10 



The upward translocation of strontium and calcium relative to that of phosphorus, 

 sulfur, iodine, and rubidium is limited. The main path appears to be the central zone 

 of the vascular tissue (58). The redistribution of strontium, calcium, yttrium, and 

 other multivalent cations is much less than that observed for cesium, rubidium, and 

 potassium (35, 131). 



C. Aerial Contamination 



A principal pathway of intake of fallout nuclides, immediately following deposi- 

 tion, is through contamination of aerial plant parts. A conclusion from studies of 

 plant material near the Operation Hurricane test site (108) is that the radioactive 

 contaminants were carried by airborne soil, which lodged upon the leaves, and were 

 then partially dissolved by nocturnal dew. Most of the fission-product radioactivity 

 associated with vegetation near the Nevada tests was in external dust (83). The reten- 

 tion of the particles by foliage was enhanced by mechanical trapping by hairs, glands, 

 and stomata. Particles of less than 44 n diameter were preferentially retained on 

 foliage, whereas particles having diameters over 88 m were rarely retained. Greater 

 absorption is generally expected from contaminants in solution than from dry con- 

 taminants. 1 



It also appears that aerial contamination of plants from stratospheric fallout 

 is important in the entry of nuclides into the food chain. The cesium- 137 to 

 strontium-90 ratio in milk in 1959 and 1960 was rather constant, although cesium 

 uptake through roots is known to be much less than that of strontium. The conclu- 

 sion was reached that cesium and strontium in the forage are largely derived from 

 foliar deposition (49). By determining the specific activities (strontium-90/strontium) 

 of different parts of wheat plants, other workers concluded that over 90 per cent of 

 the strontium-90 in the grain came from current fallout in 1959 (70). About two per 

 cent of the strontium-90 fallout during the time the heads were exposed was retained 

 in the grain. The whole crop retained about three per cent of the deposited 

 strontium-90 and removed only about 0. 2 per cent of the strontium-90 in the soil. 

 Ryegrass grown in flats absorbed directly 23 per cent of the current strontium-90 

 fallout (74). This accounted for 55 to 80 per cent of the total plant strontium-90. 



Autoradiograms show that strontium enters directly through the intact epidermis 

 of the tomato fruit (58). About four per cent of the applied strontium and two per cent 

 of applied ruthenium is absorbed by tomatoes (46). The stage of maturity of the fruit 

 had some effect on the amount absorbed. 



The species of plant is important in the absorption of foliar- applied elements 

 (71), partly because of the degree of waxiness of the leaves and partly because of the 

 death of some leaves before maturity. Wheat plants absorbed 85 per cent of the 

 applied strontium and 93 per cent of the cesium, but cabbage absorbed only five or 

 six per cent of either of these elenaents. 



The time of contajnination in relation to the maturity of the plant is important 

 also, especially for the relatively nonmobile elements. Less than 0. 1 per cent of 

 the applied strontium is found in wheat grain if the surface deposition occurs before 

 head development; up to one per cent is found when the head is contaminated (71). 



