Nitrate or ammonium nutrition in french bean

Summary Bean Plants were grown in a greenhouse in sand irrigated with nutrient solutions containing either 2 mM NO ₃ ^– or 2 mM NH ₄ ⁺ . After 45 days fresh weight of NH ₄ ⁺ plants was half that of NO ₃ ^– plants. Cation concentration in NH ₄ ⁺ plants was 30% less than in NO ₃ ^– plants. Amino acids (SER, ASN, GLN) accummulated 3 to 10 times more in NH ₄ ⁺ plants. The concentration of organic acids (malic, malonic, citric) was 10 to 30 times higher in NO ₃ ^– plants. The ATP-costings for the synthesis of amino acids and organic acids in NH ₄ ⁺ plants was half that of NO ₃ ^– ones: therefore it could not account for the reduction of growth in the ammonium-fed plants.     

Effects of light quantity and quality and soil nitrogen status on nitrate reductase activity in rainforest species of the genus Piper

Summary We studied nitrate reductase (NR) activity in six species of the genus Piper (Piperaceae) growing under a broad range of light availabilities. Field measurements were made on plants growing naturally in rainforest at the Los Tuxtlas Tropical Biological Preserve, Veracruz, Mexico at high- and lowlight extremes for each species. Foliar nitrogen on an area basis was positively related to the average daily photosynthetically active photon flux density (PFD) received by the leaf (r=0.76, p<0.01). In vivo NR activity was highly correlated with PFD (r=0.95, p<0.001) and less so with total leaf nitrogen (r=0.68, p<0.05). In vivo NR activity was always higher in high-light plants than in low-light plants within a species. Similarly, gap species such as P. auritum had much higher in vivo NR activities than shade species such as P. aequale. Soil NO ₃ ^– and NH ₄ ⁺ pools and nitrogen-mineralization rates at Los Tuxtlas were similar between high- and low-light sites, indicating that the elevated NR activities in high-light plants were not the result of higher NO ₃ ^– availabilities in high-light microsites. We performed additional experiments at Stanford, California, USA on Piper plants grown at high- and low-light. Foliar NR was highly inducible by nitrate in the gap species (auritum) but not in the generalist (hispidum) or shade (aequale) species. Root NR activities were, in general, an order of magnitude lower than foliar activities. In total, these studies suggest that Piper gap species are inherently more competent to assimilate NO ₃ ^– and are better able to respond to sudden increases in NO ₃ ^– availability than are shade species.CJWDPB publication # 1097     

Temperature dependence of inorganic nitrogen uptake and assimilation in Antarctic sea-ice microalgae

Summary The influence of temperature on NO ₃ ^- and NH ₄ ⁺ uptake, and the activity of the assimilatory enzyme NO ₃ ^- reductase (NR) was compared to inorganic C uptake (photosynthesis) in natural assemblages of Antarctic sea-ice microalgae. NO ₃ ^- and NH ₄ ⁺ uptake reached a maximum between 0.5°–2.0°C and 2.0°–3.0°C, respectively, which was close to that for photosynthesis (2.5°–3.0°C). NR showed a distinctly higher temperature maximum (10.0°–12.0°C) and a lower Q₁₀ value than inorganic N and C transport. Our data imply that, owing to differential temperature characteristics between N transport and N assimilation at in situ temperature (-1.9°C), the incorporation of extracellular NO ₃ ^- into cellular macromolecules, may be limited by transport of NO ₃ ^- into the cell rather than the intracellular reduction of NO ₃ ^- to NH ₄ ⁺ . Despite differences in temperature maxima between N transport and N assimilation, the overall low temperature maxima of inorganic N metabolism characterizes Antarctic sea-ice microalgae as psychrophilic. Our study is the first to examine the temperature dependence of inorganic N uptake and assimilation in sea-ice microbial communities.     

Induction of nitrate reductase, nitrite reductase, and glutamine synthetase isoforms in sunflower cotyledons as affected by nitrate, light, and plastid integrity

Summary We investigated the inducibility of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), and glutamine synthetase (GS; EC 6.3.1.2) isoforms in cotyledons of 7-day-old seedlings of sunflower (Helianthus annuus L.) in relation to light, nitrogen source (NO ₃ ^– , NO ₂ ^– or NH ₄ ⁺ ), and the involvement of plastids. Nitrate was absolutely (and specifically) required for NR induction, and stimulated more effectively than NO ₂ ^– or NH ₄ ⁺ the synthesis of NiR and chloroplastic GS (GS₂) over the constitutive levels present in N-free-grown seedlings. In vivo inhibition of NR activity by tungsten application to seedlings and measurements of tissue NO ₃ ^– concentration indicate that NO ₃ ^– -dependent enzyme induction is elicited by NO ₃ ^– per se and not by a product of its assimilatory reduction, e.g., NO ₂ ^– or NH ₄ ⁺ . In the presence of NO ₃ ^– , light remarkably enhanced the appearance of NR, NiR, and GS₂, while the activity of the cytosolic GS isoform (GS₁) was adversely affected. Cycloheximide suppressed much more efficiently than chloramphenicol the light- and NO ₃ ^– -dependent increase of GS₂ activity, indicating that sunflower chloroplastic GS is synthesized on cytoplasmic 80S ribosomes. When the plastids were damaged by photooxidation in cotyledons made carotenoid-free by application of norflurazon, the positive action of light and NO ₃ ^– on the appearance of NR, NiR, and GS₂ isoform was greatly abolished. Therefore, it is suggested that intact chloroplasts are required for the inductive effect of light and NO ₃ ^– and/or for the accumulation of newly formed enzymes in the organelle.Abbreviations CAP chloramphenicol - CHX cycloheximide - GS glutamine synthetase - GS₁ cytosolic GS - GS₂ plastidic (chloroplastic) GS - NF norflurazon - NiR nitrite reductase - NR nitrate reductase     

Plant-mediated processess to acquire nutrients: nitrogen uptake by rice plants

The ways in which root–soil interactions can control nutrient acquisition by plants is illustrated by reference to the N nutrition of rice. Model calculations and experiments are used to assess how uptake is affected by root properties and N transport through the soil. Measurements of the kinetics of N absorption and assimilation and their regulation, and of interactions between NH₄ ⁺ and NO₃ ^– nutrition, are described. It is shown that uptake of N from the soil–-as opposed to N in ricefield floodwater which can be absorbed very rapidly but is otherwise lost by gaseous emission–-will often be limited by root uptake properties. Rice roots are particularly efficient in absorbing and assimilating NO₃ ^–, and NH₄ ⁺ absorption and assimilation are stimulated by NO₃ ^–. The uptake of NO₃ ^– formed in the rice rhizosphere by root-released O₂ may be more important than previously thought, with beneficial consequences for rice growth. Other root-induced changes in the rice rhizosphere and their consequences are discussed.     

Impact of soil environmental factors on rates of N2-fixation associated with roots of intact maize and sorghum plants

We have evaluated the effects of oxygen partial pressure (pO₂), combined nitrogen, and the availability of organic substrates on nitrogen fixation (acetylene reduction) by bacteria associated with the roots of intact maize and sorghum plants. We also investigated the possibility of enhancing associative nitrogen-fixation by inoculating the soil in which the plants were grown withAzospirillum. Acetylene reduction (AR) activity was greatest when roots of intact plants were exposed to pO₂ between 1.3 and 2.1 kPa. Field-grown and greenhouse-grown plants supported similar levels of activity. Respiration inhibitors (2,4-dinitrophenol and sodium azide) eliminated AR activity at 2 kPa O₂, whereas a fermentation inhibitor (sodium fluoride) only partially reduced the activity. Acetylene reduction activity was rapidly (1–3 h) inhibited by NH ₄ ⁺ , NO ₃ ^– , and NO ₂ ^– at concentrations of 4–20 mg Nl^–1. Rates of AR varied substantially among individual plants in each experiment and between experiments. Amendment with any of several organic substrates greatly increased AR activity when rates were low, suggesting that the lack of activity was caused by a shortage of available carbon in the rhizosphere. Inoculation withAzospirillum failed to increase rates of AR associated with maize plants. In several experiments the indigenous bacteria associated with uninoculated plants exhibited greater activity than the bacteria associated with inoculated plants.     

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO ₃ ^– deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO ₃ ^– uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO ₃ ^– additions equivalent to 30 kg N ha^–1 year^–1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used ¹⁵N to determine the kinetic parameters of NO ₃ ^– uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO ₃ ^– –N m^–2 and determined the kinetic parameters of NO ₃ ^– and NH ₄ ⁺ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO ₃ ^– fertilization (range = non-detectable to 33 nmol NO ₂ ^– g^–1 h^–1). Maximal rates of NO ₃ ^– uptake in overstory trees also were low, ranging from 0.2 to 1.0 mol g^–1 h^–1. In seedlings, the mean V _max for NO ₃ ^– uptake in fine roots (1 mol g^–1 h^–1) was approximately 30 times lower than the V _max for NH ₄ ⁺ uptake (33 mol g^–1 h^–1). Our results suggest that A. saccharum satisfies its N demand through rapid NH ₄ ⁺ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO ₃ ^– .     

A substrate cycling model for nitrate uptake byPisum sativum seedlings: A key to sensitivity of response of net flux to substrate and effectors?

Summary Net nitrate uptake (J) intoPisum sativum L. seedlings has been investigated. J was high initially, but declined with time as NO ₃ ^– efflux (E) approached that of NO ₃ ^– influx (I). Both I and E were higher in plants which had been grown without N. J could be reversibly and immediately inhibited by 5 mmol m^–3 NH ₄ ⁺ , although plants grown in the presence of nitrate were less sensitive. A theoretical model which involves substrate cycling across the plasmalemma is shown to increase the sensitivity to substrate and effectors. It predicts that during growth of Pisum in N free media the cycling rate (E/I) is increased and the sensitivity of net flux of inhibition by NH ₄ ⁺ is highest. The model also provides a means for control of cytoplasmic nitrate pool size [NO ₃ ^– ] c.     

Growth,ionic balance,proton excretion,and nitrate reductase activity in Alnus and Hippophaë supplied with different sources of nitrogen

Summary Pre-cultivated, nodulated and non-nodulated plants of black alder (Alnus glutinosa) and sea buckthorn (Hippophaë rhamnoides ssp.rhamnoides) were grown on different N sources, with and without acidity control. Dry matter yields were lowest when plants were supplied with only NO ₃ ^– and were much greater when NH ₄ ⁺ was supplied either alone or in combination with NO ₃ ^– as long as the external pH was controlled; the final yields of the N₂-fixing plants were relatively low, especially withH. rhamnoides. Without acidity control, yields were greatly reduced in the presence of NH ₄ ⁺ .Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, agreed well with measured excretion values. Without pH adjustment, the total proton efflux into the external solution was greater inA. glutinosa than inH. rhamnoides.Both species, but particularlyA. glutinosa, displayed the highest nitrate reductase activity in the roots.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Nitrate use by tobacco cells in response to N-stress and ammonium nutrition

Summary Characterization of NO ₃ ^– use by suspension cultured tobacco cells during a culture cycle is needed to take advantage of cell cultures for further study of the biochemical regulation of NO ₃ ^– uptake induction and decay processes. Tobacco (Nicotiana tabacum L., cv. Ky14) cells were cultured with media containing different N sources. Cells cultured with a mixture of NO ₃ ^– and NH ₄ ⁺ (40 mM NO ₃ ^– plus 20 mM NH ₄ ⁺ , in Murashige and Skoog media) initially grew slightly faster but attained the same maximum cell culture density as those cultured with 40 mM NO ₃ ^– only. Cells subcultured with N-free media grew at a similar rate for the first 3 d as those cells grown with N, then ceased further growth. The cessation of growth of cells subcultured with N-free media coincided with depletion of cell NO ₃ ^– . The NO ₃ ^– influx of cells subcultured with N-free media increased eleven-fold and those grown with N increased four- to five-fold before declining. Maximal NO ₃ ^– influx rates occurred at the onset of the stationary growth phase for N-stressed cells, while cells grown with N reached maximums prior to the stationary phase of cell growth. Cells grown with a mixture of NO ₃ ^– and NH ₄ ⁺ had lower NO ₃ ^– reductase (NR) activity and higher cell NO ₃ ^– levels than those of cells grown with NO ₃ ^– only. The NR activity of cells subcultured with N-free media peaked within 1 d after subculture before declining to a constitutive level when cell NO ₃ ^– was depleted. The level of cell NO ₃ ^– plays a critical role in the expression of the NO ₃ ^– uptake and reduction processes. The transitions in the expression of NO ₃ ^– uptake and reduction activities of tobacco cell suspension cultures should prove valuable for further study of the biochemical and molecular basis for the regulation of these processes.Abbreviations DTT DL-dithiothreitol - EDTA ethylenediamine tetraacetate - FW fresh weight - MS media Murashige & Skoog media - NADH ß-nicotinamide adenine dinucleotide reduced form - PMSF phenylmethyl-sulfonyl fluoride     

Ammonia-oxidizing bacteria on root biofilms and their possible contribution to N use efficiency of different rice cultivars

Ammonia-oxidizing bacteria (AOB) populations were studied on the root surface of different rice cultivars by PCR coupled with denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). PCR-DGGE of the ammonium monooxygenase gene (amoA) showed a generally greater diversity on root samples compared to rhizosphere and unplanted soil. Sequences affiliated with Nitrosomonas spp. tended to be associated with modern rice hybrid lines. Root-associated AOB observed by FISH were found within a discrete biofilm coating the root surface. Although the total abundance of AOB on root biofilms of different rice cultivars did not differ significantly, there were marked contrasts in their population structure, indicating selection of Nitrosomonas spp. on roots of a hybrid cultivar. Observations by FISH on the total bacterial community also suggested that different rice cultivars support different bacterial populations even under identical environmental conditions. The presence of active AOB in the root environment predicts that a significant proportion of the N taken up by certain rice cultivars is in the form of NO₃ ^–-N produced by the AOB. Measurement of plant growth of hydroponically grown plants showed a stronger response of hybrid cultivars to the co-provision of NH₄ ⁺ and NO₃ ^–. In soil-grown plants, N use efficiency in the hybrid was improved during ammonium fertilization compared to nitrate fertilization. Since ammonium-fertilized plants actually receive a mixture of NH₄ ⁺ and NO₃ ^– with ratios depending on root-associated nitrification activity, these results support the advantage of co-provision of ammonium and nitrate for the hybrid cultivar.     

Effect of ammonium and nitrate on growth and appearance of nitrate reductase and nitrite reductase in dark- and light-grown mustard seedlings

Nitrate-induced and phytochrome-modulated appearance of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of the mustard (Sinapis alba L.) seedling is strongly affected by externally supplied ammonium (NH ₄ ⁺ ). In short-term experiments between 60 and 78 h after sowing it was found that in darkness NH ₄ ⁺ —simultaneously given with NO ₃ ^- —strongly inhibits appearance of nitrate-inducible NR and NIR whereas in continuous far-red light—which operates exclusively via phytochrome without significant chlorophyll formation —NH ₄ ⁺ (simultaneously given with NO ₃ ^- ) strongly stimulates appearance of NR. The NIR levels are not affected. This indicates that NR and NIR levels are regulated differently. In the absence of external NO ₃ ^- appearance of NR is induced by NH₄ in darkness as well as in continuous far-red light whereas NIR levels are not affected. On the other hand, in the absence of external NO ₃ ^- , exogenous NH ₄ ⁺ strongly inhibits growth of the mustard seedling in darkness as well as in continuous far-red light. This effect can be abolished by simultaneously supplying NO ₃ ^- . The adverse effect of NH ₄ ⁺ on growth (NH ₄ ⁺ -toxicity) cannot be attributed to pH-changes in the medium since it was shown that neither the growth responses nor the changes of the enzyme levels are related to pH changes in the medium. Non-specific osmotic effects are not involved either.Abbreviations c continuous - D darkness - FR far-red light - NIR nitrite reductase (EC 1.7.7.1) - NR nitrate reductase (EC 1.6.6.1)     

Nitrate reductase activity of two leafy vegetables as affected by nickel and different nitrogen forms

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO₃ ⁻–N, NH₄ ⁺–N, NH₄NO₃). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO₃ ⁻–N) or mixed (NH₄NO₃) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO₃ ⁻–N or NH₄NO₃. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO₃ ⁻-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH₄ ⁺–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH₄ ⁺–N the enzyme activity in roots was even higher than in those supplied with NO₃ ⁻–N.     

Accumulation and reduction of nitrate in cereal plants dependent on N supply

Summary In pot experiments the NO ₃ ^– accumulation and the occurrence of nitrate reductase (NR) capacity of wheat plants were investigated depending on late N applications at tillering, shooting and heading. NO ₃ ^– is preferentially accumulated in the stems, while NR dominates in the leaves. NO ₃ ^– accumulation is enhanced by late N treatments especially if N supply at seeding is sufficient. NR capacity of the plants is stimulated by late nitrogen supply, but its increment rates decrease with increasing NO ₃ ^– accumulation.     

Effect of Sesbania,Azolla and rice straw incorporation on the kinetics of NH4, K,Fe, Mn,Zn and P in some flooded rice soils

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH ₄ ⁺ −N, K⁺, Fe²⁺, Mn²⁺, Zn²⁺, and P. Sesbania and Azolla increased NH ₄ ⁺ −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH ₄ ⁺ −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH ₄ ⁺ −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH ₄ ⁺ −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L⁻¹). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K⁺, Fe²⁺, Mn²⁺ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe²⁺ and Mn²⁺ but increased K⁺ and P. In general, rice straw, Sesbania and Azolla decreased Zn²⁺ concentration in all soils.     

Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO ₃ ^– content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO ₃ ^– content was observed in plants grown in nitrogen replete conditions at low irradiance (170 mol photons·m^–2 ·s^–1). This relationship disappeared at higher irradiance (450 mol photons·m^–2·S^–1) because the maximal extractable NR activity in the leaves of the wild type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO ₃ ^– accumulated in the leaves of plants grown at 450 mol photons·m^–2·s^–1 than in those grown at 170 mol photons·m^–2·s^–1 in N-replete conditions. The foliar NO ₃ ^– level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO ₃ ^– was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO ₃ ^– in the constitutive NR-expressors. We conclude that the NR protein is specifically targetted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.Abbreviations Chl chlorophyll - N nitrogen - NR NADH-nitrate reductase - WT wild type     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Influence of nitrogen and sulphur form on manganese acquisition by barley (<Emphasis Type="Italic">shape Hordeum vulgare</Emphasis>)

The influence of various nitrogen (N) and sulphur (S) forms on the uptake of manganese (Mn) in young spring barley (Hordeum vulgare L cv Golf) plants was examined in both a hydroponic system and in a soil-based system. The soil was a typical Danish Mn-deficient soil viz. a sandy loam soil developed on old marine sediments. Plants growing in solution culture with NO₃^– as the only N source had a higher Mn uptake than plants receiving mixtures of NO₃^– and NH₄⁺. These findings were opposite to the results obtained in the soil-based experiments, where plants fertilized with NO₃^– as the only N source accumulated much less Mn than plants fertilized with NH₄⁺. Combining the results of these experiments confirmed that NH₄⁺ acted as a powerful antagonist to Mn²⁺ during uptake but that this antagonistic effect was more than compensated for by the influence of NH₄⁺ in reducing plant-unavailable Mn(IV) to plant-available Mn(II) in the soil. Furthermore the soil experiments showed that fertilizers containing sulphur in the form of reduced S (thiosulphate) had a strong mobilizing effect on Mn, and enabled the plants to accumulate large amounts of Mn in the biomass compared with oxidized S (sulphate). Thus, fertilization with thiosulphate may be very effective in alleviating Mn-deficiency in soils developed on old marine sediments where Mn availability is limiting plant growth.     

NO 2 − Efflux and its regulation in cyanobacterium Nostoc MAC

NO ₂ ^– efflux and its regulation have been studied in the cyanobacterium Nostoc MAC. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), carbonyl cyanide-m-chlorophenyl hydrazone (CCCP), sodium azide, p-chloromercuribenzoate (PCMB), and dicyclohexylcarbodiimide (DCCD), a specific inhibitor of bacterial ATPase, inhibited the NO ₂ ^– efflux activity singificantly. No NO ₂ ^– efflux activity was observed under dark-aerobic as well as under dark-anaerobic conditions; however, the addition of ATP resulted in NO ₂ ^– efflux even under dark-aerobic condition. Maximum NO ₂ ^– efflux activity was observed when NO ₃ ^– served as the sole nitrogen source. However, NH ₄ ⁺ ions inhibited the NO ₂ ^– efflux activity when both NO ₃ ^– and NH ₄ ⁺ wer simulatneously available to the cells. The NO ₂ ^– efflux was freed from NH ₄ ⁺ repression by l-methionine-dl-sulfoximine (MSX), an irreversible inhibitor of glutamine synthetase (GS). Chloramphenicol, a protein synthesis inhibitor, inhibited the derepression of NO ₂ ^– efflux system when NH ₄ ⁺ -incubated cells were transferred to NO ₃ ^– medium. Tungstate-treated cells lacking functional NO ₃ ^– reductase but having NO ₃ ^– uptake activity also lacked NO ₂ ^– efflux activity. These results suggest that (i) NO ₂ ^– efflux in Nostoc MAC is NO ₃ ^– dependent and an energy-dependent process that can be regulated at the levels of NO ₃ ^– uptake and NO ₃ ^– reductase; (ii) NO ₂ ^– efflux system is NH ₄ ⁺ repressible; however, the product of NH ₄ ⁺ assimilation via GS is being required for repression to occur; (iii) de novo protein synthesis is required for derepression of the NO ₂ ^– efflux system; and (iv) the catalytic activity of NO ₂ ^– reductase also seems to play an important role in the regulation of NO ₂ ^– efflux system.