Nitrate nutrition enhances zinc hyperaccumulation in Noccaea caerulescens (Prayon)

Nitrate fertilization has been shown to increase Zn hyperaccumulation by Noccaea caerulescens (Prayon) (formerly Thlaspi caerulescens). However, it is unknown whether this increased hyperaccumulation is a direct result of NO₃ ^? nutrition or due to changes in rhizosphere pH as a result of NO₃ ^? uptake. This paper investigated the mechanism of NO₃ ^?-enhanced Zn hyperaccumulation in N. caerulescens by assessing the response of Zn uptake to N form and solution pH. Plants were grown in nutrient solution with 300 μM Zn and supplied with either (NH₄)₂SO₄, NH₄NO₃ or Ca(NO₃)₂. The solutions were buffered at either pH 4.5 or 6.5. The Zn concentration and content were much higher in shoots of NO₃ ^?-fed plants than in NH₄ ⁺-fed plants at pH 4.5 and 6.5. The Zn concentration in the shoots was mainly enhanced by NO₃ ^?, whereas the Zn concentration in the roots was mainly enhanced by pH 6.5. Nitrate increased Zn uptake in the roots at pH 6.5 and increased apoplastic Zn at pH 4.5. Zinc and Ca co-increased and was found co-localized in leaf cells of NO₃ ^?-fed plants. We conclude that NO₃ ^? directly enhanced Zn uptake and translocation from roots to shoots in N. caerulescens.     

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.     

Altered cell wall properties are responsible for ammonium‐reduced aluminium accumulation in rice roots

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH₄⁺) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitro Al‐binding assay revealed that neither NH₄⁺ nor NO₃^? altered the Al‐binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH₄⁺‐supplied roots displayed lower Al‐binding capacity than those from NO₃^?‐supplied roots when grown in non‐buffered solutions. Fourier‐transform infrared microspectroscopy analysis revealed that, compared with NO₃^?‐supplied roots, NH₄⁺‐supplied roots possessed fewer Al‐binding groups (‐OH and COO‐) and lower contents of pectin and hemicellulose. However, when grown in pH‐buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al‐binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH₄⁺‐reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH₄⁺ uptake rather than direct competition for the cell wall binding sites between Al³⁺ and NH₄⁺.     

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.     

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.     

Organic acids are not specifically involved in the nitrate-enhanced Zn hyperaccumulation mechanism in Noccaea caerulescens

Nitrogen form has been shown to affect Zn uptake, translocation and storage in the Zn-hyperaccumulating plant Noccaea caerulescens but the biochemical processes are not fully understood. Organic acids and amino acids have been implicated in Zn transport and storage. This study aimed to examine the effect of N form on concentrations of organic acids and amino acids and how these metabolites correlated with Zn hyperaccumulation. Plants were grown in nutrient solution with NO₃⁻, NH₄NO₃ or NH₄⁺, supplied with 50 or 300 μM Zn, and buffered at either pH 4.5 or 6.5. The metabolomic profile was determined by gas chromatography mass spectroscopy. The concentration of Zn in shoots, xylem and roots was greatest for the NO₃⁻, pH 6.5 and 300 μM Zn treatments. For all N forms, the lower growth-medium pH raised xylem sap pH but had no influence on Zn concentration or exudation rate of the xylem sap. Nitrate enhanced organic acid production while NH₄⁺ increased amino acid production. Organic acids in the xylem were more responsive to changes in growth-medium pH than N form, and did not correlate with Zn concentration in shoots, roots or xylem. Serine might be directly involved in Zn hyperaccumulation. Phosphoric acid was associated with reduced Zn accumulation in the shoots. Malic acid was not detected in the shoots but responded to cation uptake more than to Zn specifically in the roots. Citric acid responded to cation uptake more than to Zn specifically in the shoots but did not correlate with Zn concentration in the roots or the xylem sap, or any other cations in the roots. In conclusion, organic acids in N. caerulescens are not specifically involved in Zn hyperaccumulation but are involved in regulating pH in the xylem and cation–anion balance in plants.     

Acid-base regulation during ammonium assimilation in Hydrodictyon africanum

Abstract The acid-base balance during ammonium (used to mean NH ₄⁺ and/or NH₃) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m^?3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H⁺ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H⁺ accumulate in the medium per organic N synthesized. Granted a permeability (PNH₃) of some 10^?3 cm s^?1, and a finite [NH₃] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH ₄⁺. This means that most of the H ⁺ appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H⁺ accumulates in the medium equivalent to any NH₃ entry by requilibration from exogenous NH ₄⁺. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO₃⁺grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO₃⁺in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO₃⁺-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm^?3) the net influx was all attributable to NH₄⁺influx via a uniporter, probably partly short-circuited by a passive NH₃ efflux due to intrinsic membrane permeability to NH₃. The net ammonium influx was in all cases associated with H⁺ accumulation in the medium. (1.3-1.7 H ⁺ per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H⁺ fluxes.     

Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings

The effect of NO₂ fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO₂ to the plant N budget was measured during a 48 h fumigation with 100mm³m^?3 NO₂. Plants were pre-treated with various nutrient solutions containing NO₂ and NH₄⁺, NO₃^? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH₄⁺ in the solution for 6d led to an increased capacity for NO₃^? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO₃^? uptake. In fumigated plants, NO₂ uptake accounted for 20–40% of NO₃^? uptake. Root NO₃^? uptake in plants supplied with NH₄⁺plus NO₃^? solutions was decreased by NO₂ fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO₂ and NH₄⁺ and were fumigated or not with 100mm³m^?3 NO₂ for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and ¹⁵N labelling. Root NH₄⁺ uptake was approximately three times higher than NO₃^? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO₃^? nor NH₄⁺ accumulated in the shoot, showing that all the absorbed NO₂ was assimilated. Root NO₃^? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO₂-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.     

Dynamics of nitrogen remobilization in defoliated Phleum pratense and Festuca pratensis under short and long photoperiods

The impact of photoperiod on the rate and magnitude of N remobilization relative to uptake of inorganic N during the recovery of shoot growth after a severe defoliation was compared over 18 days in two temperate grass species, timothy (Phleum pratense L. cv. Bodin) and meadow fescue (Festuca pratensis Huds. cv. Salten). Plants were grown in flowing solution culture with N supplied as 20 mM NH₄NO₃ and pre-treated by extending the 11 h photosynthetically significant light period either by 1 h (short-day or SD plants) or 7 h (long-day or LD plants) of very low light intensity, during the 10 days prior to defoliation. Following a single severe defoliation, ¹⁵N-labelled NH₄⁺ or NH₄⁺+ NO₃^? was supplied over a 20-day recovery period under the same SD and LD conditions. Changes in the relative contributions of remobilized N and newly acquired mineral N to shoot regrowth were assessed by sequential harvests. Both absolute and relative rates of N remobilization from root and stubble fractions were higher in LD than SD plants of both species, with the enhancement more acute but of shorter duration in timothy than fescue. Remobilized N was the predominant source of N for shoot regrowth in all treatments between days 0 and 8 after cutting; on average more so for fescue than timothy, because the presence of NO₃^? reduced the proportional contribution of remobilized N to the regrowth of timothy but not of fescue. Net uptake of mineral N began to recover between days 4 and 6 after cutting, with NO₃^? uptake restarting 1 or 2 days earlier than NH₄⁺ uptake, even when NH₄⁺ was the only form of N supply. LD timothy plants supplied solely with NH₄⁺ were slowest to resume uptake of mineral N. Supplying NO₃^? in addition to NH₄⁺ after defoliation promoted shoot regrowth rate but not remobilization of N. Rates of regrowth (shoot dry weight production per plant) were not correlated with rates of N remobilization from stubble either over the short-term (days 0–8) or longer term (days 0–18), interpreted as evidence against a causal dependence of regrowth rate on N remobilization under these conditions. The results are discussed in relation to hypotheses for source/sink-driven rates of N remobilization and their interactions with mineral N uptake following defoliation.     

Relationships among nitrogen accumulation, nitrogen assimilation and plant growth in chrysanthemums

Relationships among growth, N accumulation and assimilation were investigated in Chrysanthemum morifolium Ramat cv. Fiesta in experiments testing the effects of varying levels of NO^–3₃supply and of increasing NH⁺₄ added to a constant level of NO^–3₃ Flowing solution culture systems were used to provide NO^?₃at concentrations of 0.03 to 5.0 mol m^–3 and NH⁺₄ levels from 0.05 to 0.3 mmol m^–3 added to 0.1 mol m^–3NO^?₃. Rates of growth, N absorption, accumulation, distribution and utilization were estimated by regression analysis of data obtained from sequential plant harvests, and rates of NO^?₃ and NH^?₄ net uptake were estimated from solution depletion. A sustained ambient NO^?₃ concentration of 0.03 mol m^–3 was evidently adequate to support growth, since relative growth rates were not affected by increasing NO^?₃ supply from 0.03 to 1.0 mol m^–3, nor from 0.25 to 5.0 mol m^–3, in separate experiments. Shoot growth rates were stimulated by NH₄ added to NO^?₃ one experiment, but not when the experiment was repeated under ambient conditions less favorable to growth. Relative accumulation rates for total N increased with increasing NO^?₃ and with NH⁺₄added to NO^?₃ A constant proportion of NO^?₃ taken up was reduced when NO^?₃ alone was supplied. Both the proportion of total N taken up as NO^?₃ and the proportion of NO^?₃ reduced decreased with increasing NH⁺₃ added to NO^?₃ NH⁺₄ uptake apparently must exceed a threshold of about 30% of the total uptake to inhibit NO^?₃ uptake. Utilization of N in chrysanthemum was apparently limited by redistribution since relative accumulation rates for total N were equal to or greater than relative growth rates, in contrast to results reported for several other species. Results of this study and other information support the postulate that NH⁺₄ added to NO^?₃might stimulate growth by increasing transport of reduced N from roots to shoots, thus increasing the supply of reduced N available to support growth of shoot meristems.     

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.     

Phytate utilization of maize mediated by different nitrogen forms in a plant–arbuscular mycorrhizal fungus–phosphate-solubilizing bacterium system

A pot experiment was conducted to investigate the organic phosphorus (P) (phytate) utilization of Zea mays L. with different nitrogen (N) forms (NH₄⁺ and NO₃^?) when both arbuscular mycorrhizal (AM) fungus (Funelliformis mosseae) and phosphate-solubilizing bacterium (PSB, Pseudomonas alcaligenes) are present. The soil was supplied with either KNO₃ or (NH₄)₂SO₄ (200 mg kg^?1 N) with or without phytin (75 mg P kg^?1). Results showed that the application of NH₄⁺ to the soil in a plant–AM fungus–PSB system decreased rhizosphere pH and increased phosphatase activity. It also enhanced the mineralization rate of phytin, which resulted in the release of more inorganic P. The application of NO₃^? promoted mycorrhizal colonization and hyphal length density in the soil. The inorganic P in the hyphosphere decreased, but more P was transferred to the plant through the mycorrhizal hyphae. Hence, in addition, the application of the two different N forms did not significantly alter the content of plant P. The plant supplied with different N fertilizers acquired P through different mechanisms associated with other microbes. NH₄⁺ application promoted phytin mineralization by decreasing soil pH, whereas NO₃^? application increased inorganic P uptake by strengthening the mycorrhizal pathway.     

PHOTOSYNTHETIC RESPONSE OF LAKE PLANKTON TO COMBINED NITROGEN ENRICHMENT1

The complex interplay between photosynthesis and the uptake of nitrogen was investigated in samples from five lakes of different size and trophic state. When enriched with ¹⁵NH₄⁺, the photosynthetic rate was often reduced for 4–5 h in samples believed to be nitrogen deficient. This implies that energy was reallocated from photosynthesis to the uptake and assimilation of N. Stimulation in C uptake at low levels of NH₄⁺ enrichment was followed by a progressive decline with further NH₄⁺ enrichment. On other occasions when ambient NH₄⁺ was undetectable, nutrient regeneration by zooplankton supplied a significant fraction of the required nitrogen. At these times and when the plankton had sufficient available N, there usually was no change in photosynthetic rate with either NH₄⁺ or NO₃^?enrichment. Typically, little NO₃^? was taken up and no photosynthetic response was observed. On two occasions, however, the uptake of NO₃^? was significant due to high NO₃^? and low NH₄⁺ levels early in the season. At one of these times there was a reduction in photosynthesis with NO₃^? enrichment. A further complication was observed when photosynthesis decreased with NH₄⁺ enrichment but increased with NO₃^? enrichment despite negligible NO₃^? uptake. These observations illustrate that the complex metabolism of these two nitrogen sources is not fully understood. At optimum light intensity, C:N uptake ratios, even under NH₄⁺ enrichment, are only sufficient to maintain the cellular C:N ratio unless much of the fixed C is respired or excreted. Three observations suggest that photosynthesis and N uptake are not coupled, (i) Photoinhibition of C uptake, but not N uptake was observed when low light adapted populations are exposed to high light conditions, (ii) The light intensity for maximum N uptake was slightly less than that for carbon. (iii) Dark N uptake was always near 50% of the maximum rate in the light whereas the C uptake was near 2% of P_opt. Certainly, there is an interconnection because dark C uptake was enhanced by NH₄⁺ enrichment.     

Inorganic nitrogen uptake kinetics of sugarcane (Saccharum spp.) varieties under in vitro conditions with varying N supply

An in vitro system was established for the characterisation of inorganic nitrogen uptake by sugarcane plantlets of variety NCo376. After multiplication and rooting, plantlets (0.27–0.3 g fresh mass) were placed on N-free medium for 4 days, and then supplied with 2–20 mM N as NO₃ ^?-N only, NH₄ ⁺-N only or NO₃ ^?-N + NH₄ ⁺-N (as 1:1). With few exceptions, on all the tested N media, the in vitro plants always had a higher V_max for NH₄ ⁺-N (28.69–66.51 μmol g^?1 h^?1) than for NO₃ ^?-N uptake (10.24–30.19 μmol g^?1 h^?1) and the K_m indicated a higher affinity for NO₃ ^?-N (0.02–7.38 mM) than for NH₄ ⁺-N (0.06–9.15 mM). When N was applied as 4 and 20 mM to varieties N12, N19 and N36, the interaction between variety, N form and concentration resulted in differences in the V_max and K_m. The high N-use efficient varieties (N12 and N19), as determined in previous pot and field trials, behaved similarly under all tested conditions and displayed a lower V_max and K_m than the low N-use efficient ones (NCo376 and N36). Based on this finding, it was suggested that the N-use efficient designation (from pot and field trials) may not be ascribed solely to N uptake. Assessment of the relative preference index (RPI) for NO₃ ^?-N and NH₄ ⁺-N uptake revealed that, at present, the RPI has no application in sugarcane due to its preferential uptake of NH₄ ⁺-N.     

Segregation of nitrogen use between ammonium and nitrate of ectomycorrhizas and beech trees

Here, we characterized nitrogen (N) uptake of beech (Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH₄⁺ and NO₃^?. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. ¹⁵N uptake was studied under various field conditions after short‐term and long‐term exposure to a pulse of equimolar NH₄⁺ and NO₃^? concentrations, where one compound was replaced by ¹⁵N. In native EM assemblages, long‐term and short‐term ¹⁵N uptake from NH₄⁺ was higher than that from NO₃^?, regardless of season, water availability and site exposure, whereas in beech long‐term ¹⁵N uptake from NO₃^? was higher than that from NH₄⁺. The transfer rates from the EM to beech were lower for ¹⁵N from NH₄⁺ than from NO₃^?. ¹⁵N content in EM was correlated with ¹⁵N uptake of the host for ¹⁵NH₄⁺, but not for ¹⁵NO₃^?‐derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH₄⁺ than from NO₃^?. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability.     

CHANGES IN INTRACELLULAR NITROGEN POOLS AND FEEDBACK CONTROLS ON NITROGEN UPTAKE IN CHAETOMORPHA LINUM (CHLOROPHYTA)1

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH₄⁺ and NO₃^?) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO₃^? > NH₄⁺ > chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletion period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within 10 days. Upon N resupply, the RON pool increased 3 × faster than either the inorganic or protein pools, suggesting that protein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for both NH₄⁺ and NO₃^? varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO₃^? showed an initial lag phase, but the initial uptake of NH₄⁺ was enhanced and was present only when the intracellular NH₄⁺ pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool size and maximum assimilation uptake rates for both NH₄⁺ and NO₃^? suggested a feedback control on assimilation uptake by the buildup and depletion of organic compounds. Enhanced uptake and the accumulation of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demand in C. linum.     

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.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).     

PHYSIOLOGICAL ACCLIMATION OF MARINE PHYTOPLANKTON TO DIFFERENT NITROGEN SOURCES1

We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH₄⁺, NO₃^?, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO₃^? and NH₄⁺ grown cells had similar growth rates and C and X quotas. Therefore, NO₃^? grown cells used up to 48% more energy than NH₄⁺ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a^?1, we suggest that NO₃^?, grown cells do not compensate for the greater energy requirements of NO₃^? reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO₃^?, compared to NH₄⁺ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh₄⁺ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO₃^?, or urea.