The Influence of Ambient Nitrate, Temperature, and Light on Nitrate Assimilation in Sudangrass Seedlings

Seedlings of Sundangrass (Sorghum Sudanese [Piper] Stapf.) were grown 10 to 13 days of age in a nutrient solution containing nitrate and then placed under treatment conditions for 24 h before assays of nitrate assimilation were begun. Nitrate uptake was determined by its disappearance from the ambient solution. In vivo reduction of nitrate was determined by the overall balance between the amount taken up and the change in tissue concentration of nitrate during the experiments. Nitrate reductase activity was determined from tissue slices. In vivo reduction was strongly regulated by uptake in response to time and ambient nitrate concentration, temperature and light. Nitrate reduction responded to the concentration of nitrate supplied by uptake and by a storage pool, since reduction often exceeded uptake. Nitrate reductase activity in tissue slices was exponential in initial response to increasing temperature. After a 24-h equilibration period at each temperature, the activity was lower at higher temperatures. In contrast, actual reduction of nitrate increased linearly with increasing temperature between 15 and 24°C in the plants equilibrated 24 h at each temperature. Nitrate uptake and reduction were greatly inhibited under low light conditions, with reduction inhibited more than uptake., The effect of ambient nitrate, temperature, and light on the nitrate assimilatory processes help to explain observations reported on nitrate accumulation by Sudangrass forage.     

The Response of Nitrate Reductase Activity and Nitrate Assimilation in Maize Roots to Growth Regulators at Acidic pH

Nitrate and total nitrogen contents, and nitrate reductase (NR) activity of the excised maize roots in buffered or unbuffered nitrate solution (at pH 6.5 or 4.5) as affected by putrescine (PUT), abscisic acid (ABA) and salicylic acid (SA) were investigated. In unbufferred solution, the NR activity was lower at pH 4.5 as compared to that at pH 6.5, but in bufferred solution the activity was higher at lower pH. Supply of 100 µM PUT or 500 µM SA, promoted NR activity and 50 µM ABA inhibited the activity at pH 6.5. However, at pH 4.5, PUT and SA inhibited NR activity and ABA had no effect. In most cases, the increase in NR activity was positively correlated with total organic nitrogen and a negatively with nitrate content. A reverse situation was found when NR activity was inhibited by the growth regulators.     

Preferential Assimilation of Ammonium Ions from Ammonium Nitrate Solutions by Apple Seedlings

Apple seedlings, Pyrus malus L., were grown in complete nutrient solutions containing nitrate, ammonium, or ammonium plus nitrate as the nitrogen source. Uptake of nitrogen was calculated from depletion measurements of the nutrient solutions and by using ¹⁵N labelled nitrate and ammonium salts. If the plants received nitrogen as ammonium only or as nitrate only, the amounts of nitrogen taken up were similar. However, if the seedlings were supplied with ammonium nitrate, the amount of nitrate-nitrogen assimilated was only half that of ammonium. Nevertheless, if ammonium and nitrate were supplied to a plant with a split-root system, with each root half receiving a different ion, the uptakes were similar. The possibility of independent inhibition by ammonium of both nitrate uptake and reduction in the roots is discussed.     

Ammonia Assimilation in Canavalia ensiformis Plants under Water and Salt Stress

Nitrogen fixation and ammonia assimilation in nodules have beenthoroughly studied under stress conditions, but the behaviorof enzymes involved in ammonia assimilation to organic compoundsin plants of the Leguminosae family subjected to stress stillremains to be conclusively established. We found that understress conditions, C. ensiformis plants can switch from theirusual pathway of assimilation to an alternative one dependingon the nature of the stress and the tissue in which the processtakes place. In roots, it switches from the glutamate dehydrogenase(GDH) pathway to the glutamine synthetase (GS)/glutamate synthase(GOGAT) cycle under water stress but not under salt stress.However, in leaves under salt stress, GDH activity is maintainedbut GS activity markedly decreases (Received March 24, 1987; Accepted March 4, 1988)     

Role of Grafting in Resistance to Water Stress in Tomato Plants: Ammonia Production and Assimilation

In general, drought depresses nutrient uptake by the root and transport to the shoot due to a restricted transpiration rate, which may contribute to growth limitation under water deprivation. Moreover, water stress may also restrict the ability of plants to reduce and assimilate nitrogen through the inhibition of enzymes implicated in nitrogen metabolism. The assimilation of nitrogen has marked effects on plant productivity, biomass, and crop yield, and nitrogen deficiency leads to a decrease in structural components. Plants produce significant quantities of NH₄ ⁺ through the reduction of NO₃ ^? and photorespiration, which must be rapidly assimilated into nontoxic organic nitrogen compounds. The aim of the present work was to determine the response of reciprocal grafts made between one tomato tolerant cultivar (Lycopersicon esculentum), Zarina, and a more sensitive cultivar, Josefina, to nitrogen reduction and ammonium assimilation under water stress conditions. Our results show that when cv. Zarina (tolerant cultivar) was used as rootstock grafted with cv. Josefina (ZarxJos), these plants showed an improved N uptake and NO₃ ^? assimilation, triggering a favorable physiological and growth response to water stress. On the other hand, when Zarina was used as the scion (JosxZar), these grafted plants showed an increase in the photorespiration cycle, which may generate amino acids and proteins and could explain their better growth under stress conditions. In conclusion, grafting improves N uptake or photorespiration, and increases leaf NO₃ ^? photoassimilation in water stress experiments in tomato plants.     

NO参与玉米幼苗对盐胁迫的应答

以玉米幼苗为材料,研究盐胁迫下其內源NO含量、NR和NOS活性的变化;NOS专一性抑制剂L-NAME和NR非专一性抑制剂NaN3对玉米幼苗內源NO含量的影响;利用激光共聚焦显微技术观测盐胁迫下玉米幼苗根部NO含量的变化及其分布特点。结果表明,盐胁迫下玉米幼苗根尖和叶片中NO含量有猝发现象,NOS活性也随之显著提高,NR活性则显著降低;L-NAME或NaN3均可降低盐胁迫所引起的玉米幼苗NO水平的增加,L-NAME对NO含量的影响比NaN3更显著。推测,NO参与玉米幼苗对盐胁迫的应答,NOS途径是盐胁迫下玉米幼苗內源NO合成的主要途径。     

The Effects of Irradiance on Uptake and Assimilation of Nitrate by Young Barley Seedlings

The nitrogen economy of barley plants growing in a range ofirradiances from full shade (less than 0·5 W m^–2)to 119 W ^m–2 has been examined by analysing levels oftotal, organic and nitrate nitrogen, and by determining nitratereductase activity in leaf extracts. It has been confirmed thatroot growth is reduced in low irradiances which are also associatedwith a lower level of total nitrogen in the plant, and hencewith a lower uptake of nitrate. In all parts of the plant thelevel of organic nitrogen is higher in high light intensitybut nitrate-nitrogen as a proportion of the total is greatestin low irradiances. In the first leaf accumulation of free nitrateis substantially greater in low irradiances. The data indicate a higher level of nitrate assimilation inhigh irradiances and nitrate reductase activity in leaf extractsis higher in such conditions. When the first leaf is shadednitrate reductase activity falls to undetectable levels afterabout 4 days, but in the case of the second leaf, where thisis shaded, some reductase activity is always found, althoughthis is substantially less than that in unshaded conditions. It is concluded that in vitro rates of nitrate reduction mayover-estimate nitrate assimilation determined as increase inorganic nitrogen.     

The Response of Cyclic Electron Flow around Photosystem I to Changes in Photorespiration and Nitrate Assimilation

Photosynthesis captures light energy to produce ATP and NADPH. These molecules are consumed in the conversion of CO₂ to sugar, photorespiration, and NO₃⁻ assimilation. The production and consumption of ATP and NADPH must be balanced to prevent photoinhibition or photodamage. This balancing may occur via cyclic electron flow around photosystem I (CEF), which increases ATP/NADPH production during photosynthetic electron transport; however, it is not clear under what conditions CEF changes with ATP/NADPH demand. Measurements of chlorophyll fluorescence and dark interval relaxation kinetics were used to determine the contribution of CEF in balancing ATP/NADPH in hydroponically grown Arabidopsis (Arabidopsis thaliana) supplied different forms of nitrogen (nitrate versus ammonium) under changes in atmospheric CO₂ and oxygen. Measurements of CEF were made under low and high light and compared with ATP/NADPH demand estimated from CO₂ gas exchange. Under low light, contributions of CEF did not shift despite an up to 17% change in modeled ATP/NADPH demand. Under high light, CEF increased under photorespiratory conditions (high oxygen and low CO₂), consistent with a primary role in energy balancing. However, nitrogen form had little impact on rates of CEF under high or low light. We conclude that, according to modeled ATP/NADPH demand, CEF responded to energy demand under high light but not low light. These findings suggest that other mechanisms, such as the malate valve and the Mehler reaction, were able to maintain energy balance when electron flow was low but that CEF was required under higher flow.Photosynthesis must balance both the amount of energy harvested by the light reactions and how it is stored to match metabolic demands. Light energy is harvested by the photosynthetic antenna complexes and stored by the electron and proton transfer complexes as ATP and NADPH. It is used primarily to meet the energy demands for assimilating carbon (from CO₂) and nitrogen (from NO₃⁻ and NH₄⁺; Keeling et al., 1976; Edwards and Walker, 1983; Miller et al., 2007). These processes require different ratios of ATP and NADPH, requiring a finely balanced output of energy in these forms. For example, if ATP were to be consumed at a greater rate than NADPH, electron transport would rapidly become limiting by the lack of NADP⁺, decreasing rates of proton translocation and ATP regeneration. Alternatively, if NADPH were consumed faster than ATP, proton translocation through ATP synthase would be reduced due to limiting ADP and the difference in pH between lumen and stroma would increase, restricting plastoquinol oxidation at the cytochrome b₆f complex and initiating nonphotochemical quenching (Kanazawa and Kramer, 2002). The stoichiometric balancing of ATP and NADPH must occur rapidly, because pool sizes of ATP and NADPH are relatively small and fluxes through primary metabolism are large (Noctor and Foyer, 2000; Avenson et al., 2005; Cruz et al., 2005; Amthor, 2010).The balancing of ATP and NADPH supply is further complicated by the rigid nature of linear electron flow (LEF). In LEF, electrons are transferred from water to NADP⁺, oxidizing water to oxygen and reducing NADP⁺ to NADPH. This electron transfer is coupled to proton translocation and generates a proton motive force, which powers the regeneration of ATP. The stoichiometry of ATP/NADPH produced by these reactions is thought to be 1.29 based on the ratio of proton pumping and the requirement for ATP synthase in the thylakoid (Sacksteder et al., 2000; Seelert et al., 2000). However, under ambient CO₂, oxygen, and temperature, the ATP/NADPH required by CO₂ fixation, photorespiration, and NO₃⁻ assimilation is approximately 1.6 (Edwards and Walker, 1983). The ATP/NADPH demand from central metabolism changes significantly from 1.6 if the ratio of CO₂ or oxygen changes, driving different rates of photosynthesis and photorespiration (see “Theory”). Such changes in energy demand require a flexible mechanism to balance ATP/NADPH that responds to environmental conditions.The difference between ATP/NADPH supply from LEF and demand from primary metabolism could be balanced via cyclic electron flow around PSI (CEF; Avenson et al., 2005; Shikanai, 2007; Joliot and Johnson, 2011; Kramer and Evans, 2011). During CEF, electrons from either NADPH or ferredoxin are cycled around PSI into the plastoquinone pool and regenerate ATP without reducing NADP⁺ (Golbeck et al., 2006). Therefore, CEF has been suggested to be important for optimal photosynthesis and plant growth, but its physiological role in energy balancing is not clear (Munekage et al., 2002, 2004; Livingston et al., 2010). For example, there was no shift in CEF in Arabidopsis (Arabidopsis thaliana) measured under low light (less than 300 μmol m⁻² s⁻¹) and different oxygen partial pressures, which would significantly change the ATP/NADPH demand of primary metabolism (Avenson et al., 2005). Similar results were seen under low light in leaves of barley (Hordeum vulgare) and Hedera helix (Genty et al., 1990). While CEF did not shift with energy demand in steady-state photosynthesis under low light, it did increase with photorespiration as expected at high light (Miyake et al., 2004, 2005). These observations could be explained if CEF becomes more important for energy balancing under high irradiances when other mechanisms become saturated.To determine under which conditions CEF responded to ATP/NADPH demand, we used biochemical models of leaf CO₂ fixation to model ATP and NADPH demand under a variety of conditions (see “Theory”). We then used in vivo spectroscopy to measure the relative response of CEF to modeled ATP/NADPH demand from CO₂ fixation and NO₃⁻ assimilation in hydroponically grown Arabidopsis. Our findings indicate that CEF responded to modeled ATP/NADPH demand under high light but not under low light or nitrate availability.     

Some Aspects of Nitrate Assimilation in the Seedlings of Normal and Opaque-2 Mutant of Maize

The effects of 10 mM nitrate on the growth and nitrogenous componentsof Zea mays L. var. W64A wild type (normal) were compared tothose on its opaque-2 (high lysine) mutant during the first10 d of seedling growth at a constant temperature of 26 °Cand with a 16 h photoperiod. Nitrate supply had no effect onthe growth of embryonic axes in both lines till day 6. Growthof both lines was enhanced slightly after that time, however.Increases in 80% (v/v) ethanolsoluble and protein nitrogen werealso observed only after day 4 when the supply of nitrogen fromthe storage proteins in the endosperm was limiting. Nitratehad no effect on the synthesis of chlorophyll during leaf developmentbut it did increase the total chlorophyll in mature and senescingprimary leaves. The increase in nitrogenous components or chlorophyllin opaque-2 was more pronounced than in the normal type. Itmight be related to the lower proline or higher lysine in themutant.     

烟草幼苗响应弱光胁迫的转录组分析

为了研究烟草幼苗对弱光胁迫反应的分子机制,以'云烟87'为材料,构建全光照和弱光处理条件下大十字期烟苗cDNA文库,利用Illumina测序技术进行转录组测序,筛选差异表达基因.结果表明:借助RNA-seq技术共筛选到2 956个DEGs,其中弱光相对于全光照表达上调的DEGs有691个,表达下调的DEGs为2 265...     

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.     

Aluminium Toxicity Modulates Nitrate to Ammonia Reduction

Two weeks-old maize (Zea mays cv. XL-72.3) plants were exposed to Al concentrations 0 (Al0), 9 (Al9), 27 (Al27) or 81 (Al81) g m-3 for 20 d in a growth medium with low ionic strength. Thereafter, the Al concentration-dependent interactions on root nitrate uptake, and its subsequent reduction to ammonia in the leaves were investigated. Al concentrations in the roots sharply increased with increasing Al concentrations while root elongation correspondingly decreased. Root fresh and dry masses, acidification capacity, and nitrate and nitrogen contents decreased from Al27 onwards, whereas leaf nitrogen, nitrate, nitrite, and ammonia concentrations decreased starting with Al9. Electrolytic conductance increased by 60 % in root tissues from Al0 to Al81 but it did not increase significantly in the leaves. In Al9, Al27, and Al81 plants a decrease in shoot fresh and dry masses was observed. Al concentrations between 0 and 27 g m-3 increased net photosynthetic rate, stomatal conductance, and the quantum yield of photosynthetic electron transport, whereas the intercellular CO2 concentration was minimum in Al27 plants. In the leaves, nitrate reductase (E.C. 1.6.6.1) activity increased until Al27, and nitrite reductase (E.C. 1.6.6.4) activity until Al81. Hence there may be an Al mediated extracellular and intracellular regulation of root net nitrate uptake. Nitrate accumulation in the roots affects the translocation rates and, therefore, the nitrate concentration in the leaves. The in vivo reducing power generated by the photosynthetic electron flow does not limit nitrate to ammonia reduction, and the increase of maximum nitrate and nitrite reductase activities parallels the decreasing nitrate, nitrite, and ammonia concentrations.     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

Nitrate Reduction in Response to CO(2)-Limited Photosynthesis : Relationship to Carbohydrate Supply and Nitrate Reductase Activity in Maize Seedlings

The effects of CO₂-limited photosynthesis on ¹⁵NO₃⁻ uptake and reduction by maize (Zea mays, DeKalb XL-45) seedlings were examined in relation to concurrent effects of CO₂ stress on carbohydrate levels and in vitro nitrate reductase activities. During a 10-hour period in CO₂-depleted air (30 microliters of CO₂/ per liter), cumulative ¹⁵NO₃⁻ uptake and reduction were restricted 22 and 82%, respectively, relative to control seedlings exposed to ambient air containing 450 microliters of CO₂ per liter. The comparable values for roots of decapitated maize seedlings, the shoots of which had previously been subjected to CO₂ stress, were 30 and 42%. The results demonstrate that reduction of entering nitrate by roots as well as shoots was regulated by concurrent photosynthesis. Although in vitro nitrate reductase activity of both tissues declined by 60% during a 10-hour period of CO₂ stress, the remaining activity was greatly in excess of that required to catalyze the measured rate of ¹⁵NO₃⁻ reduction. Root respiration and soluble carbohydrate levels in root tissue were also decreased by CO₂ stress. Collectively, the results indicate that nitrate uptake and reduction were regulated by the supply of energy and carbon skeletons required to support these processes, rather than by the potential enzymatic capacity to catalyze nitrate reduction, as measured by in vitro nitrate reductase activity.     

Influence of Paclobutrazol and Application Methods on High-temperature Stress Injury in Cucumber Seedlings

Paclobutrazol (PBZ) is a member of the triazole plant growth inhibitor group that is responsible for inducing tolerance to a number of biotic and abiotic stresses. An experiment was therefore conducted to examine whether the application of PBZ at various concentrations (0, 25, 50, and 75 mg l⁻¹) by seed soaking or foliar spray would protect cucumber (Cucumis sativus) seedlings subjected to high-temperature stress. Thirty-five-day-old seedlings were exposed to heat stress at 40°C for 4 h per day for 5 days. PBZ improved the majority of the physiological (for example, relative chlorophyll content and chlorophyll fluorescence ratio) and morphological parameters (for example, shoot and root fresh and dry weights) measured in cucumber seedlings subjected to high-temperature stress. PBZ ameliorated the injuries caused by heat stress by increasing leaf proline content and preventing an increase in leaf electrolyte leakage. PBZ was more effective at increasing the heat tolerance of cucumber seedlings when using the seed-soaking method rather than the foliar spray method. The best protection was obtained when seeds were soaked in 50 mg l⁻¹ PBZ.     

The Assimilation of Ammonia by Nitrogen-starved Cells of Chlorella vulgaris: Part II. The Assimilation of Ammonia to other Compounds

Ammonia is rapidly assimilated by nitrogen-starved Chlorellacells and converted into soluble organic nitrogenous compounds.During the first 20 minutes of assimilation most of the ammoniawhich has been used can be accounted for as free or combined-amino-nitrogen and amide-nitrogen. Later a smaller proportionof the assimilated ammonia is found in these fractions whileappreciable quantities of basic amino-acids are formed. Theassimilation of ammonia is accompanied by an increase of therates of oxygen absorption and carbon dioxide production; thevalue of the respiratory quotient decreases. Non-reducing sugarand acid-hydrolysable polysaccharide are metabolized rapidly. Possible interpretations of these facts are discussed.     

Inhibition of Nitrogen and Photosynthetic Carbon Assimilation of Maize Seedlings by Exposure to a Combination of Salt Stress and Potassium-Deficient Stress

The main aim of this work is to identify how the combined stresses affect the interdependent nitrogen and photosynthetic carbon assimilations in maize. Maize plants were cultivated in Meider's solution. They were subjected to salt stress and potassium deficiency in the K-present Meider's media and K-deficient Meider's media. After 5?weeks, we measured chlorophyll a fluorescence and the activities of several enzymes in metabolic checkpoints coordinating primary nitrogen and carbon assimilation in the leaves of maize. The study showed that the combination of salt stress and potassium-deficient stress more significantly decreased nitrate uptake, plant growth, the activities of nitrate reductase, glutamate dehydrogenase, glutamate synthase, urease, glutamic-pyruvic transaminase, glutamic-oxaloace transaminase, sucrose-phosphate synthase, phosphoenolpyruvate carboxylase, and the synthesis of free amino acids, chlorophyll, and protein than those of each individual stress, respectively. However, the combined stresses significantly increased the accumulation of ammonium and carbohydrate products. The combined stresses also significantly decreased the oxygen evolution, the electron transport, and the efficiency of photochemical energy conversion by photosystem II in maize seedlings. Taken together, a combination of salt stress and potassium-deficient stress impaired the assimilations of both nitrogen and carbon and decreased the photosystem II activity in maize.     

Nitrate Reductase Activity of Wheat Seedlings during Exposure to and Recovery from Water Stress and Salinity

Nitrate reductase activity was inhibited as a result of reduced soil moisture potentials or application of NaCI to nutrient solutions. The decrease in enzyme activity of wheat seedlings exposed to salinity, was found 24 hours after exposure to stress. The effect of stress on nitrate reductase was found in cell-free extracts as well as in riro in assays of intact leaf sections. A recovery in enzyme activity was found after irrigation or after removal of seedlings from salinity. While relative water content of the leaves was restored within 3 hours after removal of stress, full recovery of enzyme activity occurred only after 24 hours. Cycloheximide and chloramphenicol suppressed the activity of nitrate reductase in non-stressed seedlings, but had no effect on the activity of plants exposed to salinity. However, during removal of stress, cycloheximide prevented completely the recovery of nitrate reductase, while chloramphenicol did not interfere with the recovery of the inhibited enzyme activity. It is concluded that a fraction of nitrate reductase may be located in the cytoplasm and lost activity during stress, probably due to inhibited protein synthesis. Another fraction which may be associated with chloroplasts, was inhibited by stress due to conformational changes or partial denaturation.