Bottle gourd rootstock-grafting affects nitrogen metabolism in NaCl-stressed watermelon leaves and enhances short-term salt tolerance

The plant growth, nitrogen absorption, and assimilation in watermelon (Citrullus lanatus [Thunb.] Mansf.) were investigated in self-grafted and grafted seedlings using the salt-tolerant bottle gourd rootstock Chaofeng Kangshengwang (Lagenaria siceraria Standl.) exposed to 100 mM NaCl for 3 d. The biomass and NO₃⁻ uptake rate were significantly increased by rootstock while these values were remarkably decreased by salt stress. However, compared with self-grafted plants, rootstock-grafted plants showed higher salt tolerance with higher biomass and NO₃⁻ uptake rate under salt stress. Salinity induced strong accumulation of nitrate, ammonium and protein contents and a significant decrease of nitrogen content and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT) in leaves of self-grafted seedlings. In contrast, salt stress caused a remarkable decrease in nitrate content and the activities of GS and GOGAT, and a significant increase of ammonium, protein, and nitrogen contents and NR activity, in leaves of rootstock-grafted seedlings. Compared with that of self-grafted seedlings, the ammonium content in leaves of rootstock-grafted seedlings was much lower under salt stress. Glutamate dehydrogenase (GDH) activity was notably enhanced in leaves of rootstock-grafted seedlings, whereas it was significantly inhibited in leaves of self-grafted seedlings, under salinity stress. Three GDH isozymes were isolated by native gel electrophoresis and their expressions were greatly enhanced in leaves of rootstock-grafted seedlings than those of self-grafted seedlings under both normal and salt-stress conditions. These results indicated that the salt tolerance of rootstock-grafted seedlings might (be enhanced) owing to the higher nitrogen absorption and the higher activities of enzymes for nitrogen assimilation induced by the rootstock. Furthermore, the detoxification of ammonium by GDH when the GS/GOGAT pathway was inhibited under salt stress might play an important role in the release of salt stress in rootstock-grafted seedlings.     

Development of nitrogen-assimilating enzymes in sunflower cotyledons during germination as affected by the exogenous nitrogen source

Activities of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.3) were measured in cotyledons of sunflower (Helianthus annuus L. cv Peredovic) seedlings during germination and early growth under various external nitrogen sources. The presence of NO ₃ ^- in the medium promoted a gradual increase in the levels of NR and NiR activities during the first 7 d of germination. Neither NR nor NiR activities were increased in a nitrogen-free medium or in media with either NH ₄ ⁺ or urea as nitrogen sources. Moreover, the presence of NH ₄ ⁺ did not abolish the NO ₃ ^- -dependent appearance of NR and NiR activities. The increase of NR activity was impaired both by cycloheximide and chloramphenicol, which indicates that both cytoplasmic 80S and plastidic 70S ribosomes are involved in the synthesis of the NR molecule. By contrast, the appearance of NiR activity was only inhibited by cycloheximide, indicating that NiR seems to be exclusively synthesized on the cytoplasmic 80S ribosomes. Glutamine-synthetase activity was also strongly increased by external NO ₃ ^- but not by NH ₄ ⁺ or urea. The appearance of GS activity was more efficiently suppressed by cycloheximide than chloramphenicol. This indicates that GS is mostly synthesized in the cytoplasm. The cotyledons of the dry seed contain high levels of GDH activity which decline during germination independently of the presence or absence of a nitrogen source. Cycloheximide, but not chloramphenicol, greatly prevented the decrease of GDH activity.Abbreviations GDH glutamate dehydrogenase - GS glutamine synthetase - NiR nitrite reductase - NR nitrate reductase     

Root and Nodule Enzymes of Ammonia Assimilation in Two Plant-Conditioned Symbiotically Ineffective Genotypes of Alfalfa (Medicago sativa L.)

Biochemical and physiological parameters associated with nitrogen metabolism were measured in nodules and roots of glasshouse-grown clones of two symbiotically ineffective alfalfa (Medicago sativa L.) genotypes supplied with either NO₃⁻ or NH₄⁺. Significant differences were observed between genotypes for nodule soluble protein concentrations and glutamine synthetase (GS) and glutamate synthase (GOGAT) specific activities, both in untreated controls and in response to applied N. Nodule soluble protein of both genotypes declined in response to applied N, while nodule GS, GOGAT, and glutamate dehydrogenase (GDH) specific activities either decreased or remained relatively constant. In contrast, no genotype differences were observed in roots for soluble protein concentrations and GS, GOGAT, and GDH specific activities, either in untreated controls or in response to applied N. Root soluble protein levels and GS and GOGAT specific activities of N-treated plants increased 2- to 4-fold within 4 days and then decreased between days 13 and 24. Root GDH specific activity of NH₄⁺-treated plants increased steadily throughout the experiment and was 50 times greater than root GS or GOGAT specific activities by day 24.     

Gamma-Aminobutyric Acid (GABA) Modulates Nitrate Concentrations and Metabolism in the Leaves of Pakchoi (<Emphasis Type="Italic">Brassica campestris</Emphasis> ssp. <Emphasis Type="Italic">chinensis</Emphasis> Makino) Treated with a Nitrogen-Rich Solution

Pakchoi plants were grown in 32 mM NO₃^? nutrient solution with or without 2.5 mM γ-aminobutyric acid (GABA) to investigate metabolite changes, gene and protein expression levels, and the activities of key enzymes related to nitrate metabolism in the leaves over a period of 0–12 days. High-nitrogen treatment enhanced plant growth and the NO₃^?, NO₂^?, NH₄⁺, Gln, and Glu contents in the leaves; promoted the gene and protein expression of nitrate reductase (NR) and glutamate decarboxylase (GAD); and increased the activities of NR, nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and GAD. The endogenous GABA concentration in the leaves was enhanced in parallel with the increase in GAD activity. The GABA-treated leaves displayed the greatest increases in the gene and protein expression levels of NR and GAD and in the activities of NR, NiR, GS, GOGAT, and GAD. In addition, accelerated rates of nitrate reduction and assimilation were detected, and these changes occurred concurrently with the observed increases in gene or protein expression and enzyme activity. As a result, the concentrations of NH₄⁺, Gln, Glu, and endogenous GABA were significantly elevated, and the NO₃^? and NO₂^? contents were significantly decreased, in GABA-treated leaves compared with plants exposed to nitrogen-rich conditions. Our results reveal a potential positive that GABA may act as a nitrogen source to improve the plant growth and the most prominent effect of decreasing nitrate contents by accelerating NO₃^? reduction and assimilation. Exogenous GABA plays an important role in reducing the NO₃^? content of leaves, and thereby improves the ability to harvest leafy vegetables containing higher levels of endogenous GABA.     

纯化腐植酸对氮胁迫下黄瓜幼苗生长和氮代谢的影响

为研究纯化腐植酸(PHA)在不同水平氮胁迫下对黄瓜植株生长和氮代谢的影响,探明PHA对逆境胁迫的缓解作用机制,采用水培方式,选用新泰密刺为供试品种,以正常氮水平(11 mmol·L^-1 NO₃^-)为对照,进行低氮(1.0 mmol·L^-1 NO₃^-)和高氮(101 mmol·L^-1 NO₃^-)胁迫处理,研究纯化腐植酸对氮胁迫下黄瓜幼苗生长和氮代谢的影响.结果表明: 低氮和高氮胁迫均抑制了黄瓜幼苗生长,株高、茎粗、叶面积、干物质积累量均低于正常氮水平处理.施用纯化腐植酸促进了正常氮水平及低氮胁迫下黄瓜干物质积累,在高氮胁迫下差异不显著.PHA影响了黄瓜幼苗NO₃^-的吸收,呈低氮下促进、高氮下抑制吸收的趋势;PHA显著降低了低氮、高氮胁迫下根系和叶片中的铵态氮含量;与正常氮水平相比,低氮、高氮胁迫下根系和叶片的硝酸还原酶(NR)、谷氨酰胺合成酶(GS)、谷氨酸合酶(GOGAT)、谷氨酸脱氢酶(GDH)活性及根系中亚硝酸还原酶(NiR)活性均显著降低,PHA不同程度地提高了NR、NiR、GS、GOGAT、GDH活性,还提高了根系和叶片中游离氨基酸、可溶性蛋白的含量.综上,添加PHA缓解了氮胁迫对黄瓜幼苗生长的抑制作用.     

Exogenous proline significantly affects the plant growth and nitrogen assimilation enzymes activities in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis>) under salt stress

Salinity has been shown to be a major factor contributing to low nitrogen availability in plants. To verify the changes in nitrogen metabolism activity as affected by the exogenous application of proline under salt stress and its relation to salt tolerance, in vitro rice shoot apices were used as a model to study the growth performance and changes in nitrogen assimilation activities in two Malaysian rice cultivars MR 220 and MR 253. Results revealed that salt stress greatly reduced the plant height, shoot nitrate (NO₃ ^?) content, shoot glutamine synthetase (GS), and root nitrate reductase (NR) activities in both cultivars. Supplementation of proline significantly increased the plant height, number of roots, root NO₃ ^? content, root NR, and root GS activities under salt stress in both cultivars with greater enhancement in MR 253 than MR 220. The results also indicated that MR 253 possessed higher nitrite reductase (NiR) and glutamate synthase (NADH–GOGAT) activities as compared with MR 220 in all tested treatments. It was suggested that the NO₃ ^? content, NR, and GS activities played important roles in regulating nitrogen metabolism under salt stress. Taken together, it was concluded that the ability of proline in mitigating salt stress-induced damages was correlated with the changes in nitrogen assimilation activities.     

Photorespiration Process and Nitrogen Metabolism in Lettuce Plants (Lactuca sativa L.): Induced Changes in Response to Iodine Biofortification

Iodine is vital to human health, and iodine biofortification programs help improve human intake through plant consumption. There is no research on whether iodine biofortification influences basic plant physiological processes. Because nitrogen (N) uptake, utilization, and accumulation are determining factors in crop yield, the aim of this work was to establish the effect of the application of different doses (20, 40, and 80 μM) and forms of iodine (iodate [IO₃ ⁻] vs. Iodide [I⁻]) on N metabolism and photorespiration. For this study we analyzed shoot biomass and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AAT), glutamate dehydrogenase (GDH), glycolate oxidase (GO), glutamate:glyoxylate aminotransferase (GGAT), serine:glyoxylate aminotransferase (SGAT), hydroxypyruvate reductase (HR) and catalase (CAT), nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), organic and total N, amino acids, proteins, serine (ser), malate, and α-ketoglutaric acid in edible lettuce leaves. Application of I⁻ at doses of at least 40 μM reduced the foliar concentration of NO₃ ⁻ with no decrease in biomass production, which may improve the nutritional quality of lettuce plants. In contrast, the application of 80 μM of I⁻ is phytotoxic for lettuce plants, reducing the biomass, foliar concentration of organic N and NO₃ ⁻, and NR and GDH activities. HR activity is significantly inhibited with all doses of I⁻; the least inhibition was at 80 μM. This may involve a decrease in the incorporation of carbonated skeletons from photorespiration into the Calvin cycle, which may be partially associated with the biomass decrease. Finally, the application of IO₃ ⁻ increases biomass production, stimulates NO₃ ⁻ reduction and NH₄ ⁺ incorporation (GS/GOGAT), and optimizes the photorespiratory process. Hence, this appears to be the most appropriate form of iodine from an agronomic standpoint.     

Isolation of auxotrophic mutants in support of ammonia assimilation via glutamine synthetase in Methanobacterium ivanovii

Various enzymes involved in the initial metabolic pathway for ammonia assimilation by Methanobacterium ivanovii were examined. M. ivanovii showed significant activity of glutamine synthetase (GS). Glutamate synthase (GOGAT) and alanine dehydrogenase (ADH) were present, wheras, glutamate dehydrogenase (GDH) was not detected. When M. ivanovii was grown with different levels of NH ₊ ⁴ (i.e. 2, 20 or 200 mM), GS, GOGAT and ADH activities varied in response to NH ₊ ⁴ concentration. ADH was not detected at 2 mM level, but its activity increased with increased levels of NH ₊ ⁴ in the medium. Both GS and GOGAT activities increased with decreasing concentrations of NH ₊ ⁴ and were maximum when ammonia was limiting, suggesting that at low NH ₊ ⁴ levels, GS and GOGAT are responsible for ammonia assimilation and at higher NH ₊ ⁴ levels, ADH might play a role. Metabolic mutants of M. ivanovii that were auxotrophic for glutamine were obtained and analyzed for GS activity. Results indicate two categories of mutants: i) GS-deficient auxotrophic mutants and ii) GS-impaired auxotrophic mutants.Abbreviations GS Glutamine synthetase - GOGAT glutamate synthase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

Response of Ammonia Assimilation in Cucumber Seedlings to Nitrate Stress

The influence of increased nitrate concentration—14 (control) and 140 mmol L⁻¹ (T)—in hydroponic culture on ammonia assimilation in cucumber (Cucumis sativus L. cv. Xintaimici) seedlings was investigated. The results showed that NH₃ accumulation in the roots and leaves of T seedlings increased significantly, indicating that NH₃ toxicity might be involved in nitrate stress. Under control conditions, GS and GOGAT activity were much higher in the leaves than in the roots, whereas GDH activity was much higher in the roots than in the leaves. Correlation analysis showed that NH₃ concentration had a strong negative linear relationship with GDH activity in the roots but had a strong negative linear relationship with GS and GOGAT activity in the leaves. These results indicate that NH₃ might be assimilated primarily via GDH reaction in the roots and via GS/GOGAT cycle in the leaves. Short-term nitrate stress resulted in the increase of GS and GOGAT activity in the roots and GDH activity in the leaves of T seedlings, indicating possible shifts in ammonia assimilation from the normal GDH pathway to GS/GOGAT pathway in the roots and from the normal GS/GOGAT pathway to the GDH pathway in the leaves under nitrate stress, but with the increase of treatment time, GS, GOGAT, and GDH activity in the roots and leaves of T seedlings decreased possibly due to low water potential and NH₃ toxicity.     

Nitrogen metabolism of Frankia strain,Cc01, Mg+, At4 and Hr18

The composition and levels of amino acids in four Frankia strains isolated from different actinorhizal plants, were determined. Minor differences in the amino acid profiles were noted with GLN (GLU) being the major amino acid in all four strains. Enzyme actives of ammonia metabolism, GS (glutamine synthetase), GOGAT (glutamate synthetase), and GDH (glutamate dehydrogenase), were also measured. In strains At4 and Hr18, GS and GOGAT activity levels were elevated in N₂-grown cells but significant amounts of GDH activity were present in ammonia-grown cells. No GDH was detected in strain Cc01 and Mg⁺. The characters of heat-stable and heat-labile GSs were described. In N₂-fixing cells, the ATP and amino acid content was much lower, but ammonia content was higher than in NH _inf4 ^sup+ -grown cells.     

Biochar improved nodulation and nitrogen metabolism of soybean under salt stress

To investigate salt stress and biochar application effects on nodulation and nitrogen metabolism of soybeans (Glycine max cv. M7), an experiment was conducted under the control condition. The treatments comprised three biochar rates (non, 50 and 100 g kg^?1 soil) and three salinities (0, 5 and 10 dS m^?1 NaCl), with four replications of treatments. Salt stress diminished the number of nodules and their weights in the soybean roots. Nitrogen content and metabolism decreased in nodules, roots and shoots, while reducing the activity of glutamate dehydrogenase (GDH), glutamine synthetase (GS), glutamine oxoglutarate aminotransferase (GOGAT) and nitrate reductase (NR). Also, salinity brought down root and shoot weight, total plant biomass, chlorophyll content, leaf area (LA) and rubisco activity in the soybean. On the other hand, application of biochar improved nodulation, nitrogen content, rubisco activity, GDH, GS, GOGAT and NR activities in different parts of the soybean and nodules under salt stress, and consequently improved chlorophyll content, LA, root and shoot weight. Both the 50 and 100 g kg^?1 biochar rates showed similar effects in improving nitrogen metabolism and plant performance under salt stress. Generally, biochar increased nodulation and nitrogen metabolism of the soybean under saline conditions.     

Enzymes of ammonium assimilation inStreptomyces avermitilis

Glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine dehydrogenase (ADH) and alanine aminotransferase (GPT) were detected in the cell-free homogenate ofStreptomyces avermitilis grown in a defined medium containing ammonium sulfate as the only nitrogen source. At an initial NH₄ ⁺ concentration of 7.5 mmol/L, high activities of GS, GOGAT and GDH were found while that of ADH was low. The ADH activity was markedly increased at initially millimolar NH₄ ⁺ concentrations. In some characteristics of its NH₄ ⁺-assimilating system (e.g. control of some enzyme activities, the NADPH specificity of GOGAT, the presence of alanine aminotransferase),S. avermitilis differs from other known streptomycetes.     

Oscillations in the Activities of Enzymes of Nitrate Reduction and Ammonia Assimilation in Glycine max and Zea mays

The activities of glutamate dehydrogenase (GDH), glutamine synthetase (GS), and nitrate reductase (NR) and the levels of soluble protein and NO^-₃ were assayed in soybean (Glycine max [L.] Merr.) leaves over a 48-h period with the initial 24 h under a light-dark cycle (LD 16:8) followed by 24 h of continuous light (LL). Plants had been entrained for 30 days under the LD regime. Maize (Zea mays) leaves (10 days old) under a LD 15:9 cycle were assayed only for NR and nitrite reductase (NiR). Data were subjected to frequency analysis by the least squares method to determine probabilities for cosine function periods (τ's) between 10 and 30 h. NR activities for both soybean and Zea leaves had 24 h τ's with P values < 0.05 indicating circadian periodicity. GDH in soybeans had a 24-h rhythm under LD conditions which lengthened under LL conditions. The 24-h rhythm of GDH displayed maximal activity toward the end of the dark period of the LD cycle whereas the highest activity of NR was early in the light period. Total soluble protein displayed a rhythm with a best fitting τ of greater than 24 h under both LD and LL. GDH, GS, NR, NO₃, and soluble protein in soybeans and NiR in Zea, all displayed that were ultradian (10–18 h), indicating that a τ of about one half a circadian periodicity may be a common characteristic of the enzymes of primary nitrogen metabolism in higher plants. These data also demonstrate that although both NR and GDH are circadian in their activity, the 24-h rhythm may be greatly influenced by ultradian oscillations in activity.     

Nickel-induced depression of nitrogen assimilation in wheat roots

The influence of 50 and 100 μM Ni on the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine aminotransferase (AlaAT) and aspartate aminotransferase (AspAT) was studied in the wheat roots. Root fresh weight, tissue Ni, nitrate, ammonium, glutamate and protein concentrations were also determined. Exposure to Ni resulted in a marked reduction in fresh weight of the roots accompanied by a rapid accumulation of Ni in these organs. Both nitrate and ammonium contents in the root tissue were considerably enhanced by Ni stress. While protein content was not significantly influenced by Ni application, glutamate concentration was slightly reduced on the first day after treatment with the higher Ni dose. Treatment of the wheat seedlings with 100 μM Ni led to a decrease in NR activity; however, it did not alter the activation state of this enzyme. Decline in NiR activity observed after application of 100 μM Ni was more pronounced than that in NR. The activities of GS and NADH-GOGAT also showed substantial decreases in response to Ni stress with the latter being more susceptible to this metal. Starting from the fourth day, both aminating and deaminating GDH activities in the roots of the seedlings supplemented with Ni were lower in comparison to the control. While the activity of AspAT remained unaltered after Ni application that of AlaAT showed a considerable enhancement. The results indicate that exposure of the wheat seedlings to Ni resulted in a general depression of nitrogen assimilation in the roots. Increase in the glutamate-producing activity of AlaAT may suggest its involvement in supplying the wheat roots with this amino acid under Ni stress.     

Appearance of nitrate reductase, nitrite reductase and glutamine synthetase in different organs of the Scots pine (Pinus sylvestris) seedling as affected by light, nitrate and ammonium

Appearance of nitrate reductase (NR, EC 1.6.6.1–3), nitrite reductase (NiR, EC 1.7.7.1) and glutamine synthetase (GS, EC 6.3.1.2) under the control of nitrate, ammonium and light was studied in roots, hypocotyls and needles (cotyledonary whorl) of the Scots pine ( Pinus sylvestris L.) seedling. It was found that appearance of NiR was mainly controlled by nitrate whereas appearance of GS was strongly controlled by light. In principle, the NR activity level showed the same dependency on nitrate and light as that of NiR. In the root, both nitrate and ammonium had a stimulatory effect on GS activity whereas in the whorl the induction was minor. The level of NiR (NR) activity is high in the root and hypocotyl and low in the cotyledonary whorl, whereas the GS activity level per organ increases strongly from the root to the whorl. Thus, in any particular organ the operation of the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle is not closely connected to the operation of the nitrate reduction pathway. The strong control of GS/GOGAT by light and the minor sensitivity to induction by nitrate or ammonium indicate a major role of the GS/GOGAT cycle in reassimilation of endogeniously generated ammonium.     

Modulations in key enzymes of nitrogen metabolism in two high yielding genotypes of mulberry (Morus alba L.) with differential sensitivity to salt stress

Effect of salinity stress on the performance of nitrogen metabolism was studied in two high yielding genotypes of mulberry with differential sensitivity to NaCl (S1 and ATP, salt tolerant and susceptible, respectively). Three-month-old healthy mulberry plants were subjected to different regimes of NaCl stress [0.0 (control), 0.5, 1.0 and 1.5% NaCl] and leaf samples were collected on 4, 8 and 12 DAT (days after treatment) for the analysis. The activities of nitrate reductase (NR: EC 1.6.6.1), nitrite reductase (NiR: EC 1.6.6.4), protease, glutamine synthetase (GS: EC 6.3.1.2) and its accumulation pattern, glutamate synthase (GOGAT: EC 1.4.1.13), glutamate dehydrogenase (NADH-GDH: EC 1.4.1.2 and NADPH-GDH: EC 1.4.1.4), aspartate aminotransferase (AAT: EC 2.6.1.1) and alanine aminotransferase (ALAT: EC 2.6.1.2) coupled with total protein content, free amino acid level and ammonia content were studied in leaves of both genotypes of mulberry. The total protein content in leaves of both genotypes declined with progressive accumulation of free amino acid levels. Further, the decrease in protein content was less in S1 than ATP, and it was correlated with protease activity, ammonia content and accumulation of free amino acid levels. Higher free amino acid levels were registered for S1 than ATP at 1.0 and 1.5% NaCl stress and on all days of sampling. Ammonia content was increased in both genotypes and comparatively higher ammonia levels were recorded for ATP. Increased NaCl concentrations lead to a decrease in the activity of NR and NiR in both the genotypes, the decrease was more pronounced in ATP than S1. The enhanced activity of GDH (NADH and NADPH) was noticed in both genotypes, whereas the NADPH-GDH activity was found relatively higher in S1. The immunoblot analysis with GS-45 antibodies revealed a specific cross-reaction with 42 and 45 kDa proteins in S1, and only 45 kDa protein in ATP genotype. However, increased GS protein accumulation pattern (both 42 and 45 kDa) was observed in S1 under high NaCl. Whereas, accumulation of 45 kDa protein was unchanged at all levels of stress and slight accumulation in 42 kDa protein at 1.5% NaCl was observed for ATP. Elevation in the enzyme activities of GS, GOGAT were coupled with AAT and ALAT observed in both the genotypes. Higher enzymatic activities of S1 than ATP under salinity stress may be due to efficient capacity of ammonia detoxification. Salt tolerance of S1 supports the higher metabolic activity under salinity leading to lesser amount of ammonia accumulation and higher levels of free amino acid in the tissue. In agreement with these results the physiological significance of enzymatic changes and ammonia assimilation during salt stress in relevance to plant nitrogen metabolism was discussed.     

Role of 5-aminolevulinic acid on growth,photosynthetic parameters and antioxidant enzyme activity in NaCl-stressed <Emphasis Type="Italic">Isatis indigotica</Emphasis> Fort.

5-aminolevulinic acid (ALA) is a key precursor for the biosynthesis of porphyrins such as heme and chlorophyll. ALA alleviates salinity stress damage in germinating seeds and improves seedling growth. Exogenous application of ALA at low concentrations has been shown to enhance salt tolerance in a number of plants. In the present study, we studied the effect of exogenous application of ALA on enhancing salt stress tolerance in Isatis indigotica Fort. (Anhui population as S1, Shanxi population as S2). A foliar application of 0, 12.5, 16.7, 25.0, and 50.0 mg/L ALA was given to the leaves of I. indigotica plants treated with 100 mmol/L NaCl. The fresh weight of leaves and roots; chlorophyll relative content (SPAD value); photosynthetic parameters, such as net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci) and water use efficiency of the treated plants were determined. The third leaf of each treated plant was used to determine the activities of antioxidant enzymes. Superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutamate synthase (GOGAT), nitrate reductase (NR) activities and the malondialdehyde (MDA) content increased in response to 100 mmol/L NaCl in both S1 and S2 plants. However, the fresh weight of leaf and root, chlorophyll relative content, Pn, Gs, Ci decreased in response to salt stress in both S1 and S2 plants. In all foliar application of ALA in S1 plants, the MDA content, and the activities of SOD and POD were the highest in response to 50.0 mg/L foliar application of ALA. GOGAT and NR activities were the highest in response to 16.7 mg/L foliar ALA. Chlorophyll content and Pn were the highest in S1 plants treated with by 25.0 mg/L ALA. In S2 plants, plant fresh weight, chlorophyll relative content, SOD, CAT, NR activities and Pn treated with 16.7 mg/L ALA were higher than that of the control (CK0). POD, MDA, GOGAT activities in S2 plants treated with 25.0 mg/L ALA were the highest among all treatments. Thus, our results showed that the optimal concentration of ALA (16.7 ~ 25.0 mmol/L) increases the activity of antioxidant enzymes, which in turn helps to abate the damage caused by salt stress in I. indigotica seedlings. Furthermore, ALA also results in an increase in chlorophyll content, Pn and the activities of GOGAT and NR.