Insights into the growth response and nitrogen accumulation and use efficiency of the Poaceae grass <Emphasis Type="Italic">Brachypodium distachyon</Emphasis> to high nitrogen availability

Optimization of nitrogen (N) use by grasses is a central issue of the current work. The effects of different N concentrations (0, 0.25, 0.5, 1.0, 2.5, and 5.0 mM NH₄NO₃) on growth of Brachypodium distachyon were assessed on controlled hydroponic culture. Maximal growth (132% of control) was obtained at 0.5 mM NH₄NO₃, critical N level, and was maintained at higher N concentrations. The highest N level (5.0 mM) has a similar effect on growth as 0.5 mM NH₄NO₃. It has no significant effects on water status, and total and reduced N contents in shoots while, increased those in roots, compared to plants receiving 0.5 mM NH₄NO₃. The high N availability, however, increased nitrate contents in shoots and roots by 3- and 20-folds, respectively, compared to those of plant receiving 0.5 mM NH₄NO₃. In addition, high N availability reduced the nitrogen use efficiency (NUE) by 18% compared to that of plant receiving only 0.5 NH₄NO₃. In view of B. distachyon productivity and environmental concerns, it is concluded that the critical level of N application should be 0.5 mM NH₄NO₃ and the excess fertilization led to a high nitrate accumulation.     

Plasma membrane H+-ATPase in sorghum roots as affected by potassium deficiency and nitrogen sources

We studied the influence of inorganic nitrogen sources (NO₃ ^? or NH₄ ⁺) and potassium deficiency on expression and activity of plasma membrane (PM) H⁺-ATPase in sorghum roots. After 15 d of cultivation at 0.2 mM K⁺, the plants were transferred to solutions lacking K⁺ for 2 d. Then, K⁺ depletion assays were performed in the presence or absence of vanadate. Further, PMs from K⁺-starved roots were extracted and used for the kinetic characterization of ATP hydrolytic activity and the immunodetection of PM H⁺-ATPase. Two major genes coding PM H⁺-ATPase (SBA1 and SBA2) were analyzed by real-time PCR. PM H⁺-ATPase exhibited a higher V_max and K_m in NH₄ ⁺-fed roots compared with NO₃ ^? -fed roots. The optimum pH of the enzyme was slightly lower in NO₃ ^? -fed roots than in NH₄ ⁺-fed roots. The vanadate sensitivity was similar. The expressions of SBA1 and SBA2 increased in roots grown under NH₄ ⁺. Concomitantly, an increased content of the enzyme in PM was observed. The initial rate of K⁺ uptake did not differ between plants grown with NO₃ ^? or NH₄ ⁺, but it was significantly reduced by vanadate in NH₄ ⁺-grown plants.     

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 ⁺ .     

Alteration of nitrogen metabolism in rice variety 'Nipponbare' induced by alkali stress

Aims

Alkali stress (AS) is an important agricultural contaminant and has complex effects on plant metabolism, specifically root physiology. The aim of this study was to test the role of nitrogen metabolism regulation in alkali tolerance of rice variety 'Nipponbare'.

Methods

In this study, the rice seedlings were subjected to salinity stress (SS) or AS. Growth, the contents of inorganic ions, NH ₄ ⁺ -nitrogen (free amino acids), and NO ₃ ^? -nitrogen in the stressed seedlings were then measured. The expression of some critical genes involved in nitrogen metabolism were also assayed to test their roles in the regulation of nitrogen metabolism during adaptation of rice variety 'Nipponbare' to AS.

Results

AS showed a stronger inhibiting effect on rice variety 'Nipponbare' growth than SS. AS may have more complex effects on nitrogen metabolism than SS.

Conclusions

Effects of AS on the nitrogen metabolism of rice variety 'Nipponbare' mainly comprised two mechanisms. Firstly, in roots, AS caused the reduction of NO ₃ ^? content, which caused two harmful consequences, the large downregulation of OsNR1 expression and the subsequent reduction of NH ₄ ⁺ production in roots. On the other hand, under AS (pH, 9.11), almost all the NH ₄ ⁺ was changed to NH₃, which caused a severe deficiency of NH ₄ ⁺ surrounding the roots. Both events might cause a severe deficiency of NH ₄ ⁺ in roots. Under AS, the increased expression of several OsAMT family members in roots might be an adaptative response to the reduction of NH ₄ ⁺ content in roots or the NH ₄ ⁺ deficiency in rhizosphere. Also, the down-regulation of OsNADH-GOGAT and OsGS1;2 in roots might be due to NH ₄ ⁺ deficiency in roots. Secondly, in shoots, AS caused a larger acuumulatiuon of Na⁺, which possibly affected photorespiration and led to a continuous decrease of NH ₄ ⁺ production in shoots, and inhibited the expression of OsFd-GOGAT and OsGS2 in chloroplasts.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

Effect of form and level of applied nitrogen on nitrogenase and nitrate reductase activities in faba beans

The effects of nitrogen applied at increasing levels of 0, 4, 8, 16 and 32 mM N (KNO₃ or NH₄Cl) were studied in faba bean (Vicia faba) nodulated byRhizobium leguminosarum bv.viceae RCR lool. Nitrogenase activity was higher at 4 and 8 mM N than the zero N treatment (control), but 16 and 32 mM N significantly reduced the efficiency of nodule functions. Nitrate reductase activities (NRA) of leaves, stems, roots, nodules and nodule fractions (bacteroid and cytosol) were increased with rising the NO₃ ^? or NH₄ ⁺ levels. NRA decreased in the order of nodules>leaves>stems>roots. Cytosolic NR was markedly higher than that recorded in the bacteroid fractions. Nitrate levels were linearly correlated to NRA of nodules. Accumulation of NO₂ ^? within nodules suggests that NO₂ ^? inhibits nodule’s activity after feeding plants with NO₃ ^? or NH₄ ⁺.     

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.     

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.     

Comparison of nitrogen uptake in the roots and rhizomes of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Leymus chinensis (Trin.) Tzvel is a rhizomatous grass species in the Eastern Eurasian steppe zone that is often limited by low soil nitrogen availability. Although a previous study showed that the rhizomes of L. chinensis have the capacity to take up nitrogen, the importance of such uptake for nitrogen nutrition is unclear. Moreover, little is known regarding the inorganic nitrogen uptake kinetics of roots and rhizomes in response to nitrogen status. Here, we first found that ammonium is preferred over nitrate and glycine for L. chinensis growth. Using the ¹⁵N-labelling method, we found that the rate of ion influx into roots was approximately five-fold higher than into rhizomes under the same nitrogen content, and the ion influxes into roots and rhizomes under 0.05 mM N were greater than in the presence of 3 mM N, especially in the form of NH₄⁺. Using a non-invasive micro-test technique, we characterised the patterns of NH₄⁺ and NO₃^– fluxes in the root mature zone, root tip, rhizome mature zone, and rhizome tip following incubation in the solution with different N compounds and different N concentrations. These results suggest a dynamic balance between the uptake, utilisation, and excretion of nitrogen in L. chinensis.     

Nitrogen uptake and allocation in <Emphasis Type="Italic">Populus simonii</Emphasis> in different seasons supplied with isotopically labeled ammonium or nitrate

Key message

The total uptake of ¹⁵ NO ₃ -N was twofold higher than that of ¹⁵ NH ₄ -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO ₃ -N had good growth with high photosynthetic rate and total biomass.

Abstract

Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of ¹⁵NH₄-N, ¹⁵NO₃-N, ¹⁵NH₄NO₃, and NH ₄ ¹⁵ NO₃ in May, July, and September. The total ¹⁵N uptake was twofold higher when supplied with sole ¹⁵NO₃-N compared to sole ¹⁵NH₄-N in all the investigated seasons. In the presence of ammonium nitrate (¹⁵NH₄NO₃ and NH ₄ ¹⁵ NO₃), the total ¹⁵N uptake was two times higher when supplied with NH ₄ ¹⁵ NO₃ compared to ¹⁵NH₄NO₃. Per unit biomass, the ¹⁵N-uptake ability of fine roots was higher in May and July compared to that in September. ¹⁵N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH₄-N, NO₃-N, and NH₄NO₃ for 8 weeks. The seedlings fertilized with NO₃-N had good growth with high photosynthetic rate and total biomass indicating that NO₃-N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.

     

Positive feedback between acidification and organic phosphate mineralization in the rhizosphere of maize (Zea mays L.)

To test the hypothesis that rhizosphere acidification would enhance the hydrolyzation of organic phosphates by increasing phosphatase activity. A Petri dish experiment with sterile agar and a pot experiment with a low P soil were used. In the Petri dish experiment, roots of each plant were cultured in two compartments, each of which contained agar with one of three nitrogen combinations: NH ₄ ⁺ /N0 (N0 = nitrogen free), NH ₄ ⁺ /NO ₃ ^- , and NO ₃ ^- /N0. Phytin was supplied as the sole phosphorus (P) source to all compartments. In the pot experiment, the soil in each pot was treated with N0, KNO₃, or (NH₄)₂SO₄) together with 0 or 75 mg kg^?1 phytin-P. Dry weight, P concentration, and P content of roots were highest in the NH ₄ ⁺ compartments in the Petri dish experiment. In the pot experiment, dry weight, P concentration, and P content of both shoots and roots were higher with NH ₄ ⁺ than with NO ₃ ^- . NH ₄ ⁺ treatments reduced rhizosphere pH, promoted the hydrolization of phytin, enhanced acid phosphatase activity in the rhizosphere, and increased phytin-P utilization relative to N0 and NO ₃ ^- treatments. Phosphatase activity was negatively correlated with rhizosphere pH but was positively correlated with plant P content in both experiments. Rhizosphere acidification optimized the activity of acid phosphatase excreted by maize roots and promoted phytin mineralization. NH ₄ ⁺ -induced acidification in the maize rhizosphere improved the growth of maize roots by improving P uptake from phytin; the improved growth, in turn, increased NH ₄ ⁺ uptake and acidification.     

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.     

Nitrogen use efficiency (NUE) in rice links to NH4 + toxicity and futile NH4 + cycling in roots

Aims

Rice is known as an ammonium (NH₄ ⁺)-tolerant species. Nevertheless, rice can suffer NH₄ ⁺ toxicity, and excessive use of nitrogen (N) fertilizer has raised NH₄ ⁺ in many paddy soils to levels that reduce vegetative biomass and yield. Examining whether thresholds of NH₄ ⁺ toxicity in rice are related to nitrogen-use efficiency (NUE) is the aim of this study.

Methods

A high-NUE (Wuyunjing 23, W23) and a low-NUE (Guidan 4, GD) rice cultivar were cultivated hydroponically, and growth, root morphology, total N and NH₄ ⁺ concentration, root oxygen consumption, and transmembrane NH₄ ⁺ fluxes in the root meristem and elongation zones were determined.

Results

We show that W23 possesses greater capacity to resist NH₄ ⁺ toxicity, while GD is more susceptible. We furthermore show that tissue NH₄ ⁺ accumulation and futile NH₄ ⁺ cycling across the root-cell plasma membrane, previously linked to inhibited plant development under elevated NH₄ ⁺, are more pronounced in GD. NH₄ ⁺ efflux in the root elongation zone, measured by SIET, was nearly sevenfold greater in GD than in W23, and this was coupled to strongly stimulated root respiration. In both cultivars, root growth was affected more severely by high NH₄ ⁺ than shoot growth. High NH₄ ⁺ mainly inhibited the development of total root length and root area, while the formation of lateral roots was unaffected.

Conclusions

It is concluded that the larger degree of seedling growth inhibition in low- vs. high-NUE rice genotypes is associated with significantly enhanced NH₄ ⁺ cycling and tissue accumulation in the elongation zone of the root.     

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.     

Culture medium optimization for improved puerarin production by cell suspension cultures of Pueraria tuberosa (Roxb. ex Willd.) DC.

The effects of carbon, nitrogen, phosphate, and copper on cell growth and production of the isoflavone puerarin by suspension cultures of Pueraria tuberosa (Roxb. ex. Willd.) DC were investigated. Among the various sugars evaluated (glucose, galactose, fructose, maltose, and sucrose), use of sucrose in the medium led to the maximum accumulation of puerarin. A sucrose-feeding strategy in which additional sucrose was added to the flasks 15?d into the culture cycle stimulated both cell biomass and puerarin production. The maximum production of puerarin was obtained when a concentration balance of 20:60?mM NH ₄ ⁺ /NO ₃ ^? was used as the nitrogen source. Alteration in the concentration balance of nitrogen components (NH ₄ ⁺ /NO ₃ ^? 60:20?mM) or the use of either NH ₄ ⁺ or NO ₃ ^? alone decreased biomass production and puerarin accumulation compared with the control culture (NH ₄ ⁺ /NO ₃ ^? 20:20?mM). High amounts of phosphate (2.5 and 5?mM) in the medium inhibited puerarin production whereas 0.625?mM phosphate promoted puerarin production (68.3???g/g DW on day?25). An increase in Cu²⁺ concentration from 0.025 to 0.05?mg/l in the P. tuberosa cell culture medium resulted in a 2.2-fold increase in puerarin production (up to 141???g/g DW on day?25) but reduced cell culture biomass.     

Involvement of phosphoenolpyruvate carboxylase and antioxydants enzymes in nitrogen nutrition tolerance in <Emphasis Type="Italic">Sorghum bicolor</Emphasis> plants

Plants of Sorghum bicolor (C₄ species) were grown at different nitrate or ammonium concentrations (0.5, 5, 20 and 50 mM) in order to examine the effect of nitrogen nutrition on growth, phosphoenolpyruvate carboxylase (PEPC) and antioxidant enzymes activities in both roots and leaves of 30-day-old plants. At high NO₃^? levels (20 and 50 mM) the fresh weight was significantly higher. When the nitrogen source was in ammonium form, the leaf and root mass increased drastically at low concentration 5 mM and significantly at 20 mM, however similar fresh weight was found at high level of ammonium (50 mM). The leaves catalase (CAT), guaiacol peroxidase (POD), glutathione reductase (GR), and glutathione S-transferase (GST) activities and the roots glutathione reductase and glutathione S-transferase activities were significantly higher in the NH₄⁺-fed plants than those grown in the nitrate medium. Activity and proteins levels of phosphoenolpyruvate carboxylase in both leaves and roots of sorghum plants were increased progressively with increasing external nitrogen concentration. This increase was more pronounced at high level of ammonium (50 mM), being 2-fold at 50 mM of NO₃^? and 3-fold at 50 mM of NH₄⁺. Our results suggested that antioxidant enzymes activities and PEPC play a key role in ammonium detoxification and tolerance in sorghum plants.