Tropical bloom-forming mesoalgae Cladophoropsis sp. and Laurencia sp.-responses to ammonium enrichment and a simulated heatwave

Algal blooms are increasing worldwide, driven by elevated nutrient inputs. However, it is still unknown how tropical benthic algae will respond to heatwaves, which are expected to be more frequent under global warming. In the present study, a multifactorial experiment was carried out to investigate the potential synergistic effects of increased ammonium inputs (25 μM, control at 2.5 μM) and a heatwave (31°C, control at 25°C) on the growth and physiology (e.g., ammonium uptake, nutrient assimilation, photosynthetic performance, and pigment concentrations) of two bloom-forming algal species, Cladophoropsis sp. and Laurencia sp. Both algae positively responded to elevated ammonium concentrations with higher growth and chlorophyll a and lutein concentrations. Increased temperature was generally a less important driver, interacting with elevated ammonium by decreasing the algaes' %N content and N:P ratios. Interestingly, this stress response was not captured by the photosynthetic yield (Fv/Fm) nor by the carbon assimilation (%C), which increased for both algae at higher temperatures. The negative effects of higher temperature were, however, buffered by nutrient inputs, showing an antagonistic response in the combined treatment for the concentration of VAZ (violaxanthin, antheraxanthin, zeaxanthin) and thalli growth. Ammonium uptake was initially higher for Cladophoropsis sp. and increased for Laurencia sp. over experimental time, showing an acclimation capacity even in a short time interval. This experiment shows that both algae benefited from increased ammonium pulses and were able to overcome the otherwise detrimental stress of increasingly emerging temperature anomalies, which provide them a strong competitive advantage and might support their further expansions in tropical marine systems.

Genome-wide identification, expression profiling, and functional analysis of ammonium transporter 2 (AMT2) gene family in cassava (Manihot esculenta crantz)

Background: Nitrogen (N), absorbed primarily as ammonium (NH₄ ⁺) from soil by plant, is a necessary macronutrient in plant growth and development. Ammonium transporter (AMT) plays a vital role in the absorption and transport of ammonium (NH₄ ⁺). Cassava (Manihot esculenta Crantz) has a strong adaptability to nitrogen deprivation. However, little is known about the functions of ammonium transporter AMT2 in cassava. Methods: The cassava AMT2-type genes were identified and their characteristics were analyzed using bioinformatic techniques. The spatial expression patterns were analyzed based on the public RNA-seq data and their expression profiles under low ammonium treatment were studied using Real-time quantitative PCR (RT-qPCR) method. The cassava AMT2 genes were transformed into yeast mutant strain TM31019b by PEG/LiAc method to investigate their functions. Results: Seven AMT2-type genes (MeAMT2.1-2.7) were identified in cassava and they were distributed on 6 chromosomes and included two segmental duplication events (MeAMT2.2/MeAMT2.4 and MeAMT2.3/MeAMT2.5). Based on their amino acid sequences, seven MeAMT2 were further divided into four subgroups, and each subgroup contained similar motif constitution and protein structure. Synteny analysis showed that two and four MeAMT2 genes in cassava were collinear with those in the Arabidopsis and soybean genomes, respectively. Sixteen types of cis-elements were identified in the MeAMT2 promoters, and they were related to light-, hormone-, stress-, and plant growth and development-responsive elements, respectively. Most of the MeAMT2 genes displayed tissue-specific expression patterns according to the RNA-seq data, of them, three MeAMT2 (MeAMT2.3, MeAMT2.5, and MeATM2.6) expressions were up-regulated under ammonium deficiency. Complementation experiments showed that yeast mutant strain TM31019b transformed with MeAMT2.3, MeAMT2.5, or MeATM2.6 grew better than untransgenic yeast cells under ammonium deficiency, suggesting that MeAMT2.3, MeAMT2.5, and MeATM2.6 might be the main contributors in response to ammonium deficiency in cassava. Conclusion: This study provides a basis for further study of nitrogen efficient utilization in cassava.

Functional characterization of the sugarcane (Saccharum spp.) ammonium transporter AMT2;1 suggests a role in ammonium root-to-shoot translocation

AMMONIUM TRANSPORTER/METHYLAMMONIUM PERMEASE/RHESUS (AMT) family members transport ammonium across membranes in all life domains. Plant AMTs can be categorized into AMT1 and AMT2 subfamilies. Functional studies of AMTs, particularly AMT1-type, have been conducted using model plants but little is known about the function of AMTs from crops. Sugarcane (Saccharum spp.) is a major bioenergy crop that requires heavy nitrogen fertilization but depends on a low carbon-footprint for competitive sustainability. Here, we identified and functionally characterized sugarcane ScAMT2;1 by complementing ammonium uptake-defective mutants of Saccharomyces cerevisiae and Arabidopsis thaliana. Reporter gene driven by the ScAMT2;1 promoter in A. thaliana revealed preferential expression in the shoot vasculature and root endodermis/pericycle according to nitrogen availability and source. Arabidopsis quadruple mutant plants expressing ScAMT2;1 driven by the CaMV35S promoter or by a sugarcane endogenous promoter produced significantly more biomass than mutant plants when grown in NH₄ ⁺ and showed more ¹⁵N-ammonium uptake by roots and nitrogen translocation to shoots. In A. thaliana, ScAMT2;1 displayed a K_m of 90.17 µM and V_max of 338.99 µmoles h^-1 g^-1 root DW. Altogether, our results suggest that ScAMT2;1 is a functional high-affinity ammonium transporter that might contribute to ammonium uptake and presumably to root-to-shoot translocation under high NH₄ ⁺ conditions.

The Above-Ground Part of Submerged Macrophytes Plays an Important Role in Ammonium Utilization

As a paradoxical nutrient in water ecosystems, ammonium can promote plants growth under moderate concentration, but excess of it causes phytotoxic effects. Previous research has revealed that glutamate dehydrogenase in the above-ground part of submerged macrophytes plays an important role in ammonium detoxification. However, the strategies of ammonium utilization at the whole plant level of submerged macrophytes are still unclear and the role of the above-ground part in nutrient utilization has not been clearly elucidated in previous studies, hence, directly influencing the application of previous theory to practice. In the present research, we combined the methods of isotopic labeling and enzyme estimation to investigate strategies of ammonium utilization by the submerged macrophytes. The results showed that when [NH4 +-N] was 50 mg L-1, 15N taken up through the above-ground parts was 13.24 and 17.52 mg g-1 DW, while that of the below-ground parts was 4.24 and 8.54 mg g-1 DW in Potamogeton lucens and Myriophyllum spicatum, respectively. The ratios of 15N acropetal translocation to uptake were 25.75 and 35.69%, while those of basipetal translocation to uptake were 1.93 and 4.09% in P. lucens and M. spicatum, respectively. Our results indicated that the above-ground part was not only the main part for ammonium uptake, but also the major pool of exogenous ammonium. Besides, the dose-response curve of GDH (increased by 20.9 and 50.2% under 15 and 50 mg L-1 [NH4 +-N], respectively) exhibited by the above-ground parts of M. spicatum indicates that it is the main site for ammonium assimilation of the tolerant species. This study identifies the ammonium utilization strategy of submerged macrophytes and reveals the important role of the above-ground part in nutrient utilization providing new insight into the researches of nutrient utilization by plants and theoretical supports for water restoration by phytoremediation.

Three polarly localized ammonium transporter 1 members are cooperatively responsible for ammonium uptake in rice under low ammonium condition

Ammonium is a preferential nitrogen form for rice (Oryza sativa) grown in paddy field, but the molecular mechanisms for ammonium uptake have not been well understood. We functionally characterized three members belonging to ammonium transporter 1 (AMT1) and investigated their contributions to ammonium uptake. Spatial expression analysis showed that the upregulated expression of OsAMT1;1 and OsAMT1;2 and downregulated expression of OsAMT1;3 by ammonium were higher in the root mature region than in the root tips. All OsAMT1 members were polarly localized at the distal side of exodermis in the mature region of crown roots and lateral roots. Upon exposure to ammonium, localization of OsAMT1;1 and OsAMT1;2 was also observed in the endoplasmic reticulum, but their abundance in the plasma membrane was not changed. Single knockout of either gene did not affect ammonium uptake, but knockout of all three genes resulted in 95% reduction of ammonium uptake. However, the nitrogen uptake did not differ between the wild-type rice and triple mutants at high ammonium and nitrate supply. Our results indicate that three OsAMT1 members are cooperatively required for uptake of low ammonium in rice roots and that they undergo a distinct regulatory mechanism in response to ammonium.

Elimination of GlnKAmtB affects serine biosynthesis and improves growth and stress tolerance of Escherichia coli under nutrient-rich conditions

Nitrogen is a most important nutrient resource for Escherichia coli and other bacteria that harbor the glnKamtB operon, a high-affinity ammonium uptake system highly interconnected with cellular metabolism. Although this system confers an advantage to bacteria when growing under nitrogen-limiting conditions, little is known about the impact of these genes on microbial fitness under nutrient-rich conditions. Here, the genetically tractable E. coli BW25113 strain and its glnKamtB-null mutant (JW0441) were used to analyze the impact of GlnK-AmtB on growth rates and oxidative stress tolerance. Strain JW0441 showed a shorter initial lag phase, higher growth rate, higher citrate synthase activity, higher oxidative stress tolerance and lower expression of serA than strain BW25113 under nutrient-rich conditions, suggesting a fitness cost to increase metabolic plasticity associated with serine metabolism. The overexpression of serA in strain JW0441 resulted in a decreased growth rate and stress tolerance in nutrient-rich conditions similar to that of strain BW25113, suggesting that the negative influence on bacterial fitness imposed by GlnK-AmtB can be traced to the control of serine biosynthesis. Finally, we discuss the potential applications of glnKamtB mutants in bioproduction processes.

BZR1 Regulates Brassinosteroid-Mediated Activation of AMT1;2 in Rice

Although it is known that brassinosteroids (BRs) play pleiotropic roles in plant growth and development, their roles in plant nutrient uptake remain unknown. Here, we hypothesized that BRs directly regulate ammonium uptake by activating the expression of rice AMT1-type genes. Exogenous BR treatment upregulated both AMT1;1 and AMT1;2 expression, while this induction was impaired in the BR-receptor gene BRI1 mutant d61-1. We then focused on brassinazole-resistant 1 (BZR1), a central hub of the BR signaling pathway, demonstrating the important role of this signaling pathway in regulating AMT1 expression and rice roots NH₄ ⁺ uptake. The results showed that BR-induced expression of AMT1;2 was suppressed in BZR1 RNAi plants but was increased in bzr1-D, a gain-of-function BZR1 mutant. Further EMSA and ChIP analyses showed that BZR1 bound directly to the BRRE motif located in the promoter region of AMT1;2. Moreover, cellular ammonium contents, ¹⁵NH₄ ⁺ uptake, and the regulatory effect of methyl-ammonium on root growth are strongly dependent on the levels of BZR1. Overexpression lines of BRI1 and BZR1 and Genetic combination of them mutants showed that BZR1 activates AMT1;2 expression downstream of BRI1. In conclusion, the findings suggest that BRs regulation of NH4⁺ uptake in rice involves transcription regulation of ammonium transporters.

Gene Expression of Haloferax volcanii on Intermediate and Abundant Sources of Fixed Nitrogen

Haloferax volcanii, a well-developed model archaeon for genomic, transcriptomic, and proteomic analyses, can grow on a defined medium of abundant and intermediate levels of fixed nitrogen. Here we report a global profiling of gene expression of H. volcanii grown on ammonium as an abundant source of fixed nitrogen compared to l-alanine, the latter of which exemplifies an intermediate source of nitrogen that can be obtained from dead cells in natural habitats. By comparing the two growth conditions, 30 genes were found to be differentially expressed, including 16 genes associated with amino acid metabolism and transport. The gene expression profiles contributed to mapping ammonium and l-alanine usage with respect to transporters and metabolic pathways. In addition, conserved DNA motifs were identified in the putative promoter regions and transcription factors were found to be in synteny with the differentially expressed genes, leading us to propose regulons of transcriptionally co-regulated operons. This study provides insight to how H. volcanii responds to and utilizes intermediate vs. abundant sources of fixed nitrogen for growth, with implications for conserved functions in related halophilic archaea.

Ammonium and nitrate regulate NH4+ uptake activity of Arabidopsis ammonium transporter AtAMT1;3 via phosphorylation at multiple C-terminal sites

In plants, nutrient transporters require tight regulation to ensure optimal uptake in complex environments. The activities of many nutrient transporters are post-translationally regulated by reversible phosphorylation, allowing rapid adaptation to variable environmental conditions. Here, we show that the Arabidopsis root epidermis-expressed ammonium transporter AtAMT1;3 was dynamically (de-)phosphorylated at multiple sites in the cytosolic C-terminal region (CTR) responding to ammonium and nitrate signals. Under ammonium resupply rapid phosphorylation of a Thr residue (T464) in the conserved part of the CTR (CTRC) effectively inhibited AtAMT1;3-dependent NH4+ uptake. Moreover, phosphorylation of Thr (T494), one of three phosphorylation sites in the non-conserved part of the CTR (CRTNC), moderately decreased the NH4+ transport activity of AtAMT1;3, as deduced from functional analysis of phospho-mimic mutants in yeast, oocytes, and transgenic Arabidopsis. Double phospho-mutants indicated a role of T494 in fine-tuning the NH4+ transport activity when T464 was non-phosphorylated. Transient dephosphorylation of T494 with nitrate resupply closely paralleled a transient increase in ammonium uptake. These results suggest that T464 phosphorylation at the CTRC acts as a prime switch to prevent excess ammonium influx, while T494 phosphorylation at the CTRNC fine tunes ammonium uptake in response to nitrate. This provides a sophisticated regulatory mechanism for plant ammonium transporters to achieve optimal ammonium uptake in response to various nitrogen forms.

The Signal Transduction Protein PII Controls Ammonium, Nitrate and Urea Uptake in Cyanobacteria

P_II signal transduction proteins are widely spread among all domains of life where they regulate a multitude of carbon and nitrogen metabolism related processes. Non-diazotrophic cyanobacteria can utilize a high variety of organic and inorganic nitrogen sources. In recent years, several physiological studies indicated an involvement of the cyanobacterial P_II protein in regulation of ammonium, nitrate/nitrite, and cyanate uptake. However, direct interaction of P_II has not been demonstrated so far. In this study, we used biochemical, molecular genetic and physiological approaches to demonstrate that P_II regulates all relevant nitrogen uptake systems in Synechocystis sp. strain PCC 6803: P_II controls ammonium uptake by interacting with the Amt1 ammonium permease, probably similar to the known regulation of E. coli ammonium permease AmtB by the P_II homolog GlnK. We could further clarify that P_II mediates the ammonium- and dark-induced inhibition of nitrate uptake by interacting with the NrtC and NrtD subunits of the nitrate/nitrite transporter NrtABCD. We further identified the ABC-type urea transporter UrtABCDE as novel P_II target. P_II interacts with the UrtE subunit without involving the standard interaction surface of P_II interactions. The deregulation of urea uptake in a P_II deletion mutant causes ammonium excretion when urea is provided as nitrogen source. Furthermore, the urea hydrolyzing urease enzyme complex appears to be coupled to urea uptake. Overall, this study underlines the great importance of the P_II signal transduction protein in the regulation of nitrogen utilization in cyanobacteria.

Lack of ACTPK1, an STY kinase, enhances ammonium uptake and use, and promotes growth of rice seedlings under sufficient external ammonium

Ammonium influx into plant roots via the high-affinity transport system (HATS) is down-modulated under elevated external ammonium, preventing ammonium toxicity. In ammonium-fed Arabidopsis, ammonium transporter 1 (AMT1) trimers responsible for HATS activity are allosterically inactivated in a dose-dependent manner via phosphorylation of the conserved threonine at the carboxyl-tail by the calcineurin B-like protein 1-calcineurin B-like protein-interacting protein kinase 23 complex and other yet unidentified protein kinases. Using transcriptome and reverse genetics in ammonium-preferring rice, we revealed the role of the serine/threonine/tyrosine protein kinase gene OsACTPK1 in down-modulation of HATS under sufficient ammonium. In wild-type roots, ACTPK1 mRNA and protein accumulated dose-dependently under sufficient ammonium. To determine the function of ACTPK1, two independent mutants lacking ACTPK1 were produced by retrotransposon Tos17 insertion. Compared with segregants lacking insertions, the two mutants showed decreased root growth and increased shoot growth under 1 mm ammonium due to enhanced ammonium acquisition, via aberrantly high HATS activity, and use. Furthermore, introduction of OsACTPK1 cDNA fused to the synthetic green fluorescence protein under its own promoter complemented growth and the HATS influx, and suggested plasma membrane localization. Root cellular expression of OsACTPK1 also overlapped with that of ammonium-induced OsAMT1;1 and OsAMT1;2. Meanwhile, threonine-phosphorylated AMT1 levels were substantially decreased in roots of ACTPK1-deficient mutants grown under sufficient ammonium. Bimolecular fluorescence complementation assay further confirmed interaction between ACTPK1 and AMT1;2 at the cell plasma membrane. Overall, these findings suggest that ACTPK1 directly phosphorylates and inactivates AMT1;2 in rice seedling roots under sufficient ammonium.

Interactions between nitrate and ammonium in their uptake, allocation, assimilation, and signaling in plants

Nitrogen (N) availability is a major factor determining plant growth and productivity. Plants acquire inorganic N from the soil, mainly in the form of nitrate and ammonium. To date, researchers have focused on these N sources, and demonstrated that plants exhibit elaborate responses at both physiological and morphological levels. Mixtures of nitrate and ammonium are beneficial in terms of plant growth, as compared to nitrate or ammonium alone, and therefore synergistic responses to both N sources are predicted at different steps ranging from acquisition to assimilation. In this review, we summarize interactions between nitrate and ammonium with respect to uptake, allocation, assimilation, and signaling. Given that cultivated land often contains both nitrate and ammonium, a better understanding of the synergism between these N sources should help to identify targets with the potential to improve crop productivity.

Impact of streambed morphology on the abundance and activity of ammonia-oxidizing bacteria

Ammonia oxidizers catalyze the first step of nitrification. Combined microbial nitrification-denitrification activities are essential for the removal of excess nitrogen from water bodies. In sandy streambeds, bed form structures are created by water flow and lead to the creation of heterogeneous microenvironments. The objective of this study, therefore, was to investigate the effect of bed form morphology on the abundance and activity of ammonia-oxidizing bacteria (AOB) within a benthic biofilm. An 8-month-old benthic biofilm was established in a recirculating laboratory flume under controlled flow conditions and frequent amendment with ammonium. The sand bed was arranged into bed form structures. The highest concentrations of chlorophyll a (indicative of algae) were measured on the upstream side of the bed forms. The biofilm was dominated by Nitrosospira species, and amoA gene abundance was higher on the downstream sides of the bed forms with no significant difference in oxygen consumption between the upstream and downstream sections of the bed form. In contrast, potential ammonium oxidation rates were higher on the upstream sides of the bed forms. The results suggest that bed form morphology can affect the spatial distribution and activity of AOB, possibly through the creation of distinct microhabitats. These results contribute to our understanding of nitrogen transformations and removal from streams.

AtAMT1;4, a pollen-specific high-affinity ammonium transporter of the plasma membrane in Arabidopsis

Pollen represents an important nitrogen sink in flowers to ensure pollen viability. Since pollen cells are symplasmically isolated during maturation and germination, membrane transporters are required for nitrogen import across the pollen plasma membrane. This study describes the characterization of the ammonium transporter AtAMT1;4, a so far uncharacterized member of the Arabidopsis AMT1 family, which is suggested to be involved in transporting ammonium into pollen. The AtAMT1;4 gene encodes a functional ammonium transporter when heterologously expressed in yeast or when overexpressed in Arabidopsis roots. Concentration-dependent analysis of (15)N-labeled ammonium influx into roots of AtAMT1;4-transformed plants allowed characterization of AtAMT1;4 as a high-affinity transporter with a K(m) of 17 microM. RNA and protein gel blot analysis showed expression of AtAMT1;4 in flowers, and promoter-gene fusions to the green fluorescent protein (GFP) further defined its exclusive expression in pollen grains and pollen tubes. The AtAMT1;4 protein appeared to be localized to the plasma membrane as indicated by protein gel blot analysis of plasma membrane-enriched membrane fractions and by visualization of GFP-tagged AtAMT1;4 protein in pollen grains and pollen tubes. However, no phenotype related to pollen function could be observed in a transposon-tagged line, in which AtAMT1;4 expression is disrupted. These results suggest that AtAMT1;4 mediates ammonium uptake across the plasma membrane of pollen to contribute to nitrogen nutrition of pollen via ammonium uptake or retrieval.

Effects of a stay-green mutation on plant nitrogen relations in Lolium perenne during N starvation and after defoliation

The stay-green mutation of the nuclear gene sid results in inhibition of chlorophyll degradation during leaf senescence in grasses, reducing N remobilization from senescing leaves. Effects on growth of Lolium perenne L. were investigated during N starvation (over 18 d) and after severe defoliation, when leaf growth depends on the remobilization of internal N. Rates of dry mater production, partitioning between shoots and roots, and re-partitioning of N from shoots to roots were very similar in stay-green and normal plants under N starvation. Km and Vmax for net uptake of NH4+ were also similar for both genotypes, and Vmax increased with the duration of N deprivation. The mutation had little effect on recovery of leaf growth following severe defoliation, but stay-green plants recommenced NO3- and K+ uptake 1 d later than normal plants. Import of remobilized N into new leaves was generally similar in both lines. However, stay-green plants remobilized less N from stubble compared with normal plants. It was concluded that the sid locus stay-green mutation has no significant adverse effect on the growth of L perenne during N starvation, or recovery from severe defoliation when plants are grown under an optimal regime of NO3- supply both before and after defoliation. The absence of any effect on leaf dry matter production implies that the difference in foliar N availability attributable to this mutation has little bearing on productivity, at least in the short to medium term.

INFLUENCE OF UV-B RADIATION ON NITROGEN UTILIZATION BY A NATURAL ASSEMBLAGE OF PHYTOPLANKTON

A 7-day mesocosm experiment was conducted in July 1996 to investigate the effects of ambient UV-B radiation (UVBR) exclusion and two UVBR enhancements above ambient levels on NO₃^- , NH₄⁺ and urea utilization in a natural plankton community (<240 μm) from the Lower St. Lawrence Estuary. The phytoplankton community was dominated by diatoms during the first 3 days and, afterward, by flagellates and dinoflagellates. The results of 4-h incubations just below the water surface show that, compared with ambient UVBR conditions, UVBR exclusion generally increased NO₃^- , NH₄⁺ , and urea uptakes. During the last 4 days of the experiment, the percent increase in the specific uptake rate of urea under excluded UVBR conditions varied between 17% and 130% and was a linear function of the ambient UVBR dose removed. During the first 3 days, the phytoplankton community dominated by diatoms was able to withstand UVBR enhancements without any perceptible effect on nitrogen uptake. However, during the post-diatom bloom period, UVBR enhancements resulted in decreases in NO₃^- , NH₄⁺ , and urea uptake compared with ambient UVBR conditions. The reduction of urea uptake under UVBR enhancements during the last 3 days varied between 23% and 64% and was linearly related to the enhanced UVBR dose. However, the different UVBR treatments did not affect the internal organic nitrogen composition (internal urea, free amino acids, and proteins) of the phytoplankton community experiencing vertical mixing in the mesocosms. The discrepancy between short-term uptake measurements at the surface and long-term effects in the mesocosms emphasizes the importance of vertical mixing on UVBR effects in natural ecosystems. This suggests that an increase in ambient UVBR would have a minimal effect on nitrogen utilization by natural phytoplankton assemblages if these are vertically mixed.

Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N ('Höglwald', Germany). In addition, uptake experiments were carried out under controlled conditions with young spruce and beech trees grown at normal N supply. In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site. Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15°C, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16% and 11%, respectively, of maximum uptake at 25°C. By contrast, net uptake of ammonium at 10°C reached 73% and 31% of the maximum uptake for spruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate∶ammonium ratios found in the soil solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and Gln in the roots correlated with a decrease in net uptake of nitrate. The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.

Nitrate and ammonium uptake by maize: adaptation during relief from nitrogen suppression

Initial (0-1 h) net rates of nitrate and ammonium uptake from 200 μM NH₄ NO₃ were progressively increased as 8-d-old maize (Zea mays L.) seedlings, grown on 5 mM nitrate, were exposed to nitrogen-free solutions for up to 48 h. Further nitrogen deprivation to 72 h resulted in a decline in the nitrate uptake rate. Nitrate uptake rates of plants at all stages of nitrogen deprivation increased steadily during an 8 h exposure to 200 μM NH₄ NO₃ . The pattern of the response of ammonium uptake during the 8 h adaptation period was considerably different. In nitrogen-replete plants the ammonium uptake rate increased steadily, but deprivation of nitrogen for 12 h and longer resulted in complex responses in which the initial rate was followed by a decline, a subsequent increase, and another decline. The responses of the nitrate uptake system are considered to reflect a lifting of the suppressive effects of nitrate and a product of nitrate assimilation during nitrogen deprivation, a concomitant degradation of an induced component of the nitrate uptake system during that time, and reinduction of the uptake system during the adaptation period. The responses of the ammonium uptake system are considered to reflect the interplay of suppression by a product of ammonium assimilation, the accumulation of root ammonium and associated ammonium efflux, and a stimulation by ammonium of its own uptake. As a consequence of the differential responses of the two uptake systems, nitrate and ammonium uptake rates were positively correlated, largely independent, or negatively correlated as the plants progressed through the 8 h adaptation period.

VESICULAR-ARBUSCULAR MYCORRHIZA IMPROVE BOTH SYMBIOTIC N2 FIXATION AND N UPTAKE FROM SOIL AS ASSESSED WITH A 15 N TECHNIQUE UNDER FIELD CONDITIONS

A technique using ¹⁵ N-labelled inorganic fertilizer was applied to estimate N₂ fixation by the forage legume Hedysarum coronarium L. and to ascertain the role of vesicular-arbuscular (VA) mycorrhizas in plant N nutrition throughout a growing season under field conditions. The absence of the specific Rhizobium for the forage legume in the test soil allowed us the use of ¹⁵ N methodology with the same legume as reference 'non-fixing' crop. At the first harvest, mycorrhizal inoculation behaved similarly to the phosphate addition in improving the percentage (70 %) and the total amount of N derived from fixation. But thereafter, mycorrhizal inoculation not only enhanced dry matter yield, N concentration and total N yield but also the amount of N derived from soil and from fixation, as compared with either phosphate-added or control plants. This indicated that mycorrhizas acted both by a P-mediated mechanism to improve N₂ fixation and by enhancing N uptake from soil. The latter agrees with recent findings by others that VA mycorrhizal hyphae can translocate and assimilate ammonium, a fact of physiological and ecological interest.