Nodulation and Nitrogen Fixation of Faba Bean (Vicia faba L.) as Affected by Removal of the Cotyledons and Nitrate Supply

Cotyledons of faba bean (^{Vicia faba} L. cv. Fiord) were removedto determine whether an apparent delay in nodulation of thiscultivar could be attributed to an inhibitor from these organs.Cotyledons were left intact or excised from seedling plants14 and 18 d after sowing and plants grown with or without 2·5mm NO₃^–. Seedling growth was depressed when cotyledons were removed onday 14 but not when removed on day 18. Removal of the cotyledonsat day 14 reduced nodule number and nodule weight in the absenceof NO₃^–, but in the presence of NO₂^–, nodule numberwas unaffected and only nodule weight was reduced. Cotyledonremoval at day 18 increased both nodule number and nodule weightwith +NO₃^– but not with –NO₂^–. Acetylene reduction(AR) was markedly depressed by NO₃^–. Cotyledon removalat day 14 decreased AR but removal at day 18 resulted in anincrease in AR. We suggest from these results that faba beancotyledons have an inhibitory effect on nodule activity andon nodulation and this interacts with NO₃^–. This can beexplained through a ‘feed-back’ regulation of N₂fixation by soluble N in the seedling. Vicia faba, faba bean, nodule number, nodulation, nodule activity, acetylene reduction, N₂ fixation, cotyledon removal, nitrate     

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     

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     

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     

The effect of salinity on N fixation and assimilation in Vicia faba

Production of the faba bean in semi-arid and coastal areas maybe limited by the salt sensitivity of faba bean symbiosis. Accordingly,this study was done to analyse the effects of salt on the effectivesymbiosis of faba bean (Vicia faba L. var. minor cultivar Alborea)and salt-tolerant Rhizobium leguminosarum biovar. viciae strainGRA19. After 4 weeks of growth, the nutrient solutions weresupplemented with 50, 75 and 100 mM NaCl for 21 d. Plants wereharvested four times at weekly intervals, beginning at 4 weeks.Vicia faba tolerated low (50 mM NaCl) but not higher levels(75 and 100 mM NaCl) of salt stress. Salinity affected shootgrowth more than root growth. At the end of the culture, thetotal nitrogen content in the shoot was affected more than plantgrowth; conversely, in the root, growth was influenced morethan total nitrogen content. In nodules, nitrogen fixation (acetylenereduction activity) was more sensitive to salinity than ammoniumassimilation (glutamine synthetase and glutamate synthase). Key words: Glutamate synthase, glutamine synthetase, N₂ fixation, Rhizobium leguminosarum, salinity     

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     

Transport of Nitrate and Calcium into Legume Root Nodules

Nitrate transport into nodulated plants of soybean (Glycinemax), cowpea (Vigna unguiculata) and faba bean (Vicia faba)was investigated. Nitrate entering the root system of soybeandid not pass out of the vascular system into nodular tissuesin detectable quantities. On the other hand, nitrate could passfrom soil through the outer surface of nodules but did not penetratethe infected tissue. Similarly, nitrate was restricted to corticaltissues of cowpea and faba bean. Thus, nitrate cannot inhibitnitrogen fixation as a result of reduction to nitrite by nitratereductase within the bacteroid zone. These results are, however,consistent with an effect of nitrate on an oxygen diffusionresistance located in the nodule cortex. Unlike nitrate, measurable quantities of ⁴⁵calcium were transportedvia the xylem into infected and cortical tissues of soybeannodules: it also passed from the soil into the free space ofthe nodule cortex. Key words: Nitrate, legume nodules, calcium     

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     

A Mutation in Vicia faba Results in Ineffective Nodules with Impaired Bacteroid Differentiation and Reduced Synthesis of Late Nodulins

Although numerous reports have documented the effect of bacterially-inducedineffectiveness on root nodule structure, function, and plantgene expression, few studies have detailed the effect of theplant genome on similar parameters. In this report effective(N₂-fixing) broadbean {Vicia faba L.) and plant-controlled ineffective(non-N₂-fixing) broadbean recessive for the sym-1 gene werecompared for nodule structure, developmental expression of noduleenzyme activities, enzyme proteins, and mRNAs involved in Nassimilation, leghemoglobin (Lb) synthesis, and acetylene reductionactivity (ARA). During development of effective wild-type nodules,glutamine synthetase (GS), aspartate aminotransferase (AAT),phosphoenolpyruvate carboxylase (PEPC) and NADH-glutamate synthase(GOGAT) activities and enzyme proteins increased coincidentwith nodule ARA. The increases in GS, AAT, and PEPC were associatedwith increased synthesis of mRNAs for these proteins. Synthesisof Lb polypeptides and mRNAs during development of effectivenodules was similar to that of GS, AAT, and PEPC. By contrast,ineffective sym-1 nodules displayed little or no ARA and hadneither the increases in enzyme activities nor enzyme proteinsand mRNAs as seen for effective nodules. The effect of the sym-1gene appeared to occur late in nodule development at eitherthe stage of bacterial release from infection threads or differentiationof bacteria into bacteroids. High in vitro enzyme activities,enzyme polypeptides, and mRNA levels in parental effective noduleswere dependent upon a signal associated with effective bacteroidsthat was lacking in sym-1 nodules. Nodule organogenesis didnot appear to be a signal for the induction of GS, PEPC, AAT,and Lb expression in sym-1 nodules. Key words: Vicia faba, mutation, sym-1 gene, nodules     

Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation

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

Use of the Expolinear Growth Model to Analyse the Growth of Faba bean, Peas and Lentils at Three Densities: Fitting the Model

The expolinear equation for crop growth (Goudriaan and MonteithAnnalsof Botany66: 695–701, 1990) was fitted to measurementsof above ground dry weight made on two cultivars of each ofthree species, faba bean (Vicia fabaL.), peas (Pisum sativumL.)and lentils (Lens culinarsMedic.), each grown at three densitiesat the University of Reading, UK in 1992 and 1993. The expolinearequation fitted the data well but required frequent samplingto obtain good estimates of the parameters. The equation hasthree parameters,R_mthe maximum relative growth rate,C_ma maximumcrop growth rate, andt_bthe time at which the crop effectivelyreaches a linear phase of growth.R_mdid not differ between densities,cultivars or species but differed between years.C_mincreasedwith increased density and was lower for lentils than for fababeans or peas.t_bdecreased with increased density for faba beanbut not for the other species. Incorporating an extinction coefficientfor solar radiation and the maximum fraction of radiation interceptedenabled reasonably accurate time courses of leaf area indexto be derived, as suggested by Goudriaan (1994. In: MontiethJL, Scott RK, Unsworth MH, eds.Resource capture by crops. Nottingham:Nottingham University Press, 99–110).Copyright 1998 Annalsof Botany Company Expolinear equation, grain legumes, crop growth rate, crop density, relative growth rate, growth modelling, faba bean,Vicia fabaL., peas,Pisum sativumL., lentils,Lens culinarsMedic.     

Effects of Water Stress on the Respiratory and Nitrogen-fixing Activity of Soybean Root Nodules

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.     

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.     

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     

Antigenic Similarity in Urate Oxidases of Major Ureide Producing Legumes and Its Correlation with the Type of Peroxisome in Uninfected Cells of Nodules

Structural, biochemical, and immunological comparisons of nodulesfrom ten species of plants were made to determine if a correlationexists between nodule structure, ureide production, urate oxidaseactivity, and antigenic similarity in urate oxidase. In specieswith high urate oxidase activity and cross-reaction with soybeananti-urate oxidase [soybean (Glycine max), green bean (Phaseolusvulgaris), mung bean (Vigna radiata), cowpea (Vigna unguiculata)],the nodules were determinate and contained numerous interstitialcells, interspersed among the infected cells. Within the interstitialcells of the ureide producing nodules numerous peroxisomes werenoted and these peroxisomes appear to be structurally similar,viz. a large electron opaque core surrounded by a less electronopaque rim. Although hemp sesbania (Sesbania exaltata) noduleswere similar in ultrastructure to other ureide producers withdetectable urate oxidase activity, no cross-reactivity was observedwith anti-soybean urate oxidase. Amide producing nodules eithercontained no interstitial cells [e.g. Indian jointvetch (Aeschynomeneindica), showy crotalaria (Crotalaria spectabilis)} or interstitialcells with few peroxisomes [e.g. alfalfa (Medicago saliva),broad bean (Vicia faba), pea (Pisum sativum)] with little urateoxidase activity, exhibiting no cross-reaction with soybeananti-urate oxidase. These data indicate that the urate oxidasein most ureide producing nodules is very similar and, structurally,ureide producing nodules are organized in a specialized wayto carry out ureide assimilation in the uninfected interstitialcells. (Received June 19, 1986; Accepted January 12, 1987)     

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     

Reaction of faba bean plants to soil and foliar N application and K nutrition

The effect of different rates and methods of fertilizer nitrogen application and potassium nutrition on the root nodule formation of faba bean plants var. Nadwiślański and on their nitrogenase activity was studied. It was found that fertilizer N depressed the nodule formation and nitrogenase activity, but inhibitory effect of N was smaller when it was supplied to the leaves instead to the soil. Plants growing at higher K level were in a position allowing on better development of nodules and consequently higher N₂- fixation.     

Variations of Natural 15N Abundances in Leguminous Plants and Nodule Fractions

Natural ¹⁵N abundances of various tissues from twelve speciesof leguminous plants and those of nodule fractions from threespecies grown in Japan were determined by mass spectrometry.The ¹⁵N concentrations of non-nodular tissues of nodulated plantswere close to the value of atmospheric N₂. The nodules of soybean,mung bean, cowpea, kidney bean, adzuki bean, sword bean, whiteclover, Leucaena leucocephala, and horsegram showed highly elevated¹⁵N concentrations, and those of pea had a slightly enriched¹⁵N concentration compared to other tissue, while the nodulesof peanut and lucerne had ¹⁵N values similar to those of theirshoot tissues. The bacteroid fractions from horsegram, swordbean and peanut had higher ¹⁵N concentrations than the cytosolfractions. (Received March 17, 1984; Accepted August 21, 1984)     

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

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