Comparative metabolic behavior and interrelationships of Tc and S in soybean plants.

The comparative behavior of sulfur (S) and technetium (Tc) in soybean seedlings shows gross subcellular distributions to be similar for these oxyanions. More than 75% of the tissue-deposited Tc remains soluble and extractable. Differences in Tc fixation/incorporation were noted for the nuclear and chloroplast fractions of leaf and root cells. Pulse studies showed that soluble protein and nitrate reductase levels rose in response to Tc accumulation by sink leaves but not source leaves. In vitro assay of chloroplast-based S reduction and incorporation systems showed Tc to be reduced and incorporated into amino nitrogen-containing products. A hypothesis related to the metabolic behavior of Tc in plants is presented.

Organic Constituents and Complexation of Nickel(II), Iron(III), Cadmium(II), and plutonium(IV) in Soybean Xylem Exudates.

The xylem exudates of soybean (Glycine max cv Williams), provided with fixed N, were characterized as to their organic constituents and in vivo and in vitro complexation of plutonium, iron, cadmium, and nickel. Ion exchange fractionation of whole exudates into their compound classes (organic acid, neutral, amino acid, and polyphosphate), followed by thinlayer electrophoresis, permitted evaluation of the types of ligands which stabilize each element. The polyvalent elements plutonium(IV) and iron(III) are found primarily as organic acid complexes, while the divalent elements nickel(II) and cadmium(II) are associated primarily with components of the amino acid/peptide fraction. For plutonium and cadmium, it was not possible to fully duplicate complexes formed in vivo by back reaction with whole exudates or individual class fractions, indicating the possible importance of plant induction processes, reaction kinetics, and/or the formation of mixed ligand complexes. The number and distribution of specific iron- and nickel-containing complexes varies with plant age and appears to be related to the relative concentration of organic acids and amino acids/peptides being produced and transported in the xylem as the plant matures.

Concentration of orally administered and chronically fed 95mTc in Japanese quail eggs.

A chronic feeding study using 95mTc incorporated into alfalfa and an acute study where 95mTc was amended to alfalfa showed that about 8.4% of ingested Tc was transferred to eggs. After 10 days of chronic feeding, 80% of the Tc was in yolk, 20% in albumin and less than 1% in shell and associated membranes. At necropsy, technetium concentrations in the three largest oocytes were nearly equal. The biological half-time for Tc was about one to two days in acute studies. Results from the chronic feeding study also indicated that Tc levels in albumin reach a maximum between three and five days while maximum yolk concentration is attained in about six to seven days. Albumin concentrations declined about 20-50% after Day 6.

Cadmium uptake kinetics in intact soybean plants.

The absorption characteristics of Cd(2+) 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 Cu(2+), Fe(2+), Mn(2+), and Zn(2+). 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' fraction 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 0.3, root absorption exhibited two isotherms with K(2) values of 0.08 and 1.2 micromolar. Root absorption and transfer from root to shoot of Cd(2+) was inhibited by Cu(2+), Fe(2+), Mn(2+), and Zn(2+). Analyses of kinetic interaction of these nutrient cations with Cd(2+) indicated that Cu(2+), Fe(2+), Zn(2+), and possibly Mn(2+) inhibited Cd absorption competitively suggesting an involvement of a common transport site or process.

Root absorption and transport behavior of technetium in soybean.

The absorption characteristics and mechanisms of pertechnetate (TcO(4) (-)) uptake by hydroponically grown soybean seedlings (Glycine max cv Williams) were determined. Absorption from 10 micromolar solutions was linear for at least 6 hours, with 30% of the absorbed TcO(4) (-) being transferred to the shoot. Evaluation of concentration-dependent absorption rates from solutions containing 0.02 to 10 micromolar TcO(4) (-) shows the presence of multiphasic absorption isotherms with calculated K(s) values of 0.09, 8.9, and 54 micromolar for intact seedlings. The uptake of TcO(4) (-) was inhibited by a 4-fold concentration excess of sulfate, phosphate, selenate, molybdate, and permanganate; no reduction was noted with borate, nitrate, tungstate, perrhenate, iodate, or vanadate. Analyses of the kinetics of interaction between TcO(4) (-) and inhibiting anions show permanganate to be a noncompetitive inhibitor, while sulfate, phosphate, and selenate, and molybdate exhibit characteristics of competitive inhibitors of TcO(4) (-) transport suggesting involvement of a common transport process.

The role of soil and plant metabolic processes in controlling trace element behavior and bioavailability to animals.

Metabolic and physiological processes play important roles in regulating the transfer and behavior of trace elements in the soil/plant/animal system. The behaviors of Ni, Cd, Cr, T1, Np, Pu and Tc are used to illustrate important aspects of these processes. Microbial metabolism has both indirect and direct effects on trace element solubility in soils. Once non-nutrient trace elements are solubilized, the ability of plant roots to actively accumulate them is dependent on chemical activity of the element in soil solution, the presence of competing ions and the redox potential and absorption capacity of the root. After absorption in the plant, trace elements are translocated, metabolized and stored; fate and behavior varies with the properties of the element, but is generally analogous to nutrient elements. These processes can dramatically affect the availability of individual elements to animals consuming plants.

Cadmium distribution and chemical fate in soybean plants.

The distribution and chemical behavior of Cd(2+) in tissues and its chemical form in xylem water of soybean plants (cv. Williams) were investigated. Following root absorption, Cd is strongly retained by roots, with only 2% of the accumulated Cd being transported to leaves; as much as 8% was transported to seeds during seed filling. In vivo xylem exudates contained two anionic Cd complexes in addition to inorganic forms of Cd. Once accumulated in root and leaf tissues, Cd rapidly equilibrated between the insoluble, soluble, and organelle fractions. Of the solubles, which contain 50% of the Cd, >50% was associated with components of >10,000 molecular weight, and <8% was associated with <500 molecular weight components. Cadmium accumulated in soybean seeds was primarily associated with cotyledons. Fractionation of seeds showed the soy proteinate and soy whey to contain 32 and 50% of the accumulated Cd, respectively.

Soil and plant factors influencing the accumulation of heavy metals by plants.

The use of plants to monitor heavy metal pollution in the terrestrial environment must be based on a cognizance of the complicated, integrated effects of pollutant source and soil-plant variables. To be detectable in plants, pollutant sources must significantly increase the plant available metal concentration in soil. The major factor governing metal availability to plants in soils is the solubility of the metal associated with the solid phase, since in order for root uptake to occur, a soluble species must exist adjacent to the root membrane for some finite period. The rate of release and form of this soluble species will have a strong influence on the rate and extent of uptake and, perhaps, mobility and toxicity in the plant and consuming animals. The factors influencing solubility and form of available metal species in soil vary widely geographically and include the concentration and chemical form of the element entering soil, soil properties (endogenous metal concentration, mineralogy, particle size distribution), and soil processes (e.g., mineral weathering, microbial activity), as these influence the kinetics of sorption reactions, metal concentration in solution and the form of soluble and insoluble chemical species. The plant root represents the first barrier to the selective accumulation of ions present in soil solution. Uptake and kinetic data for nutrient ions and chemically related nonnutrient analogs suggest that metabolic processes associated with root absorption of nutrients regulate both the affinity and rate of absorption of specific nonnutrient ions. Detailed kinetic studies of Ni, Cd, and Tl uptake by intact plants demonstrate multiphasic root absorption processes over a broad concentration range, and the use of transport mechanisms in place for the nutrient ions Cu, Zn, and K. Advantages and limitations of higher plants as indicators of increased levels of metal pollution are discussed in terms of these soil and plant phenomena.

Nickel in plants: I. Uptake kinetics using intact soybean seedlings.

The absorption of Ni(2+) by 21-day-old soybean plants (Glycine max cv. Williams) was investigated with respect to its concentration dependence, transport kinetics, and interactions with various nutrient cations. Nickel absorption, measured as a function of concentration (0.02 to 100 mum), demonstrated the presence of multiple absorption isotherms. Each of the three isotherms conforms to Michaelis-Menten kinetics; kinetic constants are reported for uptake by the intact plant and for transfer from root to shoot tissues. The absorption of Ni(2+) by the intact plant and its transfer from root to shoot were inhibited by the presence of Cu(2+), Zn(2+), Fe(2+), and Co(2+). Competition kinetic studies showed Cu(2+) and Zn(2+) to inhibit Ni(2+) absorption competitively, suggesting that Ni(2+), Cu(2+), and Zn(2+) are absorbed using the same carrier site. Calculated K(m) and K(i) constants for Ni(2+) in the presence and absence of Cu(2+) were 6.1 and 9.2 mum, respectively, whereas K(m) and K(i) constants were calculated to be 6.7 and 24.4 mum, respectively, for Ni(2+) in the presence and absence of Zn(2+). The mechanism of inhibition of Ni(2+) in the presence of Fe(2+) and Co(2+) was not resolved by classical kinetic relationships.

Nickel in Plants: II. Distribution and Chemical Form in Soybean Plants.

The gross tissue distribution, intracellular fate, and chemical behavior of Ni(2+) in soybean plants (Glycine max cv. Williams) were investigated. Following root absorption, Ni was highly mobile in the plant, with leaves being the major sink in the shoots for Ni during vegetative growth. A senescence >70% of the Ni present in the shoot was remobilized to seeds. Fractionation of root and leaf tissues showed >90% of the Ni to be associated with the soluble fraction of tissues; ultrafiltration of the solubles showed >77% of the Ni to be associated with the 10,000 to 500 molecular weight components of both roots and leaves. Chemical characterization of the soluble components (10,000 to 500 and >500 molecular weight) by thin layer chromatography and electrophoresis resolved a number of Ni-containing organic complexes. Major Ni-containing components formed in the root are transported in the xylem stream, and undergo partial modification on deposition in leaves. Nickel accumulated in seeds is primarily associated with the cotyledons. Chemical fractionation of cotyledon components showed 80% of the Ni to be associated with the soluble whey fraction, while 70% of this fraction was composed of Ni-containing components with molecular weight <10,000.

Vein Loading: The Role of the Symplast in Intercellular Transport of Carbohydrate between the Mesophyll and Minor Veins of Tobacco Leaves.

Enzymatically separated leaf tissues of Nicotiana tabacum L., exhibiting good metabolic integrity, were used to evaluate the kinetics of sugar accumulation over the concentration range of 10 to 100 mm. Mesophyll cells exhibited Km values of 16 and 30 mm for glucose and sucrose, respectively; minor veins showed a reverse relationship, with Km values of 58 and 16 mm for glucose and sucrose, respectively. This would suggest that sucrose is preferentially absorbed by the minor vein net. Analysis of V(max) data indicates a reduction in the ability of isolated minor veins to accumulate substrate, implicating a symplastic rather than apoplastic route for intercellular transport. Competition studies demonstrate a common carrier for sucrose and glucose in both tissue types and suggest the presence of a "transport compartment," entry to which is regulated by a critical intracellular sucrose concentration.

Use of protein in extraction and stabilization of nitrate reductase.

The in vitro instability of nitrate reductase (EC 1.6.6.1) activity from leaves of several species of higher plants was investigated. Decay of activity was exponential with time, suggesting that an enzyme-catalyzed reaction was involved. The rate of decay of nitrate reductase activity increased as leaf age increased in all species studied. Activity was relatively stable in certain genotypes of Zea mays L., but extremely unstable in others. In all genotypes of Avena sativa L. and Nicotiana tabacum L. studied, nitrate reductase was unstable. Addition of 3% (w/v) bovine serum albumin or casein to extraction media prevented or retarded the decay of nitrate reductase activity for several hours. In addition, the presence of bovine serum albumin or casein in the enzyme homogenate markedly increased nitrate reductase activity (up to 15-fold), especially in older leaf tissue.

The rapid non-polar transport of auxin in the phloem of intact Coleus plants.

Indole-3-acetic acid (IAA) is transported from a nearly mature leaf throughout an intact Coleus blumei Benth. plant in the phloem. A buffered solution of both (14)C-methylene-labeled indoleacetic acid ([(14)C]IAA) and [6-(3)H]glucose was supplied in a glass capillary to the distal end of a severed main lateral vein of the leaf. Both labeled sugar and auxin move rapidly through the plant at velocities of ca. 16-20 cm h(-1) with closely similar, exponential profiles. This translocation is nonpolar; both auxin and sugar move upwards to the apex and young expanding leaves as well as downwards to the base of the shoot. Neither tracer appears in mature leaves; this eliminates the possibility that they enter the xylem. At the end of the transport period, 80-90% of the radioactivity recovered from various portions of the plants supplied with [(14)C]IAA is still identical chromatographically with IAA. In microautoradiographs prepared by techniques that minimize loss and redistribution of soluble compounds, radioactivity from [(3)H]IAA is concentrated in the phloem of the midrib and petiole of the fed leaf. A ring of triiodobenzoic acid (TIBA) strongly inhibits the polar auxin transport in sections isolated from the ringed region but does not significantly affect auxin translocation in the phloem of intact plants. TIBA does, however, reduce the entry of auxin into the collecting veins of the leaf. Thus steps in auxin transport sensitive to TIBA may occur during transfer through the leaf or into the phloem, but not during long distance translocation in the phloem.

Carbohydrate translocation in sugar beet petioles in relation to petiolar respiration and adenosine 5'-triphosphate.

Earlier studies have shown that the retarding effect of low petiolar temperatures on sucrose transport through sugar beet (Beta vulgaris L.) petioles is markedly time-dependent. Although the initial effect of chilling the petiole to near 0 C is severely inhibitory, translocation rates soon recover (usually within about 2 hours) to values at or near the control rate. In the present studies, selected metabolic parameters were measured simultaneously with translocation. No stoichiometric relationships among petiolar sucrose transport, petiolar respiration (CO(2) production), and calculated petiolar ATP turnover rates were evident. It appears that the major sources of energy input energizing carbohydrate transport in sieve tubes function mainly at either loading or unloading sites and not at the level of individual sieve-tube elements.

Solution-flow in the Phloem: I. Theoretical considerations.

A mathematical model for the reversible exchange of THO between the sieve tube lumen and its surrounding phloem tissue is used to explain the difference between the apparent velocities of THO and (14)C-sucrose transport observed when both are supplied simultaneously. Theoretically predicted results show a close correlation with those obtained experimentally. This model may be used in evaluating previous work in which THO was used as a tracer. The calculations support the existence of a mass flow of sugars in aqueous solution along the path.