The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

High Nitrogen and Phosphorous Acquisition by Belowground Parts of Caulerpa prolifera (Chlorophyta) Contribute to the Species' Rapid Spread in Ria Formosa Lagoon,Southern Portugal

Despite worldwide proliferation of the genus Caulerpa and subsequent effects on benthic communities, little is known about the nutritional physiology of the Caulerpales. Here, we investigated the uptake rates of ammonium, nitrate, amino acids, and phosphate through the fronds and rhizoids + stolon, the internal translocation of nitrogen, and developed a nitrogen budget for the rapidly spreading Caulerpa prolifera in Ria Formosa lagoon, southern Portugal. Caulerpa prolifera acquired nutrients by both aboveground and belowground parts at similar rates, except nitrate, for which fronds showed 2-fold higher uptake rates. Ammonium was the preferential nitrogen source (81% of the total nitrogen acquisition), and amino acids, which accounted for a significant fraction of total N acquisition (19%), were taken up at faster rates than nitrate. Basipetal translocation of ¹⁵N incorporated as ammonium was nearly 3-fold higher than acropetal translocation, whereas ¹⁵N translocation as nitrate and amino acids was smaller but equal in either direction. The estimated total nitrogen acquisition by C. prolifera was 689 μmol · m⁻² · h⁻¹, whereas the total nitrogen requirement for growth was 672 μmol · m⁻² · h⁻¹. The uptake of ammonium and amino acids by belowground parts accounted for the larger fraction of the total nitrogen acquisition of C. prolifera and is sufficient to satisfy the species nitrogen requirements for growth. This may be one reason explaining the fast spreading of the seaweed in the bare sediments of Ria Formosa where it does not have any macrophyte competitors and the concentration of nutrients is high.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Replacement of nitrate by ammonium as the nitrogen source increases the salt sensitivity of pea plants. I. Ion concentrations in roots and leaves

Pea seedlings (Pisum sativum L. cv ‘Kleine Rheinlän-derin’) were grown hydroponically in solutions containing either nitrate (3 or 14 mol m⁻³) or ammonium (3 mol m⁻³) as the nitrogen source. Ammonium nutrition as such had no negative effect on plant biomass production, but drastically increased the sensitivity to moderate salinity (50 mol m⁻³ NaCl). The reasons for this effect are investigated here and in a subsequent paper. The appearance of visible symptoms of salt damage (wilting of marginal leaf areas followed by progressive necrosis) was paralleled by the development of several characteristic modifications in the solute and metabolite contents. Major changes were: (i) high salt (NaCl) accumulation in leaves; (ii) accumulation of ammonium (up to 20 mol m⁻³) and amino acids (up to 110 mol m⁻³) in leaves, but at decreased ammonium uptake rates; and (iii) decreased protein content. In a comparison paper we report on the subcellular distribution of salts, ammonium and metabolites under the above conditions.     

The effect of ammonium ions on membrane potential and anion flux in roots of barley and tomato

The effects of ammonium (0–5 mol m^?3) on root hair membrane potential and on the influx of nitrate and phosphate were investigated in roots of intact barley and tomato plants. In both species, addition of ammonium to the medium bathing the roots caused an almost immediate depolarization of the membrane potential; the depolarization was greater at higher concentrations of ammonium. Influx of ¹³NC₃^? and ³²Pi was inhibited over the same time scale and concentration range. In tomato roots, there was little further depolarization of the membrane potential or inhibition of anion influx at ammonium concentrations above 0.4 mol m^?3. In barley roots, the inhibition of nitrate influx and the depolarization of the membrane potential did not saturate below 5 mol m^?3 ammonium.     

Seasonal nitrate physiology of Macrocystis integrifolia Bory

Ambient sea-water nitrate and tissue nitrogen (ethanol soluble nitrate and amino acids, as well as total nitrogen) of Macrocystis integrifolia Bory were monitored over a 2-yr period in Bamfield, Vancouver Island, British Columbia. Sea-water nitrate varied from a high of 12 μmol · 1^?1 (individual values as high as 23 μmol · 1^?1 were recorded) in late winter to below detection limits for most of the summer. Tissue nitrate and total nitrogen paralleled the ambient nitrate levels and showed summer minima and winter maxima (from 0 to 70 μmol · g fresh wt^?1 for nitrate and from 0.8 to 2.9% of dry wt for total N). The nitrate uptake capacity was inversely proportional to tissue nitrate concentration and, furthermore, was much higher for subapical surface blades (60–70 nmol · cm^?2 · h^?1) than for older, deeper blades (5–10 nmol · cm^?2 · h^?1). Nitrate uptake by subapical blade disks in summer is apparently higher in dark (1.0–1.7 μmol · g fresh wt^?1 · h^?1) than in light (0.6–1.3 μmol · g fresh wt^?1 · h^?1) and the data obtained in 36–108 h experiments indicate nitrate pool sizes of 30–90 μmol · g fresh wt^?1. These pools are $\text{2}\text{3}$ to nearly full in winter. Ammonium does not inhibit nitrate uptake. It is taken up and apparently utilized much faster than nitrate and it may well be an important source of nitrogen for marine macrophytes.     

Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: nitrogen and carbon relations of the parasitic association Cuscuta reflexa–Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m^?3 NO₃^?, or in liquid culture with 0.022, 0.05 or 0.5 mol m^?3 NO₃^?. Some of the plants were infected with Cuscuta reflexa Roxb. As occurred for the host, dry matter production and growth of C. reflexa were severely depressed with decreasing N supply to the host. When parasitized by C. reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al. 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C. reflexa was the same or even somewhat larger than that of uninfected plants. This indicated a sink-dependent stimulation of nitrate uptake. As a result of decreased root weights, nitrate uptake g^?1 FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m^?3 nitrate supply. Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants. This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply. Besides these effects on N relations, C. reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration. This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host. The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C. reflexa. The stimulating effects of C. reflexa in the Ricinus-Cuscuta association are compared with the strongly inhibitory effects occurring in the tripartite association L. albus–Rhizobium–Cuscuta reflexa.     

What limits nitrate uptake from soil?

Abstract. An accepted view, that unless nitrate concentrations in the soil solution are very low (e.g. below 0.1–0.2 mol m^?3) the growth of high-yielding crops is not limited by the availability of nitrogen, is challenged. Conventional analyses of nutrient supply and demand, based on calculations of apparent inflow rates (uptake rates per unit total root length) are invalid. Apparent inflow rates are inversely proportional to root length. The convention of using total root length grossly overestimates the fraction of the root system active in nutrient uptake. Consequently, inflow rates based on total root lengths underestimate the true values, indicating unrealistically low nutrient concentration differentials between bulk soil and root surfaces required to drive uptake. An alternative method of analysis is suggested. This is based on total nutrient uptake rather than on inflow rate. Measurements of the former do not depend on estimates of active root length and can be made directly and reliably. The method was applied to data obtained from a pot experiment using spring wheat (Triticum aestivum L., cv. Wembley) grown in soil without nitrogen fertilizer (N₀) or with nitrogen fertilizer equivalent to 200kg N ha^?1 (N⁺). Soil nitrate concentrations calculated using the conventional method based on total root length, suggested that any increases in concentration above those measured in the N⁰ treatment should not have resulted in increased uptake and growth. However, the N⁺ plants were always bigger than those in the No treatment, refuting this suggestion. Theoretical uptakes of nitrogen (calculated initially on the basis of a fully active root system) were adjusted, by reducing the effective root length incrementally, until the theoretical uptake matched the measured net uptake of nitrogen. The mean fractions of the root systems likely to have been involved in nitrate uptake were 11% and 3.5% of the total lengths of root in the N⁰ and N⁺ treatments, respectively.     

Difficulties in location and acceptance of phloem sap combined with reduced concentration of phloem amino acids explain lowered performance of the aphid Rhopalosiphum padi on nitrogen deficient barley (Hordeum vulgare) seedlings

Effects of nitrogen deficiency in hydroponically grown barley seedlings (Hordeum vulgare L.) on the development and reproduction of the aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae) were investigated.Plant growth was significantly reduced in seedlings grown without nitrogen. Aphid intrinsic rate of increase (r _m) was also significantly lower on these plants compared with that on plants grown with 8 mol m^–3 nitrogen. Phloem sap was collected from seedling stems by aphid stylectomy and amino acids quantified by HPLC. There was a significant reduction in the concentration of non-essential amino acids as a group, but not of essential amino acids. Electrical penetration graphs (EPG) indicated that aphids reached the phloem more quickly and fed for longer on plants grown with nitrogen. This is the first reported study in which this combination of techniques has been used to understand the interactions of an aphid and plant under different environmental conditions.     

Enriched rhizosphere CO2 concentrations can ameliorate the influence of salinity on hydroponically grown tomato plants

Our previous work indicated that salinity caused a shift in the predominant site of nitrate reduction and assimilation from the shoot to the root in tomato plants. In the present work we tested whether an enhanced supply of dissolved inorganic carbon (DIC, CO₂+ HCO₃) to the root solution could increase anaplerotic provision of carbon compounds for the increased nitrogen assimilation in the root of salinity-stressed Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were grown in hydroponic culture with 0 or 100mM NaCl and aeration of the root solution with either ambient or CO₂-enriched air (5000 μmol mol^?1). The salinity-treated plants accumulated more dry weight and higher total N when the roots were supplied with CO₂-enriched aeration than when aerated with ambient air. Plants grown with salinity and enriched DIC also had higher rates of NO^?₃ uptake and translocated more NO^?₃ and reduced N in the xylem sap than did equivalent plants grown with ambient DIC. Incorporation of DIC was measured by supplying a 1 -h pulse of H¹⁴CO^?₃ to the roots followed by extraction with 80% ethanol. Enriched DIC increased root incorporation of DIC 10-fold in both salinized and non-salinized plants. In salinity-stressed plants, the products of dissolved inorganic ¹⁴C were preferentially diverted into amino acid synthesis to a greater extent than in non-salinized plants in which label was accumulated in organic acids. It was concluded that enriched DIC can increase the supply of N and anaplerotic carbon for amino acid synthesis in roots of salinized plants. Thus enriched DIC could relieve the limitation of carbon supply for ammonium assimilation and thus ameliorate the influence of salinity on NO^?₃ uptake and assimilation as well as on plant growth.     

Regulation of nitrate uptake by amino acids in maize cell suspension culture and intact roots

The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO₃ ^- strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO₃)₂ or 1 mM CaCl₂. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.     

Effects of Branched-Chain L-Amino Acids, L-Phenylalanine, and L-Methionine on the Transport of L-Glutamine in Rat Brain Cortex In Vitro. Influence of Cations

Abstract: Uptake of L-glutamine (2 mM) by rat brain cortex slices against a concentration gradient is markedly inhibited (40%) by branched-chain Lamino acids (1 mM), L-phenylalanine (1 mM), or L-methionine (1 mM); that of L-asparagine (2 mM) is much less affected by these amino acids. Other amino acids investigated have little or no effect on cerebral L-glutamine uptake. The suppressions of L-glutamine uptake by the inhibitory amino acids are apparently blocked by high [K⁺], which itself has little or no effect on glutamine uptake. This abolition of suppression is partly explained by high [K⁺] retention of endogenous glutamine; in the absence of Ca²⁺ such retention disappears. The inhibitory amino acids (1 mM) also enhance the release of endogenous glutamine, exogenous glutamine with which slices have been loaded, or glutamine synthesized in the slices from exogenous glutamate. The enhanced release of endogenous glutamine is diminished by high [K⁺]. The suppression of glutamine uptake by the branched-chain amino acids is independent of the concentration of glutamine at low concentrations (0.25–0.5 mM), indicating non-competition, but is reduced with high concentration of glutamine. The inhibition by L-phenylalanine is noncompetitive. L-Glutamine (2 mM) exerts no inhibition of the cerebral uptakes of the branched-chain L-amino acids or Lphenylalanine (0.25–2 mM). The inhibitory amino acids are as active in suppressing L-glutamine uptake with immature rat brain slices as with adult, although the uptake, against a gradient, of L-glutamine in the infant rat brain is about one-half that in the adult. They are also just as inhibitory on the concentrative uptake of L-glutamine by a crude synaptosomal preparation derived from rat brain cortex. Such a nerve ending preparation takes up L-glutamine (0.25 mM), against a gradient, at about ninefold the rate at which it is taken up by cortex slices (for equal amounts of protein), and the uptake process is markedly suppressed by high [K⁺] in contrast to the effects of high [K⁺] with slices. The possible physiological and pathological consequences of the suppression of glutamine uptake are discussed.     

Effects of atrazine on the assimilation of inorganic nitrogen in cereals

The inclusion of sub-lethal amounts ofthe herbicide atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] in the nutrient solution supplied to maize and barley increased the growth of the root and shoot and the uptake of nitrate. The activities of nitrate and nitrite reductases, glutamine synthetase and glutamate synthase were enhanced and the amino acid and nitrate contents of the xylem sap increased. All these effects of atrazine were found only in plants grown with nitrate as the nitrogen source. The uptake of ¹⁵NO₃^? and its incorporation into protein in the root and shoot of maize and barley seedlings was significantly greater in the atrazine treated plants. However, a stimulation in the incorporation of leucine-[¹⁴C] into TCA-precipitable protein of detached leaves from 7-day-old barley seedlings was obtained only in the absence of a supply of combined nitrogen either in the culture medium or in the in vitro incubation mixture containing the labelled amino acid.     

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     

Interaction of nitrate uptake and nitrogen fixation in faba beans

An experiment was carried out to determine the relationship between nitrate uptake and nitrogen fixation of faba beans. Therefore inoculated and uninoculated faba beans were grown in nutrient solution with different nitrate concentrations. Nitrate uptake was measured every two days during the growing period. At the end of the experiment the nitrate uptake kinetics were determined with a short time depletion technique and nitrogen fixation was measured with the acetylene reduction method. A limitation of nitrate uptake due to nitrogen fixation was relatively small. Nitrate concentrations of approximately 1 mol m^–3 and 5 mol m^–3 decreased nitrogen fixation to values of 16% and 1% of the control plants which received no nitrate nitrogen. A reduction of nitrogen fixation was mainly due to a decrease of specific nitrogen fixation per unit nodule weight and to a lesser extent due to a reduction of nodule growth. Only the maximum nitrate influx (I_max) seemed to be influenced by nitrogen fixation. Michaelis-Menten constants (K_m) and minimum NO _inf3 ^– -concentrations (C_min) were not significantly influenced by nitrogen fixation.     

Interaction of phloem-translocated amino compounds with nitrate net uptake by the roots of beech (Fagus sylvatica) seedlings

In the present study two experimental approaches were used to investigate the influence of changes in the allocation of amino compounds in the phloem of beech (Fagus sylvatica L.) seedlings on nitrate net uptake by the roots. In a first set of experiments Gin or Asp were directly fed into the phloem of the epicotyl via bark flaps. These compounds were previously found to be allocated in the phloem of adult beech trees and were shown to inhibit nitrate net uptake when supplied to beech roots. Feeding of solutions containing 100 mM of Gin or Asp plus 10 mM EDTA into the phloem resulted in a significant enrichment of the fine root tissue with the amino compound fed as compared to the roots of control plants supplied with amino acid-free EDTA solutions. Nitrate net uptake by the roots decreased by 61% (Gin) and 79% (Asp) as compared to the controls. In a second approach, shoots of young beech seedlings were exposed to 40g NH3 m^-3. NH3 uptake by shoots, nitrate net uptake by roots, and the contents and composition of total soluble non-protein nitrogen (TSNN) in leaves, phloem, and fine roots were determined and were compared to results gained with control plants exposed to charcoal-filtered air. NH3 fumigation of the shoots of beech seedlings resulted in a 35% reduction of nitrate net uptake by the roots as compared to controls. TSNN contents in leaves and phloem exudate of NH3-fumigated plants increased by 56% and 37%, respectively. This enrichment was mainly due to Arg and Glu in the leaves and Asp, Asn, Glu, and Gin, but not to Arg, in phloem exudate. The TSNN content of the fine roots was not changed by NH3 fumigation, but a significant increase in the Gin content was observed. From these results it is concluded that phloem transport of amino compounds, especially of Gin and Asp, from the shoot to the roots mediates regulation of nitrate net uptake by the roots of beech trees in order to adapt this process to the nitrogen demand of the whole plant.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

Interactions in the uptake of amino acids, ammonium and nitrate ions in the Arctic salt-marsh grass, Puccinellia phryganodes

The uptake of amino acids and inorganic nitrogen by roots of Puccinellia phryganodes was examined to assess the potential contribution of soluble organic nitrogen to plant nitrogen uptake in Arctic coastal marshes, where free amino acids constitute a substantial fraction of the soil‐soluble N pool. Short‐term excised root uptake experiments were performed using tillers grown hydroponically under controlled conditions in the field. The percentage reductions in ammonium uptake at moderate salinity (150 mm NaCl) compared with uptake at low salinity (50 mm NaCl) were double those of glycine, but glycine uptake was more adversely affected than ammonium uptake by low temperatures. Glycine uptake was higher at pH 5·7 than at pH 7·0 or 8·2. The glycine uptake was up‐regulated in response to glycine, whereas ammonium uptake was up‐regulated in response to ammonium starvation. Nitrate uptake was strongly down‐regulated when tillers were grown on either ammonium or glycine. In contrast to N‐starved roots, which absorbed ammonium ions more rapidly than glycine, the roots grown on glycine, ammonium and nitrate and not N‐starved prior to uptake absorbed glycine as rapidly as ammonium and nitrate ions combined. Overall, the results indicate that amino acids are probably an important source of nitrogen for P. phryganodes in Arctic coastal marshes.