Hydrogen Effect on Nitrogenase (C2H2 Reduction) Response to Oxygen in Bradyrhizobium japonicum-Phaseoleae Symbioses

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     

Regulation of Nodule Efficiency by the Undisturbed Soybean Plant1

Acetylene reduction activity (ARA) has been widely used to estimatecomparative rates of nitrogen fixation in soybean [Glycine max(L.) Merr.]. To use the non-destructive closed-container ARAassay to study plant-mediated regulation of N₂ fixation, itwas first necessary to determine the validity of the ARA procedure.Thus, well-nodulated, hydroponically-grown soybean plants wereindividually assayed for ARA twice weekly for 5 weeks. Plantswere subsequently analysed for total nitrogen by the Kjeldahlprocedure and the values obtained were compared with the N-accumulationvalues estimated by ARA. For the 168 plants examined, ARA measurementsunderestimated N₂ fixation by approximately 50%. However, N₂fixation was underestimated by approximately 80% in plants witha very low ARA whereas N₂ fixation was underestimated by only20% in plants with a high ARA. Because of the accuracy and reliabilityof the Kjeldahl measurements, these results show that nodulatedsoybean plants can exhibit different efficiencies in acetylenereduction. Thus, vigorous, well-nodulated, hydroponically-grownsoybean plants readily control nodule efficiency and the N fixationprocess.     

Phosphorus Deficiency Impairs Early Nodule Functioning and Enhances Proton Release in Roots of Medicago truncatula L.

The effect of phosphorus supply (1–15 µM) on protonrelease and the role of P in symbiotic nitrogen fixation inmedic (Medicago truncatula L. ‘Jemalong’) was investigated.As P concentration in the nutrient solution increased, shootand root growth increased by 19 and 15%, respectively by day35, with maximal growth at 4 µM P. A P concentration of15 µM appeared to be toxic to plants. Phosphorus supplyhad no influence on nodule formation by day 12 but increasednodule number by day 35. Nitrogenase activity was estimatedby in situ measurement of acetylene reduction activity (ARA)in an open-flow system. During the assay, a C₂H₂-induced declineof ARA was observed under all P concentrations except 4 µM.Specific ARA (per unit nodule weight) doubled when P supplywas increased from 1 to 8 µM. This effect of P was muchgreater than the effects of P on nodulation and host plant growth.Concentrations of excess cations in plants decreased with increasingP concentration in the nutrient solution. Phosphorus deficiencystimulated uptake of excess cations over anions by the plantsand hence enhanced proton release. The results suggest thatP plays a direct role in nodule functioning in medic and thatP deficiency increases acidification which may facilitate Pacquisition. Copyright 2001 Annals of Botany Company Medicago truncatula L. (medic), P deficiency, C₂H₂-ID, nitrogenase activity, proton release, cation-anion balance, open-flow system     

Phosphorus deficiency increases the argon-induced decline of nodule nitrogenase activity in soybean and alfalfa

Open-flow assays of H₂ evolution in Ar:O₂ (80:20, v/v) by nodulated roots were performed in situ with soybean [Glycine max (L.) Merr.] and alfalfa [Medicago sativa L.) grown in sand with orthophosphate (Pi) nutrition either limiting (low-P) or non-limiting (control) for plant growth. Nodule growth was more limited than shoot growth by P deficiency. Phosphorus concentration was less affected in nodules than in other parts of the low-P plants. During assays, nitrogenase activity declined a few minutes after exposure of the nodulated roots to Ar. The magnitude of this argon-induced decline (Ar-ID) was less in alfalfa than in soybean. In both symbioses the magnitude of the Ar-ID was larger in low-P than control plants. Moreover, the minimum H₂ evolution after the Ar-ID, was reached earlier in low-P plants. The Ar-ID was partly reversed by raising the external partial pressure of O₂ in the rhizosphere. The magnitude of the Ar-ID in soybean was correlated negatively to nodule and shoot mass per plant, individual nodule mass, H₂ evolution in air prior to the assay, and nodule N and P concentrations. Possible reasons, including nodule size and nodule O₂ permeability, for the increase in Ar-ID in P-deficient plants are discussed and an interpretation of the P effect on nodule respiration and energetic metabolism is proposed. Received: 17 May 1996 / Accepted: 16 September 1996     

Partitioning and Utilization of Photosynthate Produced at Different Growth Stages after Anthesis in Soybean (Glycine max L. Merr.): Analysis by Long-term 13C-Labelling Experiments

Yamagata, M., Kouchi, H. and Yoneyama, T. 1987. Partitioningand utilization of photosynthate produced at different growthstages after anthesis in soybean (Glycine max L. Merr.): Analysisby long term ¹³C-labelling experiments.—J. exp. Bot. 38:1247–1259. Soybean (Glycine max L. Merr. var. Akishirome) plants were allowedto assimilate ¹³CO₂ with a constant specific activity for 10h at different growth stages (a total of seven times at aboutone week intervals) after anthesis. The plants were harvestedperiodically until the time of full maturity and the partitioningof ¹³C into individual plant parts was investigated with anemphasis on the contribution of carbon assimilated at differentgrowth stages to the seed formation. Carbon assimilated at the middle to late seed-filling stagecontributed most to the seed production; one day contributionaccounted for 3–4% in total carbon of the seed at fullmaturity. Integrated contribution of carbon assimilated afteranthesis was estimated as 96% of the final seed carbon. An approximationbased on the temporal data of the incorporation of labelledcarbon into the seeds indicates that 77% of the final seed carboncame from direct transfer of current photosynthate from sourceleaves, which occurred within a few days after the photosyntheticfixation, while the rest originated from remobilization of carbonreserved mainly in leaves and stems plus petioles. In comparison with the total carbon accumulation in the seeds,protein carbon in the seeds was relatively more dependent onphotosynthate produced during the early period of reproductivegrowth stage, whereas lipid carbon was more dependent on photosynthateproduced during the later reproductive stage. Key words: Photosynthate partitioning, soybean (Glycine max L. Merr.), ¹³CO₂ assimilation, seed formation     

Short-Term Nitrate Effects on Hydroponically-Grown Soybean cv. Bragg and its Supernodulating Mutant: II. DISTRIBUTION AND RESPIRATION OF RECENTLY-FIXED 13C-LABELLED PHOTOSYNTHATE

Fully symbiotic or nitrate treated (3 d, 4·0 mol m^–3)soybean (Glycine max [L.] Merr.) cv. Bragg and a nitrate tolerantsupernodulating soybean mutant nts 1007 were exposed to ¹³Cenriched CO₂ for a period of 10 h. During this period and forthe subsequent 24 h, continuous measurements of ¹³CO₂ and ¹²CO₂evolution of their root systems were undertaken. Three harvestsduring the experiment allowed determinations of the distributionof recently fixed carbon in different plant organs. These measurementsindicated higher dependence of N₂ fixation in nts 1007 on recentlyfixed carbon (RFC) by showing elevated RFC concentrations innodules as well as their augmented respiration. Root respirationof both genotypes was generally more reliant on stored carbon. Nitrate induced in all measured parameters a clear responsein the mutant analogous to the wild type, but quantitative differencesremained throughout. Nodule respiratory activity, the relativespecific activity (RSA), and the utilization of RFC were substantiallyreduced, but remained higher in nts 1007 than in Bragg, whilethe demand of roots for RFC increased in Bragg more than inthe supernodulator. The elevated carbon requirement of the nodulecomplement of the mutant and a high dependence on recently fixedcarbon could be attributed to higher nodule growth and maintenancecosts of the supernodulating genotype and were not associatedwith augmented nitrogen fixation activity. This less efficientutilization of carbon and the associated almost parasitic characterof the nodule complement of nts 1007 is considered to be thecause of reduced growth of the mutant. No evidence was foundfor a physiologically based nitrate tolerance in terms of nitrogenfixation. Key words: Glycine max, nitrate, N₂fixation, respiration, carbon partitioning, steady-state labelling     

Root respiration associated with nitrogenase activity (c(2)h(2)) of soybean, and a comparison of estimates

Root respiration associated with symbiotic fixation in soybean (Glycine max [L.] Merr.) was estimated by four methods.

Averaged over the life of the plant, the root respires 5.8 milligrams C per milligram N accumulated from fixation. When nitrogenase (C₂H₂) activity and root respiration were decreased by treating roots briefly with 1.0 atmosphere O₂, the respiration associated with nitrogenase was estimated as 2.10 micromoles CO₂ per micromole C₂H₄.

When nitrogenase activity and respiration were decreased by addition of nitrate, the respiration associated with fixation was calculated as 2.90 micromoles CO₂ per micromole C₂H₄. Removing nodules from roots decreased fixation and root respiration, and the ratio was 4.08 micromoles CO₂ per micromole C₂H₄. When soybean plants were kept in prolonged darkness, then returned to light, the associated drop and recovery of respiration and nitrogenase activity had a ratio of 4.36 micromoles CO₂ per micromole C₂H₂.

     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Carbon Exchange Rate of Intact Individual Soya Bean Pods. 2. Ontogeny of Temperature Sensitivity

Diurnal temperature fluctuations induced change in soya bean-pod[Glycine max (L.) Merr.] carbon exchange rate (CER, where positiveCER represents CO₂ evolution). CER appeared to depend linearlyon temperature. Linear regressions of CER on temperature interceptedthe temperature axis at 5°C (i.e. zero CER at 5°C).Slopes of these regressions (i.e. temperature sensitivity) changedover the season. The CER-temperature sensitivity coefficient,K, (calculated from observed values of CER. pod temperatureand temperature intercept) rose from less than 0·02 mgCO₂ h^–1 pod^–1 °C^–1 during early pod-flll,peaked at over 0·04 mg CO₂ h^–1 pod^–1 °C^–1at mid pod-fill, and then declined during late pod-fill andmaturation. Glycine max (L.) Merr., Soya bean, carbon exchange rate, temperature     

Inhibition of nitrogenase activity and nodule oxygen permeability by water deficit

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 (P_o).These experiments showed a rapid decrease in acetylene reductionactivity (ARA) and nodule respiration. A consequence of thedecreased respiration rate was that P_o calculated by Fick'sLaw also decreased. However, these results following PEG treatmentwere in direct conflict with a previous report of stabilityin P_o 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     

Carbon Costs of Nitrogenase Activity in Legume Root Nodules determined using Acetylene and Oxygen

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     

Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: I. CARBOHYDRATE STATUS AND SHORT-TERM RECOVERY OF NODULATED ROOT RESPIRATION

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     

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     

The Influence of Carbohydrate Reserves on the Response of Nodulated White Clover to Defoliation

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     

Morphological Alterations and Root Nodule Formation inAgrobacterium rhizogenes-mediated Transgenic Hairy Roots of Peanut (Arachis hypogaeaL.)

Transformed hairy roots were induced at the excised site ofthe epicotyl of dry mature seed of a Spanish type peanut(Arachishypogaea)cv. Java 13 2 weeks after inoculation with a wild typestrain ofAgrobacterium rhizogenes,MAFF-02-10266. Composite plantsconsisting of transformed roots with non-transformed shootswere cultured using pouches. Forty days after inoculation, thecomposite plant showed a root system with abundant root mass,more lateral branching and high fractal dimension compared tothe control. No differences were observed in production of rosette-typeroot hairs or the cross sectional structure between transformedand non-transformed roots. The inoculation ofBradyrhizobiumsp.A2R1 strain to the composite plants led to the induction oftransformed root nodules. These transformed root nodules showedproduction of leghaemoglobin in the bacterial zone and nitrogenaseactivity as assayed by C₂H₂reduction, and exhibited enlargementof the nodule cortex region andde novoroot formation from thenodule cortex.Copyright 1998 Annals of Botany Company Agrobacterium rhizogenes;Arachis hypogaeaL.; composite plant; peanut; transformation; root nodule.     

Bacteroid oxalate oxidase and soluble oxalate in nodules of faba beans (Vicia faba L.) submitted to water restricted conditions: possible involvement in nitrogen fixation

The inhibitory effect exerted by water stress on acetylene reductionactivity (ARA) by nodulated roots of faba beans (Vicia fabaL.) was correlated with a 40% decline in the organic acid poolof nodule cytosol. Oxalate concentration was lowered (–55%)whereas a stimulation of the bacteroid oxalate oxidase concomitantlyoccurred. This enzyme was characterized by an optimal activityat pH 8 but, as in higher plants, exhibited a K_m for oxalateof 1.4 mM and an inhibition by substrate excess. Oxalate providedto bacteroid incubations supported C₂H₂ reduction up to 2.5mM whereas higher concentrations were strongly inhibitory. Incontrast, purified symbiosomes incubated with oxyleghaemoglobinreduced C₂H₂ in the presence of oxalate concentrations up to10 mM. The peribacteroid membrane (PBM), in controlling theoxalate flux to the bacteroids avoided the substrate inhibitionwhich would limit its efficiency. Thus, oxalate present in highconcentration in faba bean nodules could play a role as complementarysubstrate for bacteroids slowing down the nitrogen fixationdecline induced by water restricted conditions. Key words: Faba bean, water stress, oxalate, acetylene reduction, bacteroid     

Effects of low root zone temperatures on the early stages of symbiosis establishment between soybean [Glycine max (L.) Merr.] and Bradyrhizobium japonicum

It is known that low root zone temperatures (RZT) have moreeffect on infection and early nodule development than on nitrogenfixation by soybean [Glycine max (L.) Merr.]. However, therehave been no studies regarding how the low RZT inhibit the infectionstages of soybean. Two controlled environment experiments wereconducted to examine the effect of low RZT on bacterial attachmentto, and infection thread penetration of, soybean root hairs.The experimental designs were (1) plants maintained at 25, 17.5or 15C RZT, or transferred from 25 or 17.5 to 15C RZT at either0.5, 1, 2, or 7d after inoculation (DAI), (2) early symbioticestablishment between soybean and Bradyrhizobium japonicum wasexamined microscopically under three RZT (15, 17.5 and 25C).These results indicated that (1) keeping plants at 25C only0.5 DAI prior to transfer to a 15C RZT accelerates the onsetof N₂ fixation at 15C RZT by 6 d, (2) at RZT between 25 and17.5C the infection processes were progressively delayed astemperature declined, (3) RZT less than 17C strongly inhibitedinfection steps, such that when RZT dropped 8.5C from 25 to17.5C infection initiation was delayed 1 d, while when RZTdropped only 2.5C from 17.5 to 15C, infection initiation wasdelayed another 2 d. Key words: Bradyrhizobium japonicum, low temperature, nodulation, soybean     

Oxygen Influence on Foliar Nitrogen Loss From Soyabean and Sorghum Plants

An understanding of volatilization of nitrogen (N) from leavesof crop and weed species may be important to the improvementof crop production. Foliar N loss (both reduced and oxidizedforms), net CO₂ uptake, and transpiration rates were measuredconcomitantly at 30°C on soyabean (Glycine max (L.) Merr.)and sorghum (Sorghum bicolor (L.) Moench) leaves at low (1 percent), ambient (20 per cent), and high (40 per cent) levelsof oxygen. In soyabeans, maximum reduced and total N losseswere found at the highest O₂ concentration, and the lowest Nlosses were measured at the lowest O₂ level. Net CO₂ assimilationwas significantly reduced with increasing O₂ during two of threesamplings. Quantities of oxidized N lost were not altered. Sorghumshowed no significant effects from O₂ on N loss or net CO₂ assimilation.The increased ammonia released from soyabean foliage in thepresence of higher concentration of O₂ probably affects metabolicpathways that contribute to the total reduced N volatilization. Glycine max, Sorghum bicolor, CO₂ assimilation, nitrogen loss     

Short- and Long-Term Inhibition of Respiratory Carbon Dioxide Efflux by Elevated Carbon Dioxide

Dark carbon dioxide efflux rates of recently fully expandedleaves and whole plants of Amaranthus hypochondriacus L., Glycinemax (L.) Merr., and Lycopersicon esculentum Mill. grown in controlledenvironments at 35 and 70 Pa carbon dioxide pressure were measuredat 35 and 70 Pa carbon dioxide pressure. Harvest data and whole-plant24-h carbon dioxide exchange were used to determine relativegrowth rates, net assimilation rates, leaf area ratios, andthe ratio of respiration to photosynthesis under the growthconditions. Biomass at a given time after planting was greaterat the higher carbon dioxide pressure in G. max and L. esculentum,but not the C₄ species, A. hypochondriacus. Relative growthrates for the same range of masses were not different betweencarbon dioxide treatments in the two C₃ species, because highernet assimilation rates at the higher carbon dioxide pressurewere offset by lower leaf area ratios. Whole plant carbon dioxideefflux rates per unit of mass were lower in plants grown andmeasured at the higher carbon dioxide pressure in both G. maxand L. esculentum, and were also smaller in relation to daytimenet carbon dioxide influx. Short-term responses of respirationrate to carbon dioxide pressure were found in all species, withcarbon dioxide efflux rates of leaves and whole plants lowerwhen measured at higher carbon dioxide pressure in almost allcases. Amaranthus hypochondriacus L., Glycine max L. Merr., Lycopersicon esculentum Mill., soybean, tomato, carbon dioxide, respiration, growth     

Photosynthesis and photorespiration in soybean [Glycine max (L.) Merr.] chronically exposed to elevated carbon dioxide and ozone

The effects of elevated carbon dioxide (CO₂ and ozone (O₃) onsoybean (Glycine max (L.) Merr.] photosynthesis and photorespiration-relatedparameters were determined periodically during the growing seasonby measurements of gas exchange, photorespiratory enzyme activitiesand amino acid levels. Plants were treated in open-top fieldchambers from emergence to harvest maturity with seasonal meanconcentrations of either 364 or 726 µmol mol^–1 CO₂in combination with either 19 or 73 nmol mol^–1 O₃ (12h daily averages). On average at growth CO₂ concentrations,net photosynthesis (A) increased 56% and photorespiration decreased36% in terminal mainstem leaves with CO₂ enrichment. Net photosynthesisand photorespiration were suppressed 30% and 41%, respectively,by elevated O₃ during late reproductive growth in the ambientCO₂ treatment, but not in the elevated CO₂ treatment. The ratioof photorespiration to A at growth CO₂ was decreased 61% byelevated CO₂ There was no statistically significant effect ofelevated O₃ on the ratio of photorespiration to A. Activitiesof glycolate oxidase, hydroxypyruvate reductase and catalasewere decreased 10–25% by elevated CO₂ and by 46–66%by elevated O₃ at late reproductive growth. The treatments hadno significant effect on total amino acid or glycine levels,although serine concentration was lower in the elevated CO₂and O₃ treatments at several sampling dates. The inhibitoryeffects of elevated O₃ on photorespiration-related parameterswere generally commensurate with the O₃-induced decline in A.The results suggest that elevated CO₂ could promote productivityboth through increased photoassimilation and suppressed photorespiration. Key words: Photorespiration, CO₂-enrichment, ozone, climate change, air pollution