Amino acid uptake by Ricinus communis roots: characterization and physiological significance

Abstract Roots of sterile-grown, intact 6-day-old seedlings of Ricinus communis possess at least two independent active amino acid uptake systems, one for neutral and one for basic amino acids. The kinetics of uptake of L-proline and L-arginine, which were taken as representative substrates for the two systems, are biphasic. At low concentrations (0.01–0.5 mol m^?3) Michaelis -Menten kinetics prevail, changing to a linear concentration dependence at higher substrate concentrations (1–50 mol m^?3). L-glutamate uptake velocity is linear over the whole substrate concentration range. For comparison the uptake kinetics of nitrate and ammonium were determined as well as interactions among the different nitrogen sources. The K_m value for nitrate uptake was 0.4 mol m^?3, and for ammonium 0.1 mol m^?3. The uptake capacity for nitrate or ammonium was approximately the same as for amino acids. The interaction between the uptake systems for organic and inorganic nitrogen is small. Two hypotheses for the physiological significance of amino acid uptake by roots were considered: (i) Uptake of amino acids from the soil-determination of amino acids in soil and in soil water indicates that they might contribute 15–25% to the nitrogen nutrition of the plant. (ii) Amino acid uptake systems of root cells serve primarily as retrieval of amino acids delivered from the phloem- it was found that ¹⁴C L-glutamine, which was delivered to the cotyledon and transported to the root via the phloem, was not lost by the roots, whereas it appeared in the bathing medium if L-glutamine was applied externally to the root to compete for the uptake sites; this suggests that an apoplastic pool of amino acids in the root exists due to their efflux from the phloem.     

Effects of sodium and potassium ions on the uptake of noradrenaline and its precursor tyrosine

The effects of the monovalent cations Na⁺ and K⁺ were studied on the uptake of noradrenaline and tyrosine by a crude synaptosomal fraction in vitro. Sodium ions produced opposite effects on the uptake of noradrenaline and the uptake of tyrosine viz. an increase in noradrenaline uptake and a decrease in the uptake of its precursor tyrosine. A low concentration of K⁺ stimulated the uptake of noradrenaline in the presence of Na⁺, while in the absence of Na⁺ K⁺ had no effect. However, the uptake of tyrosine could be stimulated by low K⁺ in the absence of Na⁺. Besides the increased uptake in the absence of Na⁺, a second uptake was found which was Na⁺, K⁺ activated ATPase dependent. The contribution of this uptake system to the total uptake of tyrosine was about 20%. No evidence was obtained for the involvement of a Na⁺, K⁺ activated ATPase in noradrenaline uptake. It is suggested that another ATPase might be involved in the latter uptake system.     

Effect of divalent and other ions on potassium uptake by barley roots

Summary Potassium uptake as affected by the presence of divalent and other ions was studied from KCl solutions to which other salts were added. The study also includes the effect of the initial Ca-content of roots on the uptake of K⁺ from KCl and KCl + CaCl₂ solutions. The effect of time on K-uptake from electrolyte mixtures containing Mg-ions was also included. The experimental results seem to fit well a model of K⁺-uptake based on diffusion of ions in mixed electrolytes.     

Measurements of pH, sucrose and potassium ions in the phloem sap of castor bean (Ricinus communis) plants

The composition of phloem sap, sampled at different heights along, the stem of castor bean ( Ricinus communis L. cv. Gibsonii) plants, was determined. A gradient in pH was observed; the highest pH values occurred near the shoot apex, decreasing towards the base of the stem. The sucrose content of the exudate exhibited a similar gradient. The concentration of potassium ions was highest near the uppermost, full-grown leaves, decreasing towards the apex and the base of the stem. The importance of these findings for the understanding of phloem translocation and unloading is discussed.     

The influence of calcium on potassium fluxes across the root of Ricinus communis

Summary The effect of calcium on the flux of potassium to the exudate of detached root systems of Ricinus communis has been investigated. Previous analyses have indicated the presence of a water dependent and a water independent flux of potassium which vary with the concentration of potassium in the bathing medium, in the presence of 0.1 mM CaCl₂. In the present study it has been observed that at a higher concentration of calcium in the bathing medium (2.5 mM CaCl₂) the water dependent flux of potassium is greatly reduced while the water independent flux is not affected. It is proposed that the differential effect of calcium on these two fluxes is a reflection of the degree of dependence of these fluxes on the permeability of the plasmamembranes within the root.     

Graviresponsiveness and columella cell structure in primary and secondary roots of Ricinus communis

In order to determine what structural changes are associated with the onset of graviresponsiveness by plant roots, we have monitored the quantitative ultrastructures of columella (i.e., graviperceptive) cells in primary and secondary roots of Ricinus communis. The relative volumes of cellular components in lateral (i.e., minimally graviresponsive) roots were not significantly different from those of primary roots. The relative volumes of cellular components in secondary roots growing laterally were not significantly different from those of graviresponsive secondary roots. Therefore, the onset of graviresponsiveness by secondary roots of R. communis is not correlated with changes in organellar concentrations in columella cells. These results are discussed relative to a model for the differential graviresponsiveness of plant roots.     

Flows of elements, ions and abscisic acid in Ricinus communis and site of nitrate reduction under potassium limitation.

In a pot experiment Ricinus communis plants were cultivated in quartz sand and supplied daily with a nutrient solution which contained 4 mol m(-3) nitrate as the nitrogen source and either full strength potassium (1.3 mol m(-3), control) or 8% potassium (0.1 mol m(-3), K(+)-limitation). Although the final fresh weight of the whole plant was not affected by K(+)-limitation, the root-shoot ratio was increased due to a relatively increased root growth and inhibited development of younger shoot parts. Owing to K(+)-limitation, photosynthesis was slightly decreased, while dark respiration of the shoot markedly decreased and root respiration was nearly doubled. The transport of carbon in the phloem, and to some extent in the xylem, was greater and the root was favoured in the partitioning of carbon. This was also true for nitrogen and potassium which were both taken up at lower rates, particularly potassium. In these two cases a high remobilization and recycling from the old part of the shoot was observed. By contrast, uptake of sodium was 2.4-fold higher under K(+)-limitation and this resulted in increased flows in the plants, which was discussed generally as a means for charge balance (in combination with a slight increase in uptake of magnesium and calcium). Nitrate reduction took place in the same portion in the root and shoot. This was a shift to the root compared to the control and points to an inhibition of xylem transport caused by limitation of K(+) as an easily permeating countercation. Low K(+) supply also resulted in an increased biosynthesis of ABA in the roots (265%). This caused a slightly increased deposition of ABA in the roots (193%) and a 4.6-fold higher root-to-shoot and a doubled shoot-to-root ABA signal in the xylem or phloem, respectively. The high degradation of ABA in the shoots prevented ABA accumulation there.     

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.     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. II. Grown with low or high nitrate supply

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 0.2 molm^–3     

Characterization of amino-acid transport in Ricinus communis roots using isolated membrane vesicles

The mechanism and specificity of amino-acid transport at the plasma membrane of Ricinus communis L. roots was investigated using membrane vesicles isolated by phase partitioning. The transport of glutamine, isoleucine, glutamic acid and aspartic acid was driven by both a pH gradient and a membrane potential (internally alkaline and negative), created artificially across the plasma membrane. This is consistent with transport via a proton symport. In contrast, the transport of the basic amino acids, lysine and arginine, was driven by a negative internal membrane potential but not by a pH gradient, suggesting that these amino acids may be taken up via a voltage-driven uniport. The energized uptake of all of the amino acids tested showed a saturable phase, consistent with carrier-mediated transport. In addition, the membrane-potential-driven transport of all the amino acids was greater at pH 5.5 than at pH 7.5, which suggests that there could be a direct pH effect on the carrier. Several amino-acid carriers could be resolved, based on competition studies: a carrier with a high affinity for a range of neutral amino acids (apart from asparagine) but with a low affinity for basic and acidic amino acids; a carrier which has a high affinity for a range of neutral amino acids except isoleucine and valine, but with a low affinity for basic and acidic amino acids; and a carrier which has a higher affinity for basic and some neutral amino acids but has a lower affinity for acidic amino acids. The existence of a separate carrier for acidic amino acids is discussed.Abbreviations PM plasma membrane - TPP⁺ tetraphenylphosphonium ion - pH pH gradient - membrane potential This work was supported by the Agricultural and Food Research Council and The Royal Society. We would like to thank Mrs. Sue Nelson for help with some of the membrane preparations.     

Effects of drought on nutrient and ABA transport in Ricinus communis

We studied the effects of variations of water flux through the plant, of diurnal variation of water flux, and of variation of vapour pressure deficit at the leaf on compensation pressure in the Passioura-type pressure chamber, the composition of the xylem sap and leaf conductance in Ricinus communis. The diurnal pattern of compensation pressure showed stress relaxation during the night hours, while stress increased during the day, when water limitation increased. Thus compensation pressure was a good measure of the momentary water status of the root throughout the day and during drought. The bulk soil water content at which predawn compensation pressure and abscisic acid concentration in the xylem sap increased and leaf conductance decreased, was high when the water usage of the plant was high. For all xylem sap constituents analysed, variations in concentrations during the day were larger than changes in mean concentrations with drought. Mean concentrations of phosphate and the pH of the xylem sap declined with drought, while nitrate concentration remained constant. When the measurement leaf was exposed to a different VPD from the rest of the plant, leaf conductance declined by 400mmol m^?2 s^?1 when compensation pressure increased by 1 MPa in all treatments. The compensation pressure needed to keep the shoot turgid, leaf conductance and the abscisic acid concentration in the xylem were linearly related. This was also the case when the highly dynamic development of stress was taken into account.     

Transport of magnesium ions in the phloem of Ricinus communis L. seedlings

During growth of Ricinus communis seedlings, magnesium ions are mobilized in the endosperm, taken up by and accumulated to very high levels (150 μmol·g FW^?1) in the cotyledons, and translocated to hypocotyl and roots. The magnesium gain from days 6 to 7 in the cotyledons and the seedling axis necessitates a total up-take rate of 600 nmol·h^?1-seedling^?1 and the phloem translocation rate must amount to 200 nmol·h^?1. seedling^?1. The phloem loading of magnesium and the regulatory properties of this process were investigated, making specific use of the ability to collect pure phloem sap from the cut hypocotyl of 6-d-old Ricinus seedlings. The concentration of magnesium in sieve-tube sap (5 mM) was fairly constant under many incubation conditions, e.g. incubation in magnesium-free buffer, incubation with different cations (K⁺, Na⁺, NH ₄ ⁺ ) or anions (Cl^?, NO ₄ ^- , SO ₄ ^2- ), or incubation with sucrose and amino acids. Even addition of magnesium chloride to the cotyledons did not enhance phloem loading of magnesium ions. Therefore the high magnesium content of the cotyledons was sufficient for continuous phloem loading of magnesium, irrespective of external ionic conditions. Also, the flow rate of sieve-tube sap did not influence the magnesium concentration in the sap. Only the incubation with sulfate and phosphate ions increased the magnesium-ion concentration in the phloem. Magnesium sulfate offered to the cotyledons caused a threefold increase of magnesium ions in the sieve-tube sap, which was inhibited by Na⁺, NH ₄ ⁺ and Ca²⁺ in rising order, but not by K⁺. Incubation with phosphate for a prolonged period (8 h) led to an increased mobilization of intra-cotyle-donary magnesium and an enhanced phloem loading of mobilized magnesium. It is concluded that phosphate availability is a decisive factor for mobilization and translocation of magnesium ions within the plant.     

Net uptake of potassium in Neurospora. Exchange for sodium and hydrogen ions

Net uptake of potassium by low K, high Na cells of Neurospora at pH 5.8 is accompanied by net extrusion of sodium and hydrogen ions. The amount of potassium taken up by the cells is matched by the sum of sodium and hydrogen ions lost, under a variety of conditions: prolonged preincubation, partial respiratory inhibition (DNP), and lowered [K]_o. All three fluxes are exponential with time and obey Michaelis kinetics as functions of [K]_o. The V _max for net potassium uptake, 22.7 mmoles/kg cell water/min, is very close to that for K/K exchange reported previously (20 mmoles/kg cell water/min). However, the apparent K_m for net potassium uptake, 11.8 mM [K]_o, is an order of magnitude larger than the value (1 mM) for K/K exchange. It is suggested that a single transport system handles both net K uptake and K/K exchange, but that the affinity of the external site for potassium is influenced by the species of ion being extruded.     

Response of sugar metabolism in the cotyledons and roots of Ricinus communis subjected to salt stress

• Ricinus communis is an important oilseed crop worldwide and is also considered one of the best potential plants for salt-affected soil improvement in northeast China. However, little is known about photosynthesis and carbohydrate metabolism in this plant, nor the distribution of carbohydrates in cotyledons and roots under salinity stress.
• In the present study, seedling growth, gas exchange parameters (P_N, E, g_s and C_i), carbohydrate (fructose, sucrose, glucose, soluble sugar and starch) metabolism and related enzymes and genes were measured in Ricinus plants.
• Under salt stress, P_N of cotyledons decreased significantly (P < 0.05), resulting in weak photosynthetic capacity. Furthermore, salt stress increased sucrose and glucose content in cotyledons, but decreased soluble sugar and starch content. However, sucrose increased and starch decreased in roots. This may be correlated with the increasing sugar metabolism under salinity, including notable changes in sugar-related enzyme activities (SPS, SuSy, α-amylase and β-amylase) and gene expression of RcINV, RcSUS, RcAmY, RcBAM and RcGBE1.
• The results suggest that salinity reduces photosynthesis of cotyledons, alters carbohydrate allocation between cotyledons and roots and also promotes starch utilization in cotyledons and starch biosynthesis in roots, leading to a functional imbalance between cotyledons and roots. Together, these findings provide insights into the crucial role of sugar metabolism in improving salt-tolerance of Ricinus during the early seedling growth stage.

     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. III. Long-distance transport of abscisic acid depending on nitrogen nutrition and salt stress

Seedlings of Ricinus communis L. were cultivated in quartz sandand supplied with media which contained either different concentrationsof nitrate or ammonium nitrogen and were treated with a lowsalt stress. The concentration of ABA was determined in tissuesand in xylem and phloem saps. Between 41 and 51 day after sowing,abscisic acid (ABA) flows between roots and shoots were modelled.Long-distance transport of ABA was not stimulated under conditionsof nitrate deficiency (0.2 mol m^–3). However, when ammoniumwas given as the only N source (1.0 mol m^–3), ABA transportin both xylem and phloem was increased significantly. Mild saltstress (40 mol m^–3 NaCl) increased ABA transport in nitrate-fedplants, but not in ammonium-fed plants. The leaf conductancewas lowered by salt treatment with both nitrogen sources, butit was always lower in ammonium-fed compared to nitrate-fedplants. A negative correlation of leaf conductance to ABA levelsin leaves or flow in xylem was found only in comparison of ammonium-fedto nitrate-fed plants. Key words: Abscisic acid, ammonium, Ricinus communis, phloem, xylem, transport, nitrate, nitrogen nutrition     

Relation between permeability to potassium and sodium ions and fusicoccin-stimulated hydrogen-ion efflux in barley roots

Summary Measurements are described of fusicoccin (FC)-stimulated H⁺ efflux in barley (Hordeum vulgare L.) roots when K⁺ and Na⁺ concentrations were varied. In low-salt roots H⁺ efflux was stimulated in both 5 mM KCl and NaCl. In salt-saturated roots H⁺ efflux was stimulated more effectively in KCl than in NaCl solution. The stimulation of H⁺ efflux thus is parallel with the selectivity of these different root preparations for K⁺ and Na⁺ and with estimates of permeability ratios (P _Na/P _K) determined from electrical measurements. It is suggested that the results support electrogenic coupling between FC-stimulated H⁺ efflux and cation uptake.     

Transfer of amino acids and nitrate from the roots into the xylem of Ricinus communis seedlings

The loading of amino acids and nitrate into the xylem was investigated by collection and analysis of root-pressure exudate from the cut hypocotyl stumps of seedlings of Ricinus communis L. Glutamine was found to be the dominant amino acid in the exudate and also to be the amino acid which is transferred to the xylem most rapidly and accumulated to the greatest extent. The comparison between uptake and xylem loading showed significant differences in specificity between these two transport reactions, indicating a different set of transport systems. Nitrate is transferred to the xylem at a higher relative rate than any amino acid despite the great nitrate-storage capacity of the root system. Thus the supply of nitrate to Ricinus plants leads to enhanced nitrogen allocation to the shoots.