294 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



retention half-time, differs markedly from that commonly reported for fission products 

 and larger particles {>\Q ^m). The tenacity of plutonium retention observed by Romney 

 et al. (1975) and Iranzo (1968) tends to reinforce the laboratory studies with plutonium 

 described here. 



Availability of Foliar Deposits for Uptake and Transport to Other Plant Tissues 



Foliar structures are a source of many organic and inorganic substances that either 

 migrate to the surface by diffusion and mass flow or are actively exuded by secretory 

 structures. This may provide a chemical environment on the leaf surface which enables 

 readily hydrolyzable species to be complexed or chemically stabilized and therefore made 

 more available for foliar absorption. Since foliar surfaces represent an efficient absorptive 

 structure (Wittwer, Bukovac, and Tukey, 1963; Franke, 1967; 1971), fohar application of 

 micronutrients to correct nutrient deficiencies is effective, especially in situations where a 

 specific nutrient tends to be immobile and not as available for plant uptake from soil 

 (Krantz et al., 1962; Franke, 1967). The actual mechanisms involved in foliar absorption 

 are not totally understood. Available information indicates that, although the cuticle of 

 the leaf is hydrophobic in nature, penetration is facilitated via intermolecular spaces 

 (Fisher and Boyer, 1972), modification in cutin composition at anticlinal epidermal walls, 

 and the presence of ectodesmata (Franke, 1967; 1971) and trichomes (Benzing and Burt, 

 1970). The role of stomates as a route of foliar penetration under normal conditions is in 

 question and is currently considered of negligible importance (Greene and Bukovac, 

 1974). 



The relative importance of foliar absorption as compared to root absorption as a 

 route of entry into plant tissues depends on several factors. For soluble species that 

 remain relatively available in soil solution, root-absorption processes are as effective as, or 

 more effective than, foliar-absorption processes. This does not imply that foliar surfaces 

 are not effective sites of absorption. Elements reported to be absorbed and transported 

 from fohar surfaces include inorganic N, Rb, K, Na, Cs, P, CI, S, Zn, Cu, B, Mn, Fe, and 

 Mo (Wittwer, Bukovac, and Tukey, 1963). For specific nutrient deficiencies, foliar 

 application is sometimes the method of choice (Bradford, 1966; Labanauskas, 1966). 

 Tliis is especially true for nutrilites that tend to hydrolyze readily in soil solution or are 

 rapidly adsorbed to soil particles and therefore are not so available for root absorption. 



The fate, with respect to foliar absorption, of relatively insoluble elements, such as 

 plutonium, which make up or are carried on discrete particles, is in some way analogous 

 to the behavior of micronutrients, such as iron, which tend to form relatively insoluble 

 products in aqueous environments. If we can assume that small particles containing 

 plutonium (<1.0Mm) can be retained in foliar surfaces over an extended period of time, 

 the question arises as to the absorptive capacity of foliar surfaces for available plutonium. 

 Since absorption of a particular element is a function of tlie concentration of the 

 available or soluble component, an extended residence time on plant foliage may provide 

 the time necessary for soluble components to be chemically modified and/or absorbed by 

 internal tissues. This may represent a more efficient route of entry than root absorption 

 because in root absorption the same finite amount of plutonium deposited in soil may be 

 insolubilized and adsorbed to soil particles, which, of course, reduces the concentration 

 available for root absorption. 



Absorption data from laboratory studies with bush bean plants contaminated with 

 aerosolized plutonium are given in Tables 3 and 4; the protocol for this study was 



