Host range and variation in virulence of Mycosphaerella ligulicola

A soil-borne inoculum of Mycosphaerella ligulicola was found to be mildly pathogenic to leaves or stems of globe artichoke, rudbeckia, zinnia, sunflower and dahlia, but severely pathogenic to lettuce. This is the first report of an infection by this fungus of plants other than chrysanthemum or pyrethrum. It was found that, with successive passages of the fungus through lettuce and chrysanthemum respectively, an increase in virulence to these hosts occurred. After a single passage through lettuce a reduction in virulence to chrysanthemum resulted, but with further passages through lettuce there was no further reduction in virulence to chrysanthemum.     

The survival of Mycosphaerella pinodes on pea haulm buried in soil

Where haulm was left on the soil surface the mycelium of Mycosphaerella pinodes went through cycles of active saprophytic growth, followed by inactive phases during the earlier parts of the year. Burial at either 15 or 25 cm brought about a decrease in survival with time. The majority of the mycelium was on the outer portions of the stem pieces, with only a small amount of penetration into the plant remains. Experiments on root surface colonization showed that mycelium of M. pinodes was capable of growing saprophytically on the host roots in competition with the normal soil microflora.     

Studies on the influence of leaf washings on infection by Mycosphaerella ligulicola

Infection of chrysanthemum leaf discs by spores of Mycosphaerella ligulicola was increased in the presence of concentrated leaf washings obtained from five plant species tested. On separation of leaf washings by dialysis the residue stimulated infection and caused aggregation of hyphae on leaf and cellophane surfaces. In the presence of the diffusible fraction an extensive branching network of hyphae developed but there was no increase in infection except in the presence of the chrysanthemum diffusate. Infection of leaf discs of certain plants normally resistant to M. ligulicola occurred in the presence of concentrated leaf washings. Leaf washings from a resistant variety of chrysanthemum had the same effects as those from a susceptible variety. An extract of chrysanthemum leaves was less effective in stimulating infection than was a leaf washing. Defined nutrient media and potato extract stimulated infection but caused the development of different growth habits on leaf surfaces. The active components of the dialysis residue of chrysanthemum leaf washings were heat-stable but apparently volatile. Eluates from strips of certain regions of paper chromatographs of leaf washings and potato extract caused increased infection together with development of characteristic growth habits of the fungus on the leaf.     

Differential benefits of arbuscular mycorrhizal and ectomycorrhizal infection of Salix repens

 The functional significance of arbuscular mycorrhiza (AM) and ectomycorrhiza (EcM) for Salix repens, a dual mycorrhizal plant, was investigated over three harvest periods (12, 20 and 30 weeks). Cuttings of S. repens were collected in December (low shoot P) and March (high shoot P). Glomus mosseae (an arbuscular mycorrhizal fungus, AMF) resulted in low AM colonization (<5%), but showed large short-term (<12 weeks) effects on shoot growth and root length. Hebeloma leucosarx (an ectomycorrhizal fungus, EcMF) resulted in high EcM colonization (70%), but benefits occurred over a longer term (>12 weeks). Furthermore, G. mosseae colonization resulted in higher shoot P uptake, shoot growth, root growth and response duration for S. repens collected in December than for those collected in March, whereas with H. leucosarx and the non-mycorrhizal treatment there were no differences between cuttings collected on different dates. Low AMF colonization was effective in the short term for cuttings at both collecting dates. Low AMF colonization of S. repens occurred irrespective of the amount of AMF inoculum used. The intensities and relative amounts of AMF structures in S. repens and Trifolium repens were compared over three harvest periods (12, 20 and 30 weeks) to assess plant species effects on AM colonization patterns. Accepted: 13 October 2000     

Effects of anthracene on development of an arbuscular mycorrhizal fungus and contribution of the symbiotic association to pollutant dissipation

The influence of anthracene, a low molecular weight polycyclic aromatic hydrocarbon (PAH), on chicory root colonization by Glomus intraradices and the effect of the root colonization on PAH degradation were investigated in vitro. The fungus presented a reduced development of extraradical mycelium and a decrease in sporulation, root colonization, and spore germination when exposed to anthracene. Mycorrhization improved the growth of the roots in the medium supplemented containing 140 mg l⁻¹ anthracene, suggesting a positive contribution of G. intraradices to the PAH tolerance of roots. Anthracene disappearance from the culture medium was quantified; results suggested that nonmycorrhizal chicory roots growing in vitro were able to contribute to anthracene dissipation, and in addition, that mycorrhization significantly enhanced anthracene dissipation. These monoxenic experiments demonstrated a positive contribution of the symbiotic association to anthracene dissipation in the absence of other microorganisms. In addition to anthracene dissipation, intracellular accumulation of anthracene was detected in lipid bodies of plant cells and fungal hyphae, indicating intracellular storage capacity of the pollutant by the roots and the mycorrhizal fungus.     

In vitro mycorrhization of the rubber tree Hevea brasiliensis Müll Arg

In vitro cultivation systems of arbuscular mycorrhizal fungi are useful tools to study the interaction between plants and their fungal symbiont, and also to develop new biotechnologies. Plantlets of the latex-producing species Hevea brasiliensis clone PB 260 were grown in a dense extraradical mycelium network of the arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 41833 developed from a mycelium donor plant (Medicago truncatula A17). The factors indole-3-butyric acid (IBA), 2-morpholineoethanesulfonic acid monohydrate (MES) buffer, and carbon dioxide (CO₂) were tested on root development and colonization by the fungus. No colonization was observed in the presence of plantlets pre-treated with IBA. The highest levels of root colonization were obtained when plantlets were mycorrhized under a high CO₂ concentration (1,000 μmol?mol^?1) with MES (10 mM) added to the growth medium. Widespread root colonization (with presence of arbuscules, intraradical mycelium, and spores/vesicles) was predominantly observed in newly produced roots. Therefore, it appears essential to improve root initiation and growth for improving in vitro mycorrhization of H. brasiliensis. We demonstrated the potential of the “mycelium donor plant” in vitro culture system to produce colonized H. brasiliensis plantlets before their transfer to ex vitro conditions.     

Limited transfer of nitrogen between wood decomposing and ectomycorrhizal mycelia when studied in the field

Transfer of ¹⁵N between interacting mycelia of a wood-decomposing fungus (Hypholoma fasciculare) and an ectomycorrhizal fungus (Tomentellopsis submollis) was studied in a mature beech (Fagus sylvatica) forest. The amount of ¹⁵N transferred from the wood decomposer to the ectomycorrhizal fungus was compared to the amount of ¹⁵N released from the wood-decomposing mycelia into the soil solution as ¹⁵N-NH₄. The study was performed in peat-filled plastic containers placed in forest soil in the field. The wood-decomposing mycelium was growing from an inoculated wood piece and the ectomycorrhizal mycelium from an introduced root from a mature tree. The containers were harvested after 41 weeks when physical contact between the two foraging mycelia was established. At harvest, ¹⁵N content was analyzed in the peat (total N and ¹⁵NH₄ ⁺) and in the mycorrhizal roots. A limited amount of ¹⁵N was transferred to the ectomycorrhizal fungus and this transfer could be explained by ¹⁵NH₄ ⁺ released from the wood-decomposing fungus without involving any antagonistic interactions between the two mycelia. Using our approach, it was possible to study nutritional interactions between basidiomycete mycelia under field conditions and this and earlier studies suggest that the outcomes of such interactions are highly species-specific and depend on environmental conditions such as resource availability.     

A mutant in Lycopersicon esculentum Mill. with highly reduced VA mycorrhizal colonization: isolation and preliminary characterisation

This paper reports the successful isolation and preliminary characterisation of a mutant of Lycopersicon esculentum Mill. with highly reduced vesicular-arbuscular (VA) mycorrhizal colonization. The mutation is recessive and has been designated rmc . Colonization by G. mosseae is characterised by poor development of external mycelium and a few abnormal appressoria. Vesicles were never formed by this fungus in association with the mutant. Gi. margarita formed large amounts of external mycelium, complex branched structures and occasional auxiliary cells. Small amounts of internal colonization also occurred. Laser scanning confocal microscopy (LSCM) gave a clear picture of the differences in development of G. intraradices and Gi. margarita in mutant and wild-type roots and confirmed that the fungus is restricted to the root surface of the mutants. The amenability of tomato for molecular genetic characterisation should enable us to map and clone the mutated gene, and thus identify one of the biochemical bases for inability to establish a normal mycorrhizal symbiosis. The mutant represents a key advance in molecular research on VA mycorrhizal symbiosis.     

Colonization of cucumber plants by the biocontrol fungus Clonostachys rosea f. catenulata

Clonostachys rosea f. catenulata (Gliocladium catenulatum) strain J1446 (formulated as Prestop WP) suppressed Fusarium root and stem rot caused by Fusarium oxysporum f. sp. radicis-cucumerinum (Forc) on cucumber plants grown hydroponically in rockwool medium. Sixty days following application at seeding, the biocontrol agent had proliferated through the rockwool blocks and was present on cucumber roots and the crown region of the stem at populations >1 × 10⁵ CFU/g fresh weight. Scanning electron micrographs showed that C. rosea had rapidly colonized the root surface and was associated with root hairs and epidermal cell junctions. Following transformation of the fungus with Agrobacterium tumefaciens strain AGL-1 containing the hygromycin resistance (hph) and β-glucuronidase (uidA) genes, blue-stained mycelia could be seen growing on the surface and within epidermal and cortical cells of roots, stems and shoots 3 weeks after treatment. Quantification of GUS activity by fluorometric assays showed that fungal biomass was highest in the roots and crown area, while the extent of colonization of upper stems and true leaves was variable. Higher population levels resulted following application to rockwool blocks compared to seed treatment. Application of C. rosea preceding inoculation with Forc significantly reduced pathogen populations on roots compared to plants inoculated with Forc alone. Colonization of infection sites in the root zone reduced pathogen development and disease incidence. Densities of the biocontrol agent appeared to increase in the presence of the pathogen.     

Effect of glucose on the development ofGlomus fistulosum colonization and extraradical mycelium on maize roots

Various doses of glucose were added weekly to pots with maize growing in a mixture of soil and Perlite inoculated with the arbuscular mycorrhizal (AM) fungusGlomus fistulosum to define the effects of an additional carbon source on plant growth, mycorrhizae and other microbial features of the cultivation substratum. Higher doses of glucose (100 and 300 mg per pot) decreased plant growth and abundance of root hairs after 6 weeks of cultivation. Lower doses of glucose (10 and 30 mg per pot) had a positive effect on some characteristics of the development of the arbuscular fungus,e.g. root colonization, abundance of arbuscules, the length of extraradical mycelium associated with root surface, length of mycelium in the substratum as well as the length of both mycelia showing dehydrogenase activity as compared with variants not supplied with glucose solution. Glucose did not affect the number of spores of AM fungus. No effect of glucose was found on substratum respiration but glucose amendment increased microbial biomass and particularly the occurrence of saprophytic fungi. In a subsequent experiment focused on nonsymbiotic phase of fungal life cycle, the mycorrhizal root segments were incubated for 6 weeks in Petri dishes on membranes covered with a soil layer and supplied weekly with four glucose concentrations from 0.3 to 10 mg. Highest total length of hyphae associated with the root surface and the length of hyphae showing dehydrogenase activity was found when the lowest dose of 0.3 mg glucose was added to the soil weekly, whereas a 10 mg dose increased the length and activity of hyphae associated with a membrane. The possible mechanisms of the effects of additional labile carbon pool on the development of mycorrhizal fungus are discussed.     

Litter N retention over winter for a low and a high phenolic species in the alpine tundra

Mycorrhizal fungus colonization of roots may modify plant metal acquisition and tolerance. In the present study, the contribution of the extraradical mycelium of an arbuscular mycorrhizal (AM) fungus, Glomus mosseae (BEG 107), to the uptake of metal cations (Cu, Zn, Cd and Ni) by cucumber (Cucumis sativus) plants was determined. The influence of the amount of P supplied to the hyphae on the acquisition and partitioning of metal cations in the mycorrhizal plants was also investigated. Pots with three compartments were used to separate root and root-free hyphal growing zones. The shoot concentration of Cd and Ni was decreased in mycorrhizal plants compared to non-mycorrhizal plants. In contrast, shoot Zn and Cu concentrations were increased in mycorrhizal plants. High P supply to hyphae resulted in decreased root Cu concentrations and shoot Cd and Ni concentrations in mycorrhizal plants. These results confirm that some elements required for plant growth (P, Zn, Cu) are taken up by mycorrhizal hyphae and are then transported to the plants. Conversely, Cd and Ni were transported in much smaller amounts by hyphae to the plant, so that arbuscular mycorrhizal fungus colonization could partly protect plants from toxic effects of these elements. Selective uptake and transport of plant essential elements over non-essential elements by AM hyphae, increased growth of mycorrhizal plants, and metal accumulation in the root may all contribute to the successful growth of mycorrhizal plants on metal-rich substrates. These effects are stimulated when hyphae can access sufficient P in soil.     

Colonization of barley roots by Fusarium culmorum and influence of Pseudomonas fluorescens on the process

The stages of barley root colonization by Fusarium culmorum were studied in sterile vermiculite by the method of fluorescent antibodies. The influence of the antagonistic bacterium Pseudomonas fluorescens on the process of root colonization by F. culmorum was demonstrated. In vermiculite inoculated with F. culmorum, the fungus density on the roots increased gradually. In the case of joint inoculation of vermiculite with the fungus and the bacterium, the F. culmorum density on the roots changed abruptly. It was shown that the site of primary colonization of the roots by the fungus was mainly the zone of root hairs. When Pseudomonas fluorescens was present on the roots, F. culmorum colonized not only root hairs, but also the elongation zone, during the first two days. Introduction of Pseudomonas fluorescens into vermiculite resulted in lower intensity of barley root rot.     

Influence of phosphorus and formononetin on isozyme expression in the Zea mays‐Glomus intraradices symbiosis

Maize (Zea mays L. cv. Great Lakes 586) plants were either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith, or grown in the presence of the isoflavone formononetin or were provided with both G. intraradices and formononetin. All plants were grown in soil containing one of five levels of inorganic P (between 8 and 110 µg g^?1 soil). By 3 weeks there were significant differences in a number of enzyme activities and in the pattern of isoenzymes in roots colonized by the VAM fungus or treated with formononetin. One NAD-malate dehydrogenase (MDH) isozyme was expressed only in mycorrhizal roots, whether treated or not with formononetin. Despite differences in the soil P level, the expression of this isozyme was not observed in non-mycorrhizal roots, indicating specific expression in the mycorrhizae. We suggest that MDH isozyme could serve as a specific, early indicator of the Zea-Glomus symbiosis. Differences in the esterase (EST) isozyme pattern were not detectable between VAM and non-VAM roots, suggesting that this enzyme system is not a good parameter for the evaluation of mycorrhizal colonization. As available P in the soil increased, total EST activity appeared to increase as well. Interestingly, total peroxidase (POX) activity increased along with P suggesting that as plant P nutrition improved, both cell wall ramification and the quantity of defense peroxidases increased as well. Total POX activity from mycorrhizal roots was inversely correlated with root colonization, indicating that there was suppression of POX activity by the host under low soil P. Most interestingly, formononetin further decreased POX activity regardless of the level of P or mycorrhizal status. This may suggest one mechanism by which formononetin enhances root VAM colonization. The presence of this isoflavone suppressed POX activity in mycorrhizal roots allowing a rapid penetration and spread of the fungus in the root cortex. The interplay between host root, soil P levels, secondary metabolites and endogenous host enzyme activities and a particular VAM fungus has a profound effect on the efficiency, duration and functioning of an endomycorrhizal symbiosis.     

Effect of activated charcoal and pruning of the taproot on the in vitro mycorrhization of Hevea brasiliensis Müll. Arg.

In vitro mycorrhization of Hevea brasiliensis under autotrophic culture conditions is a promising methodology to produce plantlets adapted to overcome stresses during acclimatization. However, to succeed in the in vitro production of mycorrhizal plantlets, root production and subsequent colonization by the mycorrhizal fungus need to be optimized. Plantlets of H. brasiliensis clone PB 260 were grown in contact with the extraradical mycelium network of the arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 41833. Addition of activated charcoal to the medium and pruning of the taproot were evaluated for their effects on root growth and colonization. None of the treatments stimulated the early formation of new roots. However, total root length, total root colonization, and production of arbuscules and intraradical spores/vesicles were significantly higher in plantlets grown in the presence of activated charcoal (especially after 13 wk of culture). In contrast, total root colonization was significantly lower in the pruned plantlets, while total root length and arbuscule formation were not affected. None of the treatments affected activities of succinate dehydrogenase and alkaline phosphatase measured in the extraradical mycelium of the fungus. It appeared that the addition of activated charcoal to the culture medium favored root growth and mycorrhization of rubber plantlets under in vitro culture conditions, while taproot pruning did not favor these parameters.     

Transmission of Pythium aphanidermatum to greenhouse cucumber by the fungus gnat Bradysia impatiens (Diptera: Sciaridae)

The fungus gnat (Bradysia impatiens) was examined for its ability to transmit Pythium aphanidermatum to cucumber plants. Larvae that had ingested oospores and mycelium, and then fed on the roots of young cucumber plants growing in rockwool readily introduced the fungus to them. Trans-stadial transmission of oospores from the larval to the adult stage of B. impatiens was demonstrated, although decreasing to a very low level (1.67%) in adults. However, external transmission of P. aphanidermatum on the surface of adults could not be shown. These results suggest that the larval stages of fungus gnats play a role in the dissemination of the fungus between cucumber plants but that adults probably play only a minor role.     

Factors influencing colonization of cucumber roots by Clonostachys rosea f. catenulata,a biological disease control agent

The biocontrol fungus Clonostachys rosea f. catenulata (Gliocladium catenulatum) strain J1446, commercially available as Prestop® (Verdera Oy, Finland), is an effective antagonist against several root and foliar greenhouse pathogens. The biocontrol agent forms dense networks of hyphae on plant roots, grows internally in root epidermal cells, and produces hydrolytic enzymes, all of which lead to a reduction in pathogen propagules. An understanding of the environmental and host factors that influence root colonization by C. rosea f. catenulata is important to maximize disease control efficacy. Cucumber roots grown in nutrient solution in containers were inoculated with conidia of a GUS-transformed strain of C. rosea f. catenulata. Population levels associated with roots over time were assessed by colony-plate counts, GUS staining and enzymatic assays to determine GUS activity. Variables such as pH, temperature and growing medium were major factors that influenced population levels, while cucumber cultivar, addition of nutrients, and wounding of roots did not appear to significantly affect colonization. Population density of C. rosea f. catenulata on roots was highest when the nutrient solution was maintained at pH 5, 6, or 7, and at temperatures of 18–22°C. Lowest colonization levels were observed on roots of plants grown in potting mix or in field soil. Measurement of GUS activity provided a slightly more accurate assessment of root colonization levels compared to colony-plate counts. These results illustrate the optimal environmental conditions which can ensure maximum root colonization by C. rosea f. catenulata and enhance disease control by the biocontrol agent.     

The co-occurrence of ectomycorrhizal,arbuscular mycorrhizal,and dark septate fungi in seedlings of four members of the Pinaceae

Although roots of species in the Pinaceae are usually colonized by ectomycorrhizal (EM) fungi, there are increasing reports of the presence of arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi in these species. The objective of this study was to determine the colonization patterns in seedlings of three Pinus (pine) species (Pinus banksiana, Pinus strobus, Pinus contorta) and Picea glauca x Picea engelmannii (hybrid spruce) grown in soil collected from a disturbed forest site. Seedlings of all three pine species and hybrid spruce became colonized by EM, AM, and DSE fungi. The dominant EM morphotype belonged to the E-strain category; limited colonization by a Tuber sp. was found on roots of Pinus strobus and an unknown morphotype (cf. Suillus–Rhizopogon group) with thick, cottony white mycelium was present on short roots of all species. The three fungal categories tended to occupy different niches in a single root system. No correlation was found between the percent root colonized by EM and percent colonization by either AM or DSE, although there was a positive correlation between percent root length colonized by AM and DSE. Hyphae and vesicles were the only AM intracellular structures found in roots of all species; arbuscules were not observed in any roots.     

Lignosulfonate promotes the interaction between Scots pine and an ectomycorrhizal fungus Pisolithus tinctorius in vitro

Lignosulfonate (LS) is a lignin-based polymer obtained as a by-product from paper industry, which may have potential as an amendment with macronutrients. We studied effects of LS on the interaction between Scots pine (Pinus sylvestris L.) seedlings and hypocotyl cuttings and the ectomycorrhizal (ECM) fungusPisolithus tinctorius (Pers.) Coker and Couch. The experiments were performed in vitroon the MMN agar medium containing Fe–LS chelate at the concentrations of 0, 5, 10 and 25 mg/L. Inoculation with P. tinctoriusincreased root growth of the seedlings. Fe–LS enhanced P. tinctorius induced formation of lateral roots and had a dose-dependent positive effect on the establishment of mycorrhizas on the seedlings. The growth of the fungal mycelium was improved by Fe–LS, which might cause faster and more intensive contact with the roots and, thus, better root growth and mycorrhiza formation. P.tinctorius enhanced also adventitious root formation and subsequent root growth of the hypocotyl cuttings but without any synergistic effect with Fe–LS. Our study with P. tinctorius and Scots pine in vitro indicates that a low-cost by-product Fe–LS, obtained from paper industry, may be a potential tool to improve the efficiency of fungal inoculations, thus, facilitating the early interaction between an ECM fungus and host seedling.     

Effect of the ectomycorrhizal fungus Hebeloma cylindrosporum on in vitro rooting of micropropagated cuttings of arbuscular mycorrhiza-forming Prunus avium and Prunus cerasus

 The effect of different genotypes of the ectomycorrhizal fungus Hebeloma cylindrosporum on in vitro rooting of micropropagated cuttings of Prunus avium and P. cerasus was studied in an attempt to determine whether ectomycorrhizal fungi could enhance in vitro adventitious root formation in plants which form arbuscular endomycorrhizas. The rooting percentage of P. avium cuttings was approximately 16% in the absence of hormonal treatment; it increased up to 30% in the presence of 5.7 μM IAA which was the most favourable auxin concentration. The rooting percentage of cuttings cultivated in the absence of IAA was enhanced by all the studied strains of H. cylindrosporum. It ranged from 50 to 60% with the IAA-overproducing mutant D 111 or the wild-type dikaryon D1, to 100% in the presence of the mutants 331 or D 117. The cuttings of P. cerasus showed a higher rooting ability than those of P. avium since approximately 40% of them were able to root in the absence of hormonal treatment. Except for the mutant D117, their rooting percentage was not significantly improved by H. cylindrosporum. Fungal inoculation also affected the survival of cuttings at acclimatization: 50% of the uninoculated P. avium cuttings survived whereas the survival percentage of inoculated cuttings ranged from 30 to 100% depending on the fungal genotype. With P. cerasus, the percentage of survival of uninoculated cuttings ranged from 85 to 100% and fungi either did not significantly improve it or lowered it. At acclimatization fungal hyphae could be observed in close contact with adventitious roots, but they did not establish mycorrhizal association. The shoot height of P. avium plantlets obtained from inoculated cuttings was not significantly different from that of plantlets originating from uninoculated ones. By contrast, fungal inoculation generally depressed the growth of acclimatized P. cerasus plantlets. The possibility of using ectomycorrhizal fungi as a tool to enhance rooting of micropropagated cuttings of plants which do not form ectomycorrhizas is discussed. Received: 25 November 1996 / Accepted: 2 June 1997     

The effect of soil phosphorus on the external mycelium growth of arbuscular mycorrhizal fungi during the early stages of mycorrhiza formation

The effects of soil P amendments and time of application on the formation of external mycelium by different arbuscular mycorrhizal (AM) fungi were studied. In the first experiment the external mycelium produced in the soil by the AM fungus Glomus etunicatum Beck. and Gerd., during the early stages of root colonization (7 and 14 days after inoculation), was quantified by the soil-agar film technique. A Brazilian Oxisol was used with three different phosphate levels, varying from deficient to supra-optimal for the plant. Significant differences were observed in the phosphate and inoculation treatments for plant dry weight, P content in the tissue, root length and root colonization, at fourteen days after planting. At 7 days, mycelium growth, root colonization and their relationship were reduced at supra-optimal P concentrations. Applications of P one week after planting reduced mycelium growth and root colonization more than when applied to the soil before planting. In a second experiment the arbuscular mycorrhizal (AM) fungi, Scutellospora heterogama (Nicol. and Gerd.) Walker and Sanders and E3 were tested and compared with Glomus etunicatum. For the species studied, the length of external hyphae per unit of colonized root length was affected by small P additions but no further significant differences were observed at high P levels. The three AM endophytes showed marked differences in their response to P in the soil: Scutellospora heterogama, although producing external mycelium more profusely than the Glomus spp., showed a higher sensitivity to soil P supply.