Multiphasic Uptake of Sulfate by Barley Roots

Uptake of sulfate by excised barley roots increases upon their washing in aerated water or dilute CaCl₂ solutions. Washing increases the values for V_max and the sulfate concentrations required for transition between the lower phases, but the K_M-values remain essentially constant. At low sulfate concentrations, phase transitions do not occur in the absence of calcium or other divalent cations. These ions are about equally effective in enhancing short-term sulfate uptake. Phase transitions were not principally altered by sulfhydryl or protein reagents. These concentration-dependent transitions appear unrelated to temperature-dependent phase transitions as evidenced by similar multiphasic patterns at low and high temperature.     

Multiphasic Uptake of Ammonium by Soybean Roots

Uptake of ammonium by intact soybean (Glycine max Merr. cv. Amsoy) plants can be represented by 3 phases of a single, multiphasic mechanism in the range 1.78 × 10^-5-3.57 × 10^-3M. Each phase covers a limited concentration range and obeys Michaelis-Menten kinetics. The multiphasic pattern of NH₄⁺ uptake is remarkably consistent at all stages of soybean growth (20, 40, 60 and 80 days).     

Uptake of Sulfate by Roots and Leaf Slices of Barley: Mediated by Single, Multiphasic Mechanisms

The uptake of 10^?1M–2.5 × 10^?1M sulfate by roots and leaf slices of barley can be described by a single isotherm having, respectively, 8 and 5 phases with regularly increasing kinetic constants. Each phase covers a limited concentration range and obeys Michaelis-Menten kinetics. The uptake of sulfate is proposed to be rate-limited by a single mechanism or structure which is located in the plasmalemma (or cytoplasm) and which changes characteristics at certain discrete external concentrations (inflection points). Examination of published data indicates that the uptake of other inorganic ions by higher plant cells is also mediated by single, multi-phasic mechanisms.     

Boron Uptake by Excised Barley Roots

Active uptake of boron (B) by excised barley roots is linear with time for at least 1.5 h. Although no evidence was found for accumulation of B against a concentration gradient. this component of B uptake does satisfy other criteria for an active transport process. Transport is inhibited by 0.05 mM 2,4-dinitrophenol, 0.05 mM azide, 5 mM arsenate and 5 mM dicoumarol. Also, uptake is temperature-sensitive, being nil at 2°C and maximal at 34 to 38°C. Boron uptake by barley roots increases with time when they are washed in aerated 0.5 mM CaSO₄ solution. A double reciprocal plot of the B uptake data manifests a series of phases separated by sharp transitions or “jumps”, and is compatible with the concept of multiphasic uptake mechanisms. Kinetic constants and transition points for the various phases were calculated accordingly. The fit of these data was compared statistically to three other relevant models, viz, the dual model, the “single + diffusion” model (a Michaelis–Menten term and a diffusion term), and the negative cooperativity model. In each case, the data were better represented by the multiphasic model.     

Multiphasic Uptake of Sulfate by Barley Roots I. Effects of Analogues, Phosphate, and pH

For sulfate uptake by barley roots, competition studies reveal that uptake and phase transitions are caused by interaction of ions with separate sites on or in the plasmalemma. Uptake is competitively, and unequally, inhibited by sulfate analogues but not by other divalent anions. In contrast, divalent phosphate and di- and trivalent pyrophosphate are equally effective in causing transitions. Phosphate is taken up mainly or entirely as H₂PO₄^? by a similar but separate multiphasic mechanism. At pH 8, sulfate uptake is mediated by fewer phases than at low and intermediate pH.     

Characteristics of Zinc Uptake by Barley Roots

The linear uptake of zinc by excised barley roots (Hordeum distichon L.) in the time range from 20–120 min is not continued over periods of 20–28 h. In concentration dependent uptake experiments with intact barley roots three phases in the range up to 1.38 mM zinc could be detected independent of the tested uptake period. The kinetic constants increased with higher phases and the transition points were lowered with increasing time. The presence of copper did not inhibit the uptake of zinc competitively. On the contrary a slight stimulation in the uptake rates was observed indicating an interaction with the transition site. It is concluded that zinc and copper are taken up by separate mechanisms.     

Uptake of Amino Acids by Barley Leaf Slices: Kinetics, Specificity, and Energetics

Uptake of ¹⁴C-labelled _L-lysine. _L-arginine, _L-glutamic acid, _L-aspartic acid, and glycine was studied in 0.75 mm wide barley (Hordeum vulgare L. cv. Lise) leaf slices. After an initial period (10 min) of rapid accumulation amino acid uptake proceeded at a steady, lower rate for several hours. Uptake was stimulated by 10^?4M Ca^?2+ ions. Uptake was strongly _pH dependent with the following optima: aspartic acid _pH 3.5. glutamic acid _pH 4.1. glycine _pH 5.8, lysine _pH 6–7, and arginine _pH 5–8 (a broad plateau). The optimal temperature was about 30°C. and the temperature coefficient in the range 0–20gGC was 2.3–2.5. Concentration-dependence data gave uptake isotherms which appeared to be multiphasic for all the amino acids used. The amino acids inhibited each other in a competitive fashion, indicating that they were all transported by a single carrier system. Uptake of lysinc was strongly inhibited by 10^?4M 2.4-dinitrophenol. Lysine uptake was not stimulated by light under aerobic conditions. However, it was much reduced in the dark under anaerobic conditions. This reduction was almost compensated for by light. The light-stimulation of uptake under anaerobic conditions was abolished by 10^?5M 3-(3,4-dichlorophenyl) 1.1-dimethylurea.     

Uptake of Branched-Chain Amino Acids by Streptococcus thermophilus

The transport of branched-chain amino acids in Streptococcus thermophilus was energy dependent. The metabolic inhibitors of glycolysis and ATPase enzymes were active, but the proton-conducting uncouplers were not. Transport was optimal at temperatures of between 30 and 45°C and at pH 7.0 for the three amino acids leucine, valine, and isoleucine; a second peak existed at pH 5.0 with valine and isoleucine. By competition and kinetics studies, the branched-chain amino acids were found to share at least a common transport system.     

Mechanisms of Sodium and Rubidium Uptake by Excised Barley Roots

Accumulation of sodium and rubidium by excised barley roots was investigated. The concentration isotherm yielded one absorption shoulder. Nevertheless, it is suggested that two mechanisms take part in the uptake of sodium and rubidium: One non-metabolic mechanism with an apparent participation at low external salt concentrations (< 1 mM) and at high concentrations (> 20 mM). Such a mechanism is almost unaffected by low temperature conditions and by metabolic inhibitors. Rubidium possesses a high affinity toward this non-metabolic system. The second mechanism is sensitive to metabolic inhibitors and to low temperature conditions. It dominates at intermediate external concentrations (1–20 mM). Sodium possesses high affinity towards this mechanism. The two mechanisms operate in a parallel manner beyond a diffusion barrier (= plasmamembrane) surrounding the cells. It is assumed that both the metabolic and the non-metabolic mechanisms operate in the entire concentration spectrum, but their relative contribution to the total uptake varies at different ranges.     

The effect of Aging on Ion Uptake by Excised Barley Roots

When excised barley (Hodeum Vulgare L.) roots were aged, the rate of uptake of K and C1 increased raching a maximum in about 14 to 18 h. At its maximum the uptake rate was approxiamately twice that of the freshly excised root materila. Respiratory activity declined markedly during the aging period. Although excision was an essential requirement for uptake enhancement, washing was not. Both the uptake and aging processes were shown ot be unresponsive to the presence or absence of Ca in the tratment solutions. Because of cation exchange properties, the change in the total cation content of the root material was a more meaningful measure of the metabolically mediated cation uptake than was the change in content of the test cation itself. The pluggin of xylem vessels with protein and pectin-like substances was observed to increase with aging. It is proposed that the occlusion of the vessels may account for an apparent increaase in uptake since the obstruction could reduce a concurrent loss of ions through the cut ends of the root segments.     

Varietal Differences in Rubidium Uptake Efficiency of Barley Roots

The rate of light saturated photosynthesis of Nitzschia palea was reduced by crude oil, naphthalene and benzene. A decrease in the rate of photosynthesis at weak irradiance was also found with crude oil and naphthalene and high concentrations of benzene. On a mg/1 basis naphthalene decreased photosynthesis to a greater extent than did crude oil and crude oil to a greater extent than did benzene. A linear proportionality was found between the decrease in light saturated photosynthesis and the concentrations of aromatic hydrocarbons. The effects on photosynthesis were generally reversible, but a concentration of 700 mg benzene/1 stopped photosynthesis completely and irreversibly.     

Constitutive Uptake of Choline Sulfate by Roots and Leaf Slices of Barley

Barley roots take up choline sulfate constitutively via 3 separate, homogeneous mechanisms which obey Michaelis-Menten kinetics and have, respectively, low, high, and very high affinity for the sulfate ester. Leaf slices possess only the low-and the high-affinity mechanism. These are both inhibited by dini-trophenol and NaF but are differently affected by solute analogues. Uptake via the high-affinity mechanism is active; the low-affinity mechanism has also characteristics of facilitated diffusion.     

The Interaction of pH and Aeration in CI Uptake by Barley Roots

The uptake of Cl by excised roots of barley (Hordeum vulgare L.) from KC1 solution maintained at high pH was markedly reduced by high rates of aeration, whereas K uptake was scarcely affected. Aeration rate had relatively minor effects at low pH. The effect of high aeration rate at pH 9 could be overcome by the use of buffered solutions. In unbuffered solutions the H resulting from the excess cation uptake together with that produced from respiratory CO₂ was sufficient to materially reduce the pH of the solution. The reduction in pH favored the uptake of Cl which is adversely affected by high pH. The effect of aeration rate could be explained in terms of root induced pH changes and film diffusion involving the solution film adjacent to the root surface.     

Simulation of Cl Uptake by Low-salt Barley Roots as a Test of Models of Salt Uptake

Computing techniques are used to simulate the course of uptake of K⁺, Na⁺, and Cl⁻ by low-salt roots. Measurements of the fluxes of these ions in high-salt roots are used to calculate membrane permeabilities, which are then used to calculate cell uptake. In this way it is possible to test the predictive value of different models for the location of sites of salt uptake in the cell.     

Uptake and Incorporation of Amino Acids and Peptides by Bacteroides amylophilus

Bacteroides amylophilus H-18 demonstrated a higher growth yield, a slightly higher growth rate, and a diminished lag period when Tryptose was added to the basal medium. This uptake of labeled amino acids was concentration-dependent, as the contribution of exogenous amino acid to the cell protein increased from 15.4 to 24.1% when the concentration of Casamino Acids in the medium was increased from 1.4 to 2.8 mg/ml. There was considerable redistribution of (14)C-label to other amino acids. Tryptic peptides of casein competed effectively with the amino acids for uptake. The (14)C-label from a protein was incorporated into B. amylophilus H-18 cells presumably after breakdown of the protein by the B. amylophilus H-18 protease.     

Variations in Uptake of Sodium and Rubidium Along Barley Roots

Variations in uptake of sodium and rubidium were ob) served along the apical 8 cm segments of barley roots. Dif) ferent patterns of uptake were obtained for low- and for high-salt roots. The validity of using batches of entire barley roots for kinetic analysis of ion uptake is questioned.     

Membrane ATPase Activity of Barley Roots and Potassium Uptake by Isolated Stele and Cortex

Uptake of potassium ions by isolated stelar tissues of barley from 0.5 and 10 mM K⁺ was respectively 13 and 3.6% of that of the cortical tissues. 0.1 mM H₂PO₄, LO mM ATP and 10 mM Ca(NO₃)₂ did not increase the potassium uptake of either stele or cortex during 5 h of uptake period. A time-course incubation for histological demonstration of the ATPase activity of the plasmalemma and tonoplast of the matured sections of the roots demonstrated a greater activity for the cortical than the stelar tissue. In the stelar parenchyma cells, the plasma lemma showed a higher activity than the tonoplast. These results, which support the “leakiness hypothesis” of the stele, are discussed in relation to the proposed mechanisms of radial ion transport in roots.