UPTAKE OF TRANSURANIC NUCLIDES FROM SOIL BY PLANTS 343 



blade < dead blade. Morphologically, the green blades were located in the top portion of 

 the plant and the dead blades in the lower portion. Thus americium appeared to 

 accumulate in the older leaves to a much greater extent than in the newer leaves, which 

 developed in later stages of growth. The ratio of americium concentrations in dead and 

 green blades ranged from 48 to 140 with DTPA and from 8 to 44 without DTPA. 

 However, most of the americium accumulated in the sheath when this isotope was applied 

 in water and most notably when chelated. The accumulation in the sheath could be 

 attributed to physical absorption of the isotope rather than to physiological assimilation, 

 as in the case for leaf blades. Chelated americium applied to the ponded water was more 

 readily absorbed, as the leaf-blade data suggest, but it was not more readily absorbed 

 when applied to soil. 



The relative magnitudes of americium in various parts are easily discernible from the 

 CR values. Plants that received chelated-americium through water application had 

 americium contents 10 times higher than plants supplied with nonchelated americiuni. 

 Dead blades had americium contents one to two orders of magnitude higher than green 

 blades. The CR values can be used to determine the relative availability of an element 

 from a substrate and the translocation pattern of this element within the plant. The CR 

 values indicate that americium was less available when added to soil and was not readUy 

 translocated to younger leaves. 



In all CR calculations, 400 pCi/g dry soil was used, taking into account the total soil 

 mass per pot (5 kg). However, if based on only the top 1 kg of spiked soil, 2000 pCi/g 

 should be used. Thus, in the latter case, the reported CR values in Table 3 should be 

 multiplied by a factor of 0.20. With water application it is difficult to assign a conversion 

 factor. It should be pointed out that CRis probably not valid with water appUcation and 

 should be used with caution since traditionally it is used where the radionuclide was 

 applied to the soil and the total soil mass is taken into account in calculating the average 

 radionuclide concentration in the soil. Recently, however, conversion factors have been 

 introduced to consider also the fraction of the soil mass spiked (Lipton and Goldin, 

 1976). As would be expected, there is some question concerning the use of soil 

 concentration for determining the CR for water application, but the CR would 

 demonstrate the relative translocation or redistribution of ^"^^ Am in various rice parts 

 and could serve as a basis for calculating the plant concentration of '^^^ Am. 



In the flood variety* the radioactivity in the grain was below the detection Umit 

 (Table 4). There was also Uttle translocation from old leaves to green leaves. The chelate 

 DTPA mixed with the soil slightly reduced ^^^ Am uptake. Apparently the chelate level 

 (40 ppm as acid) was harmful to the rice plants, retarding and reducing growth. Organic 

 matter did not have a clear-cut effect, although it tended to suppress the uptake by the 

 nonflood variety. It should be pointed out that the OM retarded growth of the rice 

 seedlings in early stages of growth, presumably because the organic acids inhibited root 

 development (Takijima, 1964). In general, the plant tissues of the nonflood variety had 

 higher '^^^ Am levels. 



It appeared that the ^"^ ' Am content of the grain could be increased slightly by adding 

 chelated ^"^^ Am to the standing water. Thus the method of ^"^^Am placement would 



*The flood variety was a dwarfed "miracle rice" variety from southeast Asia and was ponded with 

 water all the time. The nonflood variety, also from Asia, was not ponded and was taller than the flood 

 variety. 



