296 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



reported by Cataldo, Klepper, and Craig (1976). The objective was to evaluate, on the 

 basis of chemical form supplied and the presence or absence of a solution vector 

 (simulated rainfall), the extent of absorption and translocation of foliarly applied 

 plutonium. All plants were exposed to plutonium at 20 days from planting (preflowering) 

 and were held for an additional 28 days to allow time for both absorption of plutonium 

 and seed filling. During this 28-day period, contaminated plants were either maintained in 

 the absence of a simulated rainfall or subjected to a simulated rainfall at 1,7, 14, or 21 

 days after contamination. This simulated rainfall provided a solution vector on the 

 surfaces of contaminated leaves to enable diffusion and absorption of mobile plutonium 

 forms. The pots containing soil and root were double bagged with polyethylene and 

 sealed at the lower stem; the seed tissue was contained in pods that were formed after 

 exposure. The effect of both the chemical form of the plutonium and the timing of 

 simulated rainfall on the absorption of plutonium from foliar surfaces and its 

 translocation was determined by analysis of uncontaminated seed and root tissues. 

 Quantitation was by means of transport ratios: TR = picocuries of ^'^^Pu per gram (dry 

 weight) of seed or root ^ picocuries of ^ ^ ^ Pu per gram (dry weight) of contaminated leaf 

 tissue. Although there is a tendency to compare TR values with classical concentration 

 ratios derived from plants grown on contaminated soils, this comparison is inappropriate. 

 At harvest the dry weights of contaminated leaves and uncontaminated roots and seeds of 

 individual plants were approximately 0.6, 0.5, and 0.3 g, respectively. Plants were 

 contaminated with 0.25 to 1.0 x 10^ d/min ■^^^Pu. Transport ratios (TR values) for root 

 and seed tissues from plants not subjected to leaching (solution vector) ranged from 

 < 4.5 X 10~^ to 3.3 X 10~'^. Application of a simulated rainfall to provide a solution 

 vector for diffusion and absorption of soluble components on the leaf surface increased 

 uptake and transport of plutonium to seed and root tissues for all compounds of 

 plutonium studied (1.5 X 10~^ to 4.2 X 10"''). Apparent differences in translocation 

 between the various plutonium forms may result from the relative size of the soluble 

 fraction. The fresh plutonium dioxide was truly particulate at the time of contamination, 

 the aged oxide consisted of particles with a fractured crystal lattice (Park et al., 1974), 

 the citrate represented a relatively stable soluble complex, and the nitrate represented an 

 unstable complex that rapidly hydrolyzed on dilution to form colloidal hydroxides. The 

 order of bioavailability for transport to seed and root was plutonium nitrate 

 (hydroxide) > plutonium citrate > aged oxide > fresh oxide. (Tables 3 and 4 show 

 average values for leaching treatment.) 



An interesting aspect of these data is that maximum TR values are obtained when the 

 simulated rainfall occurs at day 7 or 14, the time of rapid seed development. This 

 phenomenon is of interest from the standpoint of the mobility of plutonium within the 

 plant and the chemical form of the plutonium. It is generally accepted that materials 

 must be transported out of mature leaves in the phloem. The entry of molecules into this 

 transport conduit is metabolically regulated, and the loading process is highly specific for 

 individual organic metabolites and inorganic elements (Crafts and Crisp, 1971). There is 

 growing evidence that many inorganic nutrilites, especially multivalent cations, are 

 transported as organic complexes in both the xylem (Tiffin, 1967; 1971 ; Bradfield, 1976) 

 and phloem (van Goor and Wiersma, 1976). By analogy to the behavior of nutrilites, 

 plutonium must be transported out of the contaminated leaves via the phloem. Similarly, 

 it is unUkely that inorganic plutonium could remain soluble at the pH of phloem cell sap 

 (pH 7.2 to 8.5) (Ziegler, 1975). Therefore it is possible that the mobile plutonium which 

 was deposited in seed and root tissues may have been complexed with phloem-mobile 



