Ammonium uptake capacity and response of cytosolic glutamine synthetase 1;2 to ammonium supply are key factors for the adaptation of ammonium nutrition in Arabidopsis thaliana

Plant requires nitrogen for the growth, and it use nitrate and ammonium from the environment. Plant suffers from the toxicity when excess ammonium is supplied as a sole nitrogen, although it could be a good nitrogen source for plant growth. We hypothesized that the different responses of ecotypes to ammonium nutrient could partly account for the adaptation of Arabidopsis to an ammonium environment. The purpose of this study is to understand the different responses of ecotypes in ammonium environment. The growth of Arabidopsis thaliana ecotypes, Columbia was compared to those of Arabidopsis thaliana ecotypes, Landsberg erecta in ammonium nutrient. The ratio of shoot dry weight to root dry weight was compared to evaluate the adaptation of two ecotypes. The shoot:root ratio of Landsberg was significantly higher than that of Columbia. T-DNA insertion in cytosolic glutamine synthetase 1;2, one of the essential ammonium assimilatory enzymes, led a decrease of shoot:root ratio. We also measured the isotope-labeled ammonium uptake and the expression levels of ammonium transporter genes, and also the expression of ammonium assimilatory genes, glutamine synthetase genes and glutamate synthase genes, in roots after ammonium re-supply using real-time polymerase chain reaction analysis. We found that (1) ammonium uptake of Landsberg erecta was higher than that of Columbia, when ammonium was supplied at higher concentration, and (2) cytosolic glutamine synthetase 1;2 was highly increased by ammonium supply in the root of Landsberg erecta. The present study suggested the importance of these two factors for adaptation of Arabidopsis to an ammonium-rich environment.     

Resistance of Medicago truncatula to salt stress is related to glutamine synthetase activity and sodium sequestration

The response to salinity may largely vary not only among species but even cultivars or lines of the same species. Knowledge of the reasons underlying these differential responses can be critical in breeding programs to obtain lines with enhanced performance under salinity. In this work, the responses to salt stress of three Medicago truncatula lines with contrasting salt resistance, TN6.18 (sensitive), Jemalong (reference line), and TN8.20 (resistant), have been assessed by analyzing a full array of nodule parameters (water relations, carbohydrates, ion concentrations, and enzyme activities). The aim of this work was to look for the most important criteria conferring resistance to the M. truncatula‐Sinorhizobium symbiosis under salinity. The resistance of M. truncatula to salt stress was related to nodule osmotic adjustment due to both sequestration of sodium and accumulation of soluble carbohydrates and free amino acids following protein degradation, together with an adequate nitrogen metabolism due to maintaining relatively high glutamine synthetase activity (GS) and stimulation of NADH‐dependent glutamate dehydrogenase (GDH). Glutamine synthetase activity differed clearly between the three studied lines. Thus, it may have a key role in the resistance of Medicago truncatula to salt stress.     

Enzymes of Nitrogen Metabolism and Proteases Activity in Hairy Roots of Clover and Carrots,with and without Arbuscular Mycorrhizal Fungi

Hairy roots can be used for metabolic studies and also as a substrate for arbuscular mycorrhizal fungi (AMF). However, little is known about the behavior of these roots when infected with AMF. The metabolism of hairy roots grown with or without AMF, on two culture media, was investigated. An increase in protease activity was observed when the roots were cultured on medium with high nitrogen (N)-content. Glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) activities increased along the development in both carrot and clover hairy roots. With the advancement of root senescence (at 132 d) a greater degradation of root tissue occurred, which was characterized by an increase in catabolic enzymes activity (proteases and GDH deamination). The results indicate that the glutamine synthetase/GOGAT pathway, characteristic of plant tissues, ceases to act in synchronism and is replaced by the GDH/GOGAT system under conditions of stress or senescence when excess ammonium is present.     

Response of wheat and maize to different nitrogen sources: II. Nitrate reductase and glutamine synthetase enzyme activities,and plastid pigment content

Glutamine synthetase and nitrate reductase enzyme activities occurred both in roots and leaves of maize (Zea mays L., hybrid Pioneer 3737) and wheat (Triticum aestivum L., cultivar Jantar) plants grown on different nitrogen (N) sources. Enzyme activities and plastid pigment content in maize plants were higher in the treatments with a mixture of nitrate (NO₃) and ammonium (NH₄) than with either N source alone. In wheat plants, plastid pigment content, nitrate reductase activity, and root glutamine snynthetase activity were higher in the treatments where NO₃ alone was applied to the nutrient medium.     

Nitrogen Assimilatory Enzymes and Amino Acid Content in Inoculated Foliar Fertilized Pea Plants Grown at Reduced Molybdenum Concentration

ABSTRACT

A possibility to improve nitrogen assimilation in nitrogen fixing Molybdenum (Mo) deficient pea plants was shown. The influence of foliar supplied nutrients in addition to root nutrition resulted in reducing the unfavorable effects of inorganic nitrogen on nodule function and Mo deficiency on the nitrogen assimilatory enzymes. Inoculated pea plants were grown on liquid nutrient solution both with and without Mo. The following variants were tested: Mo supplied plants with root nutrition (F1 + Mo); Mo supplied plants with root and foliar nutrition (F2 + Mo); Mo deficient plants with root nutrition (F1 ? Mo); and Mo deficient plants with root and foliar nutrition (F2 ? Mo). Foliar application of nutrients had a positive effect on the glutamine synthetase and glutamate synthase enzyme activities in the roots and nodules of Mo deficient plants. It was found that the foliar fertilization reduced the inhibitory effect of Mo shortage on the aspartate/asparagine content in the pea shoots.     

Do reactive oxygen species (ROS) induced by NaCl contribute to ammonium accumulation in Spartina alterniflora?

Growth, activity of antioxidant enzymes viz. glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and guaiacol peroxidase (GPX), and some metabolic processes related to ammonium metabolism were investigated in a salt‐tolerant Spatina alterniflora. In comparison to 0 mM–NaCl treatment, growth of S. alterniflora plant increased significantly at 200 mM NaCl, but was highly inhibited at 500 mM NaCl. Ammonium concentration in the leaves and roots increased 2.1–3.4 times when plants were treated with 500 mM NaCl. Under 200 mM NaCl, antioxidant‐enzyme activities increased, however, at 500 mM the antioxidant system was unable to compensate reactive oxygen species induced by NaCl. At this high level of salinity, ammonium production through nitrate reductase (NR) was inhibited, but no significant changes in the activities of glutamine synthetase (GS) or glutamate dehydrogenase (GDH) were found. We conclude that the accumulation of ammonium under high salt stress was not due to inhibition of the assimilatory activities of GS or GDH. Ammonia accumulation under high salinity may result from amino acid and protein catabolism activated by reactive oxygen species (ROS) and/or a lack of carbon skeletons to incorporate ammonium into organic molecules due to a decrease in photosynthetic activity in salt‐stressed plants.     

麦套花生氮素代谢及相关酶活性变化研究

大田条件下,以花生“花育22号”为材料,研究了麦套花生的氮素代谢及相关酶活性变化情况。结果表明,麦套花生根叶游离氨基酸、氮素平均含量及根系可溶性蛋白平均含量高于单作;而叶片可溶性蛋白平均含量则低于单作。与小麦共生期间,麦套花生根叶硝酸还原酶（NRase）活性、谷氨酸脱氢酶（GDH）活性、叶片谷氨酰胺合成酶（GS）活性及谷氨酸-丙酮酸转氨酶（GPT）活性（除播后25 d）明显低于单作;整个生育期麦套花生根系GS平均活性及GPT活性高于单作花生。可见,花生苗期小麦遮荫对花生氮素代谢及酶活性有一定影响。     

氮素不同形态配比对菠菜体内游离氨基酸含量和相关酶活性的影响

采用溶液培养试验,研究了氮素不同形态配比对菠菜茎叶中游离氨基酸含量及3种主要氮代谢酶活性的影响。结果表明:1)随着营养液中铵硝比(NH4+-N/NO3--N)的降低,菠菜茎叶中游离氨基酸的总量呈下降趋势。在全硝营养下(NH4+-N/NO3--N=0∶100)下,菠菜茎叶中游离氨基酸的总量只有全铵营养(NH4+-N/NO3--N=100∶0)的34.4%。2)在全铵营养下,菠菜茎叶中游离氨基酸的主要组分是谷氨酰胺、精氨酸和谷氨酸,三者占游离氨基酸总量的百分比依次为39.8%、20.2%和8.9%;在全硝营养下,菠菜茎叶中游离氨基酸以谷氨酸、天冬氨酸和丝氨酸为主,三者占游离氨基酸总量的百分比分别为30.3%1、8.6%和8.5%。3)提高营养液中硝态氮的比例,可以显着提高菠菜茎叶中硝酸还原酶(NR)的活性,同时降低了谷氨酸脱氢酶(GDH)的活性,谷氨酰胺合成酶(GS)活性则呈现先升后降的抛物线状变化规律。4)菠菜茎叶中NR活性与谷胺酰胺含量之间存在着显著负相关关系(r=-0.968)。     

Maize growth and ammonium assimilation enzyme activity in response to nitrogen forms and pH control 1

Plant growth, glutamine synthetase and glutamine dehydrogenase activities of two maize genotypes were compared in the presence of NH₄ ⁺ and NO₃ ^‐ forms of N in sand culture. Ammonium reduced growth of the P3732 genotype 64% and the B73 x Mol7 hybrid 59% as compared to NO₃ ^‐. Both glutamine synthetase and glutamate dehydrogenase activities in roots tended to be higher with NH₄ as compared to NO₃. As the pH in the medium was increased by adding CaCO₃, glutamine synthetase and glutamate dehydrogenase activities in roots of both genotypes were reduced; however, glutamine synthetase activity in leaves of NH^‐treated plants increased at the higher pH of the growing medium.     

Ammonium assimilation in rice based on the occurrence of 15N and inhibition of glutamine synthetase activity

Assimilation of ammonium (NH₄) into free amino acids and total reduced nitrogen (N) was monitored in both roots and shoots of two‐week old rice seedlings supplied with 5 mM 99% (¹⁵NH₄)₂SO₄ in aerated hydroponic culture with or without a 2 h preincubation with 1 mM methionine sulfoximine (MSX), an inhibitor of glutamine synthetase (GS) activity. ¹⁵NH₄ was not assimilated into amino acids when the GS/GOGAT (glutamate synthase) cycle was inhibited by MSX. Inhibition of glutamine synthetase (GS) activity in roots with MSX increased both the amount of NH₄ and the abundance of ¹⁵N labeled NH₄. In contrast, the amount of Gln and Glu, and their proportions as ¹⁵N, decreased in roots when GS activity was inhibited. This research confirms the importance of GS/GOGAT in NH₄ assimilation in rice roots.

¹⁵N‐labeled studies indicate that NH₄ ions incorporated by roots of rice are transformed primarily into glutamine (Gln) and glutamic acid (Glu) before being converted to other amino acids through transamination (15). The formation of amino acids such as aspartic acid (Asp) and alanine (Ala) directly from free NH₄ in roots also has been reported (4,15). Translocation of free NH₄ to plant shoots, based on the concentration of free NH₄ in xylem exudate, has been reported in tomato (13), although NH₄ in shoots primarily originates from nitrate reduction in the shoot. Photorespiration also can contribute to the accumulation of NH₄ in leaves (7).

The GS/GOGAT cycle appears to be primarily responsible for the assimilation of exogenously supplied NH₄ and NH₄ derived from nitrate reduction in leaves, as well as NH₄ derived from photorespiration (2,3,6,8). Genetic evidence cited to support this conclusion includes the lethal effect of photorespiratory conditions on plant mutants deficient in chloroplast‐localized GS and GOGAT activities (2,3,9), and the rapid accumulation of free NH₄ in GS‐deficient mutants under photorespiratory conditions (2,3,5).

The present study was initiated to quantify the in vivo amino acid synthesis in rice roots and shoots by analysis of ¹⁵N labeling, and should provide a more complete understanding of this important system for NH₄ utilization.     

Regulation of mycorrhiza development in durum wheat by P fertilization: Effect on plant nitrogen metabolism

The aim of this work was to study the effect of arbuscular mycorrhizal fungus Glomus mosseae on growth and nitrogen (N) metabolism of durum wheat (Tritcum durum) under various P soil contents. The analyses were extended to macro and micronutrient tissue concentrations, nitrate reductase and glutamine synthetase activities, as well as protein, aminoacids, pyridine dinucleotides and adenine nucleotides. Arbuscular mycorrhiza increased wheat growth in soil in which P availability was low and nitrate was the dominant N form. The root colonization occurred at the highest level in plants grown in limiting soil P and was inversely related to soil P content. The micorrhizal wheat plants contained also the highest concentrations of macro (P, K, Ca, N) and micronutrients (Fe, Zn, Mn) as well as free amino acids, protein, NAD, NADP, AMP, ADP, ATP in roots and leaves. In particular, the micronutrient tissue concentrations (Zn, Mn) supported that mycorrhiza actively modulated their uptake limiting interferences and optimizing growth better than the plant roots, like a very efficient “rootstock”. Control plants grown at the highest soil P did not reach the same concentration as the mycorrhizal plants. Nitrate reductase activities in the roots of mycorrhizal plants were higher than in the control ones, while glutamine synthetase activities were highest in the leaves. Protein and amino acids concentrations, as well as AMP, ADP, ATP, NAD(P), and NAD(P)H were also higher than in the control. Among the free amino acids in the roots, the high levels of glutamine, asparagine, arginine, support the view that ammonium was transferred through the arbuscules to the root cells where it was re‐assimilated in the cortical cells, forming high N : C ratio‐amino acids. They were transferred to the leaves where all the other N compounds could be largely synthesized using the carbon skeletons supplied by photosynthesis.     

低氮胁迫下拟南芥低氮耐性突变体lnt1的氮代谢特征

A growth experiment on agar medium and a hydroponics experiment were carried out to study the nitrogen (N) metabolism of a low-N tolerant mutant (lnt1) of Arabidopsis thaliana under different N levels as compared with the wildtype (WT) Arabidopsis. On the agar medium, no apparent growth differences were observed between the lnt1 and WT plants under a normal N level of 9 mmol L^-1 NO₃^-. However, under a low N level of 0. 18 mmol L^-1 NO₃^-, the growth of the WT plants was greatly retarded, while the lnt1 plants were not affected by low-N stress and showed similar growth with those grown under a normal N level. In the hydroponics experiment, the lnt1 mutant had higher activities of glutamine synthetase (GS) and NADH-dependent glutamate synthase (NADH-GOGAT) in both leaves and roots under N-deficient conditions. Moreover, they accumulated less ammonium (NH₄⁺) but more free amino acids in leaves compared with the WT plants. These observations suggest that better N assimilation might contribute to the low-N tolerant phenotype of the lnt1 mutant.     

Carbon skeletons for amide synthesis during ammonium nutrition in tomato and wheat roots

Heavy application of ammonium nitrogen to plant roots results in the conversion of ammonium nitrogen to the nitrogen of amides, glutamine, and asparagine, which are stored in roots or translocated to shoot. Since the net synthesis of such amides requires the supply of corresponding carbon skeletons, the carbon metabolism in amide synthesis in response to ammonium supply was investigated in tomato and wheat roots. The content of major primary amino acids was determined in tomato and wheat roots during a 4-d period of ammonium nutrition after 1-d culture in nitrogen-free nutrient solution. Ammonium supply led to a continuous increase in the asparagine content in wheat roots, whereas in tomato roots, the glutamine content increased 1 d after ammonium supply and thereafter the glutamine content was higher than the asparagine content. The amounts of amino acids synthesized from glucose-¹⁴C increased while the amounts of organic acids decreased in tomato roots by the supply of ammonium nitrogen for 1 d, compared to the roots that did not receive nitrogen. In tomato roots, the proportion of labeled glutamine was higher than that of labeled asparagine and the C5 amino acids were more strongly labeled than the C4 amino acids. These findings were different from the previous ones in wheat roots where the proportion of asparagine was found to be extremely high (Koga and Ikeda 2000: J. Fac. Agr. Kyushu Univ45, 7–13). To examine the in vivo asparagine synthesis, aspartate-'4C was fed to the roots. The labeling of asparagine, which was the most strongly labeled amino acid among the free amino acids, was remarkably strong in wheat roots whereas the labeling of glutamine was also pronounced in tomato roots. These results indicate that the ability to replenish carbon skeletons for amide synthesis in ammonium nutrition is different between tomato and wheat roots.     

不同氮素形态比例条件下接种AMF对玉米氮同化关键酶的影响

以珍珠岩为基质,通过供应3种不同的NH4+∶NO3ˉ比例营养液,研究了接种丛枝菌根真菌对玉米氮同化关键酶活性的影响。结果看出,与不接种的玉米植株相比,接种Glomus intraradices和Glomus mosseae分别在NH4+∶NO3ˉ=3∶1和NH4+∶NO3ˉ=1∶3形态下提高了植物叶片的硝酸还原酶活性;接种AMF对叶片谷氨酰胺合成酶活性（GS）影响不大,但在NH4+∶NO3ˉ=3∶1形态下接种3种AMF处理均显著提高了根系GS活性,相对提高了铵态氮在地下部的同化比例。在铵态氮比例较高时,接种AMF的促生效应较好,且AMF提高根系GS活性作用较大。表明丛枝菌根真菌在促进宿主植物对铵态氮的利用作用较大。     

Changes in growth and nitrogen assimilation in barley seedlings under cadmium stress

Barley (Hordeum vulgare L. cv. Martin) plants grown in solution culture, were exposed to increasing cadmium (Cd) concentration (0, 5, 10, 25, 50, and 100 μM) for a duration of 12 days. The sequence of important biochemical steps of nitrate (NO₃) assimilation were studied in roots and shoots as a function of external Cd concentration. Cadmium uptake in roots and shoots increased gradually with Cd concentration in the medium. This Cd accumulation lowered substantially root and shoot biomass. The nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.6.6.4) activities declined under Cd stress. Concurrently, tissue NO₃ contents and xylem sap NO₃ concentration were also decreased in Cd‐treated plants. These results suggest that Cd could exert an inhibitory effect on the assimilatory NO₃ reducing system (NR and NiR) through a restriction of NO₃ availability in the tissues. We therefore examined, in short‐term experiments (12 h), the impact of Cd on NO₃ uptake and the two reductases in nitrogen (N)‐starved plants that were pretreated or not with Cd. It was found that Cd induced inhibition of both NO₃ uptake and activities of NR and NiR, during NO₃ induction period. The possible mechanisms of Cd action on NO₃ uptake are proposed. Further, in Cd‐grown plants, the glutamine synthetase (GS, EC 6.3.1.2) showed a decreasing activity both in shoots and roots. However, increasing external Cd concentration resulted in a marked enhancement of glutamate dehydrogenase (NADH‐GDH, EC 1.4.1.2) activity, coupled with elevated levels of ammonium (NH₄ in tissues. On the other hand, the total protein content in Cd‐treated plants declined with a progressive and substantial increase of protease activity in the tissues. These findings indicate that under Cd stress the usual pathway of NH₄ assimilation (glutamine synthetase/glutamate synthase) can switch to an alternative one (glutamate dehydrogenase). The changes in all parameters investigated were concentration‐dependent and more marked in roots than shoots. The regulation of N absorption and assimilation by Cd in relation to growth and adaptation to stress conditions are discussed.     

Provision of carbon skeletons for amide synthesis in non-nodulated soybean and pea roots in response to the source of nitrogen supply

Abstract

Soluble amino acids in roots and primary amino acids, which were involved in primary ammonium assimilation, in the metabolites of ¹⁴C-glucose fed to roots for 3 h in the dark were analyzed in the roots of non-nodulated soybean and pea plants grown in ammonium, nitrate or nitrogen-free media for 1 day. Compared with the effect of nitrate, ammonium supply strongly affected the content and synthesis of the amino acids in the roots. In both soybean and pea roots, the supply of ammonium increased considerably the concentrations of the primary amino acids, and asparagine was the most predominant amide, followed by glutamine. In nitrate-supplied soybean roots, the concentrations of asparagine, aspartate and alanine increased, but the concentration of glutamine was low. In the roots of pea plants grown in nitrate media, asparagine was the predominant amino acid, although the composition of the primary amino acids was little affected by nitrate supply. The proportion of amino acids synthesized from ¹⁴C-glucose increased and asparagine rather than glutamine was predominantly synthesized in ammonium-supplied soybean and pea roots, whereas in nitrate-supplied roots asparagine was more actively synthesized than glutamine, although asparagine was not predominant. The ratio of C4 (asparagine + aspartate) to C5 (glutamine + glutamate) amino acids was twofold higher in ammonium-supplied and nitrate-supplied soybean roots than in roots receiving no nitrogen. In contrast, in pea roots, the C4/C5 ratio was twofold higher only in ammonium nutrition. The results obtained suggest that the roots of leguminous plants might possess an indigenous ability to provide a carbon skeleton for preferential synthesis of asparagine rather than glutamine with a high intensity of ammonium supply.     

氮饥饿水稻利用不同形态氮素的差异及其生理机制

通过水培试验，研究了氮饥饿7d后，恢复供应不同形态氮源对水稻氮吸收和积累及氮同化中关键酶活性和光合色素的影响。结果表明，缺氮促进根系生长，增加根冠比。恢复供氮4d显著增加地上部生物量。铵硝混合营养促进了水稻对氮的吸收和转运，叶片和根系中全氮及叶片中铵态氮的含量以硝酸铵处理最高。与单一铵或硝营养相比，铵硝混合营养增强了根系的谷氨酰胺合成酶和叶片中硝酸还原酶的活性，提高了水稻同化和利用氮的能力。另外，与纯硝营养相比，供应铵态氮显著增加了叶片中总叶绿素，尤其是叶绿素a的含量。因此，改善水肥管理、平衡对水稻供氮的铵硝配比将提高水稻氮素的吸收和利用效率。     

Wheat growth promotion through inoculation with an ammonium-excreting mutant of <Emphasis Type="Italic">Azospirillum brasilense</Emphasis>

Azospirillum brasilense is a diazotrophic bacterium and one of the best studied plant-growth-promoting bacterium living in close association with several agronomically important crops. The production of plant-growth-regulating substances is a main mechanism of plant growth stimulation, although other mechanisms have also been proposed. Nitrogen transfer from the bacterium to the plant is one among the other possible mechanisms of plant growth stimulation. In this study, we investigated, by means of a greenhouse trial with winter wheat inoculation, the effect of a point mutation in the ammonium binding site of the A. brasilense glutamine synthetase. The glutamine synthetase is one of the main ammonium-assimilating enzymes and mutations in this enzyme generally result in the release of ammonium from the bacterium to its environment. The ammonium-excreting mutant used in this study was shown to perform better than the wild-type A. brasilense strain with respect to wheat growth parameters and yield. In the greenhouse conditions used, this effect was independent of the way fertilizer was applied. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Determining potential glutamine synthetase and glutamate dehydrogenase activity in soil

Ammonium (NH₄⁺), an important nitrogen (N) source for microorganisms, is assimilated via two major pathways. One route is catalyzed by glutamate dehydrogenase (GDH), while the other mechanism involves two enzymes, glutamine synthetase (GS) and glutamate synthase (GOGAT). The GS/GOGAT enzyme system requires more energy to operate, but has a much higher affinity for NH₄⁺ than GDH. We describe procedures to determine potential GS and GDH activity in soil samples. GS and GDH are intracellular enzymes. We used chloroform fumigation to make cell membranes permeable for substrates and products of the enzymes. Fumigation for 4 h increased GS activity almost ten-fold compared to the unfumigated control. Under optimized assay conditions, GS activity increased linearly for at least 80 min, indicating that the substrates were not limiting. In contrast to what was found for GS activity, direct addition of substrates to the soil to assay GDH activity did not result in a linear increase in GDH activity over time. A linear response for 3 h, however, resulted when the soil samples were first extracted with buffer solution and the reagents were added after centrifugation. The differences between the assays explain why fumigation for 3 d prior to the assay increased GDH activity by only 60%. In a microcosm study with glucose and NH₄⁺ addition, the activity of the two enzymes depended on the carbon (C) to N ratio of the amendment. With increasing C to N ratios from 5 to 120, GS activity doubled, while C to N ratios higher than 120 did not further increase GS activity. In contrast, GDH activity decreased by 13% with increasing C to N ratios from 5 to 200. The GDH to GS activity ratio in soil may therefore yield valuable information about the availability of N relative to C at a specific time.     

Ammonium Enables Aluminum-Induced Stimulation of Nitrogen Assimilation in Roots of Al-Tolerant Maize Genotypes

One aluminum (Al)-sensitive (B-73) and two Al-tolerant (F-2 and L-2039) maize genotypes were subjected to Al stress (100 μM Al) under two nitrogen (N) treatments [13.2 mM nitrate (NO₃^?) and 8.3 mM NO₃^? + 4.9 mM ammonium (NH₄⁺)]. Growth parameters, chlorophyll, root N and NO₃^? contents, root nicotinamide adenine dinucleotide (NADH-) and nicotinamide adenine dinucleotide phosphate (NADPH)-nitrate reductase, glutamine synthetase, and glutamate dehydrogenase activities were determined. Aluminum significantly decreased growth and chlorophyll content in Al-sensitive genotype. Nitrate accumulation in roots was increased by Al in tolerant plants. In the sensitive genotype, Al suppressed all enzymes in NO₃^? plants, while in NO₃^?/NH₄⁺ plants the suppression was less severe, and NADPH-nitrate reductase was even stimulated. In tolerant NO₃^?plants, glutamate dehydrogenase was stimulated in F-2 and glutamine synthetase suppressed in L-2039 genotype. In tolerant NO₃^?/NH₄⁺- plants, all enzymes were stimulated by Al, which may be attributed to their participation in defense mechanisms.