Proton Neutralization in the Leaves of English Oak (Quercus robur L.) Exposed to Sulphur Dioxide

Reactions to the input of acidic gases were investigated inleaves of Quercus robur L. exposed to different concentrationsof SO₂ (80, 120, and 160 nl I^–1) for 32 to 70 d. Two-year-oldoaks were grown in nutrient solutions with varied nitrogen formand were fumigated in closed chambers. An attempt was made toidentify the mechanisms of proton neutralization by consideringthe uptake of nitrogen, the increase in sulphur and carboxylatecontents, and the excretion of hydroxyl ions or protons. Inaddition, nitrate reductase activity was determined in the leaves. The reduction of sulphur was not involved in the neutralizationof protons generated by SO₂-uptake, whereas organic acid metabolismplayed a decisive role. Depending on SO₂-concentration, durationof fumigation and nitrogen supply, oaks reacted with a reductionin the size of the carboxylate pool in the leaves, and/or withan increase in proton excretion (or a decrease in hydroxyl ionexcretion). Nitrate reductase activity increased in the leavesof nitrate-grown oaks exposed to the highest SO₂-concentration(160 nl l^–1) for 42 d. The capacity of the mechanismsconsidered is sufficient for the neutralization of the calculatedamounts of protons resulting from SO₂-uptake. Key words: Leaves, neutralization, protons, Quercus, sulphur dioxide     

Sulphate supply and its regulation of transport in roots of a tropical legume Macroptilium atropurpureum cv. Sirato

During a period of sulphate deprivation, roots of Macroptiliumatropurpureum responded by increasing their uptake capacityat the plasma membrane. This effect was apparent both in intactplants and in tissues excised prior to uptake. In experiments using excised root systems previousy labelledwith ³⁵SO₄^2- the rate of tracer transport to the xylem was muchgreater in roots subsequently deprived of external sulphatethan in those supplied with unlabelled sulphate. Removing theexternal sulphate to the external solution. Additionally, compartmentalanalysis of tracer exchange kinetics showed that the flux ofsulphate from the cytoplasm to the xylem(     

Regulation of Sulphate Transport in a Tropical Legume, Macroptilium atropurpureum, cv. Siratro

When young plants of Macroptilium atropurpureum, cv. Siratrowere deprived of external sulphate (-S plants) growth of shootsand roots continued at rates comparable to those in plants wellsupplied with sulphate (control) for 3 d and 5 d respectively.Dilution of internal sulphur therefore took place and redistributionof sulphur occurred between inorganic and organic forms andbetween roots and younger leaves. Even when S-deficiency limitedgrowth, plants contained 16% of their total sulphur as sulphate,but most of this was retained in old leaves and redistributedslowly to growing zones. The capacity for sulphate uptake increased in roots of –Splants very soon after they were deprived of external sulphate;within 24 h the absorption from 0.25 mol m^–3 SO₄^2–was more than five times that of control roots. Maximum increasedcapacity was reached after 2–3 d stress when the V_maxof system 1 was 1948 nmol h^–1g^–1root fr. wt. in–S plants and 337 nmol h^–1g^–1root fr. wt.in controls. The K^mfor system 1 did not change significantlywith S-stress being between 5–8 µM in both setsof plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growthrate and the rate at which sulphate uptake capacity was enhancedby withholding sulphate from culture solutions. When –S plants were replaced in sulphate-containing solutiontheir capacity for SO₄^2– declined to the control levelwithin 24 h. Very marked repression of capacity was also foundwhen –S plants were treated with L-cysteine, but therewas no immediate effect with methionine. Roots of this species appear to have a very active system fordegrading L-cysteine to sulphate, 30% of the label in ³⁵S-cysteineabsorbed by roots was recovered in ³⁵SO₄^2– after 20 minor 2 h incubation. By contrast, roots had a very weak abilityto reduce sulphate. When part of the root system was in solution lacking sulphatethere was enhanced uptake of sulphate by other parts which themselveswere amply supplied with sulphate. This is seen as an exampleof compensatory absorption. The response to S-stress is specific and there were no positiveinteractions between S-stress and the absorption of phosphate,or P-stress and the uptake of sulphate. The results are discussed in relation to the close control ofsulphate uptake by internal sulphate concentration, redistributionof forms of sulphur during stress and mobility of sulphate inthe phloem. Key words: Kinetics, Amino-S, Sulpholipid, Repression;, Deficiency     

Compartmental analysis of 35SO2-4exchange kinetics in roots and leaves of a tropical legume Macroptilium atropurpureum cv. Siratro

Compartmental analysis of ³⁵SO^2-₄ tracer exchange kinetics hasbeen used to estimate unidirectional fluxes and compartmentcontents in excised root and leaf tissue of the tropical legume,Macroptilium atropurpureum. In excised root tissue only 5% ofthe sulphate taken up across the plasmalemma was reduced toorganic forms whereas in excised leaf tissue approximately 20%was reduced. It was necessary, therefore, to incorporate themetabolism of sulphate during the course of the experiment intothe compartmental models. In root tissue, wash-out data was fitted by three exponentials,assumed to correspond to exchange in the extracellular spaces,cytoplasm and vacuole, but in leaf tissue two large, slowlyexchanging compartments have been postulated in order to achievea fit to the data. It is likely that differences in leaf cellpopulations cause the ‘anomalous’ tracer exchangekinetics and the justification of this assumption is discussed. The fluxes of sulphate at the plasmalemma were greater thanthe corresponding fluxes at the tonoplast in both roots andleaves. The flux of SO^2-₄ from the cytoplasm to the externalsolution did not appear to limit the loss of SO^2-₄ from thevacuole. At an external SO^2-₄concentration of 0.25 mol m^-3 therate constants for exchange in the vacuole were two orders ofmagnitude greater in roots than in the slowest exchanging leafcell population. It is possible, therefore, that the slow lossof SO^2-₄ from leaf cell vacuoles may limit the redistributionof sulphate during S-stress. Key words: Compartmental analysis, sulphate, deficiency, Macroptilium atropurpureum     

Flavine nucleotide nitrate reductase from broad bean leaves

Hydrosulfite-reduced FMN served as an electron donor for nitratereductase purified from broad bean leaves. FMN was successfullyreplaced with BV. The flavine nucleotide nitrate reductase hadits pH optima at about 7.8 with phosphate buffer and at about7.4 with Tris-HCl buffer. The Km's for nitrate and FMN were3.7 ? 10^–4 M and 3.7 ? 10^–5 M, respectively. NADH₂: nitrate reductase activity was completely inhibited by0.1 mM p-CMB, whereas FMNH₂: nitrate reductase activity wasnot. Inhibited activity was restored by the addition of cysteine.A sulfhydryl enzyme is involved in the NADH₂: nitrate reductasesystem but not in the FMNH₂ : nitrate reductase system. NADH₂and FMNH₂ probably feed electrons into the electron transportchain at different sites. The nitrate reductase preparationhad an NADH₂-specific diaphorase activity which was almost completelyinhibited by 0.1 mM p-CMB. The NADH₂-specific diaphorase mayform the sulfhydryl enzyme which mediates electron transferbetween NADH₂ and nitrate. (Received May 6, 1969; )     

The Effect of SO2 on Lolium perenne L. Grown at Different Levels of Sulphur and Nitrogen Nutrition

The effects of exposure to SO₂ (55 m^–3) on the growthand sulphur nutrition of perennial ryegrass (Lolium perenneL.) cv. S23 were examined using a system of specially constructedgrowth chambers. The plants were grown in soil with two ratesof added nitrogen, either with or without added sulphate. Theywere harvested twice during the exposure period of 85 d. The high rate of added nitrogen gave a four-fold increase inthe yield of plants given added sulphate but resulted in sulphurdeficiency, with reduced yield and number of tillers, when nosulphate was given. Exposure to SO₂ alleviated the effects ofdeficiency but was without effect on the yield of plants givensulphate. With the low rate of added nitrogen, the yield ofplants and the number of tillers were unaffected by the additionof sulphate to the soil or of SO₂ to the air. The only indicationof possible injury through exposure of these plants to SO₂ wasa small increase in the proportion of dead leaves at the secondharvest. Analysis of the leaves for total S and sulphate-S gave valuesconsistent with sulphur deficiency and its alleviation throughexposure to SO₂; the contents of 0?045% S and 42 parts 10^–6sulphate-S in deficient plants were raised to 0?102% and 153parts 10^–6 respectively. The organic-S content of theleaves was always increased through exposure to SO₂. There wasno evidence that exposure to SO₂ increased the ‘transpirationcoefficient’ for plants having an adequate supply of sulphurfrom the soil.     

Nitrate reductase inactivating factor from rice seedlings

A nitrate reductase inactivating factor was found in extractsof leaf blades, leaf sheaths, and roots of rice seedlings. Thefactor was nondialyzable, precipitable with (NH₄)₂SO₄, and heatlabile. The factor from rice roots inactivated NADH nitratereductase, FMNH2 nitrate reductase, and NADH cytochrome c reductasefrom rice shoots, but had no effect on the activities of NADHdiaphorase and nitrite reductase. The factors from rice shoots,rice roots, and maize roots inactivated NADH nitrate reductaseprepared from cultured rice cells. The factor from culturedrice cells also inactivated rice shoot NADH nitrate reductase. The activity of the inactivating factor showed a diurnal changein shoots of rice seedlings grown with NO₃– medium, althoughthe fluctuation was not large compared to that of NADH nitratereductase activity. When the seedlings were placed in darkness,the activity of the factor did not change during 20 hr withNO₃– medium. However, the activity of the factor fluctuatedwith NO₃– -free medium in light; its activity startedto increase at the 8th hour after transfer. NADH nitrate reductaseactivity from rice shoots declined rapidly during the first8 hr and gradually thereafter in both types of culture. (Received August 24, 1977; )     

Growth of Lucerne (Medicago sativa L.) Exposed to Sulphur Dioxide

The effect of exposure to sulphur dioxide (0 or 96 µgm^–3 SO₂) on the growth and sulphur content of lucerne(Medicago sativa L.) was examined in a period of 135 d duringwhich the plants were harvested four times. The lucerne wasgrown in pots of soil with and without the addition of sulphateand of nitrate. Evidence of sulphur deficiency, including areduction in the weight, the number and the sulphur contentof shoots, was found in plants grown without added sulphate.Deficiency was alleviated through exposure of plants to SO₂.Apart from reducing shoot weight at one harvest and generallyincreasimg the concentration of sulphur in the shoots, exposureto SO₂ had no significant effect on plants grown with addedsulphate. The yield of shoots was greater, and was reduced toa lesser extent with sulphur deficiency, in plants grown withadded nitrate than in those dependent on rhizobia. Whilst thetranspiration coefficient increased greatly in sulphur-deficientplants, it was reduced where otherwise similarly treated plantswere exposed to SO₂; this treatment did not alter the coefficientin plants with an adequate supply of sulphur from the soil.     

The Effect of Salinity Changes Upon the Physiology of Eulittoral Green Macroalgae from Antarctica and Southern Chile: II INTRACELLULAR INORGANIC IONS AND ORGANIC COMPOUNDS

The effects of hypo- and hypersaline treatments ranging from7–68% on the intracellular inorganic ion and organic soluteconcentrations were determined in the eulittoral green macroalgaeUlothrix implexa, Ulothrix subflaccida, Enteromorpha bulbosa,Acrosiphonia arcta, and Ulva rigida from Antarctica and SouthernChile. The main inorganic cations were K⁺, Na⁺, and Mg²⁺ inall species. The major osmolyte in E. bulbosa, A. arcta, andU. rigida was K⁺ at increasing salinities. In both Ulothrixspecies, however, K⁺ levels declined during hypersaline stressand Na⁺ concentrations rose significantly. The main inorganicanions were Cl^-, SO²_4-, and PO³_4- in all algae, while E. bulbosaand U. rigida also contained NO⁺₃. A. arcta showed an extremelyhigh SO^2-₄ content. The organic solutes proline, sucrose, andß-dimethylsulphoniopropionate (DMSP) played an importantrole in osmotic acclimation. The occurrence of three organicosmolytes suggests an additional function of these solutes ascryoprotectants in the cold-water macroalgae investigated.     

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     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

野生和栽培马蹄金抗旱性比较及其抗旱机制初探

 为了鉴别野生和栽培马蹄金（Dichondra repens）的抗旱性,探讨其抗旱适应性的生理机制,对野生和栽培马蹄金进行了土壤水分胁迫处理,系统测定了野生和栽培马蹄金叶片内超氧化物歧化酶（SOD）、过氧化物酶（POD）、硝酸还原酶的活性和游离脯氨酸、可溶性糖、可溶性蛋白、NO₂^- /NO₃^-含量以及总DNA片断化程度。结果表明野生和栽培马蹄金在抗旱适应性上存在显著差异。随水分胁迫强度的增加,野生马蹄金叶片内抗氧化酶活性及其升高幅度,渗透调节物质的积累量及积累速度均高于栽培马蹄金,而叶片含水量下降的程度、DNA片断化程度低于栽培马蹄金,其中野生和栽培马蹄金叶片内NO₂^- /NO₃^-含量,硝酸还原酶的活性变化差异尤其显著,野生马蹄金叶片内的NO₂^- /NO₃^-含量、硝酸还原酶的活性明显高于栽培马蹄金,二者最大含量分别相差10和2.2倍。研究结果说明了野生马蹄金对干旱环境的适应性强于栽培品种;在干旱逆境下马蹄金叶片内的NO₂^－ /NO₃^－含量的变化在一定程度上可能反映了内源NO的变化,内源NO浓度的高低可能是野生和栽培马蹄金不同抗性的真正原因。     

Incorporation of radioactive molybdenum into protein during nitrate reductase formation and effect of molybdenum on nitrate reductase and diaphorase activities of spinach (Spinacea oleracea L.)

Spinach plants grown without molybdenum lack nitrate reductaseand when plants are deprived of nitrate existing activity islost. Transfer of molybdenum-deficient plants to a solutioncontaining (NH₄)₂⁹⁹MoO₄) or nitrate-starved plants to NaNO₃solution induced enzyme activity in 24 hr. After purificationby selective adsorption, precipitation and disc electrophoresis,the protein from molybdenum-deficient plants given ⁹⁹Mo showedradioactivity only where nitrate reductase was revealed on theacrylamide gel. Molybdenum was similarly selectively concentratedinto the enzyme as a result of induction by nitrate in plantsgrown with sub-optimal molybdenum supply in order to minimizeeffects of isotope dilution on measurement of ⁹⁹Mo incorporation. There was no exchange in vitro between ⁹⁹Mo and purified activeenzyme in the resting state over 18 hr at 4°C, or with functioningenzyme held at room temperature for 24 hr. There was evidenceeither for possible in vivo exchange of ⁹⁹Mo andenzyme boundMo or for slight synthesis of fresh enzyme under conditionsof net loss of enzyme in nitrate starved plants. Five NADH₂ and two NADPH₂ reactive diaphorases which could beseparated by electrophoresis were present in extracts. Onlyone of these having strong NADH₂ and weak NADPH₂ activity wasdirectly associated with nitrate reductase. The same complexalso showed the only benzyl viologen (BV.) reactive nitratereductase. Nitrate reductase in spinach is therefore considered to be amolybdenum-dependant and molybdenum-containing protein in whichNADH₂ (with weak NADPH₂) and BVelectron donor functions anddiaphorase/reductase activities remain closely associated duringpurification and electrophoresis. The techniques provide a simple means for the production andpurification of enzyme containing radioactively labelled Moapplicable to investigations on the structure of the enzyme. (Received January 16, 1971; )     

Nitrogen utilization by plant species from acid heathland soils: II. Growth and shoot/root partitioning of NO3- assimilation at constant low pH and varying NO3-/NH4+ ratio

The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m^–3N, varying the N0₃^–/NH₄⁺ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N0₃^–/NH₄⁺ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH₄⁺ by NO₃^– tended to enhance the plant's growth ratewhen compared to NH₄⁺ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO₃^–increasing from 0 to 100%. In the presence of NH₄⁺, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH₄⁺. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO₃^–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO₃^– [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO₃^– <0.20), D. flexuosashowed a strong preference for NO₃^– at low external nitrate(the relative uptake ratio for N0₃^– reaching a value of2.0 at 10% NO₃^–). The ecological significance of thisprominent high preference for NO₃^– at low NO₃^–/NH₄⁺ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N0₃^–/NH₄⁺ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate     

Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea mays L.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Two experiments were conducted to evaluate the effects of phenotypicrecurrent selection for high and low post-anthesis leaf-laminain vivo NRA on nitrate uptake, nitrate partitioning and in vitroNRA of seedling roots and leaves. In Experiment 1, intact plantsof cycle 0, 4, and 6 of the high and low NRA strains were grownon NH₄-N for 11 d, then exposed to 1.0 mol m^–3 KNO₃, andcultures sampled at 6 h and 28 h (induction and post-inductionperiods). Nitrate uptake, tissue nitrate concentration and invitro NRA were determined. The pattern of response to selectionin seedling leaf NRA was similar to that observed for in vivoNRA of field grown plants. Leaf NRA increased between 6 h and28 h. Root NRA was not affected by selection or sampling time.Treatments differed in total fresh weight but not in reductionor uptake of nitrate per unit weight, indicating a lack of correspondencebetween NRA and reduction and supporting the idea that concomitantreduction by NR is not obligatorily linked to nitrate influxin the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of thehigh and low NRA strains were cultured without N, detopped onday 6, transferred the following day to 0-75 mol m^–3 KNO₃and sampled at 6 h and 28 h. In contrast to Experiment 1, selectionpopulations differed in nitrate reduction and root NRA, whichby 28 h reached higher average levels than root NRA of intactplants. Translocation and reduction were inversely related amongstrains within each sampling time. The high level of translocationin detopped plants of the low NRA strain was difficult to reconcilewith its low leaf NRA level of Experiment 1. It is suggestedthat nitrate transport in detopped roots is altered relativeto the intact system in a way which permits greater NRA inductionand nitrate reduction. The results indicate that nitrate partitioningby detopped root systems should be interpreted with caution. Key words: Zea, nitrate reductase activity, nitrate uptake, nitrate reduction, nitrate partitioning, selection     

Nitrate Stimulation of Mobilization of Seed Reserves in Temperate Cereals: Importance of Water Uptake

Relationships between nitrate (NO^-₃) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (15–30%) with increased applied NO^-₃concentrationfrom 0 to 5–20 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH⁺₄) or 5 mMNO^-₃but NH⁺₄did not affect the rate of mobilization of seedreserves. Chloride (Cl^-) was similar to NO^-₃in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO^-₃or Cl^-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH⁺₄did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO^-₃or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO^-₃did not affect total seedling water or residualseed water content until 10–14 DAS. The effects of seedN and NO^-₃on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO^-₃is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization     

The Partitioning of Nitrate Assimilation Between Root and Shoot of a Range of Temperate Cereals and Pasture Grasses

Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO^-₃) content of root and shoot and NO^-₃ and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO^-₃ concentrations (0·1–20mol m^–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m^–3 NO^-₃ For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO^-₃ concentration on the partitioning of NO^-₃ assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m^–3 NO^-₃ and that for all cereals, shoot assimilationincreased in importance as applied NO^-₃ concentration increasedfrom 1.0 to 20 mol m^–3 At 5.0–20 mol m^–3 NO3,the data indicated that the shoot played an important if notmajor role in NO^-₃ assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO^-₃ assimilation at 0.5 mol m^–3NO^-₃ but shoot assimilation was predominant at 5.0 mol m^–3NO^-₃ Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m^–3 NO^-₃ Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap     

Growth of Perennial Ryegrass (Lolium perenne L.) Exposed to a Low Concentration of Sulphur Dioxide

The influence of a low concentration of SO₂ on the growth andsulphur status of perennial ryegrass (Lolium perenne L.) cv.S23 was examined in a system of specially designed growth chambers.For plants grown without additions of sulphate to the soil,erposure to ifitered air containing 50 µg m^–3 SO₂alleviated sulphur deficiency that developed during a sequenceof successive harvests, and gave increased yield. The plantshad higher contents of organic and inorganic sulphur than thosegrown with neither sulphate nor SO₂. Plants grown with addedsulphate showed no signs of sulphur deficiency and althoughexposure to SO₂ increased their sulphur content it had no effecton yield. The coefficient of transpiration (g water transpiredg^–1 dry wt. of shoots) was reduced when plants which otherwisehad an inadequate supply of sulphur were exposed to SO₂; forplants with adequate supplies of sulphate, it was similar whetherthey were grown with or without the addition of SO₂ to the air.The results are discussed in relation to other known reactionsof plants to SO₂.     

White Clover N2-Fixation in Response to Root Temperature and Nitrate: II. N2-FIXATION, RESPIRATION, AND NITRATE REDUCTASE ACTIVITY

Established, nodulated white clover plants were transferredto eight tanks of a flowing culture apparatus with solutiontemperatures of 5, 11, 17, and 25 ?C (two tanks per temperature).Shoot temperature and light environment were common to all plants.After 7 d, (10 mmol m^–3) was continuouslysupplied to one tank at each temperature while in the remainingfour tanks (one at each temperature) the plants were completelydependent on nodule N₂-fixation. Plants were randomly selected at intervals during the following14 d period in order to measure root and nodule respirationand acetylene reduction activity (ARA) in a flow-through systemset at the adapted root temperature. Additional plants wereassayed for in vitro nitrate reductase activity in leaves, roots,and nodules. Apparent nitrogenase activity (ARA) and respiration associatedwith it were each markedly affected by temperature in two ways;(1) Activity per unit weight of nodule was reduced at lowertemperatures; (2) Development of the plant, and thus also nodulemass, was restricted at lower temperatures which, in turn, restrictedtotal nodule activity per plant. The presence of nitrate significantly reduced ARA of nodules,particularly at higher temperatures. However, significant discrepancieswere found when N₂-fixation rates, estimated from the acetylenereduction assay, were compared with N₂-fixation rates calculatedfrom curves fitted to N accumulation data (minus the rate of uptake in the case of nitrate-treated plants). Carbon use efficiency (CO² respired per C₂H₄ produced) was notsignificantly affected by temperature or the presence of nitrate. Nitrate reductase activity (NRA) developed in all plant partsat the three highest temperatures, but not at 5 ?C. We calculatethat leaf NRA may account for 82, 75, and 68% of total nitratereduction at 11, 17, and 25 ?C respectively. Key words: Trifolium repens, white clover, N₂ fixation, root temperature, acetylene reduction assay, nitrate, nitrate reductase     

Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

Imsande, J. 1986. Inhibition of nodule development in soybeanby nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max(L.) Merr.] is inhibited by continuous growth in the presenceof 4· mol m^–3 KNO₃ The presence of 4·0 molm^–3 ‘starter nitrate’ for 3-6 d during noduledevelopment, however, subsequently stimulates nodule dry weightaccumulation and nitrogenase activity. These stimulations occureven though 4· mol m^–3 nitrate temporarily delaysnodule development, i.e. the late steps of nodule developmentare reversibly inhibited by a short-term exposure to 4·0mol m^–3 nitrate. On the other hand, treatment with 4·0mol m^–3 nitrate in excess of 14 d significantly reducesnodule dry weight Thus, extended growth in the presence of 4·0mol m^–3 KNO₃ seems to block both early and late stepsof nodule development. Nodulation of hydroponically grown soybeansis also inhibited by continuous growth in the presence of 2·0mol m^–3 (NH₄)₂SO₄ This inhibition is not caused by acidityof the growth medium. On the other hand, nodule development6 d after inoculation with Rhizoblum japonicum is not delayedby a 7-d exposure to 2·0 mol m^–3 (NH₄)₂SO₄ butis partially inhibited by a prolonged exposure to (NH₄)₂SO₄Because repression of nodulation by 4·0 mol m^–3KNO₃ is more severe than that by 2·0 mol m^–3 (NH₄)₂SO₄and because ammonium taken up by the soybean plant is not activelyoxidized to nitrate, it is suggested that there are at leasttwo mechanisms by which nitrate utilization represses noduleformation in soybean. Key words: Glycine max, nitrogen, nitrogen fixation, nodulation