Effects of NH4+ concentration on growth,morphology and NH4+ uptake kinetics of Salvinia natans

Many plants develop toxicity symptoms and have reduced growth rates when supplied with ammonium (NH₄⁺) as the only source of inorganic nitrogen. In the present study, the growth, morphology, NH₄⁺ uptake kinetics and mineral concentrations in the tissues of the free-floating aquatic plant Salvinia natans (water fern) supplied exclusively with NH₄⁺–N at concentrations of 0.25–15 mM were investigated. S. natans grew well, with relative growth rates of c. 0.25 g g^?1 d^?1 at external NH₄⁺ concentrations up to 5 mM, but at higher levels growth was suppressed and the plants had small leaves and short roots with stunted growth. The high-affinity transport system (HATS) that mediate NH₄⁺ uptake at dilute NH₄⁺ levels was downregulated at high NH₄⁺ concentrations with lower velocities of maximum uptake (V_max) and higher half-saturation constants (K_1/2). High NH₄⁺ levels also barely affected the concentrations of mineral cations and anions in the plant tissue. It is concluded that S. natans can be characterized as NH₄⁺-tolerant in line with a number of other species of wetland plants as growth was unaffected at NH₄⁺ concentrations as high as 5 mM and as symptoms of toxicity at higher concentrations were relatively mild. Depolarization of the plasma membrane to the equilibrium potential for NH₄⁺ at high external concentrations may be a mechanism used by the plant to avoid excessive futile transmembrane cycling. S. natans is tolerant to the high NH₄⁺ levels that prevail in domestic and agricultural wastewaters, and the inherent high growth rate and the ease of biomass harvesting make S. natans a primary candidate for use in constructed wetland systems for the treatment of various types of nitrogen-rich wastewaters.     

Nitrogen nutrition of Salvinia natans: Effects of inorganic nitrogen form on growth,morphology, nitrate reductase activity and uptake kinetics of ammonium and nitrate

In this study we assessed the growth, morphological responses, and N uptake kinetics of Salvinia natans when supplied with nitrogen as NO₃⁻, NH₄⁺, or both at equimolar concentrations (500 μM). Plants supplied with only NO₃⁻ had lower growth rates (0.17 ± 0.01 g g⁻¹ d⁻¹), shorter roots, smaller leaves with less chlorophyll than plants supplied with NH₄⁺ alone or in combination with NO₃⁻ (RGR = 0.28 ± 0.01 g g⁻¹ d⁻¹). Ammonium was the preferred form of N taken up. The maximal rate of NH₄⁺ uptake (V_max) was 6–14 times higher than the maximal uptake rate of NO₃⁻ and the minimum concentration for uptake (C_min) was lower for NH₄⁺ than for NO₃⁻. Plants supplied with NO₃⁻ had elevated nitrate reductase activity (NRA) particularly in the roots showing that NO₃⁻ was primarily reduced in the roots, but NRA levels were generally low (<4 μmol NO₂⁻ g⁻¹ DW h⁻¹). Under natural growth conditions NH₄⁺ is probably the main N source for S. natans, but plants probably also exploit NO₃⁻ when NH₄⁺ concentrations are low. This is suggested based on the observation that the plants maintain high NRA in the roots at relatively high NH₄⁺ levels in the water, even though the uptake capacity for NO₃⁻ is reduced under these conditions.     

Nitrogen Acquisition from Atmospheric NH3 by Lolium perenne

Ammonia (NH₃) is the third most abundant N species in the atmosphere and, due to various natural and anthropogenic sources, can reach high concentrations in some areas. While some plants show effects of toxicity, others are capable of using this N-form and grow well without any utilization of soil-N. Acquisition of atmospheric NH₃ will affect the acid-base balance of the plants as absorption and dissolution causes an alkalinisation (production of OH^?) and assimilation of NH₃ results in an acidification (generation of H⁺). As there is only a limited capacity for biochemical disposal of excess H⁺ in shoots, pH regulation may involve H⁺/OH^? extrusion into the media via roots and transport of (in)organic ions between roots and above-ground parts of the plant. Our aim therefore was to assess NH₃ acquisition by Lolium perenne and to study the effects of gas phase NH₃ on growth, acid-base balance and mineral composition of the plants. The experiments therefore included application of a range of ¹⁴NH₃ to the shoots and of ¹⁵N as NO₃^?, NH₄⁺ or NH₄NO₃ to the roots, from which the amount of gas phase NH₃ acquisition could be quantified. Analysis of the mineral composition provided data for calculation of acid-base balance as well as for water use efficiencies of the plants. The results indicate that over the range of NH₃ supplied, plants from all treatments could utilize gas-phase NH₃ as demonstrated by increases in growth and in N and C use efficiencies. Plants receiving NO₃^? via their roots had a higher capacity to use gaseous NH₃ than those growing with NH₄⁺. NH₃ assimilation in shoots reduced both the acid load with NH₄⁺ nutrition and the alkaline load with NO₃^? supply to the roots. The results of the experiments are discussed in relation to possible acid-base regulation mechanisms of the whole plant.     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

Absorption of nitrate and ammonium ions by Lolium perenne from flowing solution cultures at low root temperatures

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49 d to low root temperature (5°C) prior to treatment at root temperatures of 3, 5, 7 and 9°C for 41 d with common air temperature of 20/15°C day/night and solution pH 5·0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH₄ or NO3 at 10 mmol m^?3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5°C had very high dry matter contents as reflected by the fresh weight: freeze-dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9°C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NHJ but at any given temperature, the [N] in plants grown with NHJ was significantly greater than in those grown with NO₃. The specific absorption rate (SAR) of NH⁺₄ was greater at all temperatures than SAR-NO₃. In plants grown with NH⁺, 3–5% of the total N was recovered as NH⁺₄, whereas in those grown with NO^?₃ the unassimilated NO^?₃ rose sharply between 7 and 9°C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH⁺₄ lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO^?₃-grown plants. Plants grown with NH⁺₄ had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in Concentration to those in NO^?₃ grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH⁺₄ or NO^?₃. The additional residual N (protein) in NH⁺₄ grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.     

Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH₄⁺ and NO₃^? uptake from mixed sources, NH₄⁺ and NO₃^? were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH₄⁺-fed axis than in the NO₃^?-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH₄⁺-fed axis can be accounted for by the retransport of CH₂O equivalents from the root back to the shoot with amino acids produced by NH₄⁺ assimilation. Uptake of NH₄⁺ or NO₃^? by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH₄⁺ and NO₃^? were supplied to separate halves of the split-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH₄⁺ to NO₃^? uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH₄⁺ and NO₃^? uptake by roots. For whole roots supplied with both NH₄⁺ and NO₃^?, the restriction in uptake of NO₃^? may involve a stimulation of NO₃^? efflux rather than an inhibition of NO₃^? influx. While only the net uptake of NH₄⁺ and NO₃^? was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH₄⁺, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH₄⁺ was supplied with NO₃^? to whole-root systems or separately to an axis of the split-root system. In the case of NO₃^?, however, the number of net efflux events increased from 8 when NO₃^? was supplied to a separate axis of the split-root system to between 19 and 24 when NO₃^? was supplied with NH₄⁺ to whole-root systems.     

Nitrate nutrition enhances nickel accumulation and toxicity in Arabidopsis plants

Background and aims

Nickel (Ni) has become a major heavy metal contaminant. The form of nitrogen nutrition remarkably affects IRT1 expression in roots. IRT1 has an activity of transporting Ni²⁺ into root cells. Therefore, nitrogen-form may affect Ni accumulation and toxicity in plants. The assumption was investigated in this study.

Methods

The Arabidopsis plants were treated in Ni-contained growth solutions with either nitrate (NO₃ ^?) or ammonium (NH₄ ⁺) as the sole N source. After 7-day treatments, Ni concentration, IRT1 expression, Ni-induced toxic symptoms and oxidative stress in plants were analyzed.

Results

The NO₃ ^?-fed plants contained a higher Ni concentration, had a greater IRT1 expression in roots, and developed more severe toxic symptoms in the youngest fully expanded leaves, compared with the NH₄ ⁺-fed plants. The Ni-induced growth inhibition was also more significant in NO₃ ^?-fed plants. Interestingly, Ni exposure resulted in greater hydrogen peroxide (H₂O₂) and superoxide radical (O₂ ^{. ?}) accumulations, more severe lipid peroxidation and more cell death in NO₃ ^?-fed plants, whereas the opposite was true for NH₄ ⁺-fed plants. Furthermore, the Ni-enhanced peroxidase (POD) and superoxide dismutase (SOD) activities were greater in NO₃ ^?-fed plants

Conclusion

NO₃ ^? nutrition promotes Ni uptake, and enhances Ni-induced growth inhibition and oxidative stress in plants compared with NH₄ ⁺ nutrition.     

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃^? + NH₄⁺) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃^? nutrition, mixed‐N and NH₄⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root‐shoot ratios in NH₄⁺‐ and mixed‐N‐supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄⁺‐ and mixed‐N‐supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄⁺‐ and mixed‐N‐supplied plants were significantly higher than that in NO₃^?‐supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed‐N and NH₄⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.     

Growth of Salvinia molesta as affected by water temperature and nutrition I. Effects of nitrogen level and nitrogen compounds

The growth of Salvinia molesta D.S. Mitchell was studied in a greenhouse using controlled-temperature water-baths at 16, 19 and 22°C and 4 different nitrogen compounds (NO₃^?, NH₄⁺, NH₄NO₃ and urea) at levels up to 60 mg N l^?1. Little growth occurred at 16°C even if 20 mg N l^?1 was supplied together with other nutrients including phosphorus (2 mg H₂PO₄-P l^?1). The highest relative growth rate and total dry matter production occurred at 22°C when plants were supplied with 20 mg NH₄-N l^?1. At this temperature, the NH₄⁺ ion was superior to the NO₃^? ion or urea as a nitrogen source (almost doubling the biomass), but was not significantly better than NH₄NO₃. Over a period of 19 days for plants receiving 0.02 mg NH₄-N l^?, biomass increased 4-fold at 16°C, 9-fold at 19°C and 10-fold at 22°C. In contrast, for plants receiving 20 mg NH₄-N l^?1, biomass increased 4-fold at 16°C, 18-fold at 19°C and 38-fold at 22°C.     

Influence of different mineral nitrogen sources (NO 3 − -N vs. NH 4 + -N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices–cowpea symbiosis

Labeled nitrogen (¹⁵?N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO ₃ ^? vs. NH ₄ ⁺ ) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO ₃ ^? and NH ₄ ⁺ . However, the amount of N transferred from the FC to the plant was higher when NO ₃ ^? was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher ¹⁵?N enrichment when the FC was supplied with ¹⁵NH ₄ ⁺ compared with ¹⁵NO ₃ ^? . The ¹⁵?N shoot/root ratio of plants supplied with ¹⁵NO ₃ ^? was much higher than that of plants supplied with ¹⁵NH ₄ ⁺ , indicative of a faster transfer of ¹⁵NO ₃ ^? from the root to the shoot and a higher accumulation of ¹⁵NH ₄ ⁺ in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH ₄ ⁺ preferentially over NO ₃ ^? but that export of N from the hyphae to the root and shoot may be greater following NO ₃ ^? uptake. The need for NH ₄ ⁺ to be assimilated into organically bound N prior to transport into the plant is discussed.     

Influence of inorganic nitrogen sources on K+/Na+ homeostasis and salt tolerance in sorghum plants

This work aimed to study the regulation of K⁺/Na⁺ homeostasis and the physiological responses of salt-treated sorghum plants [Sorghum bicolor (L.) Moench] grown with different inorganic nitrogen (N) sources. Four days after sowing (DAS), the plants were transferred to complete nutrient solutions containing 0.75 mM K⁺ and 5 mM N, supplied as either NO₃ ^? or NH₄ ⁺. Twelve DAS, the plants were subjected to salt stress with 75 mM NaCl, which was applied in two doses of 37.5 mM. The plants were harvested on the third and seventh days after the exposure to NaCl. Under the salt stress conditions, the reduction of K⁺ concentrations in the shoot and roots was higher in the culture with NO₃ ^? than with NH₄ ⁺. However, the more conspicuous effect of N was on the Na⁺ accumulation, which was severely limited in the presence of NH₄ ⁺. This ionic regulation had a positive influence on the K⁺/Na⁺ ratio and the selective absorption and transport of K⁺ in the plants grown with NH₄ ⁺. Under control and salt stress conditions, higher accumulation of free amino acids and soluble proteins was promoted in NH₄ ⁺ grown roots than NO₃ ^? grown roots at both harvesting time, whereas higher accumulation of soluble sugars was observed only at 7 days of salt stress exposure. Unlike the NH₄ ⁺ grown plants, the gas exchanges of the NO₃ ^? grown plants were reduced after 7 days of salt stress. These results suggest that external NH₄ ⁺ may limit Na⁺ accumulation in sorghum, which could contribute to improving its physiological and metabolic responses to salt stress.     

Different Tolerance to Light Stress in NO3−- and NH4+-Grown Phaseolus vulgaris L.

Abstract: NH₄⁺‐grown plants are more sensitive to light stress than NO₃^?‐grown plants, as indicated by reduced growth and intervenal chlorosis of French bean (Phaseolus vulgaris L.). Measuring the time course of F_v/F_m ratios under photoinhibitory light regimes did not reveal any difference in PS II damage between NO₃^?‐ and NH₄⁺‐grown plants, in spite of some indications of higher energy quenching in NO₃^?‐grown plants. Also, a direct action of NH₄⁺ as an uncoupler at the thylakoid membrane could be excluded. Instead, biochemical analysis revealed enhanced lipid peroxidation and higher activity of scavenging enzymes in NH₄⁺‐grown plants indicating that these plants make use of metabolic pathways with stronger radical formation. Evidence for higher rates of photorespiration in NH₄⁺‐grown plants came from experiments showing that electron flux and O₂ evolution were decreased by SHAM in NH₄⁺‐grown plants, and by antimycin A in NO₃^?‐grown plants. Further, the comparison of electron flux and of photoacoustic measurements of O₂ evolution suggested that in NH₄⁺‐grown plants the Mehler reaction was also increased, at least in the induction phase. However, the major cause of N form‐dependent stress sensitivity is assumed to be in the coupling between photosynthesis and respiration, i.e., NO₃^?‐grown plants can utilize the TCA cycle for the generation of C skeletons for amino acid synthesis, thus improving the ATP: reductant balance, whereas NH₄⁺‐grown plants have enhanced rates of photorespiration.     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

Estimation of Nitrification and Denitrification from Microprofiles of Oxygen and Nitrate in Model Sediment Systems

The coupling between nitrification and denitrification and the regulation of these processes by oxygen were studied in freshwater sediment microcosms with O₂ and NO₃^- microsensors. Depth profiles of nitrification (indicated as NO₃^- production), denitrification (indicated as NO₃^- consumption), and O₂ consumption activities within the sediment were calculated from the measured concentration profiles. From the concentration profiles, it was furthermore possible to distinguish between the rate of denitrification based on the diffusional supply of NO₃^- from the overlying water and the rate based on NO₃^- supplied by benthic nitrification (D_w and D_n, respectively). An increase in O₂ concentration caused a deeper O₂ penetration while a decrease in D_w and an increase in D_n were observed. The relative importance for total denitrification of NO₃^- produced by nitrification thus increased compared with NO₃^- supplied from the water phase. The decrease in D_w at high oxygen was due to an increase in diffusion path for NO₃^- from the overlying water to the denitrifying layers in the anoxic sediment. At high O₂ concentrations, nitrifying activity was restricted to the lower part of the oxic zone where there was a continuous diffusional supply of NH₄⁺ from deeper mineralization processes, and the long diffusion path from the nitrification zone to the overlying water compared with the path to the denitrifying layers led to a stimulation in D_n.     

Differential responses of root uptake kinetics of NH4+ and NO3− to enriched atmospheric CO2 concentration in field-grown loblolly pine

The nitrogen requirement of plants is predominantly supplied by NH₄⁺ and/or NO₃^? from the soil solution, but the energetic cost of uptake and assimilation is generally higher for NO₃^? than for NH₄⁺. We found that CO₂ enrichment of the atmosphere enhanced the root uptake capacity for NO₃^?, but not for NH₄⁺, in field-grown loblolly pine saplings. Increased preference for NO₃^? at the elevated CO₂ concentration was accompanied by increased carbohydrate levels in roots. The results have important implications for the potential consequences of global climate change on plant-and ecosystem-level processes in many temperate forest ecosystems.     

Nitrogen nutrition of Canna indica: Effects of ammonium versus nitrate on growth, biomass allocation, photosynthesis, nitrate reductase activity and N uptake rates

The effects of inorganic nitrogen (N) source (NH₄⁺, NO₃⁻ or both) on growth, biomass allocation, photosynthesis, N uptake rate, nitrate reductase activity and mineral composition of Canna indica were studied in hydroponic culture. The relative growth rates (0.05-0.06 g g⁻¹ d⁻¹), biomass allocation and plant morphology of C. indica were indifferent to N nutrition. However, NH₄⁺ fed plants had higher concentrations of N in the tissues, lower concentrations of mineral cations and higher contents of chlorophylls in the leaves compared to NO₃⁻ fed plants suggesting a slight advantage of NH₄⁺ nutrition. The NO₃⁻ fed plants had lower light-saturated rates of photosynthesis (22.5 μmol m⁻² s⁻¹) than NH₄⁺ and NH₄⁺/NO₃⁻ fed plants (24.4-25.6 μmol m⁻² s⁻¹) when expressed per unit leaf area, but similar rates when expressed on a chlorophyll basis. Maximum uptake rates (V_max) of NO₃⁻ did not differ between treatments (24-35 μmol N g⁻¹ root DW h⁻¹), but V_max for NH₄⁺ was highest in NH₄⁺ fed plants (81 μmol N g⁻¹ root DW h⁻¹), intermediate in the NH₄NO₃ fed plants (52 μmol N g⁻¹ root DW h⁻¹), and lowest in the NO₃⁻ fed plants (28 μmol N g⁻¹ root DW h⁻¹). Nitrate reductase activity (NRA) was highest in leaves and was induced by NO₃⁻ in the culture solutions corresponding to the pattern seen in fast growing terrestrial species. Plants fed with only NO₃⁻ had high NRA (22 and 8 μmol NO₂⁻ g⁻¹ DW h⁻¹ in leaves and roots, respectively) whereas NRA in NH₄⁺ fed plants was close to zero. Plants supplied with both forms of N had intermediate NRA suggesting that C. indica takes up and assimilate NO₃⁻ in the presence of NH₄⁺. Our results show that C. indica is relatively indifferent to inorganic N source, which together with its high growth rate contributes to explain the occurrence of this species in flooded wetland soils as well as on terrestrial soils. Furthermore, it is concluded that C. indica is suitable for use in different types of constructed wetlands.     

Importance of the nitrogen source in the grass species Brachiaria brizantha responses to sulfur limitation

Background and aims

Plant responses to S supply are highly dependent on N nutrition. We investigated the effect of S status on metabolic, nutritional, and production variables in Brachiaria brizantha treated with different N forms. Additionally, ¹⁵N and ³⁴S root influx were determined in plants under short- and long-term S deprivation.

Methods

Plants were submitted to soil fertilization treatments consisted of combinations of N forms [without N, ammonium (NH₄ ⁺), nitrate (NO₃ ^?) or NH₄ ⁺+NO₃ ^?] at S rates (0, 15, 30, or 45 mg dm^?3). N and S influx capacity was determined in hydroponically-grown plants.

Results

Shoot production due to S supply increased 53, 145 and 196 % with NH₄ ⁺, NH₄ ⁺+NO₃ ^? and NO₃ ^? treatments, respectively. No or low S impaired protein synthesis and led to high accumulation of N-NO₃ ^? and asparagine in NO₃ ^?-fed plants, both alone and with NH₄ ⁺. Proline accumulation was observed in NH₄ ⁺-fed plants. Short- and long-term S deprivation did not promote considerable changes in ¹⁵N influx. ³⁴S absorption decreased depending on the N form provided: NH₄ ⁺+NO₃ ^? > only NH₄ ⁺ > only NO₃ ^? > low N.

Conclusions

Including both NH₄ ⁺ and NO₃ ^? forms in fertilizer increases N and S intake potential and thereby enhances plant growth, nutritional value and production.     

Effects of pH and nitrogen on cadmium uptake in potato

This study investigated the effects of pH and nitrogen form and concentration on cadmium (Cd) uptake by potato (Solanum tuberosum L.) grown in hydroponic culture. Potato plants grown in a pH-buffered nutrient solution for 10 d were exposed for 24 h to 25 nM CdCl₂ labelled with ¹⁰⁹Cd. Plants showed a significantly higher Cd uptake and accumulation at pH 6.5 than at pH 4.5 and 5.5. Nitrogen supplied as nitrate (NO₃ ^?) generally resulted in a higher Cd uptake and accumulation than N supplied as ammonium (NH₄ ⁺). This effect was most pronounced at pH 6.5. The N concentration increasing from 6.5 to 26 mM resulted in a decreased Cd influx when either NO₃ ^? or NH₄ ⁺ was used. Cd translocation to the shoot was increased when NO₃ ^? was used as the sole N source. In conclusion, pH had a strong influence on Cd uptake by roots and N form is especially important for Cd translocation within the potato plant.