Root Absorption and Transport Behavior of Technetium in Soybean

Abstract

The absorption characteristics and mechanisms of pertechnetate (TcO47) uptake by hydroponically grown soybean seedlings (Glycinemax cv Williams) were determined. Absorption from 10 micromolar solutioDs was linear for at least 6 hours, with 30% of the absorbed TcO47 being transferred to the shoot. Evaluation of concentration-dependent absorption rates from solutions containing 0.02 to 10 micromolar Technetium-99 is currently projected to be one of the most important isotopes associated with the nuclear fuel cycle based on potential long-term dose to man. This is based on a number of criteria including fission yield (6%), long half-life (2.13 x l0 year), mobility and persistence in soils as the pertechnetate ion (TcO4-), and its comparatively high availability for plant absorption (11,19 for radiation-induced damage and suggested an interaction or substitution of Tc for an essential nutrient. Pertechnetate has been shown to be toxic to a number of algae and bacteria, and interfere with both respiration and photosynthesis (9) at concentrations in excess of 1 mm. The potential interaction of Tc with nutrient ion metabolism is supported by uptake inhibition studies which showed 9Tc4O uptake to be reduced by the presence of sulfate, selenate, phosphate, and molybdate (6).The purpose ofthe present study is to evaluate the parameters associated with TcO4-absorption by plant roots, and the kinetics of its interaction with essential nutrient elements and nonnutrient elements with chemical similarities.MATERIALS AND METHODS Plant Culture. Seeds of Glycine max cv Williams were germinated on moist filter paper and individual seedlings transferred to 600-ml beakers containing 500 ml ofaerated nutrient solution 3 d following germination. The nutrient solution contained 150 mg of KCI, 120 mg of MgSO4, 946 mg of Ca(N03)2 4H20, 68 mg of KH2PO4, 0.06 mg of ZnSO4.7H20, 0.69 mg of H3BO3, 0.017 mg of CuC12-2H20, 0.024 mg of Na2MoO4-2H20, 0.022 mg of MnCl2-4H20, and 0.60 mg of Fe+3 (as Fe EDDHA) per liter. The pH was adjusted to 6.8 with KOH and solutions changed three times a week. Plants were maintained in controlled-environment chambers with a 16-h light cycle (-500 ME m-2 s ', PAR, at leaf surface), a day/night temperature cycle of 26/22C, and 50% RH.Technetium Uptake. Evaluation of the absorption behavior of Tc was performed using 10-to 12-d-old plants. Prior to use, plants were transferred from nutrient solutions to 0.5 mM CaCl2 solutions (pH 6.8) for 12 h to allow for desorption of possible interfering ions from root surfaces. Individual plants were transferred to aerated solutions containing 500 ml 0.5 mm CaC12 (MCB, suprapure) and various concentrations of NH49Tc04. Following the absorption period, which was 2 h except as noted, plants were transferred to either complete nutrient solution or 0.5 mM CaC12 solutions containing 10 mm KH2PO4 or K2S04. Ions were desorbed from roots using three changes of solution for a total of 20 min.The influence of H+ concentration on Tc absorption and transfer from root to shoot was d...

Accumulation of technetium in floating aquatic plants

“…The second hypothesis assumes that Tc behaves like an analogue for an essential element. Accumulation of Tc in terrestrial plants is reduced in fertilized soils [5], which is assigned to the analogy of Tc0 4 " with N0 3~ in the plants metabolism [6,7], although also S0 4 2~, P0 4 3~, Se0 4 2 ", or Mo0 4 2 are reported to inhibit Tc uptake [8]. Recently, it was shown that N0 3 " also inhibit Tc accumulation in the submerged grown amphibious aroid Cryptoryna affinis [9].…”

Soil availability, plant uptake and soil to plant transfer of 99Tc—A review

“…This may help to explain the absence of competition effects noted in plants that have not been exposed to nitrate for 24 h or more and fed with NH 4 TcO 4 (e.g. Cataldo et al, 1983). However, we suggest that as NO (Forde and Clarkson, 1999).…”

“…The mechanisms that control plant uptake can therefore determine foodchain contamination. There has been some debate, as yet unresolved, about the mechanisms through which TcO Wildung et al, (1977) and Cataldo et al (1983) suggested that multiphasic uptake, including dependence on metabolic energy, indicated entry through saturable, active transport mechanisms that conformed to Michaelis-Menten kinetics. The mathematical model of 99 Tc uptake developed by Van Loon et al (1989) is also consistent with uptake as a nutrient analogue.…”

“…It is becoming clear that there are numerous transport proteins involved in anion uptake by plants and that a variety of them might be involved in TcO 4 , and might be influenced by Cl Ϫ transporters. Cataldo et al (1983) noted the dependence of 99 Tc uptake on external pH, which is to be expected of transport through H + cotransporters now known to transport many anions. Molecular studies, in particular those with electrophysiological models and knock-out mutants, such as those of White and Broadley (2000) and Broadley et al (2001), have been useful in radioecological studies of 137 Cs (Shaw, 2001) and are likely to prove so for 99 Tc.…”

Soil-to-Plant Concentration Ratios for Assessing Food Chain Pathways in Biosphere Models

“…The vials were marked with sample name and date. The tissues were then wet digested according to the method of Cataldo et al (1983). Briefly, the dried tissues were wetted with 10 mL of 3 N NH 4 OH, covered and digested for 12-to 20-h on low heat (~60°C) in B A 3-9 the hood.…”