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.     

Regulation of bound peroxidase by polyamines and guanidines in maize scutellum

Peroxidase bound to the membrane either ionically or covalently, but not the free enzyme, is inhibited by polyamines and activated by guanidines. The ionically bound peroxidase detached from the membrane by Ca²⁺, or the peroxidase present in the cytosolic fraction, can be associated with the membrane fraction from which the ionically bound enzyme is removed, by Ca²⁺. The reconstituted membrane fraction, either with the enzyme solubilized by Ca²⁺, or with the cytosolic enzyme, can again be modulated by these compounds by changing the affinity of the enzyme for its substrate.     

Kinetics of iron absorption by excised rice roots

Summary Studies on the rate of iron absorption by excised rice roots from solutions of different concentrations of FeSO₄ showed the presence of two patterns, one in the low (0.005–0.5 mM) and the other in the high (1–30 mM) concentration range. The presence of CaSO₄ or MnSO₄ at 0.5 mM enhanced Fe⁺⁺ absorption in the low concentration range, while CaSO₄ at 10 mM inhibited Fe absorption in the high concentration range in a competitive manner. Fe⁺⁺ absorption at both low and high concentrations was sensitive to metabolic inhibitors. The isotherm for Fe⁺⁺ absorption at O° exhibited an initial absorption shoulder in both low and high concentrations and was suggestive of a latent ion-transport capacity for Fe⁺⁺ in rice roots.     

Effect of lanthanum on ion absorption in corn roots

Short term (10 min) influx of (86)Rb-labeled potassium into corn (Zea mays L. WF9 x M14) root segments was inhibited by La (NO(3))(3) or LaCl(3). Half maximal inhibition of K(+) influx from 0.25 mm KCl was obtained with 0.025 mm La(3+). Kinetic analysis indicated the inhibition to be of a competitive nature. With absorption periods exceeding one hour, La(3+) no longer inhibited, but rather stimulated K(+) influx rates. La(3+) was not an inhibitor of (36)Cl or (32)P absorption. Separated cortex and stele absorbed labeled potassium (and phosphate) at comparable rates, and La(3+) inhibited potassium influx in both tissues. The effects of La(3+) on ion absorption were similar to those of Ca(2+), suggesting that the two polyvalent cations act at the same site. Based on this and the observation that La(3+) does not seem to penetrate the plasma membrane, it was concluded that La(3+) and Ca(2+) affect changes in ion transport without entering cells.     

Nitrate absorption by corn roots : inhibition by phenylglyoxal

Nitrate transport in excised corn (Zea mays L.) roots was inhibited by phenylglyoxal, but not by 4,4′-diisothiocyano-2,2′-stilbene disulfonic acid (DIDS) or fluorescein isothiocyanate (FITC). Inhibition of nitrate uptake by a 1-hour treatment with 1 millimolar phenylglyoxal was reversed after 3 hours, which was similar to the time needed for induction of nitrate uptake. If induction of nitrate uptake occurs by de novo synthesis of a nitrate carrier, then the resumption of nitrate uptake in the inhibitor-treated roots may occur because of turnover of phenylglyoxal-inactivated nitrate carrier proteins. All three chemicals inhibited chloride uptake to varying degrees, with FITC being the strongest inhibitor. While inhibition due to DIDS was reversible within 30 minutes, both FITC and phenylglyoxal showed continued inhibition of chloride uptake for up to 3 hours after removal from the uptake solution. Assuming that the anion transporter polypeptide(s) carries a positive charge density at or near the transport site, the results indicate that the nitrate carrier does not carry any lysyl residues that are accessible to DIDS or FITC, whereas the chloride carrier does. Both chloride and nitrate carriers, however, seem to possess arginyl residues that are accessible to phenylglyoxal.     

Inhibition of nitrate reductase induction by canavanine in maize roots

The induction of nitrate reductase activity in maize root tips was inhibited by canavanine and the inhibition increased with increasing concentration of canavanine between 0·1 and 1 mM. Addition of canavanine to the induced enzyme had little effect on the disappearance of the enzyme when nitrate was removed, and it is likely that the canavanine reduces the activity of the nitrate reductase by inhibiting its synthesis rather than by accelerating its breakdown.     

Sucrose absorption and synthesis by excised Lycopersicon esculentum roots

Asymmetrically-labelled sucrose was absorbed intact by excised roots of tomato, grown in sucrose. Glucose-grown roots possessed sucrose synthetase and sucrose phosphate synthetase activity.     

Multiphasic uptake of phosphate by corn roots

Abstract The concentration dependence of phosphate uptake was studied using root sections of corn (Zea mays L. cv. Ganga 5). Detailed and wide-range (57 concentrations in the range 1 μmol m^?3-75 mol m^?3), precise (average SEM < 2.5%, n= 6) and reproducible (similar patterns in three independent experiments and for 5, 10, 15, 20, 25 and 30°C) data revealed six (or seven) concentration-dependent phases separated by ‘jumps’ or sharp breaks. These transitions were independent of temperature and occurred over relatively narrow concentration ranges (0.0001–0.0004, 0.08–0.31, 1.0–3.5, (7.5–10), 18–20 and 57–59 mol m^?3). The intermediate phases obeyed Michaelis-Menten kinetics, whereas sigmoidal kinetics were observed at lower concentrations. Uptake within each of the two highest phases increased more rapidly with increasing external phosphate concentration than predicted from Michaelis-Menten kinetics but also saturated more rapidly. The latter finding is not consistent with free diffusion across the plasmalemma at high external phosphate concentrations. Kinetic models yielding continuous isotherms, e.g. the sum of one or two Michaelis-Menten terms and a diffusion term, cannot account for the data.     

Kinetics of zinc absorption by excised roots of two sugarcane clones

Summary Sugarcane clones differ in regard to susceptibility to zinc (Zn²⁺) deficiency in the field. Excised roots of clone H57-5174 actively absorbed Zn²⁺ at a rate twice that of H53-263 roots. The maximum rates of Zn²⁺ uptake (Vmax) and the Km values also differed markedly between these two clones. H53-263 roots have a 6-fold greater affinity for Zn²⁺ than do those of H57-5174. H57-5174 readily develops Zn²⁺ deficiency symptoms in the field but H53-263 rarely does so. A partial explanation for these varying responses appears to lie in these data.Journal Series No. 1536 of the Hawaii Agricultural Experiment Station.     

Oligomycin effect on ion absorption by excised barley roots and on their ATP content

Summary Chloride absorption by excised barley roots from dilute solutions is more oligomycin-sensitive than its absorption from more concentrated solutions and than K⁺ and Na⁺ absorption from dilute as well as concentrated solutions. Oligomycin decreased the ATP content of excised barley roots. The mode of oligomycin interference with ion absorption by plant cells is discussed.     

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.     

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.     

Na+ and K+ transport in excised soybean roots

Uptake, accumulation and xylem transport of K⁺ and Na⁺ in excised roots of soybean were investigated by use of a perfusion technique. This technique permitted independent quantification of, on the one hand, entry of ions into the roots and their transport through the cortex to the xylem vessels, and on the other hand reabsorption from the xylem vessels to the neighbouring cells and the external medium. Data are consistent with a low degree of selective uptake of K⁺ over Na⁺. However, Na⁺ depletion of the xylem stream by reabsorption limits, although weakly, its translocation to the shoots. Na⁺ reabsorbed is for a great part reexcreted into the external medium. The low efficiency of these processes is discussed in relation to the Na⁺ sensitivity of soybean.     

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.     

Effect of removal of the root tip on the development of enhanced rb absorption by corn roots

Samples of primary root tissue of corn (Zea mays L.) were aged either in CaSO(4) solution or in humid air, after which they were immersed for 10 minutes in a solution containing 0.1 mm(86)RbCl. Aging in solution, but not in humid air, enhanced the subsequent rate of Rb(+) absorption. Excision of roots before aging was followed by greater enhancement than when exicision followed aging. The time course of aging of 1-cm segments from different portions of the root showed decreasing response with increasing distance from the root cap. The aging response of apical segments (5-15 mm from the root cap) could be detected within 10 minutes and usually reached a maximum within 2 hours. Rb(+) absorption by apical segments (5-15 mm) aged without the tip (0-5 mm) was more than double that by apical segments whose tips were left attached until the end of the aging period. When apical segments without the tip were aged for 2 hours in the CaSO(4) solution in which seedlings had previously been grown for 24 hours, the rate of absorption was only 63% of samples aged in fresh solution. When apical segments were aged for 2 hours in fresh solution containing excised tips floating free in the solution, the rate of Rb(+) absorption was 20% less than in samples aged in solution containing no excised tips. The data presented in this study are interpreted to indicate that a water-soluble metabolite, originating in the root tip and translocated basipetally, inhibits Rb accumulation.     

Effect of calcium on the absorption and translocation of heavy metals in excised barley roots: Multi-compartment transport box experiment

Summary The effect of Ca on the absorption and translocation of Mn, Zn and Cd in excised barley roots was studied using a multi-compartment transport box technique. A radioisotope (⁵⁴Mn,⁶⁵Zn or^115mCd)-labelled test solution was supplied to the apexes of excised roots and the distribution pattern in the roots was examined in the absence or presence of Ca. Results obtained were as follows. Addition of Ca to the test solution reduced the absorption of Mn and inhibited drastically its translocation in excised roots. With increasing concentrations of Ca in test solutions, its inhibitory effects on the absorption and translocation of Mn became severe. Similar results were observed for the absorption and translocation of Zn. Ca in the test solution decreased the absorption and inhibited drastically the translocation of Zn; as in the case of Mn, higher concentrations of Ca had severe effects on these functions. It was also evident that the addition of Ca to the test solution reduced the absorption of Cd at all levels of Cd concentration (1, 10, and 100 μM). Cd absorption decreased with increasing concentrations of Ca in the test solution. However, Ca accelerated the translocation of Cd in excised roots supplied with test solutions containing up to 10μM Cd. At 100μM Cd, addition of Ca caused a negligibly small acceleration of Cd translocation. The accelerating effect of Ca on Cd translocation, especially “xylem exudation”, decreased markedly with the addition of 2,4-dinitrophenol, but not with the addition of chloramphenicol or p-chloromercuribenzene sulphonic acid. When barley plants were supplied with only CaSO₄ during the entire growing period, that is, plants were not supplied with nutrient solution on the last day of this period, Ca had no accelerating effect on Cd translocation in excised roots.