AIRBORNE PLUTONIUM 297 



organic species. This would explain the apparent increase in TR values seen during the 

 time of seed development. During this period there is a significant change in both the 

 composition and quantity of specific metabolites being produced by leaves and being 

 exported to metabolic sinks, such as seeds and roots. This change in metabolism may 

 increase the potential for soluble species of plutonium to become complexed with organic 

 metabolites and subsequently to be exported to metabolic sinks. Although this is a 

 tentative judgment and subject to substantive studies, this interpretation serves to explain 

 the observed results on the basis of known metabolic aspects of plant function. 



Conclusions 



The ability of terrestrial plants to accumulate potentially hazardous elements from soils 

 via root absorption and the relative importance of these elements in the food web to man 

 has prompted numerous studies over the past 25 years. The majority of these 

 investigations have been concerned with soil— plant transfer rates since the soil represents 

 a major repository for pollutants released to the environment and since the plant root is 

 an efficient solute-absorbing structure. Until recently the foliar portions of plants were 

 considered to play a minor, transient role at best with respect to dose-assessment 

 problems. 



Our current understanding of the aerodynamic behavior of particles and anticipated 

 reductions in particle-size distributions of materials such as plutonium through an 

 expanded nuclear energy program suggests that a reevaluation of the role of plant foliage 

 in particle interception and absorption of materials contained on airborne particles is in 

 order. 



This need is supported by both early investigations and studies currently under way. 

 Early studies of worldwide fallout and current work on contaminated soils resuspended 

 by wind indicate that foliar retention and foliar absorption may be as important as, and 

 in some cases exceed, the role of root absorption with respect to food-chain transport. A 

 critical evaluation of past literature on the leaching of foliar deposits suggests that aerosol 

 polydispersity and large particle size (e.g., 45 ^m MMD) may explain the comparatively 

 large degree of leaching or weathering reported for comparatively large particles 

 (Witherspoon and Taylor, 1969; 1970; 1971). This view is reinforced by data reported for 

 well characterized particles of lead and plutonium in laboratory studies and field 

 observations for fallout plutonium. These latter investigations indicate that a sizeable 

 fraction (>80%) of submicron-size particles deposited onto foliage are tenaciously held 

 on leaf surfaces under varied conditions (e.g., simulated rainfall and wind). Aside from 

 the potential health implications associated with increased foliar retention, the problem 

 of foliar absorption must be considered. In the reported studies, a substantial fraction of 

 the foliar plutonium deposits was transported to seed and roots. Transport ratios were 

 affected by both the presence of a solution vector (simulated rainfall) and the timing of 

 its application with respect to the stage of plant development. 



References 



Aylor, D. E., 1975, l-'orce Required to Detach Conidia o\' Ht'lniiiit/iosporiunj inaydis. Plant Physiol., 

 56: 97-99. 



, 1976, Resuspensiiin o\' Particles from Plant Surfaces by Wind, in Atmosphere -Surface Exchange 



oj Particulate and (iaseous Pollutants. DOL Symposium Series, Richland, Wash., Sept. 4-6, 1974, 

 R. J. Engelmann and G. A. Sehmel (Eds.), pp. 791-812, CONI-740921, NTIS. 



