Effects of temperature on orthophosphate absorption by excised corn roots

The uptake of orthophosphate (³²P) by excised corn roots, Zea mays L. was studied using roots grown on 0.2 mm CaSO₄. Nine concentrations of KH₂PO₄ from 1 to 256 μm were used at temperatures of 20°, 30°, and 40°. Enzyme kinetic analysis was applied to the data obtained. Two apparent mechanisms (sites) of phosphate uptake were observed, 1 dominating at high P concentrations and 1 at low P concentrations. A Km of 1.36 × 10⁻⁴ and a Vmax of 177 × 10⁻⁹ moles per gram of roots per hour at 30° was calculated for the mechanism dominating at high P concentrations. Similar calculations gave a Km of 6.09 × 10⁻⁶ and a Vmax of 162 × 10⁻⁹ moles per gram of roots per hour at 30° for the mechanism dominating at low P concentrations. The Q₁₀ for both mechanisms was approximately 2. Calculation of thermodynamic values from the data gave ΔF of − 5200 cal, ΔH of − 950 to − 1400 cal, and a enthalpy of activation (A) of 10,300 to 13,800 cal per mole for the mechanism dominating at high P concentrations. Similar calculations from the data for the mechanism dominating at low P concentrations gave a ΔF of − 7300 cal, ΔH of − 10,700 to − 8200 cal, and a A of 9300 to 18,900 cal per mole. If the dual mechanism interpretation of this kind of data adequately describes this system, then both mechanisms of P absorption by corn roots involve chemical reactions.     

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.     

Pump-leak sodium fluxes in low salt corn roots

Summary The influx and efflux of sodium from 4-hr washed, low salt corn roots (Zea mays L.) has been studied for characterization of passive and active components. Initial Na⁺ content of the roots is very low, 2.25±0.4 mol/g fresh weight. Na⁺ influx in the presence of 0.2mm Ca²⁺ and 0.002 to 20mm K⁺ is passive (a leak) based upon Goldman-type models, being determined by Na⁺ and cell potential (). Na⁺ was not transported by the K⁺ carrier and influx was unaffected by 50 m dicyclohexylcarbodiimide (DCCD). Permeability of the cells to Na⁺ was of the same order asP _k.Efflux of Na⁺ was by an efficient and rapid active transport system (a pump), thus accounting for the failure of these roots to accumulate high levels of Na⁺. In short-term loading and efflux experiments, internal Na⁺ turnover had a half-time of about 5 min. Sodium efflux was unaffected by DCCD. Net H⁺ flux was zero in the presence of DCCD regardless of sodium efflux, indicating absence of Na⁺/H⁺ antiport. Efflux of Na⁺ was equally rapid into medium containing no Na⁺ and only 0.002mm K⁺. K⁺ influx accounted for less than 4% of Na⁺ efflux, prompting the hypothesis that the Na⁺ (or cation?) efflux pump is the second electrogenic system previously defined based upon electrophysiological measurements.     

Effects of low water potential on ion uptake and loss for excised roots

Summary In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.Lowering the water potential greatly increased the rate of loss of potassium and bromide from the cytoplasm, but the increases in loss from the vacuoles were much smaller. The results suggest that the mechanisms of ion uptake are not affected by low water potential and that the decrease in ion accumulation is caused by the increased leakage from the cells.     

Electrochemical gradients and k and cl fluxes in excised corn roots

The compartmental analysis method was used to estimate the K⁺ and Cl⁻ fluxes for cells of excised roots of Zea mays L. cv. Golden Bantam. When the measured fluxes are compared to those calculated with the Ussing-Teorell flux-ratio equation, an active inward transport of Cl⁻ across the plasmalemma is indicated; the plasmalemma K⁺ fluxes are not far different from those predicted for passive diffusion, although an active inward transport cannot be precluded. Whether fluxes across the tonoplast are active or passive depends upon the vacuolar potential which is unknown. Assuming no electropotential gradient, the tracer flux ratios are fairly close to those predicted for passive movement. However, if the vacuole is positive by about 10 millivolts relative to the cytoplasm, the data suggest active inward transport for K⁺ and outward transport for Cl⁻.     

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.     

Effects of external cations and respiratory inhibitors on electrical potential of the xylem exudate of excised corn roots

Glass capillary microelectrodes were used to study the electrical potential difference (PD) between the xylem exudate of excised corn roots, Zea mays L. Golden Bantam hybrid, and the external solution. A survey of the effects of various ions on the PD was made. With 1 mm single salt solutions, the PD was between 25 and 50 mv, exudate negative. The PD responded to concentration differences in single salt solutions of K⁺, Na⁺, and Ca²⁺ in a manner suggestive of cation selectivity and cation diffusion potentials. With Ca²⁺ present, the PD was insensitive to concentration changes of other cations. Substitution of NO₃⁻ for Cl⁻ in K⁺ solutions increased the PD by 2 to 5 mv, although in general the PD showed little response to anion concentration changes. The PD was partially abolished by cyanide. The remaining fraction of the PD was sensitive to concentration changes in external K⁺, and we postulate that the PD is the result of both a diffusion potential and an electrogenic pump.     

Effects of chronic reserpine pretreatment on myocardial sodium pump and inotropic sensitivity to ouabain in guinea pig heart

The effects of chronic reserpine pretreatment (0.1 mg/kg/day, 7–9 days) on myocardial sodium pump activity, the binding of ³H-ouabain to Na⁺, K⁺-ATPase, and the positive inotropic effect of ouabain were studied in guinea pig hearts. Ouabain-sensitive ⁸⁶Rb uptake, an estimate of sodium pump activity, was significantly decreased (33.0%) in papillary muscles of chronic reserpine-pretreated guinea pigs as compared to the saline-treated controls. Kinetic analyses of the interaction of ³H-ouabain with Na⁺, K⁺-ATPase indicated that chronic reserpine pretreatment resulted in a significant decrease (24.3%) in the maximum ³H-ouabain binding site concentration when the results were expressed as pmoles per mg protein. The maximum ³H-ouabain binding sites or the number of Na⁺, K⁺-ATPase units, however, were not significantly different between the two groups when they were expressed as pmoles per mg DNA. The affinity or the dissociation rate constant (Kd) of ³H-ouabain binding was not altered after chronic reserpine pretreatment. In isolated, electrically-driven left atrial preparations, the basal contractile force was slightly higher in the reserpine-pretreated animals; the subsequent development of the positive inotropic effect and the concentration of ouabain needed to produce half-maximal inotropic response, however, were not different from the controls. Thus, it is concluded that chronic reserpine pretreatment is accompanied by a significant reduction in myocardial sodium pump activity; however, the number of sodium pump sites per cell was unchanged. The sensitivity of the reserpine-pretreated myocardium to the inotropic action of ouabain as well as its affinity for ³H-ouabain binding $\text{in vitro}$ are also unchanged.     

Erythrocyte sodium pump stimulation by ouabain and an endogenous ouabain-like factor

Cardiac glycosides inhibit the sodium pump. However, some studies suggest that nanomolar ouabain concentrations can stimulate the activity of the sodium pump. In this study, using the Na(+)/K(+)-ATPase of human erythrocytes, we compared the effect of digoxin, ouabain and an ouabain like-factor (OLF), on (86)Rb uptake. Ouabain concentrations below 10(-9) M significantly stimulate Rb(+) uptake, and the maximal increase above base-line values is 18 +/- 5% at 10(-10) M ouabain. No stimulation is observed in the same conditions by digoxin. OLF behaved like ouabain, producing an activation of Rb(+) flux at concentrations lower than 10(-9) M ouabain equivalents (14 +/- 3% at 10(-10) M). Western blot analysis revealed the presence of both alpha(1) and alpha(3) pump isoforms in human erythrocytes. Our data confirm the analogies between OLF and ouabain and suggest that Na(+)/K(+)-ATPase activation may be related to the alpha(3) isoform. In addition, we investigated whether ouabain at different concentrations was effective in altering the intracellular calcium concentration of erythrocytes. We found that ouabain at concentration lower than 10(-9) M did not affect this homeostasis.     

Effects of temperature and dinitrophenol on the uptake of potassium and sodium ions in Ricinus communis roots

Summary Detopped root systems of Ricinus communis plants were used for the study of the effects of temperature and DNP on the uptake of K and Na ions supplied as KNO₃ and NaNO₃.When K and Na ions were offered together in equivalent concentrations, the steady state uptake rates for K⁺ and Na⁺ at 23 to 25° gave a K⁺/Na⁺ ratio of 3. Increasing the Na⁺ concentration relative to K⁺ 3-fold did not alter the preferential uptake of K⁺. The uptake of K⁺ was more sensitive to temperature in the range 10 to 40° and to the application of DNP at 1.5x10^-4 M than was the uptake of Na⁺. When NaNO₃ was the only salt supplied Na⁺ uptake became more sensitive to DNP than when both K⁺ and Na⁺ nitrates were supplied. Prolonged application of DNP led to net K⁺ efflux from the roots, even when no K⁺ was being supplied to the roots. Net Na⁺ efflux under the influence of DNP occurred only in roots previously grown on Na-containing nutrient medium.The different responses of the K⁺ and Na⁺ uptake processes to temperature and DNP suggest the operation of different uptake mechanisms for K⁺ and Na⁺ These results have been considered in relation to the recent concept of dual mechanisms for the absorption of alkali cations by plant tissues.     

Variations in sodium uptake along primary roots of corn seedlings

Entry of Na⁺ into segments of the apical 8-centimeter portion of corn (Zea mays) roots was investigated and analyzed for each centimeter segment separately. Influence of temperature in the 0 C to 30 C range was well described by the Arrhenius equation [U = A exp (-Ea/RT)]. Values of A and Ea differed for each segment, tending to lessen with increasing distance from root apex. Time course of Na⁺ entry was followed up to 70 minutes. Time relations of the process fit well the expression U = m [1 - exp (-nt)]. Calculated maximal uptake capacity (m) diminished with increasing distance from the apex. The data presented indicate that sodium uptake mechanisms vary qualitatively and quantitatively along corn roots. Thus, the use of entire roots for characterization of uptake mechanisms should be reassessed.     

Short Report : Effect of amines and guanidines on Rb+ absorption by excised corn roots

The effect of alkyl-amines and -guanidines on the absorption of rubidium by the excised roots of the corn plant was tested. Inhibition of Rb⁺ absorption was observed with both amines and guanidines, where guanidines were more effective. The effect of alkylamines on Rb⁺ transport depends on their molecular structure.     

Partitioning of previously-accumulated nitrate to translocation,reduction, and efflux in corn roots

The effect of nitrate uptake, or its absence, on the utilization of nitrate previously accumulated by dark-grown, decpitated maize (Zea mays L., cv. DeKalb XL-45) seedlings was examined. Five-d-old plants that had been pretreated with 50 mM ¹⁴NO ₃ ^? for 20 h were exposed for 8 h to nutrient solutions containing either no nitrate or 50 mM ¹⁵NO ₃ ^? , 98.7 atom % ¹⁵N. The ambient solution, xylem exudate, and plant tissue were analyzed to determine the quantities of previously-accumulated (endogenous) ¹⁴NO ₃ ^? that were translocated to the xylem, lost to the solution, or reduced within the tissue during the 8-h period. Energy was continuously available to the roots from the attached endosperm. In the absence of incoming nitrate, appreciable reduction and translocation of the endogenous ¹⁴NO ₃ ^? occurred, but efflux of ¹⁴NO ₃ ^? to the external solution was minimal. In contrast, during ¹⁵NO ₃ ^? uptake, there was considerable efflux of ¹⁴NO ₃ ^? as well as translocation of ¹⁴NO ₃ ^? to the xylem, but little ¹⁴NO ₃ ^? was reduced. Thus there appeared to be an inverse relationship between ¹⁴NO ₃ ^? efflux and reduction. The data are tentatively interpreted on the basis of a model which envisages (a) two storage locations within roots, one of which primarily supplies nitrate for translocation and the other of which primarily supplies nitrate for outward passage through plasmalemma, and (b) the majority of nitrate reduction as occurring during or immediately following influx across the plasmalemma, with endogenous ¹⁴NO ₃ ^? initially moving outward being recycled inward and thereby being reduced.     

Evidence for a sodium influx pump in sunflower roots

Summary Transmembrane electrical potential differences in the cortical cells of the root of the sunflower (Helianthus annuus) have been measured using microelectrodes. The plants were grown in culture solution with a range of sodium concentrations. It was found that increasing the external sodium concentration had virtually no effect on the transmembrane potential. The vacuolar content of sodium did not change significantly with the age of the tissue indicating that sodium was in flux equilibrium in our experiments. This allowed the Nernst equation to be used to calculate the electrochemical potential gradient for sodium between the vacuole and the external solution. It was concluded that sodium was being transported into the vacuole against the electrochemical potential gradient. The location and role of the inwardly directed sodium pump implied by these results is discussed in relation to the efflux pumps for sodium reported for roots of other species. Potassium was also accumulated against the electrochemical potential gradient by these cells.Sodium was found to stimulate the growth of H. annuus when present in the culture solution at very low concentrations.