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V. PLANT RELATIONS A. Uptake The uptake by plants of ions from solution has been the subject of many in- vestigations. There have been short-term experiments with adsorption periods of minutes or hours and long-term experiments with adsorption periods of days, weeks, and months. Short-term experiments are useful in studying mechanisms of the initial steps, whereas longer-term experiments elucidate over-all effects and the general distribution within the plant. Numerous mechanisms have been proposed for the uptake of ions by plant roots. Most hypotheses state that the process of concentrating the ions within the plant root is a metabolic function. Essential in most of these mechanisms is a biological com- pound that serves as a carrier (20). Evidence has been obtained that calcium, stron- tium, and barium compete for an identical carrier, whereas potassium, rubidium, and cesium compete for a different carrier (20, 21, 28). Hydrogen appears to compete with all ions (27). In addition to the accumulation of ions within the root, there is adsorption of ions on the root. The CEC of roots can be increased by nitrogen fertilization (121). A linear correlation was observed between the CEC of different species and the uptake of strontium-90 (75). The exchange adsorption does not appear to be controlled di- rectly by metabolism (19, 45) and has been considered by some investigators (50) to be independent of active transport. The point of maximum uptake of strontium and iodine appears to be within a few mm of the root apex (45), and no enhanced uptake is observed with root hairs. Other work (140) indicates that the tips of barley roots absorb various ions readily but that the greatest translocation occurs from a region 30 mm above the tip. It has been suggested that strontium can partially substitute for calcium (133, 137) and even that strontium is an essential element (141). Sixty to 70 per cent of the strontium in the plant has been found to be water-soluble, whereas 97 per cent of the cesium and only 16 per cent of the cerium-144 was water-soluble (83). A possible error in short-term experiments is the exchange of the radioactive isotope for the stable isotope already in the plant. This is particularly true for ions of slow turnover rate, such as calcium. Some workers (73, 133) consider the first 24 hours of calcium uptake to be largely nonmetabolic exchange. B. Translocation Most investigators believe that translocation of ions is governed less by me- tabolism than by the uptake process (5). The translocation of rubidium from the root to the top has been related to the transpiration stream (29). However, no such rela- tionship was found for calcium (7).
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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 n were rarely retained. Greater absorption is generally expected from contaminants in solution than from dry con- taminants. * 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 elements. The time of contamination 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).
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11 Iodine-131 can occur in the gaseous state, and in this form it is taken up by both the mesophyll (35 to 40 per cent) and the epidermal tissue (118). The rate de- pends upon the concentration. Stable iodine does not reduce iodine-131 absorption, but it does reduce translocation. The washing effect of rain can reduce the foliar intake of strontium by a factor as large as six (71). Intake of cesium is reduced to a lesser extent. Contaminated dust is satisfactorily removed from leaves by washing with 0. 1 N HC1, but spray contamination is much more difficult to remove (4). Over 50 per cent of the iodine in foliar contamination is removed by washing, 70 to 85 per cent by different ad- hesives, and up to 97 per cent by stripping off the upper and lower epidermis (42). Not only can washing remove surface contamination, but it can also remove ions from within the plant (55). Sodium, potassium, and manganese are readily leached; calcium, magnesium, sulfur, potassium, and strontium are moderately leached; and iron, zinc, phosphorus, and chlorine are leached with difficulty (130). D. Plant-Base Absorption Some British workers have given attention to the absorption of fission products from the root mats of long-established grass pastures (107). The root mat is com- posed of roots, basal portions of stems, and organic matter. The strontium-90 in rainfall and that washed off the plants may be held in the mat long enough for con- siderable absorption to occur. This pathway bypasses soil reactions and may be very important where the root mat is present. The proposed plant-base mechanism provides a reasonable explanation for the strontium-90 concentration in pasture vegetation, which appears far too high to be accounted for from expected soil uptake and foliar absorption. E. Distribution in Plants Strontium tends to accumulate in the aboveground portions of plants (16, 47, 67, 78, 106, 123), particularly in the vascular tissues (77), although the root con- centration increases with time (58). The greatest concentration of strontium is usually found in the older leaves (85, 94), with only about one tenth as much in the seeds (78, 94, 120). However, the seeds of a few plants, e.g., Euphorbia, accumu- late strontium to a greater extent (13). Cesium and rubidium are similar to potassium, and therefore it is expected that they are distributed more uniformly throughout the plant than strontium. About 10 per cent of the total plant cesium is found in the grain of wheat and oats (47), and other work (67) indicates that there is a slight tendency for both cesium and rubidium to accumulate in the young leaves and flowers. In four plant species, the highest concentration of iodine was found in the roots, followed by older leaves and, finally, the younger leaves (119). Other radioactive constituents of fallout, which have not been specifically mentioned, concentrated in the roots, with little translocation to the top (47, 85, 94, 106).
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12 F. Species Differences The order of fission-product uptake by the different plant families is Leguminosae> Solanaceae> Compositae>Gramineae for the tops and Leguminosae >Gramineae>Compositae>Solanaceae for the roots (142). Other workers report no consistent differences between the lower and higher orders of the plant kingdom (79). The calcium and strontium content of eight legumes was about three times that found in eight grasses (138). The absorptive power of a given species for strontium is con- sidered to be proportional to its absorptive power for calcium (17, 66). Characteristics of the root system may be very important in determining the uptake of radioisotopes from soil. Russian thistle can absorb strontium from a soil depth greater than 3-1/2 feet (116). Since the plants of the grass family have rela- tively shallow root systems, they will preferentially absorb nuclides occurring near the surface rather than those placed at a greater depth. With a grass-clover mixture, it was found that both the strontium content and the strontium-to-calcium ratio were reduced 70 per cent by plowing under the surface contamination (72). More deeply rooted crops showed only small effects from this deeper placement. Bicarbonate has differential effects on plant species, with beans taking up lesser amounts of cations than barley in the presence of bicarbonate (32). Additional interactions of plant species with rate of uptake, distribution, temperature, and other factors are probably of minor importance when considering broad differences.