28 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Plant Uptake. Transuranic elements in terrestrial environments can enter plants by foliar 

 absoiption and root uptake. The route o^ entry into plants will depend on the nature of 

 the source; climatic conditions affecting ueposition, retention, and chemistry of particles 

 on leaf surfaces; the foliar surface area exposed; and soil conditions affecting resuspension 

 and solubility. 



The root is the major ion-absorbing organ of the plant. Somatic cells in the leaf 

 possess the same potential for absorption; however, they are protected by a waxy cuticle. 

 Foliar absorption is an efficient route of entry for nutrients (Bukovac and Wittwer, 1957; 

 Wittwer, Bukovac, and Tukey, 1963), fission products, and activation products (Tukey, 

 Wittwer, and Bukovac, 1961; Athalye and Mistiy, 1972). Foliar absorption of ^^^Pu and 

 ^^ ^ Am can occur and is dependent on chemical form and environmental conditions with 

 up to 10^^ and 10^^ of the foliar deposits absorbed and translocated to seeds and roots 

 (Cataldo and Vaughan, this volume; Cataldo, Garland, and Wildung, 1978). About one 

 one-millionth of the plutonium applied as oxide was absorbed by leaves; availability was 

 dependent on particle size. The availability of americium applied as the oxide was two to 

 five times as great as that of the less soluble plutonium oxide at comparable particle size. 

 Thus foHar uptake appeared to be related to transuranic solubihty. 



Ion uptake by plant roots is apparently a metaboUcally mediated process in which 

 ions are transported across the cell membrane. The process is concentration dependent 

 over a broad range (10"^ to \0~^M), exhibits a degree of ion selectivity, and may allow 

 for accumulation of ions against a concentration gradient (Nissen, 1973). Although the 

 transport process is selective, plants accumulate nonnutrient ions. Processes leading to the 

 deliveiy of soluble transuranic species to root membranes have been described. It is 

 critical to determine if discrimination occurs at the membrane level because this would 

 limit transuranic uptake by plants and incorporation into food chains. 



Because of the relatively low uptake of plutonium and americium from soil by plants 

 (CR values of 10"^ to 10~^), it has been generally assumed that marked discrimination 

 occurs. Evidence is increasing that solubility in soil rather than plant discrimination at die 

 membrane level limits transuranic uptake by plants. As expected from their respective 

 aqueous chemistries, transuranic elements are sorbed by soil in the order 

 Pu > Am ~ Cm > Np (Table 12). Uptake of these elements is apparently inversely related 

 to soil sorption. The addition of complexing agents, wliich markedly increases transuranic 

 solubility in soil (Wildung and Garland, 1975), also increases plant uptake (10- to 

 10,000-fold) (Energy Research and Development Administration, 1976). Thus indirect 

 evidence supports soil solubility as the primaiy factor governing transuranic availability to 

 plants. 



Experiments with plants grown in hydroponic solutions containing plutonium aid in 

 distinguishing soil sorption and plant root discrimination when uptake is compared with 

 uptake by plants grown in soils. Wlien hydroponically grown soybeans (Glycine max) 

 were placed in /.iM^^^Pu-DTPA solutions and permitted to accumulate plutonium for up 

 to 49 hr (Wildung et al., 1977), CR's [(^iCi/g diy plant) per (juCi/ml nutrient solution)] 

 for shoot tissues were 6 X 10"^ and 3 x 10"' after 1 and 24 hr, respectively. The 

 Pu— DTPA complex supplied in the growth medium was not detected in the exudates. 

 Similarly, leaves of bush beans (Phaseolus vulgaris) exhibited CR's in nutrient solution of 

 0.8 and 5.1 for Pu(lV) and Pu(VI), respectively. Thus plants can" accumulate soluble 

 plutonium effectively; much of the apparent discrimination found in soil-plant studies 

 resuhs from the effect of soil sorption in reducing the quantity of soluble plutonium 

 available to the plant. 



