Effects of NaCl on Growth, Nitrogen Incorporation and Chemical Composition of Inoculated and NH4NO3 Fertilized Vicia faba (L. ) Plants

Vicia faba cv. Maris Bead was grown either on fixed nitrogenor on ammonium nitrate. After 4 weeks growth, nutrient solutionswere supplemented with 50, 75 and 100 mol m^–3 NaCl for15 d. Five harvests were made at weekly intervals, beginningat 4 weeks. Effects of salinity were directly related to dose,plant growth (fresh and dry weight) being depressed in bothN-fixing and N-fertilized plants. The number of nodules perplant and the proportion of those formed which developed intothe active nitrogen fixing state were depressed by salt stress.Increased size of nodules in salt-stressed plants only partlycompensated for the lower specific nitrogenase activity. Theeffects of salinity on plant nitrogen content were more pronouncedon N-flxing than on N-fertilized plants. The former took upmore Na⁺ and Cl^– than the latter: the implications ofthis and of ionic imbalance are discussed. Key words: Vicia faba, Growth, Salt stress, Nodulation     

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 Effect of Reduction in the Number of Nodules on Nodule Activity of Faba Bean (Vicia faba cv. Fiord)

Faba bean (Vicia faba L. cv. Fiord) plants were raised in agrowth room for 5 weeks and then transplanted to a hydroponicsystem. After 48 h for acclimation, nine plants were removed(day 0) for the measurement of nitrogenase activity by acetylenereduction (AR), for determination of nodule number and noduleweight, volume of the active N₂ fixing region (VAR), and volumeof the senescent N₂, fixing region (VSR). Half the nodule populationon a further 18 plants was excised, and nine of these plantswere assayed for AR. The nine plants from which the noduleshad been removed (treated plants) and nine control plants witha full complement of nodules, were left to grow for 5 d, afterwhich they were all harvested and assayed. The average weight of nodules and VAR remained constant in thecontrol plants between day 0 and day 5, whereas the nodulesleft on the treated plants increased in weight by 1.2 timesand VAR by 2.2 times. By day 5, VAR per plant was the same inthe control plants as in the treated, whereas VSR of controland treated plants increased by 4.6 and 2.2 times, respectively.Removal of half of the nodules at day 0 halved the AR activityper plant, but specific activity remained the same. After 5d, however, the nodules of the treated plants showed the sametotal activity as those of the control. Thus the specific activityof the nodules left on the treated plants doubled after 5 din response to excision. The indeterminate nodule of faba bean appears to be able toincrease its specific activity substantially in response toincrease in the demand for fixed N. N₂ fixation per nodule wasresponsive to substrate supply, in that halving the number ofnodules on a plant induced the remaining nodules to increaseactivity, presumably because they could use the assimilate previouslydistributed over a larger number of nodules. Victa faba, faba bean, nodule number, nodule activity, acetylene reduction, volumes of active and senescent N₂ fixing regions of nodules, hydroponic system     

A Light and Scanning Electron Microscope Study of Stem Nodules in Vicia faba L

Stem nodules were observed on plants of Vicia faba L. cv. ThrowsMS grown in a variety of environmental conditions. Observationsby light and scanning electron microscopy indicated that theseorgans were morphologically similar to root nodules on the sameplants. Nodules arose following infection of stem hairs andsubsequent growth of infection threads into the stem cortex.They developed to a mature, nitrogen-fixing state.     

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     

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)     

Effect of Osmotic Stress Upon Endogenous Hormone Levels in Euphorbia lathyrus L. and Vicis faba L.

Ethylene concentrations rise in stems, lacunae and roots ofEuphorbia Iathyrus subjected to osmotic stress. In contrastto other species, however, no such changes are observed in leaves.Similarly, ABA concentrations rise in the leaves of osmotically-stressedplants but little change was observed in roots. In no case wasthere any strong evidence for the existence of a threshold ofosmotic potential above which a response will not occur. Instead,marked increases in concentrations of both ABA and ethylenewere obtained in response to osmotic stresses which did notresult in significant changes in water saturation defecit (WSD)during the experiment. In Vicia faba, time course studies at constant osmotic potentialshowed that ABA concentrations increased with duration of stressand increased WSD but the effect was most marked in youngertissues for a given level of stress. Concentrations of boundABA showed a similar pattern with time but here the change wasleast marked in the younger tissues. Euphorbia Iathyrus, Vicia faba, endogenous growth regulators, osmotic stress     

Some Effects of Abscisic Acid and Water Stress on Stomata of Vicia faba L.

Vicia faba seedlings grown under a plastic tent in the laboratorywere either watered well throughout their growth period or weresubjected to a water stress treatment for several days priorto an experimental treatment. The effects of a further waterstress treatment or an application of an aqueous solution ofabscisic acid (ABA) on the stomata of these plants were determined.Stomata of previously water-stressed plants proved to be moresensitive than stomata of well watered plants to ABA appliedthrough the petiole via the transpiration stream and sprayedonto leaf surfaces. Stomata of previously water-stressed plantsclosed more rapidly and to a greater degree than stomata ofwell watered plants. The hormone had only a small effect whenapplied directly to epidermal fragments removed from both groupsof plants. Stomata of plants which had received a water stresspretreatment were less sensitive to a subsequent period of waterstress than were stomata of previously well watered plants.It is proposed that stomatal adaptation to water stress maybe related to changes in the hormonal balance of the plant.     

The effect of different water regimes on plant growth and nodule structure of greenhouse-grown winged bean (Psophocarpus tetragonolobus)

Plants of Psophocarpus tetragonolobus cv. Chimbu Illinois were germinated under uniform conditions, after which they were transferred to three water treatments, ‘wet’, control and ‘stress’. Those plants given most water (i. e. ‘wet’) grew best and produced most root nodules. Water stress delayed nodule formation and quantitative microscopic examination showed that nodules from stress plants had more bacteroid-containing cells per unit area up to 4 wk than nodules from either of the other treatments. Bacteroid-containing cells were found to be vacuolated in P. tetragonolobus.     

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     

Water Retention Capacity in Root Segments Differing in the Degree of Exodermis Development

Water loss from roots back into drying soil is a problem ofpractical importance in plants growing under conditions of verylow substrate water potential, such as dry or saline areas.Root exodermis is relatively impermeable and has been suggestedto play a protective role against water loss. The relative waterretention ability was compared in root segments from exodermal(maize, onion, sunflower, Rhodes grass and sorghum) and non-exodermalspecies (Pisum sativum,Vicia fabaand wheat). Apical and basalsegments from exodermal roots, with different degrees of exodermisdevelopment, were also compared, as were segments from sorghumroots in which the exodermis thickness had been modified bysubjecting the plants to a 30 d water stress treatment. Waterretention was significantly higher in segments from exodermalroots. In each root, water loss was higher in apical than inbasal segments, regardless of the presence of exodermis. Insorghum, prolonged drought treatment increased exodermis thickeningin nodal roots, however, no differences in rates of water losswere observed in segments obtained from control and droughtedplants. Soil sheaths formed around roots of Rhodes grass growingin very dry soil with the epidermis adhering tightly to thesheath. In plants growing in the field, soil sheaths may bemore effective than the exodermis in preventing root water loss.Copyright1999 Annals of Botany Company. Root, exodermis, rhizosheaths, water loss.     

An Early Cytochemical Marker of Commitment to Stelar Differentiation in Meristems from Dicotyledonous Plants

A cytochemical study of root apices from Vicia faba and Pisumsativum showed esterase activity to be present in the stele,root cap and rhizodermis, but almost completely absent fromthe developing cortex and quiescent centres. The meristem cellsgiving rise to the cortex were almost negative whilst thosegiving rise to the stele were positive for esterase activity.Cambia from roots, shoots and petioles of a number of dicotyledonousspecies were all positive for esterase activity. It is proposedthat esterase activity may be used as an early marker of commitmentto differentiation into stele in roots of dicotyledonous plants,and that the cambia are fully committed meristems. Pisum sativum L., Vicia fabaL., garden pea, broad bean, meristems, stelar differentiation, esterase activity, xylem differentiation, cytochemistry, cambium     

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

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

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     

The Effect of Irradiance on the Recovery of Soybean Nodules from Sodium Chloride-Induced Senescence

Soybean (Glycine max L. Merr) cv. Clarke plants inoculated withBradyrhizobium japonicum strain RCR3407 were grown either ina greenhouse with a low irradiance (200–400)µmolm^–2 s^–1) or in a controlled-environment growth cabinetwith a higher irradiance (600 µimol m^–2 s^–1).At 42 d plants were given a nitrogen-free nutrient solutioncontaining 50 mol m^–3 sodium chloride for 2 weeks andthen allowed to recover from salt-stress for a further 2 weeks. Salt treatment reduced plant growth by at least half in bothgrowth regimes, however, the controlled environment-grown (CEG)plants were five times larger than the greenhouse-grown (GG)plants in terms of dry weight and number/weight of nodules perplant, regardless of treatment. The structure of nodules, from both growth regimes, harvestedat the end of the 2 week salt-stress was similar to unstressedcontrol nodules. However, nodules harvested 1 week later fromboth CEG and GG plants had structural changes including degradationof bacteria in vacuoles around host cell nuclei, particularlyin the outer cell layers of the infected tissue. In addition,meristematic activity was seen in the cortex of some nodulesfrom GG plants. Young cells here contained infection threadsand newly-released bacteria. Nodules harvested 2 weeks after removal of the salt-stress fromCEG plants showed an apparent recovery from the stress. However,there was a very marked increase in the amount of starch inthe cortex which was not seen in equivalent GG nodules. In contrast,nodules from GG plants contained many vacuolate infected cellsand, consequently, a lowered bacteroid population. Further,meristematic activity was seen in a zone concentric to the infectedzone, newly-formed cells contained many large infection threadsand were interspersed with intercellular bacteria. The meristematicactivity increased the relative volume of cortical to infectedcells in these nodules. Growth conditions did not affect control nodule specific nitrogenaseactivity or oxygen diffusion resistance (R) and these parameterswere also not altered in CEG nodules exposed to salt plus the14 d recovery period. However, nitrogenase activity was greatlyreduced, and R increased by more than eight times in equivalentGG nodules exposed to salt plus recovery. It is hypothesized that the gross morphological changes werean attempt to counter salt toxicity and/or oxygen damage underconditions of reduced photosynthate supply to the nodules dueto the poor light levels in the greenhouse. However, soybeannodules supplied with adequate photosynthate were able to withstandand recover from long-term salt-stress with little alterationto their structural integrity. Key words: Soybean, sodium chloride, nitrogen fixation, light intensity, oxygen diffusion resistance     

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     

Ammonia Assimilation in Canavalia ensiformis Plants under Water and Salt Stress

Nitrogen fixation and ammonia assimilation in nodules have beenthoroughly studied under stress conditions, but the behaviorof enzymes involved in ammonia assimilation to organic compoundsin plants of the Leguminosae family subjected to stress stillremains to be conclusively established. We found that understress conditions, C. ensiformis plants can switch from theirusual pathway of assimilation to an alternative one dependingon the nature of the stress and the tissue in which the processtakes place. In roots, it switches from the glutamate dehydrogenase(GDH) pathway to the glutamine synthetase (GS)/glutamate synthase(GOGAT) cycle under water stress but not under salt stress.However, in leaves under salt stress, GDH activity is maintainedbut GS activity markedly decreases (Received March 24, 1987; Accepted March 4, 1988)     

An Isotopic Study of the Fixation of Nitrogen associated with Nodulated Plants of Alnus, Myrica, and Hippophae

Nodulated plants of Alnus glutinosa, Myrica gale, and Hippophaërhamnoides, the root systems of which had been exposed to excessfree ¹⁵N, showed substantial enrichment in fixed ¹⁵N contentin all parts of the plant, but particularly in the root nodules.The data resemble closely those obtained by the present andprevious authors with legumes under comparable conditions, andsupport the conclusion already drawn from experiments of traditionaltype that the nodules of these non-legumes are similar to thoseof legumes in their function and relation to the rest of theplant. Nodulated plants of Alnus and Myrica continued to fixfree nitrogen concurrently with the uptake of combined nitrogenwhen the latter was supplied in the rooting medium in amountsunlikely to be exceeded in the field. Isotopic tests on detachednodules of one of the species (Alnus glutinosa) showed thatfixation continued, and though much reduced as compared withthat shown by attached nodules it considerably exceeded thatexperienced by the present and previous authors with detachedlegume nodules, on the basis of fixation per unit of total nodulenitrogen. It is probable that detached Alnus nodules presentconvenient material for the further study of various aspectsof the fixation process.     

Effect of Water Stress on Nodule Physiology and Biochemistry of a Drought Tolerant Cultivar of Common Bean (Phaseolus vulgarisL.)

Plants of EMGOPA-201, a drought tolerant cultivar of commonbean(Phaseolus vulgaris), were maintained either at 90% soilfield capacity (SFC) or stressed by reducing SFC to 70, 50 or30% over a 10 d period. Plant dry weight was not affected byany of these treatments although the number and weight of noduleswas reduced at 50 and 30% SFC. Nitrogenase activity, determinedby the acetylene reduction assay (ARA), was also reduced, ona plant basis, at 50% SFC and was almost stopped at 30% SFC.The latter treatment caused a marked increase in nodule O₂diffusionresistance and induced nodule senescence. A time-course analysisof the 10 d 30% SFC treatment showed a decrease in leaf waterpotential from -0.5 to -0.87 MPa by 8 d, with a cessation ofdry weight increase after 3 d, when leaf water potential was-0.65 MPa. Proteins in the host plant fraction of nodules decreasedto 50% of control values by 10 d and leghaemoglobin (Lb) contentwas also lower at this stage. The activity of sucrose synthase(SS) showed a 76% reduction between 3 and 6 d, whilst glutamatesynthase (GOGAT) activity showed a 40% reduction. The activityof other key enzymes of carbon metabolism was also reduced after10 d. Nodule sucrose content increased to double that of controlnodules by 6 d, before declining back to control levels at 10d. Starch content fell by 3 d and continued to fall throughoutthe stress period. The results are discussed in terms of droughttolerance strategies in relation to growth and metabolism inwhole plants and nodules.Copyright 1999 Annals of Botany Company. Phaseolus vulgaris,common bean, water stress, nitrogen fixation, oxygen diffusion, acetylene reduction, enzyme activity, carbon metabolism.     

Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor?

The high quality of leguminous hosts for the parasitic plantRhinanthus minor (in terms of growth and fecundity), comparedwith forbs (non-leguminous dicots) has long been assumed tobe a function of the legume's ability to fix atmospheric nitrogen(N) from the air and the potential for direct transfer of compatibleamino compounds to the parasite. Using associations betweenRhinanthus minor and Vicia faba (Fabaceae) that receive N eitherexclusively via symbiotic associations with rhizobia supplyingorganic N fixed from N₂ or exclusively through the supply ofinorganic nitrate to the substrate, the underlying reasons forthe quality of legumes as hosts for this parasite are unravelled.It is shown that sole dependence of the host, V. faba, on Nfixation results in lower growth of the attached parasite thanwhen the host is grown in a substrate supplied exclusively withinorganic N. In contrast, the host plants themselves achieveda similar biomass irrespective of their N source. The physiologicalbasis for this is investigated in terms of N and abscisic acid(ABA) partitioning, haustorial penetration, and xylem sap aminoacid profiles. It is concluded that legume N fixation does notunderpin the quality of legumes as hosts for Rhinanthus butrather the well-developed haustorium formed by the parasite,coupled with the lack of defensive response of the host tissuesto the invading haustorium and the presence of sufficient nitrogenouscompounds in the xylem sap accessible to the parasite haustoria,would appear to be the primary factors influencing host qualityof the legumes. Key words: ABA, haustorium, legume, nitrogen fixation, nodules, parasitic plant Received 14 November 2007; Revised 7 January 2008 Accepted 8 January 2008