Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown inminus nitrogen culture were each partnered with a single seedlingof the root hemiparasite Olax phyllanthi. Partitioning of fixedN between plant organs of the host and parasite was studiedfor the period 4–8 months after introducing the parasite.N fluxes through nodules of Acacia and xylem-tapping haustoriaof Olax were compared using measured xylem flows of fixed Nand anatomical information for the two organs. N₂ fixation duringthe study interval (635 µg N g FW nodules^–1 d^–1)corresponded to a xylem loading flux of 0.20 µg N mm^–2d^–1 across the secretory membranes of the pencycle parenchymaof the nodule vascular strands. A much higher flux of N (4891µg mm^–2 d^–1) exited through xylem at the junctionof nodule and root. The corresponding flux of N from host xylemacross absorptive membranes of the endophyte parenchyma of Olaxhaustorium was 1.15 µg N mm^–1 d^–1, six timesthe loading flux in nodules. The exit flux from haustorium toparasite rootlet was 20.0 pg N mm^–1 d^–1, 200-foldless than that passing through xylem elements of the nodule.Fluxes of individual amino compounds in xylem of nodule andhaustorium were assessed on a molar and N basis. N flux valuesare related to data for transpiration and partitioning of Cand N of the association recorded in a companion paper. Key words: Olax phyllanthi, host-parasite relationships, N flux, Acacia, N₂ fixation     

Effectiveness of Lotus Root Nodules: III. EFFECT OF COMBINED NITROGEN ON NODULE EFFECTIVENESS AND FLAVOLAN SYNTHESIS IN PLANT ROOTS

When grown in a nutrient solution containing combined nitrogen(NH₄NO₃), Lotus pedunculatus and L. tenuis seedlings inoculatedwith a fast-growing strain of Rhizoblum (NZP2037) did neitherdevelop root nodules nor develop flavolans in their roots. Incontrast, the roots of nodulated seedlings growing in a nitrogen-freenutrient solution contained flavolans. Flavolan synthesis coincidedwith root nodule development on these plants. When added as a single dose, high concentrations of NH₄NO₃ (5and 10 mg N per plant) stimulated the growth of L. pedunculatusplants but suppressed nodulation and nitrogen fixation. In contrastthe continued supply of a low concentration of NH₄NO₃ (1?0 mgN d^–1 per plant) stimulated nitrogen fixation by up to500%. This large increase in nitrogen fixation was associatedwith a large increase in nodule fresh weight per plant, a doublingof nodule nitrogenase activity, and a lowering of the flavolancontent of the plant roots. The close relationship between nitrogendeficiency, nodule development, and flavolan synthesis in L.pedunculatus meant that it was not possible (by nitrogen pretreatmentof plants) to alter the ineffective nodule response of a Rhizobiumstrain (NZP2213) sensitive to the flavolan present in the rootsof this plant.     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

Effect of Salinity on Growth, Nodulation and Nitrogen Yield of Chickpea (Cicer arietinum L.)

Chickpea cultivar ILC 482 was inoculated with salt-tolerantRhizobium strain Ch191 in solution culture with different saltconcentrations added either immediately with inoculation or5 d later. The inhibitory effect of salinity on nodulation ofchickpea occurred at 40 dS m^–1 (34.2 mol m^–3 NaCl)and nodulation was completely inhibited at 7 dS m^–1 (61.6mol m^–3 NaCl); the plants died at 8 dS m^–1 (71.8mol m^–3 NaCl). Chickpea cultivar ILC 482 inoculated with Rhizobium strain Ch191^spcstrwas grown in two pot experiments and irrigated with saline water.Salinity (NaCl equivalent to 1–4 dS m^–1) significantlydecreased shoot and root dry weight, total nodule number perplant, nodule weight and average nodule weight. The resultsindicate that Rhizobium strain Ch191 forms an infective andeffective symbiosis with chickpea under saline and non-salineconditions; this legume was more salt-sensitive compared tothe rhizobia, the roots were more sensitive than the shoots,and N₂ fixation was more sensitive to salinity than plant growth. Key words: Cicer arietinum, nodulation, N₂ fixation, Rhizobium, salinity     

Nitrogen fixation byAlnus viridis (Chaix) DC.

Summary InAlnus viridis nodule growth relative to plant growth was inversely related to the quantity of nitrate added to nutrient solutions. Nodulated plants showed maximum growth when grown independently of supplied nitrogen and made better growth in its absence than unnodulated plants at any level of added nitrogen. Low levels of nitrate caused a depression of growth of nodulated plants, apparently by suppressing both nitrogen fixation and nodule growth. Nodules in nitrogen-free sand culture fixed atmospheric nitrogen at a rate of 6.6 mg/day/g nodule. Phosphorus deficiency was induced by low levels of phosphate and resulted in small plants with dark-green foliage. Root and nodule growth as a percentage of total plant growth and the percentage of total accumulated plant nitrogen below ground were greater at a root temperature of 11°C than 21°C. Thus at low root temperature processes other than nitrogen fixation were limiting to plant growth. Excised nodules were exposed to an N ₂ ¹⁵ -enriched atmosphere. A positive correlation between rate of nitrogen fixation and temperature was obtained, with optimum fixation occurring at about 20°C. It was shown that in spite of decreasing mean temperatures with increase in altitude, rate of nitrogen fixation by nodules of plants growing in the field increased with increase in altitude. This latter trend was deduced to be a reflection of the extent to which the field sites were nitrogen deficient in relation to climatically possible growth.     

Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

Fiskeby V soya bean was grown from seed germination to seedmaturation with two contrasting patterns of nitrogen metabolism:either wholly dependent on dinitrogen fixation, or with an abundantsupply of nitrate nitrogen, but lacking root nodules. The carbonand nitrogen economies of the plants were assessed at frequentintervals by measurements of photosynthesis, shoot and rootrespiration, and organic and inorganic nitrogen contents. Plantsfixing atmospheric nitrogen assimilated only 25–30 percent as much nitrogen as equivalent plants given nitrate nitrogen:c. 40 per cent of the nitrogen of ‘nitrate’ plantswas assimilated after dinitrogen fixation had ceased in ‘nodulated’plants. The rates of photosynthesis and respiration of the shootsof soya bean were not markedly affected by source of nitrogen;in contrast, the roots of ‘nodulated’ plants respiredtwice as rapidly during intense dinitrogen fixation as thoseof ‘nitrate’ plants. The magnitude of this respiratoryburden was calculated to increase the daily whole-plant respiratory loss of assimilate by 10–15 per cent over thatof plants receiving abundant nitrate. It is concluded that ‘nodulated’plants grew more slowly than ‘nitrate’ plants inthese experiments for at least two reasons: firstly, the symbioticassociation fixed insufficient nitrogen for optimum growth and,secondly, the assimila tion of the nitrogen which was fixedin the root nodules was more energy-demanding in terms of assimilatethan that of plants which assimilated nitrogen by reducing nitratein their leaves.     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: IV. EFFECT OF PHOSPHATE CONCENTRATION ON THE GROWTH OF TRIFOLIUM REPENS L. SUPPLIED WITH NITRATE, OR DEPENDENT UPON SYMBIOTICALLY FIXED NITROGEN

Breeze, V. G. and Hopper, M. J. 1987. The uptake of phosphateby plants from flowing nutrient solution. IV. Effect of phosphateconcentration on the growth of Trifolium repens L. suppliedwith nitrate, or dependent upon symbiotically fixed nitrogen.—J.exp. Bot. 38: 618–630. Nodulated white clover plants were subjected to a range of phosphateconcentrations in flowing solution culture (0.32 to 8.0 mmolm^–3 P) at 41 d from sowing, either supplied with nitrateor dependent on symbiotically-fixed nitrogen. No effect of phosphateconcentration in solution on dry matter production, relativegrowth rate, root/shoot ratio, or water soluble carbohydrateconcentration of the plant tissue was observed after 24 d fromthe start of the experiment, although the plants supplied withnitrate yielded more than the others. Phosphate uptake throughoutthe experimental period was related to the solution concentration,but the source of nitrogen did not affect the phosphorus concentrationsof the shoots. However, the roots of the plants dependent onsymbiotically-fixed nitrogen had higher concentrations of phosphorusthan those supplied with nitrate, but this did not appear tobe due to an increased phosphorus requirement for nitrogen fixation,because the amount fixed was unaffected by the phosphate concentrationin solution. The cation-anion balance showed that plants dependenton nitrogen fixation had no larger requirement for calcium thanplants supplied with nitrate, but a requirement for hydroxylions equivalent to over 130 kg lime per tonne of dry shoot.It is suggested that the enhanced phosphate uptake by plantsdependent on nitrogen fixation is due to this need for a cation-chargebalancing anion. Key words: Phosphate uptake, nitrogen fixation, Trifolium repens L., repens L., cation-anion balance, flowing solution culture     

Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation

Single plants of white clover, grown in a controlled environmentand dependent for nitrogen on fixation in their root nodules,were defoliated once by removing approximately half their shoottissue. Their regrowth was compared with the growth of comparableundefoliated plants. Two similar experiments were carried out:in the first, plants were defoliated at 2.5 g, and in the secondat 1.2 g total plant d. wt. Defoliation reduced rate of N₂ fixation by > 70 per cent,rate of photosynthesis by 83–96 per cent, and rate ofplant respiration by 30–40 per cent. Nodule weights initiallydeclined following defoliation as a result of loss of carbohydratesand other unidentified components. No immediate shedding ofnodules was observed but nodules on the most severely defoliatedplants exhibited accelerated senescence. The original rates of N₂ fixation were re-attained after 5–6or 9 d regrowth, with increase in plant size at defoliation.In general, the rate of recovery of N₂ fixation was relatedto the re-establishment and increase of the plant's photosyntheticcapacity. Throughout the growth of both defoliated and undefoliatedplants nodule respiration (metabolism) accounted for at least23 ± 2 per cent of gross photosynthesis. The unit ‘cost’of fixing N₂ in root nodules, in terms of photosynthate, appearedto be unaffected by defoliation, except perhaps for plants veryrecently defoliated. Similarly, the percentage nitrogen contentsof shoot, root and nodules of defoliated plants became adaptedwithin a few days to those characteristic of undefoliated plants. Trifolium repens, white clover, N₂ fixation, defoliation, photosynthesis, respiration     

Growth of Actinorhizal Plants as Influenced by the Form of N with Special Reference to Myrica gale and Alnus incana

Growth of two actinorhizal species was studied in relation tothe form of N supply in water culture. Non-nodulated bog myrtle(Myrica gale) and grey alder (Alnus incana) were grown withNH₄⁺, NH₄NO₃^– or NO₃^– (4 mol m^–3 N). A nodulatedseries of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/rootratio were highest for the two species with the N completelysupplied as NH₄⁺. In both species, nitrate was largely reducedin the roots and the presence of NO₃^– in combined-N supplyalways affected the RGR and N utilization rate negatively. BothN₂ fixation and complete NO₃^– nutrition represented conditionsof relative N-deficiency resulting in relatively low tissue-Nconcentrations and a greater allocation of dry mass to the roots.The physiological N status of nodulated M. gale plants was comparativelygood, as indicated by a normal nodule weight ratio and a relativelyhigh N₂-fixing rate per unit nodule mass. However, whole-plantN₂-fixing capacity remained relatively low in comparison withacquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directlyexcreted as an OH^– efflux. H ⁺ /N ratios generally agreedwith the theory. In comparison with NH₄⁺ nutrition, carboxylateconcentrations were higher in N₂-fixing M. gale plants and theH ⁺ /N ratio in nodulated plants was less than unity below thevalue for ammonium plants as previously found for other actinorhizalspecies. Therefore, NH₄⁺ should be an energetically more attractiveN source for actinorhizal plants than N₂. The results agree with commonly accepted views on energeticsof N uptake and assimilation in higher plants and support theconcept of a basically similar physiological behaviour betweennon-legumes and legumes. Key words: Actinorhizal symbioses, ammonium, H⁺/OH^– efflux, nitrate, N₂ fixation, NRA     

Responses of N2 Fixation-linked Respiration to Host-plant Energy Status in White Clover Acclimated to a Controlled Environment

Ryle, G. J. A., Powell, C. E. and Gordon, A. J. 1988. Responsesof N₂ fixation-linked respiration to host-plant energy statusin white clover acclimated to a controlled environment.—J.exp. Bot. 39: 879–887. Single plants of white clover, acclimated to a controlled environmentand dependent for nitrogen on N₂ fixation in their root nodules,were darkened, defoliated or exposed to enhanced CO₂ levelsto establish the quantitative relationships between the photosynthesisof the host plant and the N₂ fixation metabolism of root nodules. The nodule respiration associated with N₂ fixation (FLR) declinedrapidly to 10–15% of its normal rate following plant darkeningearly in the photoperiod. Darkening at progressively later intervalsduring the photoperiod demonstrated a positive, apparently linearrelationship between duration of illumination and total FLRduring the photoperiod and the following night period. Completeor partial defoliation reduced FLR according to the leaf arearemoved: again, there was a strong positive correlation betweencurrent rate of photosynthesis, whether of defoliated or undefoliatedplants and the FLR of root nodules. Doubling the current rateof photosynthesis, by enhancing CO₂ levels around the shoots,promoted FLR within 1–2 h when plants were stressed bylack of light. However, enhanced CO₂ levels increased FLR onlyslowly over a period of several hours in plants entrained tothe normal growing conditions. It is concluded that, in these plants acclimated to a uniformand favourable controlled environment, the supply and utilizationof photosynthetic assimilate in N₂ fixation was finely balancedand quantitatively linked during a single diurnal period andthat nodule functioning was not depressed by lack of energysubstrate. Key words: White clover, N₂ fixation, photosynthesis.     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: I. CARBON, NITROGEN AND MINERAL ELEMENT DISTRIBUTION, RESPIRATION AND THE EFFECT OF NITRATE ON NITROGENASE ACTIVITY

A comparison between two hydroponically-grown soybean genotypes(Glycine max [L.] Merr.) cv. Bragg and the supernodulating mutantnts 1007 was made in terms of dry matter accumulation, carbon,nitrogen, and mineral element distribution, ¹⁵N natural abundanceand the effect of short-term treatment with 4·0 mol m^–3KNO₃ on nitrogenase activity and respiration. Differences weremost pronounced in nodule dry weight and plant nitrogen content,both of which were recorded to be substantially elevated inthe mutant. Mineral element concentrations in different plantparts proved to be rather similar with the exception of Ca,found to be lower in leaves of the mutant, and Mn concentrationswhich were twice as high in roots of nts 1007. The values of¹⁵N natural abundance showed that both genotypes were equallydependent on nitrogen fixation when nitrate was absent. Theresults of the acetylene reduction assays indicated similarspecific nodule activity, while on a per plant basis nitrogenaseactivity of the mutant proved to be more than twice the amountof Bragg. This effect was also reflected in higher nodule respirationwhile root respiration remained below that of Bragg. Nitrate induced a substantial reduction in nitrogenase activitynot only in Bragg, but also in nts 1007. Nodule respiratoryactivity of Bragg was reduced by nitrate from 1·27 to0·34 mg C h^–1 plant^–1. In nts 1007 correspondingvalues were 2·70 to 1·52 mg C h^–1 plant^–1.Starch concentration in nodules was decreased in both genotypes,but nevertheless remained higher in nts 1007. Values for solublesugars in nodules even increased in the mutant in response tonitrate while the same treatment caused a reduction in Bragg.The data indicate that nitrogenase activities of Bragg and nts1007 are equally sensitive to short-term application of nitrate. Key words: Glycine max, C and N distribution, nitrate, root respiration, ¹⁵N natural abundance     

Comparative Growth and Symbiotic Performance of Seedlings of Acacia spp. in Defined Pot Culture or as Natural Understorey Components of a Eucalypt Forest Ecosystem in S.W. Australia

Hansen, A. P. and Pate, J. S. 1987. Comparative growth and symbioticperformance of seedlings of Acacia spp. in defined pot cultureor as natural understorey components of a eucalypt forest ecosystemin S.W. Australia.—J. exp. Bot. 38: 13–25 Growth, nitrogen accumulation, nodulation and nitrogenase activity(C₂H₂ reduction activity) were monitored in a dense stand ofseedlings of Acacia pulchella R.Br. and A. alata R.Br. duringtwo growing seasons after fire at a jarrah (Eucalyptus marginataDonn ex Sm.) forest site at Illawarra, 35 km south-east of Perth.Nitrogen fixation, estimated by a previously calibrated C₂H₂reduction assay, was essentially restricted to the winter andspring months (July to October) and was estimated to contribute37% and 9% respectively of the total N accumulated by A. pulchelladuring the first and second seasons of growth. Comparable valuesfor A. alata were 29% and 2%. Comparisons with fully symbioticplants raised in a glasshouse in supposedly non-limiting growthconditions in minus nitrogen sand culture indicated that waterstress rather than high temperatures was responsible for lossof nodules, cessation of symbiotic activity and attenuationof growth during summer in the field. By 19 months glasshouseplants had gained 130–230 times the dry weight and accumulated110–160 times the total N of similarly aged, field grownplants. Growth, nodulation and N₂ fixation of symbiotically-dependentsand cultured plants of A. pulchella and A. alata respondedmarkedly to phosphate, and additions of water and nutrientsto 3-year plants of A. pulchella during a winter growing seasonin the field indicated that periods of reduced soil moistureand low availability of nutrients, particularly phosphorus,might limit symbiotic performance under natural conditions. Key words: Seasonal nitrogen fixation, Acacia spp     

The Physiology and Nitrogen-fixing Capability of Aquatically and Terrestrially Grown Neptunia plena: The Importance of Nodule Oxygen Supply

The aquatic legume Neptunia plena (L.) Benth. was grown in non-aeratedwater culture or vermiculite. Growth, nodulation, nitrogen fixationand nodule physiology were investigated. Over an 80-d period,plants grew and fixed nitrogen and carbon equally well in bothrooting media, although distribution of growth between plantparts varied. Total nodule dry weights and volumes were similarbut vermiculite-grown plants had three times as many (smaller)nodules than those grown in water. Oxygen diffusion resistanceof nodules exposed to 21% oxygen and 10% acetylene did not differsignificantly. Both treatments showed similar declines in rootrespiration and acetylene reduction activity (approx. 10%) whenroot systems were exposed to stepped decreases and increasesin rhizosphere oxygen concentration. However, nitrogenase activityof aquatically grown plants was irreversibly inhibited by rapidexposure of nodules to ambient air, whereas vermiculite-grownplants were unaffected. Aeration of water-cultured N. plenareduced stem length (but not mass) and number of nodules perplant. The concentration of nitrogen fixation by 163%. PossibleO₂ transport pathways from the shoot atmosphere to roots andnodules are discussed. Aquatic legume, diffusion resistance, Neptunia plena, nitrogen fixation, oxygen, root nodules     

The Uptake and Distribution of Copper in Some Forage Grasses as Affected by Nitrate-nitrogen Supply in Flowing Solution Culture

The effect of changes in nitrate-nitrogen supply on the absorptionand distribution of copper was examined in grasses grown inflowing solution culture with a maintained concentration ofcopper. Absorption by roots (µg Cu g^–1 dry root)decreased markedly when nitrogen had been depleted or was maintainedat 0.1 mg l^–1 N, but there was an immediate increase whennitrogen was maintained at 1.0 or 10.0 mg l^–1. There werealso large increases in the concentration of copper in the shootsof plants grown with 1.0 and 10.0 mg 1^–1 N. The rootsof plants grown with 0.1 or 1.0 mg 1^–1 N retained similarproportions of uptake, but a lower proportion was retained whenthe plants were grown with 10.0 mg 1^–1. Although a lowerproportion of the copper was associated with cell walls in theplants grown at 10.0 mg 1^–1 N this was the result of alower content of cell walls rather than an effect on copperitself. In a longer-term experiment in conventional solutionculture with a range of nitrogen concentration, the concentrationof copper in shoots was largely determined by shoot growth. Dactylis glomerata, Festuca arundinacea, Lolium perenne, cell walls, copper absorption, copper distribution, flowing solution culture, nitrate-nitrogen     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: III. POST-ANTHESIS KINETICS OF NET NITRATE UPTAKE AND THE ROLE OF THE RELATIVE ROOT SIZE IN DETERMINING THE CAPACITY FOR NITRATE ACQUISITION

Barley (Hordeum vulgare L., cvs Golf, Mette, and Laevigatum)was grown under nitrogen limitation in solution culture untilnear maturity. Three different nitrogen addition regimes wereused: in the ‘HN’ culture the relative rate of nitrate-Naddition (RA) was 0·08 d^–1 until day 48 and thendecreased stepwise to, finally, 0·005 d^–1 duringgrain-filling; the ‘LN’ culture received 45% ofthe nitrogen added in HN; the ‘CN’ culture was maintainedat RA 0·0375 d^–1 throughout. Kinetics of net nitrateuptake were measured during ontogeny at 30 to 150 mmol m^–3external nitrate. V_max (which is argued to reflect the maximuminflux rate in these plants) declined with age in both HN andLN cultures. A pronounced transient drop was observed just beforeanthesis, which correlated in time with a peak in root nitrateconcentration. Similar, but less pronounced, trends were observedin CN. The relative V_max (unit nitrogen taken up per unit nitrogenin plants and day) in all three cultures declined from 1·3–2·3d^–1 during vegetative growth to 0·1–0·7d^–1 during generative growth. These values are in HN andLN cultures 15- to more than 100-fold in excess of the demandset by growth rates throughout ontogeny. Predicted balancingnitrate concentrations (defined as the nitrate concentrationrequired to support the observed rate of growth) were below6·0 mmol m^–3 in HN and LN cultures before anthesisand then decreased during ontogeny. In CN cultures the balancingnitrate concentration increased during grain-filling. Apartfrom the transient decline during anthesis, most of the effectof ageing on relative V_max can be explained in terms of reducedcontribution of roots to total biomass (R:T). The loss in uptakeper unit root weight is largely compensated for by the declinewith time in average tissue nitrogen concentrations. The quantitativerelationships between relative V_max and R:T in ageing plantsare similar to those observed for vegetative plants culturedat different RAs. The data support the contention that the capacity for nitrateacquisition in N-limited plants is under general growth control,rather than controlled by specific regulation of the biochemicalpathway of nitrate assimilation. Key words: Barley, nitrogen concentration, root: total plant biomass ratio, V_max     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Oxygen Diffusion in Lupin Nodules: I.VISUALIZATION OF DIFFUSION BARRIER OPERATION

Root nodules of Lupinus albus (L.) cv. Multolupa were subjectedto short- and medium-term stresses by lowering rhizosphere temperaturefrom 25 to 16°C (2 h), detopping plants (3 h), darkeningplants (21 h) or exposing roots to 20 mol m^–3 KNO₃ for4 d. All experimental treatments produced increases in oxygendiffusion resistance, compared with control plants. These correlatedwith structural changes in the nodule cortex, which is describedin detail for the first time. The most noticeable change isthe occlusion of intercellular spaces by a glycoprotein whichwas identified using the monoclonal antibody MAC236. This glycoproteinwas also found surrounding bacteria in intercellular spacesof the cortex of control nodules. Key words: Oxygen diffusion resistance, glycoprotein, nodules, nitrogen fixation, Lupinus albus     

Effect of Contrasting Patterns of Nitrate Application on the Nitrate Uptake, N2-Fixation, Nodulation and Growth of White Clover

White clover (Trifolium repens L.) plants were grown from seedin perlite, inoculated with effective rhizobia and exposed tothe same ‘concentration x days’ of ¹⁵N-labellednitrate in four contrasting patterns of doses. Acetylene reductionwas measured at intervals using an open, continuous-flow sytem.Mean dry weight per nodule and rates of acetylene reductionfell rapidly (2–3 d) during periods of exposure to highnitrate concentrations (> 7 mM N) and rose again, equallyrapidly, when nitrate was withdrawn or substantially reduced.The fall in mean dry weight per nodule (50–66 per cent)was almost certainly too large to be accounted for by loss ofsoluble or storage carbohydrate only. No new nodules were formedduring periods of high nitrate availability. When nitrate wassupplied continuously at a moderate concentration (5.7 mM N)nodule numbers stabilised although existing nodules increasedin dry weight by almost four-fold over the 30 d measurementperiod. Treatment had no effect on the percentage nitrogen in planttissues although there were large differences in the proportionsderived from nitrate and N₂-fixation. Plants exposed continuouslyor frequently to small doses of nitrate took up more nitrate,and hence relied less heavily on N₂-fixation, than those exposedto larger doses less often. Increased reliance on nitrate broughtwith it increased total dry weight and shoot: root ratios. Possiblemechanisms involved in bringing about these differences in nitrogennutrition and growth are discussed. White clover, Trifolium repens, nitrate, N₂-fixation, nodule, acetylene reduction, ¹⁵N     

The Effects of Aluminium on the Growth of White Clover dependent upon Fixation of Atmospheric Nitrogen

Jarvis, S. C. and Hatch, D. J. 1985. The effects of aluminiumon the growth of white clover dependent upon fixation of atmosphericnitrogen.–J. exp. Bot. 36: 1075–1086. The effects of aluminium (Al^{3 +}) on the growth of white cloverdependent upon symbiotic fixation of atmospheric nitrogen wereexamined at concentrations that may be encountered in solutionsfrom soils of low pH. Well established plants were grown inflowing solution culture with carefully maintained concentrationsof Al and of P and with pH held constant at a value (4.5) atwhich insoluble precipitates are unlikely to form. After 3 weeksof treatment, there were major visual differences between treatmentsin both shoots and roots. Although added Al at 25, 50 and 100mmol m^–3 reduced dry weight, the differences between theplants were not significant. There were, however, some considerabledifferences in P and Ca contents between the treatments. Muchof the Al held by the roots was displaced when plants were transferredto solutions containing either scandium or gallium. As wellas the effects on the plant, Al had a considerable influenceon the fixation of atmospheric nitrogen. At concentrations of50 and 100 mmol m^–3 Al³⁺ nodule initiation was reducedand there was a much reduced nodule nitrogenase activity perunit of root. Even at 25 mmol m^–3 Al³⁺, when nodule numberswere not reduced, nitrogenase activity was adversely affected. Key words: —Aluminium, Trifolium repens, nitrogen fixation     

The Influence of Root Temperature, Rhizobium Strain and Host Selection on the Structure and Nitrogen-fixing Efficiency of the Root Nodules of Trifolium subterraneum

Low root temperature greatly affected the structure and N2-fixingefficiency of root nodules. More nodule tissue was formed perplant at 11 and 15 °C than at 7 and 19 °C. Low roottemperatures either prevented or slowed bacteroid differentiation;the differentiation zone was 19 per cent of the total noduletissue at 7 °C but only 5 per cent at 19 °C. The amount of bacteroid tissue formed at the different roottemperatures by the two fully effective strains TAi and SU297reflected the environment from which they originated. Both formedthe same amount at 15 and 19 °C but only TAI, which originatedfrom a cold environment formed bacteroids at 7 °C. At 7°C a bacteroid-filled cell did not degenerate until after20 days, cf. less than 10 days at 19 °C. At 7 and 11 °Call strains formed more bacteroids in the abundantly nodulatingthan in the sparse host independently of nodule number. Strain0403 was most sensitive to both temperature and host; it formedbacteroids in nodules on the sparse host at 19 °C only,but formed bacteroids in the abundant host between 7–19°C. The amount of bacteroid tissue formed by TAI and SU297 dependeddirectly on nodule number and was approximately constant between20–40 days only at 19 °C when nodule formation hadalmost stopped. The optimum temperature for maximum fixation of nitrogen wasnot necessarily that for maximum efficiency of fixation, whichfor these experiments was 51 ug N mm-3 bacteroid tissue perday.