210P010niumEntry into Plants Chester W. Francis,' Gordon Chesters, and Wilfred H. Erhardt2 Soils Department, University of Wisconsin, Madison, Wis. 53706
The possibility that root uptake of zloPbfrom soils is the principal mechanism of 210Poentry into plants was evaluated and proved to be invalid. If the zloPbin the plant is not soilderived, the second most likely source is natural radioactive fallout, of which the most important component is from rainfall. Material balances of the quantities of zloPbin tobacco over time intervals of known rainfall indicate rains deposit approximately twice as much zloPbas observed in tobacco leaves, verifying the hypothesis that deposition of 21°Pb by rainfall is the principal mechanism of zlOPoentry in plants.
ecause of the implication that loPo,an alpha-emitting and volatile radionuclide, in tobacco was an initiator of neoplasms in the bronchial epithelium of cigarette smokers (Holtzman and Ilcewicz, 1966; Radford and Hunt, 1964), it was decided to investigate the source of this radioactive element in plants. It appears that zlOPo, being volatile at the temperature of a burning cigarette, adheres readily to smoke particles, and thereby gains direct access to the lung. Polonium-210, with a half-life of 138.5 days, is a member of the 23W radioactive decay series and its immediate precursors are ZlOBi and zloPb. The chemistry of Po is not well understood, although it is known to be amphoteric and to have stable oxidation states of +2 and +4. A number of researchers have investigated the role of zlOPo as a possible initiator of bronchial cancer, but because of the lack of a satisfactory model, it has been impossible to determine the accumulation of zlOPo in the lungs of tobacco smokers and the extent of the radiation hazard (Holtzman and Ilcewicz, 1966). The studies of Hill, Osborne, et al. (1964) revealed that concentrations of zlOPoin normal bone and reproductive organs of humans accounts for mean dose
1 Present address, USAID/IRI, c/o American EmbassyBrasilia, APO, New York 09676 2 Present address, Department of Plant and Soil Sciences, University of Maine, Orono, Me. 04473
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Environmental Science and Technology
rates of several millirem per year, and 210Po often occurs independently and at considerably higher activities than its precursor 226Ra. The radioactive disequilibrium described above can only be produced by the accumulation of a radioactive isotope between 2z6Raand zlOPoin the 238Useries or of ZlOPo itself. It is suspected that this accumulation may occur by deposition of zlaPbthrough natural radioactive fallout on plant tissue which humans consume (Hill, 1960). However, zlOPoanalysis of edible portions of vegetables failed to show detectable quantities of loPo while cured tobacco samples showed relatively high concentrations-Le., 0.34 to 1.50 pCi per gram, where pCi is picocurie (Berger, Erhardt, et af., 1965). Tso, Hallden, et al. (1964) reported that the 210Poin tobacco was not in radioactive equilibrium with 226Raand hypothesized that its source was 210Pbtaken up from the soil. Francis and Chesters (1967) demonstrated conclusively that 21OPo in tobacco is in radioactive equilibrium with zlaPband the question of the mode of entry of 210Pbinto plant tissue is raised. Thus, zloPb may enter plants by uptake from soil and/or deposition of zloPband precursors from natural radioactive fallout. The phenomenon of natural radioactive fallout has long been recognized (Blifford, Lockhart, et al., 1952; Israel, 1951) and is caused by the evolution of the inert gasZzZRn (a daughter product of 226Rawith a half-life of 3.82 days) from the soil into the atmosphere where it decays to its daughter products. Lead-210, having the longest half-life (22 years), tends to accumulate and apparently attaches itself electrically to small dust particles of
