fission products usually appears to be one considered ideal for maximum crop production. These requirements in- clude high exchangeable calcium, high exchangeable potassium, high organic-matter content, and a slightly alkaline reaction.
