Effects of low temperature on respiration and uptake of rubidium ions by excised barley and corn roots

The effect of temperature upon ion uptake and respiration was investigated with excised roots of corn (Zea mays) and barley (Hordeum vulgare). A strong inhibition (Q₁₀ = 5 to 8) of ion uptake was observed at temperatures below 10 C. At higher temperatures more normal temperature dependencies (Q₁₀ = 1.3 to 2) were obtained. When the data were plotted according to the Arrhenius relationship, two different activation energies were indicated above and below 10 C. Other studies have related such changes with temperature in activation energy of processes to changes in membrane properties induced by temperature. These results suggest that such phase transitions may affect ion uptake processes. If so, then differences among species in their capacity to maintain normal root function at low soil temperature and to resist low temperature stress may be related to differences in the physical properties of cellular membranes.     

Beryllium uptake by excised barley roots

The uptake of beryllium by excised barley roots from a solutioncontaining 111 µM beryllium and 500 µM calcium wasstudied. The exchange-adsorbed portion was 15% of the totalBe uptake after 30 min of exposure. The total Be content couldbe reduced to 55% of the control in 1 hr with demineralizedwater. At pH levels above 6, Be uptake was increased dramatically.A Q₁₀ of 1.2 was found to exist between 1 and 23°C. Mg (111µM) did not influence Be absorption while Zn and Al atthe same concentration reduced it, possibly competitively, 27and 62% of the control, respectively. Dinitro-phenol (500 µM),cyanide (500µM), azide (500 µM) and m-chlorocarbonylcyanidephenylhydrazone (100 µM) at least doubled it indicatingthe existence of a possible metabolic barrier to beryllium entrywhich was disrupted by these compounds. Absorption kineticsover an external beryllium concentration range of 11 to 444µM suggested a number of binding sites. Above 444 µMa scattering effect was noted due possibly to its toxic effect. ¹Pesticide Programs, Environmental Protection Agency (P.S. -769),Washington, D.C. 20460, U.S.A. (Received August 13, 1979; )     

Inhibition of respiration and ion uptake by 2,3,5-triiodobenzoic Acid in excised barley roots

The addition of 1 micromolar 2,3,5-triiodobenzoic acid (TIBA) to solutions containing KCl resulted in the inhibition of K and Cl uptake in excised barley roots. The effectiveness of TIBA as an inhibitor increased as the pH of the treatment solution decreased and approached the pK_a of TIBA. A lag period of approximately 20 minutes existed prior to the onset of TIBA induced inhibition of ion uptake. Respiratory activity was also inhibited by TIBA. The data suggest that in this material, TIBA functions by entering the cytoplasm and inhibiting metabolism. Comparisons made on the effect of added Ca, showed that at pH 5.7 and higher, Ca had no effect on ion uptake whereas at lower pH values the presence of Ca enhanced uptake by offsetting the deleterious effects of H⁺.     

Effect of azetidine 2-carboxylic Acid on ion uptake and ion release to the xylem of excised barley roots

Azetidine 2-carboxylic acid (AZ) was used as an analog of proline to investigate further the relationship between protein synthesis and ion transport. AZ does not inhibit protein assembly, but the proteins formed are ineffective as enzymes. At relatively low concentrations (50 μM) AZ was a potent inhibitor of release of ions to the xylem of excised roots of barley (Hordeum vulgare L.) and intact plants. Uptake to the root was also inhibited but to a lesser degree. A procedure was introduced for estimating unidirectional fluxes from measurements of net tracer uptake, net transport to the xylem, and net efflux from the roots. It was shown that inhibition of release to the xylem was not caused by reduction in influx at the plasmalemma or to stimulation of influx to the vacuoles. It was suggested that AZ was acting on the process of release from symplast to the xylem. The action of AZ is compared with similar effects on ion transport produced by p-fluorophenylalanine, cycloheximide, and abscisic acid.     

Mechanism of sulfate uptake by excised maize roots

The mechanism of uptake of sulfate by excised maize roots (Zea mays L. cv. Ganga-5) was investigated. It was found that in the concentration range l × 10^?9 M ? 5 × 10^?2 M, the sulfate uptake could be represented by a single multiphasic isotherm having four phases. Each phase covered a limited concentration range and obeyed Michaelis-Menten kinetics, which indicates mediation in sulfate uptake by a structure which changes characteristics (kinetic constants) at certain discrete salts concentrations (transition points).     

Effect of adsorbed cations on phosphorus uptake by excised roots

Pretreatment of excised roots of Hordeum vulgare, Zea mays, and Glycine max with various salt solutions affected their subsequent rate of phosphorus absorption from 2 × 10⁻⁵m KH₂PO₄. The rate of absorption was greatest for roots pretreated with trivalent cations, intermediate with divalent cations and lowest with monovalent cations. It appeared that the pretreatment involved a rapid exchange reaction at the root surface which was reversible. A 1 min pretreatment was effective for more than 20 min. The acceleration of phosphorus uptake by roots produced by polyvalent cations may be due partly or entirely to a greater reduction in the electrical potential at the root surface or within the pores of the negatively charged cell wall by polyvalent cations than by monovalent cations.     

Effects of organic acids on ion uptake and retention in barley roots

Effects of several organic acids on ion uptake and retention and on respiration in barley roots having low and high KCl contents were assayed by measurements of K⁺, Na⁺, Ca²⁺, Cl⁻, and oxygen uptake. Organic acids with high pK_a values increase the permeability of roots to ions and decrease respiration when present in sufficient concentrations at pH 5 but have no inhibitory effects at pH 7. Absence of respiratory inhibition in short times and at lower organic acid concentrations, under conditions that immediately produce a permeability increase, indicate that the permeability change is not a result of respiratory inhibition. Effects of formate, acetate, propionate, and glutarate are attributed to entry of undissociated acid molecules into the effective membranes. Lack of a permeability increase with succinate, which has lower distribution coefficients to lipid solvents than do the aliphatic acids, can be explained by failure of sufficient amounts of the hydrophilic succinic acid molecules to penetrate the membranes involved. These experiments suggest that undissociated acid in root membranes can increase permeability of the roots.     

Effects of ouabain and low temperature on the sodium efflux pump in excised corn roots

Ouabain (0.05 millimolar) and low temperature (4 C) both caused the tissue Na⁺ content of excised 5-day-old corn roots to increase, indicating that there is an inhibition of the Na⁺ efflux pump. Na⁺ efflux was measured utilizing three different methods. Each method gave similar results in terms of rate and ouabain sensitivity. With one of these methods, the compartmental efflux method, it was demonstrated that rates for Na⁺ efflux increase as the external Na⁺ concentration is increased; e.g. the efflux rates are 0.529, 1.78, and 3.64 microequivalents per gram fresh weight per hour for external NaCl concentrations of 1, 10, and 30 millimolar, respectively. The data indicate that the Na⁺ efflux pump is located in the plasmalemma of root cells.     

Effects of low water potential on cortical cell length in growing regions of maize roots

Roots growing under low water potential commonly exhibit a marked decrease in growth rate and in diameter. Using median longitudinal sections of fixed maize (Zea mays L. cv WF9 × Mo 17) seedling roots, we investigated the cellular basis for these effects. Cortical cells in the shortened elongation zone of water stressed roots were longer than cortical cells in the comparable location of well-watered roots. Nearly twofold differences in cell length were seen in the region 2 to 4 millimeters behind the root apex. The shortened growth zone, however, leads to a final mean cortical cell length approximately 30% shorter in the stressed roots. These differences were present regardless of the age of the control roots. These data, and the slower growth rate seen in water-stressed roots, suggest that the water deficit causes a significant reduction in the rate of cell supply to the cortical cell files.     

pH control in nitrate uptake studies with excised roots

Summary In the investigation of NO ₃ ^– uptake by excised roots, pH of salt solutions can be effectively controlled by periodic additions of a cation exchange resin, Amberlite IR-120, in the H⁺ form. This method of pH control provides for the addition of H⁺ to systems without an accompanying anion which might influence the absorption of NO ₃ ^– . The resin appeared to provide a favorable buffering capacity during an extended period and the method was especially useful for NO ₃ ^– uptake studies conducted at high root-to-solution ratios.Contribution from the Plant Nutrition Laboratory, Department of Botany, The University of Michigan, paper number 39. This work was supported by research grant GM-07339-02 from the National Institutes of Health, U. S. Public Health Service.     

Cation uptake from bentonite suspensions by excised barley roots

Summary The net uptake of Na, K, or Ca by excised barley roots as influenced by the amount and kind of other cation was studied from bi-ionic and tri-ionic bentonite suspensions. The net uptake of Na or K from the Na-Ca or K-Ca system followed the concentration of soluble Na or K in the system. Calcium in both systems was taken up by the excised roots only at the 80 and 100 per cent Ca levels. At lower levels of Ca, the roots lost some of their initial calcium contents to the suspensions. The net uptake of Na or K from the Na-K system was in agreement with the concentrations of soluble Na or K in the system. At the various levels of Na or K, some of the initial calcium of the roots was lost to the suspensions. In the Na-K-Ca system, Na uptake gradually decreased with the increase of Ca level. Calcium was not absorbed by the roots at the various Ca levels. However, calcium greatly enhanced K uptake from this system. Part of a dissertation submitted by the senior author in partial fulfillment of requirements for the Ph. D. degree.     

Effects of ophiobolin a on ion leakage and hexose uptake by maize roots

Ophiobolin A, a sesterterpene metabolite of Helminthosporium maydis, Nisikado and Miyake, stimulates net leakage of electrolytes and glucose from maize (Zea mays L.) seedling roots. Treatment of the roots with ophiobolin A at a concentration of 10 mug/ml (25 mum) inhibits uptake of 10 mm 2-deoxyglucose by 50% and of 0.5 mm 2-deoxyglucose by 85%. Compartmental analysis of the efflux of 3-O-methylglucose failed to show a similar effect of ophiobolin A on the rate of efflux of hexose. The inhibition of uptake is not reversible by washing. There is no difference in the effects on roots from cytoplasmic male sterile or normal cytoplasm plants, and exposure of carrot (Daucus carota L.) root discs to ophiobolin A also causes inhibition of 2-deoxyglucose uptake by this tissue.     

Nitrate uptake and induction of nitrate reductase in excised corn roots

The characteristics of nitrate uptake and induction of nitrate reductase were studied in excised roots of corn (Zea mays L.). Upon initial exposure to nitrate, the low initial rate of nitrate uptake gradually increased until a steady uptake rate was achieved in 1 to 2 hours depending on the NO(3) (-) concentration. The pattern was observed over a wide range (0.2-5 mm) of nitrate concentrations and was independent of the accompanying cation.The nitrate uptake pattern as a function of increasing external nitrate concentrations (0.2-50 mm) followed saturation type kinetics. The reciprocal plot of the data was not linear but hyperbolic, indicating that more than one Km for nitrate uptake can be resolved from the data. This suggests the existence of either one carrier system with changing kinetic constants or the existence of dual uptake systems. The pattern of induction of nitrate reductase was coincident with the pattern of nitrate uptake as a function of time and increasing nitrate concentrations. The rate of induction of nitrate reductase was regulated by the rate of nitrate flux.Washing the roots for 2 hours enhances nitrate uptake by 2.5-fold over the nonwashed tissue. The presence of nitrate in the washing solution leads to further (3.5-fold over control) increases in the rate of nitrate uptake supporting the contention that nitrate plays a specific role in the induction of the inducible nitrate carrier independent of the washing effect.     

非均匀水势下作物根系吸水模型分析

根据土壤-根系统中水分守恒和水势对水分运输作用的原理, 建立了土壤中非均匀水势作物根系吸水模型。在该模型中, 分别对一次函数和指数函数两种不同的非均匀土壤水势的表达形式建立模型, 并对非均匀水势和均匀水势下模型的解析解之间的关系进行了探讨; 利用该模型讨论根系的吸收阻力和木质部传导阻力的比率对根吸水的影响; 运用阻力比率的合理生理范围确定根生长的优化长度。结果表明: 在特定情况下, 非均匀水势下的根系吸水模型可以用于均匀水势, 对Poiseuille公式进行修正后得到的根的优化长度接近实际值。     

Calcium uptake by excised maize roots and interactions with alkali cations

Ca²⁺ uptake was studied in short-term experiments using 5-day-old excised maize roots. This tissue readily absorbs Ca²⁺, and inhibition by dinitrophenol and low temperature shows that the process is metabolically mediated. The uptake of Ca²⁺, like that of other cations, is influenced by the counter ion, the pH and concentration of the ambient solution, and the presence of other cations. The rate of uptake from various salts decreases in the following order: NO₃⁻ > Cl⁻ = Br⁻ > SO₄²⁻. K⁺ and H⁺ greatly interfere with Ca²⁺ absorption, while Li⁺ and Na⁺ have only slight effects.     

Electrical potential differences in cells of barley roots and their relation to ion uptake

Single cell electropotentials of barley (Hordeum vulgare L., cv. `Compana') root cortex were measured at different external concentrations of KCl in the presence of Ca²⁺. The roots were low in salt from seedlings grown on 0.5 mm aerated CaSO₄ solution. Thus, the conditions were equivalent to those used to define the dual mechanisms found with radioactive tracer-labeled ion uptake. In 0.5 mm CaSO₄ alone, there is an increase with time of cell negativity from about -65 millivolts 15 minutes after cutting segments to about -185 millivolts in 6 to 8 hours. Two possible hypotheses, not mutually exclusive, are offered to explain this aging effect: that cutting exposes plasmodesmata which are leaky initially but which seal in time, and that some internal factors, e.g., hormones diffusing from the apex, have a regulatory effect on the cell potential, an influence which becomes dissipated in isolated segments and permits the development of a higher potential difference. In any case changes in selective ion transport must be involved. The cell potentials at KCl concentrations above 2.0 mm are more negative than would be expected for a passive diffusion potential. It is suggested that this discrepancy may be due to an electrogenic pump or to a higher K⁺ concentration in the cytoplasm than in the remainder of the cell, or perhaps to both. Whether there is a clear relationship between cell potential and mechanisms 1 and 2 of cation transport depends upon whether the cell potentials of freshly cut or of aged tissue represent the values relevant to intact roots.     

Varietal differences in potassium uptake by excised roots ofNicotiana tabacum

Summary The rate of potassium uptake by two varieties of burley tobacco (Nicotiana tabacum) differed by as much as two-fold when excised roots were absorbing K from solutions of 2.5 × 10^–4 M KCl. With increasing substrate levels of KCl the difference between varieties decreased and no difference was observed at a KCl concentration of 1.6 × 10^–2 M. The K-absorption by excised roots of several varieties from solutions of 5 × 10^–4 M KCl was compared.Contribution of the Department of Agronomy, Kentucky Agr. Exp. Sta., Lexington, and published with the approval of the Experiment Station Director.     

Further studies on cation uptake from bentonite suspensions by excised barley roots

Summary The net uptake of Na, K, Li, and Ca or Mg by excised barley roots was studied from bi-ionic and tri-ionic bentonite suspensions. The net uptake of Li from Li-Ca system progressed linearly with progressive Li levels and was related to the concentration of soluble lithium. Calcium in this system was taken up only at the 100 per cent Ca level. At lower Ca levels calcium was lost from the roots to the suspensions. In K-Mg and Na-Mg systems the net uptake of Na or K by the excised roots was related to the concentration of the cation in the solution phase. Magnesium uptake took place at 80 and 100 per cent Mg levels. It was much less than that of K or Na at similar levels. At lower levels of Mg the roots lost some of their initial Mg contents to the suspensions. In the Na-K-Mg system magnesium was not taken up by the excised roots. Sodium uptake was not practically affected by the Mg level, but K uptake was greatly enhanced by magnesium.     

Oligomycin inhibition of phosphate uptake and ATP labeling in excised maize roots

ATP labeling by newly absorbed ³²P in excised maize roots was reduced 34% by the presence of oligomycin during a 4-min uptake period with no reduction in rate of phosphorus absorption. Longer exposure to oligomycin, during pretreatment periods or longer uptake periods, reduced phosphorus absorption and further reduced ATP synthesis. In these tissues it appears that oligomycin inhibits ATP production at the mitochondria, that ATP is the energy source for phosphorus uptake at the plasmalemma, and that a depletion in the ATP supply causes a reduced rate of uptake.