Nitrogen assimilation and partitioning in two nitrogen-fixing cultivars of Phaseolus vulgaris L.

Two cultivars of Phaseolus vulgaris L., one responsive (Mexico 309) and one less-responsive (Rio Tibagi) to nodulation with Rhizobium were grown in Leonard jars in a greenhouse. Bean plants were either inoculated with a strain of Rhizobium leguminosarum bv. phaseoli (UMR-1899), a vesicular-arbuscular mycorrhizal (VAM) fungus (Glomus etunicatum) or were left non-inoculated (controls). At two harvests (21 and 28 days post-emergence), extracts containing soluble proteins and free amino acids were prepared from leaves, roots and nodules of field beans. Nodulated plants contained a significantly higher concentration of protein and amino acids in all plant parts. Nitrogen-fixing beans invested a significantly greater proportion of total N as protein-N and amino acid-N as compared to VAM or control beans. Abundant nodule-specific proteins (nodulins) were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), identified and quantified using scanning densitometry. Rio Tibagi nodules contained a significantly lower concentration of glutamine synthetase (GS) subunits than did Mexico 309 nodules. Glutamate synthase (GOGAT) and GS activities were low relative to other legumes. The transferase/synthetase ratio for GS was also low indicating that the synthetase activity was caturated and was operating at maximal level in these young N₂-fixing associations. Specific nodule activity (SNA) and the level of GS were correlated (r=0.90, p<0.05) for both cultivars at both harvests. GS activity was only 8 or 24% higher than SNA in nodules of Mexico 309 or Rio Tibagi cultivars, respectively, under conditions where substrate was not limiting. This suggests that early in the functioning of this symbiosis N assimilation by GS is the rate-limiting step in N₂ fixation by these two bean cultivars, each with a different symbiotic efficiency. Phaseolus breeding programs that attempt to improve N₂ fixation in beans should identify germplasm that expresses elevated levels of nodule-specific GS or GOGAT, and this material should be used along with effective R. leguminosarum bv. phaseoli strains that have already been selected, to determine superior host-microsymciont associations.     

Arbuscular mycorrhizal fungal-induced alteration to root architecture in strawberry and induced resistance to the root pathogenPhytophthora fragariae

Three strawberry cultivars Elsanta, Cambridge Favourite and Rhapsody were inoculated with eitherGlomus fasciculatum orGlomus etunicatum and their growth compared with non-inoculated plants. The roots of all inoculated plants were 55 to 70% colonised after 98 days. Increases in both root and shoot dry weights were measured. Root architecture was also determined and increases in branching were evident in AMF colonised root systems. The remaining plants were then inoculated with the root pathogenPhytophthora fragariae and allowed to grow for a further 58 days before harvest. In two of the cultivars, Cambridge Favourite and Elsanta, AMF reduced root necrosis by approximately 60 and 30% respectively. Only in the least susceptible cultivar, Rhapsody, was no reduction measured in AMF colonised plants. There were differences in the control conferred by the two arbuscular mycorrhizal fungi and this suggests there may be practical benefits of inoculation. Relationships between the presence of roots of different orders, on inoculation with the pathogen, and subsequent necrosis provided a mechanism for identifying root-architecture driven alteration to susceptibility. Root system necrosis was positively correlated with the proportion of the root system made up of higher order roots (3° to 4°) in non-colonised plants and negatively correlated in AMF colonised plants. These data suggest that root-architecture changes are not important per se but factors expressed concurrently may be.     

The response of field grownPhaseolus vulgaris to Rhizobium inoculation and the quantification of N2 fixation using15N

Summary A field experiment was performed to assess the effects of Rhizobium inoculation and nitrogen fertilizer (100 kg N ha^–1) on four cultivars of Phaseolus beans; Carioca, Negro Argel, Venezuela 350 and Rio Tibagi. In the inoculated treatment 2.5 kg N ha^–1 of¹⁵N labelled fertilizer was added in order to apply the isotope dilution technique to quantify the contribution of N₂ fixation to the nutrition of these cultivars.Nodulation of all cultivars in the uninoculated treatments was poor, but the cultivars Carioca and Negro Argel were well nodulated when inoculated. Even when inoculated, nodulation of the cultivars Venezuela 350 and Rio Tibagi was poor and these cultivars showed little response to inoculation in terms of nitrogen accumulation or grain yield. The estimates of the contribution of N₂ fixation estimated using the isotope dilution technique, for the Carioca and Negro Argel cultivars, amounted to 31.7 and 18.4 kg N ha^–1 respectively. These two cultivars produced 991 and 883 kg ha^–1 of grain, respectively, when inoculated and 663 and 620 kg ha^–1 with the addition of 100 kg N ha^–1 of N fertilizer. The response to nitrogen was particularly poor due to high leaching losses in the very sandy soil at the experimental site.The Venezuela 350 and Rio Tibagi cultivars only responded to N fertilizer and not to inoculation with Rhizobium which stresses the great importance of selecting plant cultivars for nitrogen fixation in the field.     

Mycorrhizal responses of barley cultivars differing in P efficiency

The purpose of this study was to investigate how barley cultivars which are different in dry matter yield at low phosphorus (P) supply (i.e. they differ in agronomic P efficiency) respond to mycorrhizal infection. In a preliminary experiment, six mycorrhizal fungi were tested for their ability to colonize barley (Hordeum vulgare L.) roots at a soil temperature of 15°C.Glomus etunicatum was the most effective species and was used in the main experiment. The main experiment was conducted under glasshouse conditions in which soil temperature was maintained at 15°C. Treatments consisted of a factorial arrangement of 8 barley cultivars, 2 mycorrhiza (inoculated and non-inoculated), and 3 rates of P (0, 10 and 20 mg kg^-1). P utilization efficiency (dry matter yield per unit of P taken up) and agronomic P efficiency among the barley cultivars was significantly negatively correlated with mycorrhizal responses. However, the response to mycorrhizal infection was positively correlated with response to P application. Poor correlation was observed between P concentration when neither mycorrhiza nor P were supplied and the percentage of root length infected. The extent of mycorrhizal infection among the barley cultivars in soil without P amendment varied from 8.6 to 28.6%. Significant interactions between cultivar and P addition, and between mycorrhiza and P addition were observed for shoot dry weight but not root dry weight.     

Development of two endomycorrhizal symbioses on soybean and comparison with phosphorus fertilization

Summary Soybean (Glycine max L. Merr. cv. Amsoy 71) plants were grown in a greenhouse in a soil very low in plant-available P, and plants were harvested 5 times over a 21-week growth period. Soybeans were inoculated with one of two species of VAM fungi or received daily one of three nutrient solutions of different P concentrations (0.0, 0.2, or 1.0mMP). Until week 9, the dry weights, leaf areas and developmental stage of soybeans inoculated withG. fasciculatum orG. mosseae were similar to the 1.0 or 0.2mMP-treated plants, respectively. Phosphorus concentrations were significantly lower in VAM plants at weeks 6 and 9 as compared to non-VAM soybeans given 1.0mMP, suggesting P input in VAM plants was immediately used for new growth. Total P input for VAM plants was linear over 21 weeks, and the average rate of P uptake for these plants was 0.19mg P d⁻¹. Estimated specific P uptake rates (SPUR) for the mycorrhizae (VAM roots) were twice that of the control (0.0mMP) roots. The calculated SPURs forG. fasciculatum andG. mosseae hyphae were 95 and 120μg P g⁻¹ VAM d⁻¹ respectively, a 4 to 5 fold increase over non-inoculated roots, indicating more attention must be paid to P assimilation by VAM fungi in P-fixing substrates. Contribution from the Western Regional Research Center, USDA-ARS (CRIS No. 5325-20580-003).     

Mycotrophy of Annona cherimola and the morphology of its mycorrhizae

The mycotrophic character of Annona cherimola (Magnoliales), a tropical/subtropical plantation crop of interest, is described for the first time. This crop seems to depend on mycorrhizae (arbuscular) for optimal growth, with Glomus deserticola being the most effective endophyte tested. Study of the morphology of the arbuscular mycorrhizae in Annona roots showed exclusively intracellular hyphal development, with cell-to-cell fungal passage and an abundance of arbuscules and coiled hyphae within cells. Intercellular distributive hyphae were not observed. The morphology and the pattern of spread of the mycorrhizal colonization were similar for the different endophytes involved and appeared to be dependent on the host root. Such features of mycorrhizal colonization are characteristic of host species lacking intercellular air channels and have been described for some species of ecological interest, but they are not commonly noted in the mycorrhizal literature, especially that dealing with crop species. Some ecophysiological consequences of this pattern of colonization are discussed.     

Effect of mycorrhization on the accumulation of rishitin and solavetivone in potato plantlets challenged with<Emphasis Type="Italic"> Rhizoctonia solani</Emphasis>

The effect of colonization with the vesicular-arbuscular mycorrhizal fungus Glomus etunicatum on the content of rishitin and solavetivone was determined in potato plants cv. Goldrush challenged with Rhizoctonia solani. Mycorrhization stimulated significantly the accumulation of both phytoalexins in roots of plantlets challenged with R. solani but did not influence phytoalexin levels in non-challenged plantlet roots. No accumulation of solavetivone or rishitin was detected in shoots. In Petri dish bioassays, rishitin and solavetivone inhibited mycelial growth of R. solani.     

Phosphorus allocation in P-efficient and inefficient barley cultivars as affected by mycorrhizal infection

A glasshouse experiment was undertaken to investigate the effect of mycorrhizal infection on the allocation of phosphorus (P) in agronomically P-efficient (i.e. high yields at low P supply) and inefficient barley (Hordeum vulgare L.) cultivars. Four barley cultivars differing in agronomic P-efficiency were inoculated or not inoculated with Glomus etunicatum. Cultivars did not differ in percentage of root length infected. The concentration of P in roots of the inefficient cultivars was higher than that of the efficient cultivars. However, because of changes in root to shoot dry weight ratio and below-ground productivity, mycorrhizal infection significantly reduced the percentage of total plant P in roots of the inefficient cultivars. The distribution of P between root and shoot of P-efficient cultivars was not affected by mycorrhizal infection. Root to shoot dry weight ratio of the P-efficient cultivars was lower than that of the inefficient cultivars, and the decrease in the ratio following infection was significant in inefficient but not in P-efficient cultivars. This study indicates that mycorrhizal infection alters the allocation of P in inefficient cultivars and effectively improves the efficiency of P utilization with respect to shoot growth.     

Vesicular arbuscular mycorrhizae in floating wetland mat communities dominated by Typha

Low levels of vesicular arbuscular mycorrhizae were present in floating mats dominated by clones of Typha angustifolia L., T. x glauca Godr., and T. latifolia L. Floating mats composed of rhizomes (underwater-ground stems with high starch accumulation), roots, decaying organic matter, and wind deposited soil, easily supported human activities. The majority of roots isolated from the root cores were connected to Typha rhizomes. Tests employing the gridline intersect method, intensity, spore counts and most probable number (MPN) were used to define the level of colonization. Mycorrhizal colonization from the T. angustifolia and T. x glauca clones averaged 4 to 5%, while the T. latifolia clone averaged 13%. When colonization was encountered, intensities varied from a high of 3.0 to a low of 0.4 on a sclae of 0 to 4. Although arbuscules were not found, abundant hyphae, vesicles and spores indicated that presumed facultative associations occurred between the vesicular arbuscular fungi and the indicated that presumed facultative associations plant communities found on floating mats. The mycorrhizal fungi identified from these communities in cluded Glomus albidum Walker & Rhodes, G. caledonium (Nicol. & Gerd.) Trappe & Gerdemann, G. etunicatum Becker & Gerdemann, and G. microcarpum Tul. & Tul. Spore counts ranged from 16 to 76 spores per gram dried organic soil. The recolonization ability of VAM propagules by way of a most probable number bioassay with maize yielded numbers that ranged from zero to 96 propagules per gram soil, with G. etunicatum the only species recovered.     

Differential host plant performance as a function of soil arbuscular mycorrhizal fungal communities: experimentally manipulating co-occurring Glomus species

In order to evaluate host plant performance relative to different soil arbuscular mycorrhizal fungal (AMF) communities, Andropogon gerardii seedlings were grown with nine different AMF communities. The communities consisted of 0, 10, or 20 spores of Glomus etunicatum and 0, 10, or 20 spores of Glomus intraradices in all possible combinations. Spores were produced by fungal cultures originating on A. gerardii in a serpentine plant community; seeds of A. gerardii were collected at the same site. The experiment was performed in the greenhouse using a mixture of sterilized serpentine soil and sand to which naturally occurring non-mycorrhizal microbes were added. There was no difference in root AMF colonization rates between single species communities of either G. etunicatum or G. intraradices, but G. intraradices enhanced plant growth and G. etunicatum did not. However, plants grew larger with some combinations of G.␣intraradices plus G. etunicatum than with the same quantity of G. intraradices alone. These results suggest the potential for niche complementarity in the mycorrhizal fungi. That G. etunicatum only increased plant growth in the presence of G. intraradices could be illustrative of why AMF that appear to be parasitic or benign when examined in isolation are maintained within multi-species mycorrhizal communities in nature.     

Cooccurring plants forming distinct arbuscular mycorrhizal morphologies harbor similar AM fungal species

Arbuscular mycorrhizal (AM) fungal spores were isolated from field transplants and rhizosphere soil of Hedera rhombea (Miq) Bean and Rubus parvifolius L., which form Paris-type and Arum-type AM, respectively. DNA from the spore isolates was used to generate molecular markers based on partial large subunit (LSU) ribosomal RNA (rDNA) sequences to determine AM fungi colonizing field-collected roots of the two plant species. Species that were isolated as spores and identified morphologically and molecularly were Gigaspora rosea and Scutellospora erythropa from H. rhombea, Acaulospora longula and Glomus etunicatum from R. parvifolius, and Glomus claroideum from both plants. The composition of the AM fungal communities with respect to plant trap cultures was highly divergent between plant species. Analysis of partial LSU rDNA sequences amplified from field-collected roots of the two plant species with PCR primers designed for the AM fungi indicated that both plants were colonized by G. claroideum, G. etunicatum, A. longula, and S. erythropa. G. rosea was not detected in the field-collected roots of either plant species. Four other AM fungal genotypes, which were not isolated as spores in trap cultures from the two plant species, were also found in the roots of both plant species; two were closely related to Glomus intraradices and Glomus clarum. One genotype, which was most closely related to Glomus microaggregatum, was confined to R. parvifolius, whereas an uncultured Glomeromycotan fungus occurred only in roots of H. rhombea. S. erythropa was the most dominant fungus found in the roots of H. rhombea. The detection of the same AM fungal species in field-collected roots of H. rhombea and R. parvifolius, which form Paris- and Arum-type AM, respectively, shows that AM morphology in these plants is strongly influenced by the host plant genotypes as appears to be the case in many plant species in natural ecosystems, although there are preferential associations between the hosts and colonizing AM fungi in this study.     

Influence of soil pH on the soybean-endomycorrhiza symbiosis

Summary Soybean (Glycine max {L.} Merr.) cultivars were inoculated withGigaspora gigantea andGlomus mosseae to determine mycorrhizal: cultivar relationships as affected by soil pH. The specific cultivarfungal response was dependent on soil pH. Overall cultivar responses in unlimed soil (pH 5.1) were greater forG. gigantea thanG. mosseae. The Bossier —G. gigantea combination was particularly responsive in unlimed soil and showed an increase of 10% in shoot length, 35% in shoot dry weight. 75% in root dry weight, and 397% in nodule dry weight over uninoculated controls. Little cultivar response was observed withG. mosseae inoculation in unlimed soil. In limed soil (pH 6.2), the larger responses were obtained withG. mosseae inoculated plants, although inoculation with eitherG. mosseae orG. gigantea appeared effective. In general, nodulation was greater on mycorrhizal roots than on control roots.     

Response ofLeucaena esculenta to Endomycorrhizae andRhizobium inoculation

Dual inoculations onLeucaena esculenta plants of eitherGlomus versiforme andRhizobium loti NGR 8 orGlomus sp. andR. loti ENCB 31, gave higher growth and phosphorus accumulation compared with treatments with a single micro-symbiont. The above combinations could be used in a re-forestation programme withL. esculenta in the highlands of Oaxaca State, Mexico. The use of endomycorrhized plants is recommended to alleviate the planting shock-phase, particularly in arid zones, and also the risk of root colonization by possibly less-efficient indigenous fungi.     

Genetic differentiation in Plantago major L. in growth and P uptake under conditions of P limitation

To study possible adaptive mechanisms inbred lines from three populations of Plantago major from sites that were found to differ in P availability were compared. In a pot experiment the growth and P uptake either in the presence or absence of Glomus fasciculatum was determined. Under these P-limited conditions it was shown by partitioning the relative growth rate (RGR, in mg g^-1 day^-1) in the components root weight ratio (RWR, in g_roots g_plant ^-1), specific P uptake rate (SPUR, in mol P g_roots ^-1 day^-1), and P-efficiency (PEFF, in mg mol P^-1), that the increase in RGR of mycorrhizal infected plants was related to an increase in SPUR, and a decrease in RWR and PEFF. P. major ssp. major had a lower RGR (related to a lower PEFF and SPUR) and a higher RWR than P. major ssp. pleiosperma. In a second experiment three inbred lines were compared upon P depletion in a nutrient solution. The P. major ssp. major line had a lower RGR and higher RWR, and a higher accumulation of P in the roots than the P. major ssp. pleiosperma lines under optimal growing conditions. There were no differences among the inbred lines in the relative contribution of inorganic P to the total P concentration in the shoot. The results are discussed in relation to the characteristics of the habitats of the investigated P. major populations.     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Host-related variability in arbuscular mycorrhizal fungal structures in roots of <Emphasis Type="Italic">Hedera rhombea</Emphasis>, <Emphasis Type="Italic">Rubus parvifolius</Emphasis>, and <Emphasis Type="Italic">Rosa multiflora</Emphasis> under controlled conditions

The arbuscular mycorrhizal (AM) morphology of three host plant species inoculated with single and mixed fungal culture and the distribution of AM fungal species in roots of the hosts treated with a mixed culture of AM fungi were determined. The aim was to investigate the effect of host plants and AM fungi on AM morphology of coexisting plant species. Noncolonized rooted cuttings of Hedera rhombea (Miq) Bean, Rubus parvifolius L., and Rosa multiflora Thunb. were inoculated with five fungal species as single and mixed culture inocula. The fungal species used were Gigaspora rosea and Scutellospora erythropa, previously isolated from H. rhombea; Acaulospora longula and Glomus etunicatum from R. parvifolius; and Glomus claroideum from both plant species. A few hyphal and arbusculate coils were seen in the mixed culture-inoculated roots of R. parvifolius; all fungal treatments produced this Paris-type AM in H. rhombea and Arum-type AM in R. parvifolius, and R. multiflora indicates that AM morphology is strongly controlled by the identity of the host plants used in this study. AM fungal rDNA was extracted separately from roots of each replicate plant species inoculated with the mixed fungal culture, amplified, cloned, sequenced, and analyzed to determine the AM fungal species and their respective proportions in roots of each plant species. Glomus etunicatum and G. claroideum of the family Glomaceae generally occurred more frequently in R. parvifolius and R. multiflora, which form Arum-types, whereas S. erythropa, of the family Gigasporaceae, was the most frequently detected species in H. rhombea, which produced Paris-type AM. Although the genotype of the plant species used appears to determine the AM morphologies formed, there was preferential association between the hosts and AM fungal inoculants.     

Studies on the diversity of the distinct phylogenetic lineage encompassing Glomus claroideum and Glomus etunicatum

Morphological and molecular characters were analysed to investigate diversity within isolates of the Glomus claroideum/Glomus etunicatum species group in the genus Glomus. The inter- and intra-isolate sequence diversity of the large subunit (LSU) rRNA gene D2 region of eight isolates of G. claroideum and G. etunicatum was studied using PCR-single strand conformational polymorphism (SSCP)-sequencing. In addition, two isolates recently obtained from Southern China were included in the analysis to allow for a wider geographic screening. Single spore DNA isolation confirmed the magnitude of gene diversity found in multispore DNA extractions. An apparent overlap of spore morphological characters was found between G. claroideum and G. etunicatum in some isolates. Analysis of the sequence frequencies in all G. etunicatum and G. claroideum isolates (ten) showed that four LSU D2 sequences, representing 32.1% of the clones analysed for multispore extraction (564) were found to be common to both species, and those sequences were the most abundant in four of the ten isolates analysed. The frequency of these sequences ranged between 23.2% and 87.5% of the clones analysed in each isolate. The implications for the use of phenotypic characters to define species in arbuscular mycorrhizal fungi are discussed. The current position of G. claroideum/G.etunicatum in the taxonomy of the Glomeromycota is also discussed.     

Fungal endosymbionts affect aphid population size by reduction of adult life span and fecundity

The symbiosis between grasses and endophytic fungi is a common phenomenon and can affect herbivore performance through acquired, chemical plant defence by fungal alkaloids. In laboratory experiments, two species of common grass aphids, Rhopalosiphum padi and Metopolophium dirhodum were tested, in a population experiment (on four plant cultivars) and individually (on one plant cultivar) for the effects of the endophyte, Neotyphodium lolii, that forms symbiotic associations with perennial ryegrass Lolium perenne. In the population experiment that lasted for four aphid generations both aphid species showed decreased population sizes when feeding on each of the four endophyte-infected cultivars. Individuals of R. padi tested individually showed reduced adult life span and fecundity when feeding on infected plants. Individuals of M. dirhodum showed no response in any of the traits measured. This suggests that R. padi individuals are more sensitive to endophyte infection than M. dirhodum individuals. However, all infected grass cultivars reduced population sizes of both aphid species over four generations. Therefore, fungal endophytes can reduce populations of aphid herbivores independent of plant cultivars.     

Effects of the root-lesion nematode Pratylenchus vulnus and the vesicular-arbuscular mycorrhizal fungus Glomus mosseae on the growth of three plum rootstocks

The effects of Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on growth of Myrobalan 605, Marianna 2624 and San Julian 655-2 plum rootstocks were measured under shadehouse conditions in the field for two growing seasons (1990–91). Shoot dry weights were higher in the majority of the vesicular-arbuscular mycorrhizal (VAM) alone inoculated plants after both growing seasons. Root weights of mycorrhizal Myrobalan and Marianna were higher than root weights of the same rootstocks lacking mycorrhizae, inoculated with P. vulnus, and VAM in combination with the nematode. Mycorrhizal Marianna inoculated with the nematode showed a considerably higher final nematode population in relation to non-inoculated VAM treatments. No correlation was found in the number of nematodes per gram of root between mycorrhizal and non-mycorrhizal treatments. P. vulnus adversely affected the mycorrhizal colonization in Marianna, but not in Myrobalan and San Julian. Marianna appears to be more mycorrhizal dependent than the two other rootstocks.     

Flavonoids in roots of white clover: interaction of arbuscular mycorrhizal fungi and a pathogenic fungus

The effects of two arbuscular mycorrhizal fungi (AMF) (Glomus mosseae and G. claroideum) and a pathogenic fungus (Pythium ultimum) on the production of eight flavonoids in roots of two white clover (Trifolium repens L.) cultivars were evaluated. Quantification of AM and pathogenic fungi in the roots showed that the AM symbiosis significantly reduced P. ultimum biomass and in some cases prevented infection. The flavonoid productions in clover roots varied depending on the presence of beneficial and/or pathogenic fungi, fungal isolate or plant cultivar. Only plants colonized with G. claroideum showed detectable concentrations of either coumestrol or kaempferol (cultivar-dependant). In addition, inoculation with G. claroideum resulted in significantly higher concentrations of coumestrol in cv. Sonja and medicarpin in cv. Milo. A low production of coumestrol and kaempferol in mycorrhizal plants may be G. mosseae-specific. Only the concentrations of formononetin and daidzein increased in clover roots in response to infection with P. ultimum. These flavonoids are supposedly stress metabolites, synthesized or produced from glycosides in response to pathogen infection. However, the presence of one or both AMF significantly lowered the formononetin and daidzein concentrations, and overruled the inductive effect of P. ultimum. Therefore the antagonistic action of AM against the pathogen must take place through another mechanism.