Symbiotic Performance of Supernoclulating Soybean (Glycine max (L.) Merrill) Mutants during Development on Different Nitrogen Regimes

Growth and symbiotic performance of soybean (Glycine max (L.)Merrill) cv. Bragg and three of its induced nodulation mutants(nod49, non-nodulating; ntsl 116, intermediate supernodulator;nts1007, extreme supernodulator) were compared throughout developmentunder different nitrogen regimes (0, 2, 5 and 10 mol nitratem^–3). Nitrogen fixation was assessed using ¹⁵N-isotopedilution and xylem sap analysis for ureide content. Both techniquesconfirmed a complete lack of N₂ fixation activity in nod49.Plant reliance on nitrogen fixation by the other genotypes wasdependent on the nitrate regime and the developmental stage.The ntsl007 and ntsl 116 mutants fixed more nitrogen than theparent cultivar in the presence of 10 mol m^–3 nitratein the nutrient solution, but higher input of symbioticallyderived nitrogen was still insufficient to offset the amountof nitrogen removed in the harvested seed. However, the mutantsutilized less nitrate for growth than Bragg. Comparison of estimatesof N₂ fixation derived from the ¹⁵N-dilution technique withthose based on relative ureide content of xylem sap indicatedthat the latter offered a simple and reliable procedure forevaluating the symbiotic performance of supernodulating plants. Key words: ¹⁵N-isotope dilution, supernodulation, ureides     

Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO₂^–, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution     

Economy of Symbiotically Fixed Nitrogen in Red Clover (Trifolium pratenseL.)

The seasonal dynamics of symbiotic fixation, distribution andfate of nitrogen (N) were studied on two successive crops ofred clover (Trifolium pratenseL.) grown outdoors in soil containersunder the Mediterranean climate of southern France. Nitrogenaseactivity was followed throughout the growing season using acetylene(C₂H₂) reduction assays. The distribution and transfer of symbioticallyfixed N were followed by periodic measurements of¹⁵N distributionin plants after exposure of the root systems to labelled dinitrogen(¹⁵N₂). In both years there were two peaks of nitrogenase activity,one in spring and one in late summer, separated by a sharp decreaseduring the flowering period. Over the entire growth cycle, symbioticallyfixed N accounted for 61 to 96% of the total plant N. Once weekafter incorporation, 60 to 90% of N derived from the atmospherewas recovered in the aerial parts of the plants. More than 50%of this was in the leaves, but there were differences in distributionaccording to the stage of development. The maximum percentage(20–28%) recovered from nodulated roots occurred in May–June,during maximum growth of the vegetative organs, and in September.Above-ground symbiotically fixed N was highly mobile with time,moving from the rosette leaves to the leaves attached to theelongated stems and then to the seeds, where 25 to 50% of Nfixed in May and June was recovered in September. Because of:(1) the high turnover rate of leaves; and (2) the relativelyhigh N content of dead leaves, as much as 50% of the symbioticallyfixed N in a year was potentially available to the soil micro-organismsas litter. The maximum transfer was in spring and winter. Ofthe remainder, 20 to 35% was recovered in living plant partsduring regrowth in March of the second year. Transfers to andfrom the root system were less pronounced, but significant decreasesin N content of the roots occurred early in the second yearjust after foliage regrowth was initiated. It is concluded that,because of its high foliage productivity and turnover rate,and high yield of symbiotically fixed N, red clover is a goodcandidate to provide substantial amounts of N to the soil throughoutthe year and therefore restore N fertility. Red clover; Trifolium pratenseL.; forage legumes; labelled dinitrogen (¹⁵N₂) reduction; acetylene reduction; nitrogen fixation; nitrogen distribution; nitrogen transfer     

Nitrogen Uptake and Plant Growth I. Effect of Nitrogen Removal on Growth of Polygonum cuspidatum

Polygonun cuspidatum was grown hydroponically to examine theeffect of nitrogen removal from the nutrient solution upon plantgrowth and the partitioning of dry matter and nitrogen amongorgans. Nitrogen removal reduced the growth rate mainly dueto the reduced growth of leaf area. Accelerated root growthwas observed only in plants which earlier had received highlevels of nitrogen. Nitrogen removal caused almost exclusiveallocation of available nitrogen to root growth. Nitrogen fluxfrom the shoot to the root occurred in plants which had receivedlow nitrogen. Not only was net assimilation rate (NAR) littleaffected by nitrogen removal, but it also was not correlatedwith the concentration of leaf nitrogen on an area basis. Light-saturatedCO₂ exchange rate (CER) was highly correlated with the concentrationof leaf nitrogen. Nitrogen use efficiency (NUE) in CER (CERdivided by leaf nitrogen) remained constant against leaf nitrogen,indicating efficient use of nitrogen under light saturation,while NUE in terms of NAR decreased with higher concentrationof leaf nitrogen. Polygonum cuspidatum Sieb. et Zuce., CO₂ exchange rate, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency, partitioning of dry matter and nitrogen     

Biological Nitrogen Fixation in Pachyrhizus ahipa (Wedd.) Parodi

The patterns of plant growth and N₂ fixation capability in Pachyrhizusahipa (Wedd) Parodi inoculated with Bradyrhizobium ‘PachyrhizusSpec 1’ strains (Lipha Tech) were investigated in a zero-Nculture system under greenhouse conditions The P ahipa plantis day-neutral with respect to reproductive development Competitionoccurred between the two storage organs (legume and tuber) andprevented high tuber yield in P ahipa The symbiotic effectivenessof the association was high, as the profuse nodulation providedthe inoculated plants with adequate amounts of N Nodules werepresent throughout the cycle of P ahipa The change in rate ofN₂ fixation (RNF) and relative growth rate (RGR) was almostparallel during ontogenesis The developmental pattern of N₂fixation activity revealed that 65% of total N₂ fixation occurredafter N began to accumulate in the reproductive (pod wall plusseed) tissue During pod filling allocation of N compounds tothe seeds exceeded N₂ fixation, the pod walls being the primarysource of redistributed N, followed by the leaves. Pachyrhizus ahipa (Wedd) Parodi, ahipa, tuber crop, dinitrogen fixation, dry matter, N partitioning, reproductive growth     

Effect of Molybdenum and Inorganic Nitrogen on Molybdenum Redistribution in Black Gram (Vigna mungo L. Hepper) with Particular Reference to Seed Fill

Seeds used to plant a crop may contain sufficient molybdenum(Mo) to prevent subsequent Mo deficiency in the crop even whenthey are sown on Mo deficient soils. However, little is knownabout either the sources of the Mo acquired by the seed, orthe timing of its redistribution during seed development. Aglasshouse experiment was set up to examine the effect of Mosupply and nitrogen source on the redistribution of Mo withinblack gram, from full flowering to seed maturity. Treatmentscomprised two sources of N (symbiotic N₂fixation, NH₄NO₃), twolevels of Mo supply [nil (-Mo), 0.64 mg Mo kg^-1soil (+Mo)] andfour harvests (full flowering, early pod setting, late pod fillingand seed maturity). The redistribution of Mo in black gram wasexamined by determining changes over time in the content ofMo in plant parts at each growth stage. Molybdenum supply and the plant growth stage strongly affectedthe redistribution of Mo to the seed. In -Mo plants relianton symbiotic N₂fixation, Mo redistributed from roots, stemsand leaves was the only source of Mo for reproductive developmentsince, from full flowering until maturity, there was no netincrease in whole plant Mo. For pod and early seed development,the roots were the major source of Mo in -Mo plants. After latepod filling, nodules replaced roots as the major source of Mofor seed fill in -Mo plants. By contrast, for +Mo plants relianton symbiotic N₂fixation, Mo taken up from the soil after fullflowering could have supplied nearly 50% of the seed Mo. Themajor sources of Mo for seed filling in +Mo plants were middlestem leaves during early podding, and middle stems and pod wallsfrom late podding. Supplying NH₄NO₃to plants from sowing had little effect on Modistribution or redistribution in +Mo black gram plants. However,in -Mo plants it accelerated the loss of Mo from middle stemsand their leaves compared to nodulated plants. Black gram; Vigna mungo L. Hepper; distribution; molybdenum; nitrogen; nodules; redistribution; seed fill     

The Relation Between the Nitrogen Concentration and Photosynthetic Capacity of Potato (Solanum tuberosum L.) Leaves

Measurements of the rate of light-saturated photosynthesis (P_max)were made on terminal leaflets of potato plants growing in cropssupplied with 0, 3, 6, 12, 24 and 36 g N m^–2. Measurementswere made between 100 and 154 d after planting. Two types ofleaf were selected—the fourth leaf on the second-levelbranch (L₄, _B1) and the youngest terminal leaflet that was measurable(L_YM). Later, the total nitrogen concentration of each leaflet(N_L) was measured. A linear regression between P_max and N_L,common to both leaf positions, explained 68.5% of the totalvariation. With L₄, _B1 leaves there was a significant improvementin the proportion of variation explained when regressions withseparate intercepts and a common slope were fitted to individualfertilizer treatments. These results suggest that an increasingproportion of leaf nitrogen was not associated with the performanceof the photosynthetic system with increasing nitrogen supply.This separation between nitrogen treatments was not as clearfor L_YM leaves. Stomatal conductance to transfer of water vapourwas neither influenced by leaf position nor directly by nitrogensupply. Rather conductance declined in parallel with the declinein photosynthetic capacity. Solanum tuberosum, potato, nitrogen, photosynthesis, stomatal conductance, leaf     

N2Fixation Response to Drought in Common Bean (Phaseolus vulgarisL.)

Nitrogen fixation activity in common bean is generally thoughtto be low and sensitive to soil drying and, consequently, droughtcan have important negative effects on N accumulation and yieldpotential. The objectives of this research were to examine theresponse of N₂fixation to drought stress in common bean, andto test the hypothesis that drought sensitivity of N₂fixationin common bean is linked to ureide levels in the plants. Twoglasshouse experiments were conducted to compare the responsesof leaf transpiration and acetylene reduction activity (ARA)to soil water contents. ARA decrease during soil dehydrationwas found to lag behind the decline in transpiration. This indicatesthat ARA is relatively less sensitive to soil dehydration comparedto leaf gas exchange. Further, in comparing two cultivars therewas no consistent difference in the relative response of ARAand transpiration to soil drying. The ureide concentrationsmeasured in common bean plants were low, ranging from 0.1 to1.0 mmol l^-1in xylem sap exudates. Ureide concentrations inthe sap exudate varied significantly among the two genotypeseven though there was no difference in ARA response to drought.It was concluded that in common bean, the lower sensitivityof N₂fixation to drought compared to leaf gas exchange couldbe related to low ureide concentrations in petioles and xylemsap.Copyright 1998 Annals of Botany Company Phaseolus vulgaris,nitrogen fixation, drought stress, nodules, ureides.     

Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

During vegetative growth in controlled environments, the patternof distribution of ¹⁴C-labelled assimilates to shoot and root,and to the meristems of the shoot, was measured in red and whiteclover plants either wholly dependent on N₂ fixation in rootnodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot wereexposed to ¹⁴CO₂, nodulated plants of both clovers generallyexported more of their labelled assimilates to root (+nodules),than equivalent plants utilizing nitrate nitrogen, and thiswas offset by reduced export to branches (red clover) or stolons(white clover). The intensity of these effects varied with experiment.The export of labelled assimilate to growing leaves at the terminalmeristem of the donor shoot was not influenced by source ofnitrogen. Internode elongation in the donor shoot utilized nolabelled assimilate. Whole plants of white clover exposed to ¹⁴CO₂ on seven occasionsover 32 days exhibited the same effect on export to root (+nodules),which increased slightly in intensity with increasing plantage. Nodulated plants had larger root: shoot ratios than theirequivalents utilizing nitrate nitrogen. Trifolium repens, Trifolium pratense, red clover, white clover, nitrogen fixation, nitrate utilization, assimilate partitioning     

Nitrogen Deficiency Slows Leaf Development and Delays Flowering in Narrow-leafed Lupin

Effects of nitrogen (N) supply on leaf and flower developmentinLupinus angustifolius L. cv Merrit were examined in a temperature-controlledglasshouse. Low N supply (0.05 or 0.4 mM N) had little effecton leaf initiation but slowed leaf emergence on the main stemcompared with plants receiving high N supply (6.0 or 6.4 mMN), or with symbiotic N₂-fixation. Plants experiencing transientN deficiency had slower leaf emergence than plants with a continuoussupply of 6.4 mM N. Nitrogen supply did not affect the timeof floral initiation, which occurred within 4 weeks of sowing,by which time nine to ten leaves had emerged. However, the floweringof low-N plants was delayed by 68 to 220 °C d (i.e. 4–14d) even though they had fewer leaves. The effect of N deficiencyon flowering time was largely a result of slower leaf emergence. Lupinus angustifolius L.; nitrogen; leaf; flower initiation; thermal time; plastochron; phyllochron     

Responses of Noogoora Burr (Xanthium occidentale Bertol.) to Nitrogen Supply and Carbon Dioxide Enrichment

We studied the responses of Xanthium occidentale (Bertol.) (cockleburor Noogoora burr), a noxious weed, to atmospheric CO₂ enrichmentand nitrate-N concentrations in the root zone ranging from 0.5to 25 mM. CO₂ enrichment (1500 cm³ m^–3) increased dry-matterproduction to about the same extent (18 per cent) at all levelsof supplied N: most of the increment in dry matter was distributedequally between leaves and roots so that there was little effecton shoot-to-root dry-weight ratios. Growth was stimulated greatlyby N and plateaued at 12 mM supplied N. Shoot-to-root dry-weightand total N ratios increased with increasing N supply. CO₂ enrichmenthad no effect on the total amount of N accumulated by plants,but increased the N-use efficiency of leaves. Enriched plantshad lower concentrations and quantities of N in their leavesthan controls, and therefore lower shoot-to-root total N ratios.Little free NO₃^– accumulated in organs of control or enrichedplants. NO₃^– was the major form of N in xylem sap fromdetopped plants at low supplied NO₃-N, but amino N was equalin importance at high supplied NO₃-N in control and enrichedplants. Concentrations of NO₃^– were lower in the xylemsap of CO₂ enriched plants. It was concluded that the betterN-use efficiency of CO₂ enriched plants could result in increasedgrowth of X. occidentale in regions of marginal soil fertilityas atmospheric levels of CO₂ increase. CO₂ enrichment, nitrogen, Xanthium, Noogoora burr, cocklebur     

Cycling of amino compounds in symbiotic lupin

The composition of amino acids was determined in the xylem andphloem sap of symbiotic lupins grown under a variety of treatmentsdesigned to alter the rate of nitrogen fixation. Asparaginewas the major amino acid in both xylem and phloem with glutamine,glutamate and aspartate also major components. GABA had a highconcentration in the xylem while valine was a major componentin the phloem. Exposure to combined nitrogen in the form ofeither ammonium or nitrate caused a reduction in specific nitrogenaseactivity and was associated with subsequent changes in bothof the translocated saps. Inhibiting nitrogen fixation by exposingnodules to oxygen produced a lower amide to amine ratio in thexylem sap (1.3:1) compared with control and nitrate ratios (2.6:1)and ammonium ratios (7.1:1). Similar ratios for amide aminewere also observed in the phloem sap. Labelling studies using¹⁵N₂ to follow nitrogen fixation, ammonium assimilation andamino acid transport have shown rapid accumulation of labelinto glutamine with subsequent enrichment in glutamate, aspartate,alanine, and GABA. Asparagine was found in high concentrationsin nodules and became slowly enriched. Labelled nitrogen fixedand assimilated in nodules was detected 40 min later in stemxylem extracts, largely as the amides glutamine and asparagine.These experiments provide evidence that large amounts of nitrogenouscompounds are cycled through the root nodules of symbiotic plants(contributing approximately 50% of xylem N) and that differencesin the composition of the phloem sap may influence nodule growthand activity. Key words: Nitrogen fixation, nitrogen translocation, isotope labelling, legumes, GC-MS     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Nitrogen Economy of Post-fire Stands of Shrub Legumes in Jarrah (Eucalyptus marginata Donn ex Sm.) Forest of S.W. Australia

Hansen, A. P., Pate, J. S., Hansen, A. and Bell, D. T. 1987.Nitrogen economy of post-fire stands of shrub legumes in jarrah(Eucalyptus marginata Donn ex Sm.) forest of S.W. Australia.—J.exp. Bot. 38: 26–41. Growth, demography and N economy of 1–6-year-old standsof Acacia pulchella, A. alata, A. extensa and Bossiaea aquifoliumwere examined using population sampling to assess annual incrementsof N as living biomass, and returns of N as litter, seed anddead plants. Dependence on nitrogen fixation was assessed fromseasonal profiles of acetylene reduction, employing data fromprevious calibrations to convert C₂H₂ reduced to N₂ fixed. After2 years of slow growth and minimal reproduction all speciesgrew rapidly to achieve maximum or near maximum size and seedproduction. Intense self-thinning of stands occurred in thesecond and third years, especially in the highly dense standsof the smallest species, A. alata. Annual turnover in standsranged from 0?3 to 127 kg N ha^–1 year^–1, dependingon species current age and density of a stand. Returns of Nas litter and shed seed comprised small proportions of the annualbudgets, but returns due to plant death equalled or exceededincrements of living biomass in years when stands were thinningrapidly. Proportional dependencies of the species on fixed N₂were relatively high (13–61%) in first year seedlings,and then declined markedly to 1?1–3?4percnt; in the second,0?3–1?6% in the third, and, with one exception, to wellbelow 1% in the fourth and sixth year stands. Mean annual ratesof N₂ fixation over the 6-year post-fire period were 1?6 kgN ha^–1 year^–1 for A. alata, 0–49 for A. pulchella,0?19 for B. aquifolium and 0-10 for A. extensa Key words: Shrub legumes, post-fire N economy     

Rhizobium meliloti Mutants Defective in Symbiotic Nitrogen Fixation Affect the Oxygen Gradient in Alfalfa (Medicago sativa) Root Nodules

Rhizobium meliloti bacteroids carrying mutations in either fdxNor fixX isolated from alfalfa root nodules were shown to containthe nitrogenase proteins NifH, NifD and NifK. In contrast toan in vitro system of N₂-fixation based on R. meliloti wild-typebacteroids, nitrogenase activity could not be restored in crudeextracts of these mutant bacteroids by the addition of an artificialelectron donor, indicating that the nitrogenase proteins werepresent but not functional. ESR-studies revealed that both mutantslacked the FeMo-cofactor of nitrogenase. To analyse the roleof free O₂ on the damage of the nitrogenase components and theFeMo-cofactor in these mutant bacteroids, microelectrode studiesof O₂ concentrations and gradients within alfalfa root noduleswere carried out. R. meliloti mutants defective in other genesnecessary for symbiotic N₂-fixation were also included in thisstudy. Four distinct types of O₂ gradients were defined by theapparent presence or absence of an O₂ diffusion barrier andby the minimum internal O₂ concentration. These data clearlydemonstrated the influence of the microsymbiont on the O₂ gradientswithin the nodules. Nodules induced by Rm0540, an R. melilotimutant with altered exopolysaccharide production, which is notable to infect plant cells, did not contain an O₂ diffusionbarrier. In contrast, nodules containing a mutant defectivein dicarboxylate transport (dctA-), produced an O₂ gradientsimilar to the wild-type. Microelectrode measurements revealedH₂ concentrations in alfalfa wild-type nodules comparable tosoyabean, whereas no hydrogen could be detected in nodules harbouringthe dctA mutant or any other mutant strain. Key words: Nitrogen fixation, Rhizobium meliloti bacteroids, ferredoxin-like proteins, microelectrode studies     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

Growth and Biomass Allocation, with Varying Nitrogen Availability, of Near-isogenic Pea Lines with Differing Foliage Structure

The single-gene mutation afila in pea (Pisum sativum L.) resultsin the replacement of proximal leaflets with branched tendrils,thereby reducing leaf area. This study investigated whethertheafila line could adjust biomass partitioning when exposedto varying nutrient regimes, to compensate for reduced leafarea, compared with wild-type plants. Wild-type and afila near-isogeniclines were grown in solution culture with nitrate-N added toinitially N-starved seedlings at relative addition rates (R_N)of 0.06, 0.12, 0.15 and 0.50 d^-1. The relative growth rate (R_W)of the whole plants closely matched R_Nat 0.06 and 0.12 d^-1,but higher R_Nresulted in a slightly higher growth rate. At agiven R_N, the wild-type line had lower plant nitrogen statusthan the afila line. R_Wof the roots of the afila line was lessthan R_Wof the roots of the wild-type at the three higher ratesof N supply despite a greater accumulation of N in the rootsof the afila plants. Consequently, plant nitrogen productivity(growth rate per unit nitrogen) was lower for afila. Dry matterallocation was strongly influenced by nitrogen status, but nodifferences in shoot–root dry matter allocation were foundbetween wild-type and afila with the same plant N status. Theseresults imply that decreased leaf area as a result of the single-genemutation afila affects dry matter allocation, but only accordingto its effect on the nitrogen status. Copyright 2000 Annalsof Botany Company Pisum sativum, pea, nitrogen limitation, growth, shoot–root allocation, relative growth rate, nitrogen productivity, isolines     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Increased Defoliation Frequency Depletes Remobilization of Nitrogen for Leaf Growth in Grasses

Plants ofLolium perenneandFestuca rubrawere grown in sand culturereceiving all nutrients as a complete nutrient solution containing1.5 mMNH₄NO₃, and subjected to one of three defoliation treatments:undefoliated, defoliated on one occasion, or defoliated weekly.¹⁵Nlabelling was used to determine the rate of N uptake, allowingthe amount of N remobilized from storage for the growth of thetwo youngest leaves (subsequently referred to as ‘newleaves’) growing over a 14 d period after defoliationto be calculated. The total plant N uptake by both species wasreduced, compared with undefoliated plants, by both a singleand repeated defoliation, although neither caused complete inhibitionof uptake. Regularly defoliatedL. perennehad a greater reductionin root mass, concomitant with a greater increase in N uptakeper g root than did regularly defoliatedF. rubra. In both species,the amount of N derived from uptake recovered in the new leaveswas unaffected by the frequency of defoliation. BothL. perenneandF.rubramobilized nitrogen to the new leaves after a single defoliation,mobilization being sufficient to supply 50 and 41%, respectively,of the total nitrogen requirement. In regularly defoliated plants,no significant nitrogen was mobilized to the new leaves inL.perenne, and only a small amount was mobilized inF. rubra. Plantsachieved greater leaf regrowth when only defoliated once. Weconclude that increasing the frequency of defoliation of bothL.perenneandF. rubrahad little effect on the uptake of nitrogenby roots which was subsequently supplied to new leaves, butdepleted their capacity for nitrogen remobilization, resultingin a reduction in the rate of growth of new leaves. Lolium perenne; Festuca rubra; defoliation frequency; mobilization; root uptake; nitrogen     

Development of the Xylem Ureide Assay for the Measurement of Nitrogen Fixation by Pigeonpea (Cajanus cajan (L.) Millsp.)

Quantification of N₂ fixation by pigeonpea (Cajanus cajan (L.)Millsp.) in the field has proved difficult using techniquessuch as ¹⁵N isotope dilution, acetylene reduction and N difference.We report experiments to develop the ureide assay of N₂ fixationbased on extraction and analysis of xylem exudate. Plants ofpigeonpea cv. Quantum, inoculated with effective Rhizobium spp.CB756, were grown in a temperature-controlled glasshouse inlarge pots filled with a sand: vermiculite mixture, in waterculture and in a slightly acidic, red-brown earth in replicatedfield plots. Xylem exudate was collected as bleeding sap fromboth nodulated and unnodulated roots, and from detached nodules.Exudate was extracted also from detached shoots and stems ofpigeonpea using a mild vacuum (60–70 kN m^–2). Largedifferences in the composition of N solutes exported from rootsof N₂-dependent and nitrate-dependent plants suggested thatshifts in plant dependence on N₂ fixation may be reflected byconcomitant changes in N solutes. Thus, nodulated plants weresupplied throughout growth with either N-free nutrients or nutrientssupplemented with 1, 2, 5, 5, 10, or 20 mol m^{–3 15}. Plants were harvested at regular intervals fordry matter and vacuum-extracted exudate. The relative abundanceof ureides ([ureide-N/ureide-N + nitrate-N + -amino-N] ? 100)in the exudate was highly correlated with the proportion ofplant N (calculated using a ¹⁵N isotope dilution technique)derived from N₂ fixation. Two distinct phases of plant growthwere recognized and standard curves were prepared for each.The relationship between proportional dependence of plants onN₂ and xylem relative ureides was unaffected by mineral-N source,i.e. nitrate or ammonium. This result is discussed in relationto interpretation of material from field-grown plants. The effectsof plant genotype, strain of rhizobia, section of stem extracted,removal of leaves, time delay between shoot detachment and extraction,and diurnal characteristics were examined in order to identifypotential sources of error and to optimize sampling procedures. Key words: Ureides, allantoin, allantoic acid, N₂ fixation, pigeonpea, Cajanus cajan