No abstract

The absorption characteristics of Cd2' by 10-to 12-day-old soybean plants (Glycine max cv Williams) were investigated with respect to influence of Cd concentration on adsorption to root surfaces, root absorption, transport kinetics and interaction with the nutrient cations Cu2+, Fe2 , Mn2 , and Zn2 . The fraction of nonexchangeable Cd bound to roots remained relatively constant at 20 to 25% of the absorbed fraction at solution concentration of 0.0025 to 0.5 micromolar, and increased to 45% at solution concentration in excess of 0.5 micromolar. The exchangeable fraction represented 1.4 to 32% of the absorbed fraction, and was concentration dependent. Using dinitrophenol as a metabolic inhibitor, the 'metabolically absorbed' frction was shown to represent 75 to 80% of the absorbed fraction at concentration less than 0.5 micromolar, and decreased to 55% at 5 micromolar. At comparatively low Cd concentrations, 0.0025 to micromolar 0. Cadmium Uptake. Evaluation of the absorption behavior of Cd 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 5.8) for 12 h to allow for desorption of possible interfering ions from root surfaces. Individual plants were then transferred to fresh 0.5 mm CaCl22 and various concentrations of CdCl2. CaCl2 was employed in all uptake and absorption solutions to provide sufficient Ca2`to maintain membrane permeability (20,22). For absorption periods of 60 min or less, 500 ml volumes were employed; for absorption periods of over 60 min, 1-L volumes were employed to limit reduction of total Cd levels in solution to less than 10% during the experiment.Solutions containing CdCl2 levels from 0.0025 to 5.0 liM were traced with carrier-free`°CdCI2, and adjusted to pH 5.8 with KOH. Following the absorption period, shoots were removed and roots were transferred to 0.5 mm CaCl2 solutions containing unlabeled CdCl2 at concentrations 20-fold higher than treatment concentrations. Roots were routinely desorbed using three changes of solution for a total of 2 h; in preliminary studies, desorbed '"Cd was measured in efflux solutions at 30-min intervals following 5-min wash in 0.5 mM CaC12.

A method is proposed for analysis of total nitrogen in plant tissues enabling predigestion (to reduce nitrate to ammonium) and digestion to be performed in a Folin‐Wu tube. Following digestion, a sensitive colorimetric assay for ammonium is used to quantitate N‐content in an aliquot of the digest. The proposed method has several advantages: 1) up to 110 tissue samples can be predigested and digested in one batch; 2) small tissue samples (5 to 100 mg) can be accommodated; 3) the colorimetric assay for ammonia is very sensitive; 4) tissues can be predigested to recover all nitrate in the samples; and 5) aliquot size from the diluted digest can be varied to effectively detect as little as 1 µg of N.

Soils amended with [14C]hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) were sampled over 60 d and subjected to exhaustive Soxhlet extraction followed by HPLC analysis. RDX was the only radiolabeled compound observed in soil extracts. Emission of volatile organics and 14CO2 from soil accounted for only 0.31% of the amended radiolabel. Mass balance for RDX‐amended soil was better than 84% throughout the two‐month study. The analytical method developed for plants involved acid hydrolysis, solvent extraction, fractionation on Florisil® adsorbent and separation by HPLC. The described methodology allowed for RDX recovery of 86 ± 3% from fortified bush bean leaf tissue. Further experiments were conducted with bush bean plants maintained on RDX‐containing hydroponic solutions. Hydroponic plants did not emit detectable amounts of 14CO2 or radiolabeled volatile organics. Analysis of the plant tissue indicated bioaccumulation of RDX in the aerial tissues of hydroponic plants exposed for either 1 or 7 d. Metabolism of RDX to polar metabolites was observed in plants exposed for 7 d.

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