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.     

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     

Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C.E. 1987. Diurnal production and utilization of photosynthatein nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state ¹⁴C-labelling technique was used to examine thediurnal carbon fixation, storage and export characteristicsof white clover leaves. Approximately 70% of fixed carbon wasexported to other organs during the photoperiod. The remainingcarbon was stored mainly as starch (80% at the end of the photoperiod)with smaller amounts of sucrose, hexoses and charged compounds.Carbon export from the leaf at night was provided by remobilizationof starch. During the photoperiod it was estimated that c.60% of carbonexported from the leaf was directed towards the nodulated root;45% to nodules and 15% to roots. The 40% directed towards theshoot was supplemented by a further 11% of carbon (in the formof amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulatedcarbohydrate which was available for use during darkness, inconjunction with a diminished supply of exported carbon fromleaves. Nodules exhibited a striking pattern of carbohydratestorage and depletion. The levels of sucrose and starch in thenodules at the end of the photoperiod were sufficient to maintainN₂ fixation for 8–9 h of the 12 h dark period. We proposethat continued import from leaves provided the additional sucrosenecessary to support undiminished nodule function throughoutthe entire dark period. Key words: White clover, photosynthate, starch, carbohydrate, nodules, N₂ fixation     

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 Interactive Effects of Carbon Dioxide Enrichment and Daylength on Growth and Development inPetunia hybrida

Plants were grown at either 350 or 1000 µl l^-1CO₂and inone of three photoperiod treatments: continuous short days (SD),continuous long days (LD), or short switched to long days atday 41 (SD–LD). All plants received 9 h of light at 450µmol m^-2s^-1and LD plants received an additional 4 h oflight at 8 µmol m^-2s^-1. Growth of SD plants respondedmore positively to elevated CO₂than did LD plants, due largelyto differences in the effect of CO₂on unit leaf rate. High CO₂increasedheight and decreased branching under SD conditions, but hadno effect under LD conditions. Elevated CO₂also increased thenumber of buds and open flowers, the effect for flower numberbeing greater in short than in long days. The specific leafarea of plants grown at 1000 µl l^-1CO₂was reduced regardlessof daylength. High CO₂also decreased leaf and increased reproductiveallocation, the magnitude of these effects being greater underSD conditions. Bud formation and flower opening was advancedunder high CO₂conditions in SD plants but bud formation wasdelayed and there was no effect on flower opening under LD conditions.The effects of CO₂on plants switched from SD to LD conditionswere largely intermediate between the two continuous treatments,but for some parameters, more closely resembled one or the other.The results illustrate that daylength is an important factorcontrolling response of plants to elevated CO₂. Petunia hybridaHort. ex Vilm; carbon dioxide; photoperiod; functional growth analysis; daylength; global change; development; phenology     

Green Light Drives CO2 Fixation Deep within Leaves

Maximal ^l4CO₂-fixation in spinach occurs in the middle of thepalisade mesophyll [Nishio et al. (1993) Plant Cell 5: 953],however, ninety percent of the blue and red light is attenuatedin the upper twenty percent of a spinach leaf [Cui et al. (1991)Plant Cell Environ. 14: 493]. In this report, we showed thatgreen light drives ¹⁴CO₂-fixation deep within spinach leavescompared to red and blue light. Blue light caused fixation mainlyin the palisade mesophyll of the leaf, whereas red light drovefixation slightly deeper into the leaf than did blue light.¹⁴CO₂-fixation measured under green light resulted in less fixationin the upper epidermal layer (guard cells) and upper most palisademesophyll compared to red and blue light, but led to more fixationdeeper in the leaf than that caused by either red or blue light.Saturating white, red, or green light resulted in similar maximal¹⁴CO₂-fixation rates, whereas under the highest irradiance ofblue light given, carbon fixation was not saturated, but itasymptotically approached the maximal ¹⁴CO₂-fixation rates attainedunder the other types of light. The importance of green lightin photosynthesis is discussed. ¹Supported in part by grants from Competitive Research GrantsOffice, U.S. Department of Agriculture (Nos. 91-37100-6672 and93-37100-8855).     

The Effects of Exogenous Amino Acid on Acetylene Reduction Activity of Vicia faba L. cv. Fiord

The feed back control mechanism proposed to explain the inhibitionof N₂ fixation by N was investigated using Vicia faba cv. Fiord.Plants were grown under controlled conditions without mineralN in coarse river sand. Asparagine was supplied to plants activelyfixing N₂ by absorption through cut roots and via a wick ordirect injection into the stem just above the bottom leaf. Responsesin N₂ fixation were measured by acetylene reduction (AR). Feedingplants with [¹⁴C]-labelled asparagine showed that the amidewas taken up when exogenously applied. Asparagine (10 mM) suppliedby the above procedures resulted in a 50-70% inhibition of ARby 48 h. Glutamine produced a similar effect. The cut root methodallowed higher levels of these amides to be supplied but theinhibition observed with 10 mM asparagine was only increasedslightly with higher levels of the amide. The antibiotic Securopenprevented bacterial contamination of root solutions of asparagineand glutamine and had no effect on nodule activity. It is concludedthat accumulation of asparagine of glutamine or the resultantincrease in the pool of soluble N in the plant cause a feedbackeffect on the activity of nitrogenase.Copyright 1993, 1999 AcademicPress Vicia faba, faba bean, asparagine, inhibition of N₂ fixation     

Evidence for carbon flux shortage and strong carbon/nitrogen interactions in pea nodules at early stages of water stress

Symbiotic N₂ fixation in legume nodules declines under a widerange of environmental stresses. A high correlation betweenN₂ fixation decline and sucrose synthase (SS; EC 2.4.1.13) activitydown-regulation has been reported, although it has still tobe elucidated whether a causal relationship between SS activitydown-regulation and N₂ fixation decline can be established.In order to study the likely C/N interactions within nodulesand the effects on N₂ fixation, pea plants (Pisum sativum L.cv. Sugar snap) were subjected to progressive water stress bywithholding irrigation. Under these conditions, nodule SS activitydeclined concomitantly with apparent nitrogenase activity. Thelevels of UDP-glucose, glucose-1-phosphate, glucose-6-phosphate,and fructose-6-phosphate decreased in water-stressed nodulescompared with unstressed nodules. Drought also had a markedeffect on nodule concentrations of malate, succinate, and     

Root Nodules of Phaseolus: Efficient Scavengers of Molybdenum for N2-fixation

Molybdenum is thought to be of intermediate mobility in thephloem and this may limit N₂-fixation by restricting the supplyof molybdenum to the nodules of legumes. When no molybdenumwas supplied to Phaseolus vulgaris nodule Mo content increasedat the expense of shoots and roots even when seed molybdenumcontent was large. Nodules sampled from plants receiving molybdenumin the feeding solution had a concentration of 21–78 µgMo g^-1. In the absence of molybdenum and with deficient seedcontent (<0.5 µg Mo seed^-1) nodule concentrations rangedfrom 1.9 to 3.5 fig Mo g^-1 in a small seeded genotype and 8.7±0.48µg Mo g^-1 in a large seeded genotype. N₂-fixation in theseplants was not impaired except in one instance where noduleconcentration was 1.9 µg Mo g^-1. Evidence that molybdenumis effectively translocated from leaves to roots and noduleswas obtained using foliar treatments. All of the 3.3 µgMo applied to a leaf was recovered in the plant after 10 d.Mo content of the nodules increased by 81%, whilst Mo contentof shoots increased by 56%. Root Mo content was eight timesgreater than that in plants not receiving a foliar treatmentof molybdenum. We conclude that when molybdenum was scarce inthe plant it was mobile and was translocated from roots andshoots to the nodules. As a result, nodule concentrations andcontents of molybdenum were frequently maintained at amountssufficient for N₂-fixation even when the plant was entirelydependent on a small seed reserve of molybdenum.     

Growth and nitrogen assimilation in nodules in response to nitrate levels in Vicia faba under salt stress

This study analyses the effects of salt on the effective symbiosisof faba bean (Vicia faba L. var. minor cv. Alborea) and salt-tolerantRhizobium leguminosarum biovar. viciae strain GRA19 grown withtwo KNO₃ levels (2 and 8 mM). The addition of 8 mM KNO₃ to thegrowth medium increases plant tolerance to salinity even witha concentration of 100 mM NaCl. This KNO₃ level in control plantsreduced the N₂ fixation. For 2 and 8 mM KNO₃ the plants treatedwith NaCl reduced N₂ fixation to identical values. The activityof the enzymes mediating ammonium assimilation in nodules (GS,NADH-GOGAT and NADH-GDH) was decreased by high KNO₃ levels.The results show that NADH-GOGAT activity was more markedlyinhibited than was GS activity by salinity, therefore NADH-GOGATlimits the ammonium assimilation by nodules in V. faba undersalt stress. The total proline content in the nodule was notrelated to salt tolerance and thus does not serve as a salttoleranceindex for V. faba. Key words: Glutamate synthase, glutamine synthetase, N₂ fixation, nitrate, salinity     

Can the 15N Dilution Technique be used to Study N2 Fixation in Tropical Tree Symbioses as Affected by Water Deficit?

Three methods were used to study N₂ fixation and effects ofwater deficit on N₂ fixation: C₂H₂ reduction assay (ARA), ¹⁵Ndilution technique and accumulated N content. In addition, ¹⁵Ndilution was calculated both in a traditional way and in a modifiedway, which takes into consideration N and ¹⁵N content for theplants before the experiment started. The three methods wereapplied on the following Rhizobium-symbioses: Acacia albidaDel (Faidherbia albida (Del) A. Chev.) and Leucaena leucocephala(Lam) de Wit., and the Frankia-symbiosis Casuarina equisetifoliaL. The plants wereabout 4-months-old when they were harvested. Nitrogen derived from N₂ fixation in control plants of Acaciaalbida was 54·2 mg as measured with ARA, while it was28·5 mg as measured with the ¹⁵N dilution technique,compared to 30·7 mg calculated as accumulated N. In comparison,L. leucocephala fixed 41·6 mg N (ARA), 53·5 mgN(15N dilution technique) and 56·3 mg N (accumulatedN). The Frankia-symbiosis had fixed 27·4 mg N as measuredby ARA, 8·1 mg N as measured by ¹⁵N dilution techniqueand 12·3 mg N as accumulated N. There were no differencesbetween the estimates based ontraditional and modified waysof calculating ¹⁵N dilution. The immediate effect of water deficit treatment on N₂ fixationwas continuously measured inall species with ARA, which startedto decrease approximately 10 d after the initiation of the treatment,and declined to less than 5% of the initial level after 21–28d. The decrease in the amount of N derived from N₂ fixation wasstudied in L. leucocephala during the period of treatment. Therewas a 26% decrease in amount of N derived from N₂ fixation asresult of water deficit (as measured with ARA), while the decreasewas 23% when measured withboth the ¹⁵N dilution method and asaccumulated N. The three different methods for measuring N₂ fixation and effectsof water deficit on N₂ fixation are discussed. Key words: Acacia albida, ARA, Casuarina equisetifolia, Leucaena leucocephala, ¹⁵N dilution, N₂N fixation, water deficit     

Some Effects of Daylength, Temperature and Exogenous Growth Regulator Application on the Growth of Actinidia chinensis Planch

The growth responses of Actinidia chinensis raised from cuttingswere compared in 8 h short days (SD) and 16 h long days (LD)at 15, 20 and 25 °C, as well as under varying day and nighttemperatures The data obtained reveal effects on stem elongation,apparent plastochrons, leaf area and shape, as well as dry matteraccumulation and water contents of different plant parts Theseinvestigations were supplemented by studies on the effects ofapplied GA₃ and ethephon Alternating day/night temperatures(thermoperiodicity) increased leaf area and d wt accumulationin LD Effects on sugar and starch contents, are described anddiscussed Unexpected effects such as very high petiole watercontents and their continuous growth, increased twisting ofthe climbing stem in SD and other findings are also reportedand discussed Actinidia chinensis, Kiwi fruit, gibberellic acid, ethephon, temperature, photoperiod, themoperiodicity     

N2 Fixation and the Respiratory Costs of Nodules, Nitrogenase Activity, and Nodule Growth and Maintenance in Fiskeby Soyabean

The respiratory effluxes of nodules and of roots of FiskebyV soyabean (Glycine max (L.) Merr.), grown in a controlled environment,were measured at intervals in air and 3% O₂ from shortly afterthe onset of N₂ fixation until plant senescence. The respiratoryburdens linked with nitrogenase plus ammonia metabolism, andnodule growth and maintenance, were calculated from gas exchangedata and related to the concurrent rates of N₂ fixation. The specific respiration rates of nodules increased to a maximumof 21 mg CO₂ g^–1 h^–1 at the time pods began development:the equivalent maximum for roots was c. 4.5 mg CO₂ g^–1h^–1. Maximum nodule and root respiration rates per plantwere attained about 25 d later at the time N₂ fixation peakedat 15 mg N d^–1 plant^–1. The relationship between nodule respiration and N₂ fixationindicated an average respiratory cost of 13.2 mg CO₂ mg^–1N until the last few days of plant development Separation ofnodule respiration into the two components: nitrogenase (+ NH₃metabolism) respiration and nodule growth and maintenance respiration,indicated that the latter efflux accounted for c. 20% of nodulerespiration while N₂ fixation was increasing and new noduletissue was being formed. When nodule growth ceased and N₂ fixationdeclined, this component of respiration also declined. The respiratorycost of nitrogenase activity plus the associated metabolismof NH₃ varied between 11 mg CO₂ mg^–1 N during vegetativeand early reproductive growth, to 12.5 mg CO₂ mg^–1 N duringthe later stages of pod development. Key words: N2 fixation, Respiration, Nodules, Nitrogenase     

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N₂ fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with ¹⁵N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N₂ fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N₂fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N₂ fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of ¹⁵N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N₂ fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N₂ fixation, N remobilization, N₂ partitioning, regrowth     

The Effect of the Accumulation of Carbohydrate and Organic Nitrogen on Nitrogen Fixation (Acetylene Reduction) of Faba Bean cv. Fiord

Plants of faba bean cv. Fiord were grown under controlled conditions,without mineral N, in coarse river sand. Twenty-five days aftersowing when plants had at least eight fully opened leaves andwere nodulated and actively fixing N₂, half were topped andkept debudded for 21 d. Changes in dry weight, N₂ fixation (acetylenereduction activity), soluble carbohydrate, starch, soluble Nand total N in plants were monitored over the period. Both debudded and control plants grew and accumulated dry matter.Debudding resulted in a significant increase in the concentrationof soluble carbohydrate, starch and soluble N. but had onlya small effect on the total N concentration. A strong positivelinear relation between total plant weight and N content ofboth control and debudded plants showed that even under conditionsof excess supply of carbohydrate, faba beans have little capacityto store N. Soluble N accumulated in debudded plants presumablybecause less N was needed for the formation of new tissues thanin control plants. AR continued to increase throughout the experimentin control plants but declined in debudded plants from 6 to13 d after debudding and remained low until the end of the experiment.The decline was associated with an increase in available carbohydrateand in soluble N. The results of this experiment are consistentwith a feed back control of N₂ fixation by the soluble poolof N.Copyright 1994, 1999 Academic Press Vicia faba, faba bean, debudding, soluble N, inhibition of N₂ fixation     

Nitrogenase Activity in Response to Darkening and Defoliation of Alnus incana

Huss-Danell, K. and Sellstedt, A. 1985. Nitrogenase activityin response to darkening and defoliation of Alnus incana. —J.exp. Bot. 36: 1352–1358 In the Alnus-Frankia symbiosis the nitrogen-fixing root nodulesare one of the sinks for carbon compounds newly formed in photosynthesisand exported from the leaves (source). The source-sink ratioof cloned plants of Alnus incana was reduced by darkening orby total or partial defoliation and the resulting nitrogenaseactivity (C₂H₂-reduction) was measured. Nitrogenase activityhad nearly ceased 5 h after total defoliation but not untilca. 5 d after total darkening. Most of the activity was lostduring the initial hours and days, respectively. When leaf areawas reduced approximately by half nitrogenase activity decreasedslightly less than by half. Removal of upper leaves seemed lessharmful than removal of lower leaves one day after defoliation.On the following 2 d the treatments appeared to be similar.Thus, nitrogenase activity was largely dependent on newly formedassimilates but could also depend on stored reserves that weremobilized. Measurements of in vitro nitrogenase activity inroot nodule homogenates from darkened plants indicated thatnitrogenase gradually became inactivated and/or depleted after1 and 2 d in darkness Key words: Carbon supply, Frankia, nitrogen fixation     

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     

A Model of the Leaf Extension Rate of Tall Fescue in Response to Nitrogen and Temperature

The leaf extension rate (LER) of tall fescue (Festuca arundinaceaSchreb.) was studied in the field under various nitrogen andtemperature regimes. The LER was closely related to temperaturewhen N was not limiting plant growth. Two distinct relationshipsbetween the LER and the temperature were obtained, one for vegetativegrowth and one for the reproductive period. These relationships,described by a Gompertz function, were exponential at temperaturesbelow 8 °C and linear at temperatures above 8 °C. Theymade possible the calculation of an optimal LER correspondingto non-limiting N conditions for plant growth. The strong influence of the temperature on the LER was stillobserved under N limiting conditions. The N status of the swardswas described by the ratio between the actual N content (N_actual)and the optimal N content (N_optimal). The N_optimal was definedas the N content experienced at a non-limiting level of N nutritionbut without N luxury consumption. The N_optimal, expressed asa function of dry matter yield, declined during growth. Theeffect of the N status of the swards on the LER was analysedby calculating the ratio between the actual LER and the optimalLER, and relating it to the ratio between N_actual and N_optimal.It was shown that these two ratios were highly correlated. Leaf extension, Festuca arundinacea, nitrogen, temperature     

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     

The Effect of Defoliation on Carbohydrate, Protein and Leghaemoglobin Content of White Clover Nodules

Single white clover plants grown in pots of Perlite in a controlledenvironment and completely dependent on N₂ fixation were defoliatedto various degrees (46–85 per cent of shoot weight removed).The soluble protein content of nodules declined by about 20per cent and leghaemoglobin content by 50 per cent in the first4–7 d after defoliation but increased again to controllevels as new leaf tissue appeared. In the short term (2–3h) carbohydrate content of nodules declined to different extentsdepending on the severity of defoliation. The initial declinein N₂ fixation and the respiration associated with it, appearednot to be related to the instantaneous carbohydrate contentof nodules but rather to the supply of current photosynthatefrom the shoot. After 24–48 h, however, the carbohydratecontent of nodules had declined to low levels, regardless ofthe severity (46 or 71 per cent shoot removed) of defoliation.As new leaf tissue appeared carbohydrate levels in all partsof the plant gradually recovered towards control levels. Microscopic examination of nodule sections indicated that onlyafter very severe defoliation (80–85 per cent shoot removed)was nodule deterioration evident. Even here, as the plant establishednew leaves, the damage to nodules was repaired and no noduleloss was apparent. Trifolium repens, white clover, defoliation, carbohydrate, protein, leghaemoglobin