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1.
Diffusion resistance to oxygen within nodules was calculatedusing the respiratory quotient (RQ) of nodules from intact plantsof subterranean clover (Trifolium subterraneum L.) cv. SeatonPark nodulated by Rhizobiun trifolii WU95. From 21 to 52% O2,the RQ remained between 0.94 and 1.04, whereas at 10% O2, theRQ was 1.65. When nodulated roots of intact plants were exposedto sub-ambient pO2 in a continuous flow-through system, respirationdeclined immediately, followed by a partial recovery within30 min. The magnitude of the final respiration rate was dependentupon the pO2 in the gas stream. Initial rates of respirationwere re-established after 24 h at sub-ambient pO2 as a resultof changes in the resistance of the variable barrier to oxygendiffusion within the nodules. Nitrogenase activity also decreasedlinearly with decreasing pO2 in the gas stream, but partialrecovery occurred after 24 h incubation at sub-ambient pO2.Maximum rates of nitrogenase activity occurred at rhizosphereoxygen concentrations between 21% and 36% O2. Resistance tothe diffusion of oxygen within the nodules increased at supra-ambientpO2 and at oxygen concentrations above 36% O2, resulted in adecrease in both nitrogenase activity and nodulated root respiration.The diffusion resistance of nodules to oxygen increased rapidlyin the presence of either supra-ambient pO2 or saturating pC2H2.Reductions in nodule diffusion resistance either during recoveryfrom exposure to 10% acetylene or to sub-ambient pO2 occurredmore slowly. It is concluded that subterranean clover is welladapted for maximum nitrogen fixation at ambient pO2. Key words: Nitrogenase activity, oxygen, subterranean clover, diffusion resistance  相似文献   

2.
A growth-chamber study was carried out to determine whetherthe response of apparent nitrogenase activity (C2 H2 reduction)to complete defoliation is influenced by the availability ofcarbohydrate reserves Reserve carbohydrate (TNC) concentrationsof 6-week-old white clover (Trifoliun repens L) plants weremodified by CO2 pretreatments There was no difference in theresponse of apparent nitrogenase activity to defoliation betweenplants with different TNC concentrations C2H2 reduction activitydeclined sharply after defoliation and then recovered similarlyin both high- and low-TNC plants Further experiments were conductedto explain the lack of response of apparent nitrogenase activityto TNC levels Bacteroid degradation was ruled out because invitro nitrogenase activity of crude nodule extracts was stillintact 24 h after defoliation Sufficient carbohydrates appearedto be available to the nodules of defoliated plants becauseadding [14C]glucose to the nutrient solution did not preventthe decline in apparent nitrogenase activity These conclusionswere supported by the finding that an increase in pO2 aroundthe nodules of defoliated plants completely restored their C2H2reduction activity The comparison of the effects of defoliationand darkness suggested that the decrease in apparent nitrogenaseactivity was not related directly to the interruption of photosynthesisIt appears that lack of photosynthates is not the immediatecause of the decline of nitrogen-fixing activity after defoliation White clover, Trifolium repens L, defoliation, nitrogen fixation, regrowth, reserves, carbohydrates, acetylene reduction, nodule extract  相似文献   

3.
Exposure of mature, nodulated plants of white clover (Trifoliumrepens) cv. Blanca to 330 mg dm–3 NO3-N for 8 d causednitrogenase activity per plant to decrease by 80%. Total nodulatedroot respiration was not significantly affected but analysisof its components showed an 81% decrease in nitrogenase-linkedrespiration and a 340% increase in growth and maintenance respiration.Carbon costs of nitrogenase activity (mol CO2 respired per molC2H4 produced) increased by 45% over the exposure period. Sucrosecontent of the nodules decreased, but the pattern of decreasedid not correlate with that of nitrogenase activity. The oxygendiffusion resistance of the nodules was increased by a factorof five. Characterization of this resistance increase suggestsan abnormal modification of the diffusion barrier and it isconcluded that alteration in the oxygen supply to the bacteroidsis involved in the effect of nitrate on nitrogenase activity. Key words: Nitrogenase activity, nitrate, oxygen  相似文献   

4.
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  相似文献   

5.
Changes in nitrogenase activity (C2H2 reduction and H2 production),nodulated root respiration and the efficiency of nitrogenasefunctioning were measured in response to progressive dehydrationof nodules on intact well-watered plants of subterranean clover(Trifolium subterraneum L.) cv. Seaton Park. The nodulated rootsof vegetative plants grown to the 14-leaf stage were incubatedin a gas exchange system through which a continuous dry airstreamwas passed over an 8 d period. The root tips were immersed inan N-free nutrient solution during this time so that water andion uptake was unimpeded. The decline in nodulated root respirationresulting from nodule drying was associated with a continualreduction in respiration coupled to nitrogenase activity. Asnodule water potential (nod) decreased, the proportion of totalnodulated root respiration which was nitrogenase-linked declinedfrom 50% (day 1) to 33% (day 8). This was accompanied by a 79%reduction in specific nitrogenase activity (from 3.79 to 0.81umol C2H4 g–1 nodule dry weight min–1). Nodule dehydrationalso induced a decline in hydrogen (H2) production in air. Therelative decline in hydrogen production exceeded that of acetylenereduction activity and this resulted in an increase in the relativeefficiency of nitrogenase functioning. However, the carbon costof nitrogenase activity progressively increased above 2.0 molCO2 respired per mol C2H4 reduced as rood decreased below –0.4to –0.5 MPa. Consecutive measurements of the rates ofhydrogen evolution, 15N2 fixation and acetylene reduction activityon intact unstressed plants resulted in a C2H4/N2 conversionfactor of 4.08 and an electron balance of 1.08. These resultsindicated that the pre-decline rate of acetylene reduction activitymeasured in a flow-through system provided a valid measure ofthe total electron flux through nitrogenase. Key words: Subterranean clover, dehydration, efficiency, nitrogenase activity  相似文献   

6.
The aim of this study was to test the effect of oxygen partialpressure as a possible limiting factor of nitrogen fixationfollowing defoliation. The response of nitrogenase activity(C2H2-reduction) of defoliated and undefoliated white and redclover plants (Trifolium repens L. and Trifolium pratense L.)to either 19 kPa oxygen or 55 kPa oxygen was investigated. Priorto defoliation, white clover plants were grown for five weeksin a growth chamber, and red clover plants for 7 or 11 weeksin a glasshouse. The results included measurements of 16N2-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severelyrestricted nitrogenase activity of undefoliated white cloverplants; however, 2 h after complete defoliation, the same treatmentcaused a significant increase. A fivefold increase in nitrogenaseactivity upon exposure to the elevated oxygen partial pressurewas found at the end of a 24 h period. This beneficial effectdecreased gradually from 1 to 5 d after defoliation. The responseof recently defoliated red clover plants to 55 kPa oxygen partialpressure was similar to that of white clover, independentlyof plant age. The gradual recovery of nitrogenase activity duringthree weeks of regrowth was associated with a simultaneous changein the response to increased oxygen partial pressure, leadingagain to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogenfixation, resulting from a dramatic increase in oxygen-diffusionresistance, is the main factor limiting nitrogenase activityfollowing defoliation. Trifolium repens L., Trifolium pratense L., white clover, red clover, defoliation, regrowth, nodules, nitrogen fixation, nitrogenase activity, oxygen limitation  相似文献   

7.
In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO2 g–1h–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N2 fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N2,fixation: 9.1 µmol CO2 µmol–1N2. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO2 µmol–1 C2H4;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N2 fixation increased with plant growthfrom 1 to 22 µmol N2 per plant h–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N2 fixation indicated a value of 2.9 µmolC2H4 µmol–1 N2, for rates of N2 fixation between1 and 22 µmol N2 per plant h–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN2 fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N2, fixation, nitrogenase  相似文献   

8.
The influence of hydrogenase in Bradyrizobum-Phaseoleae symbioseswas studied ex-planta and in-planra in soybean (Glycine max)and cowpea (Vigna unguiculata). The hydrogenase was activatedby the addition of hydrogen in the incubation gas phase whichmodified the response of nitrogenase activity of Hup+ (hydrogenuptake positive) symbiosis to the external oxygen partial pressure.For bacteroids the hydrogenase expression increased nitrogenaseactivity at supraoptimal pO2, acting possibly as a respiratoryprotection of nitrogenase. However, at suboptimal pO2, nitrogenaseactivity of Hup+ bacteroids decreased with hydrogen, a phenomenonattributed to the lower efficiency of ATP synthesis from hydrogenthan from carbon substrates oxidation. For undisturbed nodules,the hydrogenase expression in soybean increased the optimalpO2 for ARA (COP), from 35.3 to 40.3 kPa O2, and the ARA atsupraoptimal pO2; at suboptimal PO2 there was a negative effectof hydrogenase on ARA, although this inhibition was less thanon bacteroids and was not detected if plants were grown at 15°C rather than 20 °C root temperature. No H2 effectwas detected on cowpea nodules. The results on soybean nodulesare consistent with the concept that symbiotic nitrogen fixationis oxygen-limited and that hydrogenase activity has no beneficialeffect on nitrogen fixation in O2 limitation. Key words: Glycine max, hydrogenase, nitrogenase, nitrogen fixation, nodules, Vigna unguiculata  相似文献   

9.
Seventy-five per cent of the N2-fixing activity (measured asthe reduction of C2H2 to C2H4) and 50 per cent of the respiratoryactivity of detached soybean root nodules was lost when thewater potential () of the nodules was lowered from approximately–1 ? 105 Pa (turgid nodules) to –9 ? 105 Pa (moderatelystressed nodules). Severely stressed nodules ( = –1.8? 106 Pa) showed almost total loss of N2-fixing activity andup to 80 per cent loss of respiratory activity. Increasing theoxygen partial pressure (PO2) from 104 to 105 Pa completelyrestored both N2-fixation and respiration in moderately stressednodules, but only partial recovery was possible in severelystressed nodules. The activity of the stressed nodules was verylow at low PO2 (5 ? 103 and 104 Pa). The C2H2-reducing activityof nodule slices, nodule breis, and bacteroids from turgid andmoderately stressed nodules was almost identical but some activitywas lost in the breis and bacteroids from severely stressednodules. Calculations showed that at low PO2 (104 and 2 ? 104Pa), the rate of O2 diffusion into severely stressed noduleswas ten times lower than that for turgid nodules, but only fourtimes lower at a higher PO2 (4 ? 104 Pa). Carbon monoxide inhibitionof C2H2 reduction was slower in stressed nodules than in turgidnodules. The results are discussed in view of the possible developmentof a physical barrier to gaseous diffusion and/or the possiblealtered affinity of the nodule leghaemoglobin for O2 in thewater-stressed nodules.  相似文献   

10.
There is a coupled decrease in respiration and nitrogenase activityof nodules of many legume symbioses induced by exposure to acetylenein the presence of 21% O2. The respiratory costs of nitrogenaseactivity can be determined directly and distinguished from respiratorycosts for growth and maintenance of roots and nodules, usingthe linear regression of respiration on nitrogenase activity.The regression gradient represents the carbon costs for thetransfer of one pair of electrons by nitrogenase in terms ofmoles CO2 released per mole of ethylene produced. The interceptof the regression is the growth and maintenance respirationof nodules or nodulated roots. Exposure to acetylene at decreasedor increased oxygen concentrations in the range from 10% to70% resulted in a wider range of values for CO2 production andnitrogenase activity that fell on the same regression line asvalues obtained during the acetylene-induced decline at 21%oxygen. Oxygen concentrations below 10% increased significantlythe proportion of anaerobic respiration and produced changesin nitrogenase activity not correlated with CO2 production.Provided that these limits are not exceeded, oxygen-inducedchanges in nodule activity in the presence of acetylene canbe used to measure the efficiency of those symbioses which donot exhibit an acetylene-induced decline at a fixed oxygen concentration. Respiratory cost (moles CO2/mole ethylene) remained relativelyconstant with plant age for detached pea nodules (2.8), attachednodulated roots of lucerne (2.5) and detached nodulated rootsof field bean (4.2). However, for lucerne and field beans theproportion of total root respiration coupled to nitrogenasedeclined with time. A survey of 13 legume species gave values from 2 to 5 molesCO2/mole C2H4 Rhizobium strain and host-dependent variationsin efficiency were found. Key words: Nitrogenase, Legume root nodules, Respiration, Oxygen  相似文献   

11.
A flow-through gas system was used to study the effects of disturbanceon nitrogenase (acetylene reduction) activity of nodulated rootsystems of soyabean (Glycine max) and white clover (Trifoliumrepens). Detopping plus removal of the rooting medium (by shaking)produced a substantial decrease in maximum nitrogenase activity.This response is due to a reduction in oxygen flux to the bacteroidscaused by an increase in the oxygen diffusion resistance ofthe nodule. The decrease in maximum nitrogenase activity wasmuch smaller for roots subjected to detopping only. Thus, theeffect of root shaking is more important than that of shootremoval. The effect of detopping plus root shaking on nitrogenase activityoccurred whether the plants were equilibrated and assayed at25°C or 15°C. However, the effect of disturbance onthe oxygen diffusion resistance of the nodules, and thus onnitrogenase activity, was greater at the higher temperature.At the lower temperature the oxygen diffusion resistance ofthe nodules had already been increased in response to the reducedrequirement for oxygen. These nodules were less susceptibleto the effects of disturbance. Thus, comparisons of the effectsof equilibration temperature on nitrogenase activity produceddifferent results depending on whether intact or disturbed systemswere used. With intact systems activity was lower at the lowertemperature but with detopped/shaken roots the lowest activityoccurred at the higher temperature. It is concluded that the use of detopped/shaken roots can producesubstantial errors in the acetylene reduction assay, which makesthe assay invalid even when used for comparative purposes. However,comparisons with rates of 15N2 fixation and H2 production showthat accurate measurements of nitrogenase activity can be obtainedfrom maximum rates of acetylene reduction by intact plants ina flow-through gas system. The continued use of assay proceduresin which cumulated ethylene production from disturbed systemsis measured in closed vessels must be questioned. Key words: Nodules, acetylene, nitrogenase activity  相似文献   

12.
Bacteroids, formed by the same strain of Rhizobium, were isolatedanaerobically from peanut and cowpea root nodules and theirC2H2 reduction activities were measured. Measurements were startedin a pure N2 atmosphere followed by stepwise addition of smallamounts of O2. The procedures may have general application andare described in detail. With increasing O2 level, a gradualincrease in nitrogenase activity was observed which reacheda peak, presumably at the optimum availability of O2 to bacteroids,and then declined. The maximum activity attained by isolatedbacteroids of cowpea was much higher than that obtained frommeasurement of activities of intact nodules and their bacteroidcontent, whereas for peanut the two were nearly equal. The resultsindicated that intranodular conditions are probably responsiblefor the difference in nitrogenase activities of peanut and cowpeanodules rather than the unique morphological modification ofpeanut bacteroids. Key words: Root nodules, Peanut, Cowpea, Bacteroids, Nitrogenase activity  相似文献   

13.
The effect of denitrification on N2 fixation was studied ina denitrifying photosynthetic bacterium, Rhodopseudomonas sphaeroidesforma sp. denitrificans. KNO3 inhibited diazotrophic growthin light, but NC3–-dependent diazotrophic growth in darknesswas found. NO3– inhibited C2H2 reduction activity in lightin cells grown with NO3–. NO3–-dependent C2H2 reductionactivity in darkness also was present in cells grown with N2plus NO3–, but not in cells grown on glutamate with NO3–.NO3– repressed the synthesis of nitrogenase in light.This repression was not overcome by the addition of methioninesulfoximine. The inhibitory and repressive effect of NO3– was not mediatedby the NO2– produced from NC3– nor by the NH4+ excretedinto the medium. But, as N2 fixation is controlled by O2 (redoxcontrol) it seems to be mediated through the denitrificationprocesses. Much of the glutamine synthetase was adenylylatedin cells grown with NO3– and its adenylylation state closelyparallelled nitrogenase activity in the cells. (Received September 4, 1982; Accepted December 11, 1982)  相似文献   

14.
The nodule water potential (nod) of subterranean clover (Trifoliumsubterraneum L.) cv. Seaton Park incubated in a flow-throughgas-exchange system was induced to decline independently ofleaf water potential (1) by passing a continuous dry airstreamover the nodulated roots of intact well-watered plants. Reducedtranspiration by plants whose nodules had become dehydratedwas hypothesized to be related to the decline in nitrogen fixation.Whole-plant and nodule soluble carbohydrates increased as noddeclined. Throughout an 8 d period of continual nodule dehydration,the gaseous diffusion resistance of nodules increased and theoptimum pO2 for nitrogenase activity declined from 52 to 28kPa. Following rehydration of the nodulated roots between days4 and 5 and between days 7 and 8, nodulated root respirationincreased to or above pre-stress levels whereas nitrogenaseactivity did not recover. Re-establishment of initial ratesof nodulated root respiration was due to the stimulation ofgrowth and maintenance respiration, not to the respiration coupledto nitrogenase activity. Although no recovery of nitrogenaseactivity occurred, the elapsed time from the introduction ofacetylene into the gas stream flowing past the nodules untilmeasurement of the acetylene-induced decline in nitrogenaseactivity, decreased substantially. This was characteristic ofan increase in the permeability of the nodules to gaseous diffusionupon rehydration. However, calculated values of nodule diffusionresistance after the 24 h periods of rehydration did not indicateany recovery of gaseous diffusion resistance based on measurementsof the respiration coupled to nitrogenase activity. Hence, useof a diffusion analogue (i.e. Fick's Law) in conjunction withnodule respiratory CO2 efflux was unable to predict changesin permeability of the variable barrier of legume nodules followingnodule dehydration and recovery. Key words: Subterranean clover, gaseous diffusion, respiration, carbohydrates, drought  相似文献   

15.
The gas exchange characteristics of intact attached nodulated roots of pea (Pisum sativum cv. Finale X) and lupin (Lupinus albus cv. Ultra) were studied under a number of environmental conditions to determine whether or not the nodules regulate resistance to oxygen diffusion. Nitrogenase activity (H2 evolution) in both species was inhibited by an increase in rhizosphere pO2 from 20% to 30%, but recovered within 30 min without a significant increase in nodulated root respiration (CO2 evolution). These data suggest that the nodules possess a variable barrier to O2 diffusion. Also, nitrogenase activity in both species declined when the roots were either exposed to an atmosphere of Ar:O2 or when the shoots of the plants were excised. These declines could be reversed by elevating rhizosphere pO2, indicating that the inhibition of nitrogenase activity resulted from an increase in gas diffusion resistance and consequent O2-limitation of nitrogenase-linked respiration. These results indicate that nodules of pea and lupin regulate their internal O2 concentration in a manner similar to nodules of soybean, despite the distinct morphological and biochemical differences that exist between the nodules of the 3 species. Experiments in which total nitrogenase activity (TNA = H2 production in Ar:O2) in pea and lupin nodules was monitored while rhizosphere pO2 was increased gradually to 100%, showed that the resistance of the nodules to O2 diffusion maintains nitrogenase activity at about 80% of its potential activity (PNA) under normal atmospheric conditions. The O2-limitation coefficient of nitrogenase (OLCN= TNA/PNA) declined significantly with prolonged exposure to Ar:O2 or with shoot excision. Together, these results indicate a significant degree of O2-limitation of nitrogenase activity in pea and lupin nodules, and that yields may be increased by realizing full potential activity.  相似文献   

16.
This paper describes the construction and operation of H2 specificmicroelectrodes and their application to measurements of H2concentrations, H2 gradients and H2 inhibition of N2-fixationin legume root nodules. Electrode construction was similar to that of O2 specific microelectrodespreviously reported. They comprise an outer casing drawn toa 2–20 µm tip plugged with silicone rubber and aconcentric inner electrode made from glass coated platinum wire.The exposed tip of the Pt wire was placed close to the siliconeplug and polarized positively at 0?4 V with respect to an internalAg reference electrode. With an internal electrolyte of KC1/HC1current flow through the electrode was proportional to H2 concentrationand independent of CO2 and O2. With appropriate amplificationand screening the detection limit for this system was 0?0001atm H2 (4?0 µmol m–3). Within newly detached nodulesof Hup–ve symbioses of soyabean, pea and clover H2 concentrationvaried from 0?009 to 0?014atm compared with 0?021 atm in lupinnodules. In nodules formed by the Hup–ve soyabean/RCR3442symbiosis internal pH2 increased from 0?012 atm to 0?09 atmwhen external pO2 was raised to 0?60 atm. Hydrogen could notbe detected within nodules of the Hup+ve Clarke/RCR3407 symbiosiseven when N2 in the gas phase was replaced with Ar and externalpO2 was increased to 0?60 atm. An assessment of H2 inhibition of nitrogen fixation in the soyabean(Clarke/RCR3442) symbioses involved measurements of H2 productionat increasing internal H2 levels, induced by stepped increasesin gas phase H2 concentration. The initial relative efficiencyof 0?66 (calculated from the pH2 of nodules exposed to air andAr/O2 mixtures) started to decrease at an internal pH2 of 0?02to 0?03 atm and fell by 80% to 0?18 at an internal pH2 of 0?1atm. This threshold value for inhibition is above the measuredmean H2 concentration for this symbiosis of 0?01 atm. Hydrogen gradients through the nodule showed a sharp increasein the region of the inner cortex, which was reciprocal to adecrease in O2 concentration, and a shallow gradient throughthe infected zone. These results indicate that the inner airspaces in the nodule are interconnected and confirm that thebarrier to O2 diffusion is located in the inner cortex. Key words: Root nodules, hydrogen, hydrogenase, oxygen  相似文献   

17.
Short-term effects of water deficit on nitrogenase activitywere investigated with hydroponically grown soybean plants (Glycinemax L. Merr. cv. Biloxi) by adding polyethylene glycol (PEG)to the hydroponic solution and measuring nitrogenase activity,nodule respiration, and permeability to oxygen diffusion (Po).These experiments showed a rapid decrease in acetylene reductionactivity (ARA) and nodule respiration. A consequence of thedecreased respiration rate was that Po calculated by Fick'sLaw also decreased. However, these results following PEG treatmentwere in direct conflict with a previous report of stabilityin Po determined by using an alternative technique. To resolvethis conflict, an hypothesis describing a sequence of responsesto the initial PEG treatment is presented. An important findingof this study was that the response to water deficit inducedby PEG occurred in two stages. The first stage of decreasednodule activity was O2-limited and could be reversed by exposingthe nodules to elevated pO2. The second stage which developedafter 24 h of exposure to PEG resulted in substantial loss innodule activity and this activity could not be recovered withincreased pO2. Severe water deficit treatments disrupt noduleactivity to such a degree that O2 is no longer the major limitation. Key words: Glycine max, N2 fixation, soybean, oxygen permeability, water deficit  相似文献   

18.
The interaction between carbon substrates and O2 and their effects on nitrogenase activity (C2H2) were examined in detached nodules of pea (Pisum sativum L. cv “Sparkle”). The internal O2 concentration was estimated from the fractional oxygenation of leghemoglobin measured by reflectance spectroscopy. Lowering the endogenous carbohydrate content of nodules by excising the shoots 16 hours before nodule harvest or by incubating detached nodules at 100 kPa O2 for 2 hours resulted in a 2- to 10-fold increase in internal O2, and a decline in nitrogenase activity. Conversely, when detached nodules were supplied with 100 millimolar succinate, the internal O2 was lowered. Nitrogenase activity was stimulated by succinate but only at high external O2. Oxygen uptake increased linearly with external O2 but was affected only slightly by the carbon treatments. The apparent diffusion resistance in the nodule cortex was similar in all of the treatments. Carbon substrates can thus affect nitrogenase activity indirectly by affecting the O2 concentration within detached nodules.  相似文献   

19.
The effects of different NaCl concentrations on the growth andnitrogen fixation activity of white lupin (Lupinus albus [L.])was studied over a 6 d period. Plant growth parameters, photosynthesisand shoot respiration were unaffected by NaCl concentrationsup to 150 mol m–3. However, nitrogenase activity decreasedwith increased NaCl concentration up to 100 mol m–3, whilstthe O2 diffusion resistance increased with 100 mol m–3NaCl, but showed no further change when 150 mol m–3 NaClwas applied for 6 d. Increases in NaCl concentration decreasednodular starch content while increasing sucrose content, suggestingan osmotic regulation. These changes were associated with a77% decrease in sucrose synthase activity. The effect on theO2 diffusion resistance was paralleled by changes in glycoproteincontent of the nodules, as determined by immunogold localizationand ELISA. X-ray microanalysis studies of nodules showed that,following a 6 d exposure to 150 mol m–3 NaCl, Na+ ionswere largely excluded from the infected zone, whilst only lowlevels of Cl- ions penetrated into this region. Na+ entry intoroots and leaves was also at a low level. Leghaemoglobin contentdecreased with saline stress, as did superoxide dismutase; whichdecreased by 36% following exposure to 100 mol m–3 saltfor 6 d. These results are discussed in relation to the relativesalt tolerance of the Multolupa/ISLU-16 symbiosis. Key words: Salt stress, nodules, nitrogen fixation, oxygen diffusion, carbohydrates, Lupinus albus  相似文献   

20.
From homogenates prepared from surface-sterilized nodules ofseedlings of Casuarina cunninghamiana grown aeroponically, astrain of Frankia designated HFPCc13 was isolated and has beengrown in pure filamentous culture in a defined synthetic nutrientmedium. Vesicle and sporangium formation can be induced by removalof combined nitrogen from the medium.Frankia strain HFPCc13nodulates young seedlings of C. cunninghamiana and C. equisetifoliawithin three weeks of inoculation with an optimum root mediumpH of 6–7 for nodulation and optimum temperature of 30–35°C. The presence of combined nitrogen in the root mediuminhibits nodulation with NH4+ more inhibitory than NO3.Frankia HFPCc13 does not nodulate Allocasuarina species withinthe same family nor several other possible actinorhizal plantstested. Thus this strain is quite precise in its host specificity.The rate of acetylene reduction was greater in C. cunninghamianathan the closely related species C. equisetifolia. In neitherof these host species were vesicles observed to occur withinthe infected root nodules which had been demonstrated to beactively fixing dinitrogen. Root nodules were shown to be activein acetylene reduction over a range of O2 concentration in thegaseous environment with an optimum at about 20 per cent O2,the ambient PO2 of the air. The mechanism(s) for oxygen protectionof nitrogenase within the filamentous form of Frankia withinthese nodules remains to be explained. Casuarina, Frankia, nodulation, nitrogen fixation  相似文献   

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