Following a 3-wk pretreatment with 150 mM NaCl, Sorghum plants were able to survive exposure to 300 mM NaCl, a lethal concentration for nonpretreated plants. This response is termed salt adaptation. Although the population was initially homogeneous, Na-includer and Na-excluder individuals coexisted after the achievement of the salt-adaptation response, and a large diversity in size of the seeds produced was observed at the end of the life cycle. Offspring of salt-adapted plants were exposed to a new salt-adaptation treatment. Even in the absence of selection, the proportion of Na-excluder individuals in the progeny was significantly increased. Offspring germinated from small seeds differed significantly from plants first-exposed to a salt-adaptation treatment, whereas offspring from large seeds displayed intermediate characteristics. This suggests that some of the adaptive changes were transmitted through embryo imprinting. The importance of this phenomenon for emergence of a newly adapted ecotype is discussed.

Companion cells in the secondary phloem of 125 Indian dicotyledonous species belonging to 43 families were examined by light microscopy. Four types of companion cell were identified: E-, S- and L-types which were equal in length, shorter, or longer, respectively, than the associated sieve tube element; and R-type, in which two or more companion cells in a vertical row were associated with the sieve tube element. The commonest was the E-type and the rarest was the L-type. E-type companion cells were most frequently found associated with short sieve tube elements (50–250 μm) which had a high frequency of simple sieve plates, considered phylogenetically advanced. R-type companion cells were most frequently associated with long sieve tube elements (>400 μm) with a high frequency of compound sieve plates, considered phylogenetically the least advanced. A strong positive correlation was found between the average number of companion cells associated with a sieve tube element and the lengths of the sieve tube elements. There was also a strong negative correlation between the average number of companion cells associated with a sieve tube element and companion cell lengths.

Within the growth zone, salt-affected leaves of sorghum (Sorghum bicolor) had narrower protoxylem and metaxylem cells than controls. Leaf width and cross-sectional area were also reduced, so that the salt treatment had no effect on the area of protoxylem per area of leaf cross section. Dye uptake studies suggested that in controls most of the veins, but in salt-affected leaves only half of the veins, are functional in water transport. Volumetric water flow was greatly diminished in the salt-affected plants. The reduced flow rate was largely explained by the salt-induced decrease in leaf surface area. Some decreases in flow rates per unit leaf mass or area were also produced by salinity, particularly in late developmental stages. Not surprisingly, leaf conductance measured with a diffusion porometer did not appear to be correlated with the diameter of the protoxylem or metaxylem elements. By contrast, published values of water deposition rates are strongly related to the size of the protoxylem elements: the rates of water deposition into the growing leaf tissue are proportional to the square of the protoxylem radius. Thus environmentally produced change of the hydraulic architecture of monocot leaves may cause change in local growth rates.

In normal populations of the common grass Holcus lanatus there is a polymorphism for arsenate resistance, manifested as suppressed phosphate uptake (SPU), and controlled by a major gene with dominant expression. A natural population of SPU plants had greater arbuscular-mycorrhizal colonization than wild type, nonSPU plants. It was hypothesized that, in order to survive alongside plants with a normal rate of phosphate (P) uptake, SPU plants would be more dependent on mycorrhizal associations. We performed an experiment using plants with SPU phenotypes from both arsenate mine spoils and uncontaminated soils, as well as plants with a nonSPU phenotype. They were grown with and without a mycorrhizal inoculum and added N, which altered plant P requirements. We showed that grasses with SPU phenotypes accumulated more shoot P than nonSPU plants, the opposite of the expected result. SPU plants also produced considerably more flower panicles, and had greater shoot and root biomass. The persistence of SPU phenotypes in normal populations is not necessarily related to mycorrhizal colonization as there were no differences in percentage AM colonization between the phenotypes. Being mycorrhizal reduced flower biomass production, as mycorrhizal SPU plants had lower shoot P concentrations and produced fewer flower panicles than non-mycorrhizal, nonSPU plants. We now hypothesize that the SPU phenotype is brought about by a genotype that results in increased accumulation of P in shoots, and that suppression of the rate of uptake is a consequence of this high shoot P concentration, operating by means of a homeostatic feedback mechanism. We also postulate that increased flower production is linked to a high shoot P concentration. SPU plants thus allocate more resources into seed production, leading to a higher frequency of SPU genes. Increased reproductive allocation reduces vegetative allocation and may affect competitive ability and hence survival, explaining the maintenance of the polymorphism. As mycorrhizal SPU plants behave more like nonSPU plants, AM colonization itself could play a major part in the maintenance of the SPU polymorphism.

In the northeastern part of Belgium, the Centaurea jacea complex shows extensive morphological variation and is represented by a diploid (2n = 22) and a tetraploid (2n = 44) cytotype. Polysomic inheritance of allozyme markers in the tetraploids, suggesting autopolyploidy, is here demonstrated for the first time. In order to test whether the two cytotypes occupy distinct habitats and possess different gene pools, patterns of allozymic and morphological variation were investigated in relation to ploidy level and site characteristics in 26 populations from the Belgian Ardennes. The two cytotypes showed a parapatric distribution, the diploids occurring at higher elevations (mostly above 500 m) than the tetraploids (mostly below 500 m). Three mixed populations were found near the contact zone of the two cytotypes. Within the mixed populations no triploid plant and no evidence for gene flow between cytotypes were found, despite widely overlapping flowering periods. The two cytotypes can be distinguished on the basis of morphological traits and enzymatic gene pools. The congruence of morphological and allozymic variation with chromosome numbers suggests a secondary contact between the two cytotypes with limited gene flow between them. The origin and persistence of the parapatric distribution are discussed.

Patterns of photobiont diversity were examined in some Nostoc-containing lichens using the nucleotide sequence of the cyanobacterial tRNALeu (UAA) intron. Lichen specimens collected in northwestern USA were analysed and the sequence data were compared with tRNALeu(UAA) intron sequences previously obtained from lichens in northern Europe. Generally, it is the species identity of a lichen rather than the geographical origin of the specimen that determines the identity of the cyanobiont. Identical intron sequences were found in Peltigera membranacea specimens collected in Oregon (USA) and in Sweden, and very similar sequences were also found in Nephroma resupinatum thalli collected in Oregon and Finland. Furthermore, in mixed assemblages where two Peltigera species grew in physical contact with each other, the different lichen species housed different photobiont strains. There is however not a one-to-one relation between mycobiont and photobiont as some intron sequences were found in more than one lichen species, and different intron sequences were found in different samples of some lichen taxa. Peltigera venosa exhibited a higher level of photobiont diversity than any other lichen species studied, and several intron sequences could for the first time be obtained from a single thallus. It is not clear whether this is evidence of lower cyanobiont specificity, or reflects an ability to exhibit different degrees of lichenization with different Nostoc strains. In one specimen of P. venosa, which contained bipartite cyanosymbiodemes and tripartite, cephalodiate thalli, both thallus types contained the same intron sequence.

Fungal growth within reaction zones of beech (Fagus sylvatica) challenged by three basidiomycetes, Inonotus hispidus, Ganoderma adspersum, Fomitopsis pinicola, and one ascomycete, Ustulina deusta, was studied in naturally colonized and artificially inoculated wood. All the fungi, except F. pinicola, breached reaction zones, but the mechanisms involved were all somewhat different. Both I. hispidus and U. deusta bypassed blocked cell lumina by tunnelling through cell walls (soft-rot mode), but the latter caused far more decomposition of cell walls. Degradation of polyphenols was slight with I. hispidus and absent with U. deusta. By contrast, G. adspersum preferentially degraded the polyphenolic occlusions in the cell lumina. The failure of F. pinicola to invade reaction zones was typical of a brown rot fungus having limited enzymatic potential and a uniform growth pattern. Mechanisms of lesion expansion, illustrated and summarized in schematic diagrams, are consistent with earlier observations that reaction zones in beech sapwood are static boundaries, which may be successfully breached by white and soft-rot fungi.

This study examines the microtubular cytoskeleton during megasporogenesis in the Nun orchid, Phaius tankervilliae. The subepidermal cell located at the terminal end of the nucellar filament differentiates first into an archesporial cell and then enlarges to become the megasporocyte. The megasporocyte undergoes the first meiotic division, giving rise to two dyad cells of unequal size. Immunostaining reveals that microtubules become more abundant as the megasporocyte increases in size. Microtubules congregate around the nucleus forming a distinct perinuclear array and many microtubules radiate directly from the nuclear envelope. In the megasporocyte, prominent microtubules are readily detected at the chalazal end of the cell cytoplasm. After meiosis I, the chalazal dyad cell expands in size at the expense of the micropylar dyad cell. At this stage, new microtubule organizing centres can be found at the corners of the cells. The appearance of these structures is stage-specific and they are not found at any other stages of megasporogenesis. The functional dyad cell undergoes the second meiotic division, resulting in the formation of two megaspores of unequal size. The chalazal megaspore enlarges and eventually gives rise to the embryo sac. As the functional megaspore expands, the microtubules again form a distinct perinuclear array with many microtubules radiating from the nuclear envelope. A defined cortical array of microtubules has not been found in P. tankervilliae during the course of megasporogenesis.

Results presented in this paper demonstrate the relationship between two data sets from Mubwindi Swamp, south-west Uganda. The first data set is from a survey of dryland montane forest within the immediate catchment and the second is in the form of pollen spectra recovered from surface samples of sediment. Numerical analysis grouped pollen types encountered into five categories, according to their level of representivity when compared with abundances of the same taxa in the sampled vegetation: under representative, moderately under representative, representative, moderately over representative, and over representative. The northern part of Mubwindi Swamp also yielded a 5-m-long sequence of organic-rich sediments (core MB6) from which sub-fossil pollen and radiocarbon data have been previously obtained. This provides an opportunity to apply calibration factors derived from modern pollen–vegetation relationships to sub-fossil pollen data that were obtained from the same site. These calibration factors effectively down-weighted the influence of over representative pollen types and permitted the discrimination of a phase of forest disturbance that had not previously been so clearly visible in sub- fossil pollen data.

In Lake Zürich, populations of the cyanobacterium Planktothrix rubescens develop in the metalimnion during the summer and become gradually entrained in the deepening surface mixed layer during the autumn. It had previously been demonstrated that the daily integrals of photosynthetic production accounted for the growth observed in the metalimnion and greatly exceeded the smaller increase during the autumn. We have now determined the relationship between growth rate (μ) and irradiance (I) in cultures of P. rubescens strain Pla 9316 maintained at 20 °C on a 12[ratio ]12 h light[ratio ]dark cycle: the highest net growth rate averaged over the 24 h (μh) was 0.123 d−1, the dark rate (μD) was −0.020 d−1, the gross rate (ϕm = μh−μD) was 0.144 d−1, the affinity coefficient (α) was 0.0273 (d μmol m−2 s−1)−1 and the compensation point (IC) was 1.76 μmol m−2 s−1. Using the corresponding coefficients calculated for the light period (μLh = 0.267 d−1, ϕLm = 0.287 d−1 and αL = 0.0547 (d μmol m−2 s−1)−1), instantaneous growth rates could be calculated from the irradiance. Comparison with growth rates at 10 °C indicated a Q10 of 1.48. These coefficients were used in a modification of the Smith equation to calculate potential growth rates of Planktothrix from the irradiance and temperature at each time and depth in Lake Zürich. Data on irradiance, vertical light attenuation and temperature were used to calculate the daily integrals of biomass increase over a period of 136 d. These growth integrals gave a closer correspondence to the observed population increase than the photosynthetic integrals calculated previously from measurements made with lakewater samples dominated by Planktothrix. Photosynthetic measurements made with the Planktothrix culture indicated a maximum rate of carbon increase (0.467 d−1 that exceeds the maximum growth rate, which suggests that other factors limit growth over long periods.

A new stripe rust resistance gene, YrH52, derived from the unique Mount Hermon population of wild emmer wheat (Triticum dicoccoides) in Israel, was previously located on chromosome 1B. The main objectives of the present study were to increase marker density in the vicinity of the YrH52 gene using additional microsatellite markers, and to evaluate the accuracy and efficiency of marker-assisted selection on this gene in the F2 generation. By means of 70 additional microsatellite primer pairs, 150 individuals of the F2 mapping population were genotyped. Among 202 marker loci, 20 were found to be linked to the YrH52 gene with log-likelihood (LOD) scores ranging from 3.84 to 58.82, and linkage distances ranging from 0.33 to 41.39 cM. A genetic map was constructed of chromosome 1B, consisting of 23 markers and the YrH52 gene, with a total map length of 149.5 cM. Most of the markers were located in the region close to YrH52, which was flanked by Xgwm413 and Xgwm273a with map distances of 1.3 and 2.7 cM, respectively. The accuracy and efficiency of marker-assisted selection were calculated as AMAS and EMAS, respectively, for homozygous resistant genotypes of YrH52 gene in the F2 generation. AMAS and EMAS for homozygous resistant genotypes of the YrH52 gene in the F2 generation showed linear and significant negative correlation with the map distance between marker and target gene or between the two bracketing markers. It was concluded that the YrH52 region on chromosome 1B is significantly more enriched by microsatellite markers than the previously published map; that a single microsatellite marker is efficient for marker-assisted selection of homozygous resistant genotypes of YrH52 gene in the F2 generation when the map distance is <5.0 cM; and that when two markers are used AMAS can be dramatically improved and becomes relatively stable, whereas EMAS will not be obviously improved and will still vary linearly with map distance.

The development of cereal cyst nematode (CCN; Heterodera avenae) induced syncytia in the host roots of infected resistant bread wheat (Triticum aestivum cv. AUS10894), diploid wheat (Aegilops tauschii), barley (Hordeum vulgare cv. Chebec and cv. Galleon) and in the susceptible wheat cv. Meering and barley cv. Clipper were studied over a period of 13 d. The resistance to CCN in these cereal plants is conferred by the resistance genes Cre1 in the wheat cv. AUS10894, Cre3 in A. tauschii, Ha2 in barley cv. Chebec and Ha4 in barley cv. Galleon. Anatomical observations were made on the development of the syncytia in CCN-infected wheat and barley roots, which carry each of these four sources of resistance genes. Accelerated development of the syncytia in resistant plants, especially in the barley cultivars, was observed. The sites of syncytia development in susceptible wheat and barley were also closely associated with the vascular tissues in the stele, but less so in the resistant plants. The syncytia in the infected susceptible wheat and barley were also metabolically active at day 13. By contrast, the syncytia of resistant wheat plants carrying the Cre1 or Cre3 genes remained extensively vacuolated and less metabolically active. In barley plants with the Ha2 or Ha4 genes, the syncytia appeared non-functional and in early stages of degeneration by day 13 after inoculation.

Soilborne pathogens, especially Fusarium oxysporum, are responsible for damping-off and root necrosis in Eucalyptus nurseries. New technologies are increasingly considering strategies for plant disease control other than chemical fungicides. Among these, natural fungal antagonists, which are colonizers of the root cortex, are potential biocontrol agents. An in vitro system was used: (1) to test the pathogenic effects of F. oxysporum strain Foeu1 which was recovered from a forest nursery soil; (2) to explore the potential of the nonpathogenic F. oxysporum strain Fo47, which is known for its efficiency in biological control, to suppress damping-off of Eucalyptus seedlings; (3) to compare the patterns of root colonization and host response to invasion by the two Fusarium strains inoculated separately in a time-course study. Root inoculation of E. viminalis with F. oxysporum strain Foeu1 caused damping-off in young seedlings in vitro, whilst disease symptoms were not visible in plants inoculated with F. oxysporum strain Fo47 or when both strains (Foeu1 + Fo47) were inoculated simultaneously. Each strain showed similarities in patterns of root tissue colonization, and in the processes of root penetration and initial colonization. Differential effects on root tissue were observed with fungal development within the cortex: ingress of strain Foeu1 was accompanied by severe host-cell alterations whilst no tissue damage occurred with development of strain Fo47.

We studied the transport of 15N from a soil compartment separated from a plant root compartment by a hydrophobic polytetrafluoroethylene (PTFE) membrane to plants in the presence and absence of arbuscular mycorrhizal fungi (AMF). We have previously shown that this type of membrane efficiently inhibits mass flow and diffusion of mobile ions in the soil solution in an abiotic system, but can be penetrated by the hyphae of mycorrhizal fungi. Mycorrhizal tomatoes (Lycopersicon esculentum) colonized by Glomus mosseae were grown at two N fertilizer concentrations in a root compartment. A PTFE membrane was placed between the root compartment and an adjoining soil compartment that was inaccessible to the roots but accessible to the AMF hyphae (hyphal compartment). Additional N was applied to the hyphal compartment using uniformly 15N-labelled NH4NO3. There was a flux of 15N from the hyphal compartment to the plants even in the absence of mycorrhizal fungi. However, this flux was much higher in mycorrhizal plants, which had much higher N concentrations in their shoots and roots than did the non-mycorrhizal control plants. This was particularly apparent when the root compartment had a low N fertilizer concentration. Of the total N content of mycorrhizal plants, c. 42 and 24% at the low and high N fertilizer concentrations, respectively, were estimated to originate from the hyphal compartment by transport through AMF hyphae. In the presence of mycorrhizal fungi, the flux of 15N was about three times higher than in their absence. The results show that AMF can access a soil compartment separated by a PTFE membrane, and can contribute substantially to N uptake by plants.

Ectomycorrhizal seedlings of Scots pine (Pinus sylvestris) inoculated with the nitrotolerant Laccaria bicolor and the nitrophobic Suillus bovinus were exposed to ambient (350 μl l−1) and elevated (700 μl l−1) [CO2]. After 79 d the seedlings were labelled for 28 d with 14CO2, after which they were harvested. 14C was determined in shoots, roots plus mycorrhizas, soil, and below-ground respiration; nitrogen was determined in shoots and roots. Total net 14C uptake increased under elevated [CO2]. The extra carbon did not increase the shoot mass but was translocated to the roots and resulted in a decreased shoot-to-root ratio in the Suillus-inoculated seedlings. Laccaria-inoculated seedlings did not incorporate the additional carbon in root or fungal tissue but only increased below-ground respiration. S. bovinus acquired or transferred nitrogen better than L. bicolor and enabled the seedlings to perform better with regard to net carbon uptake under elevated [CO2]. This resulted in nitrogen concentrations in shoots of Suillus-inoculated seedlings that were twice as high as in Laccaria-inoculated seedlings, irrespective of [CO2]. The higher nitrogen concentration in the shoots resulted in a doubling of the 14C uptake per unit shoot mass. Our results suggest that the ability of ectomycorrhizal Scots pine seedlings to respond positively to elevated atmospheric [CO2] is strongly fungal-species specific.

In previously reported laboratory experiments, infection of Rumex obtusifolius by the rust fungus Uromyces rumicis was decreased on leaves which had prior herbivory by the beetle Gastrophysa viridula. In this paper we investigate whether this interaction is robust for natural infection by a variety of fungi in field experiments carried out in spring and autumn with plants given different levels of nitrogen fertilization. Grazing by G. viridula led to a decrease in lesion density of Ramularia rubella and Venturia rumicis in the spring and V. rumicis and U. rumicis in the autumn experiment. For V. rumicis and U. rumicis significant reductions in lesion density occurred on the undamaged leaves of damaged plants, compared with similar leaves on undamaged plants, suggesting systemic induced resistance. This induced resistance was usually independent of the amount of nitrogen fertilization, although the inhibitory effect of grazing on R. rubella in the spring and V. rumicis in the autumn experiment was enhanced by increasing nitrogen fertilization and was inhibited by increasing nitrogen fertilization for V. rumicis in the spring. In both experiments, the lesion density of V. rumicis was greater on leaves on which R. rubella was also present, and the presence of U. rumicis in the autumn experiment was linked to a similar but greater effect on V. rumicis lesion density. We found no evidence of induced resistance by fungi against fungi in these experiments. We highlight the complex interactions between inhibitory and facilitatory processes acting on leaf fungal infection. These results are compared with the proposed molecular mechanisms of induced resistance(s) and we consider the benefits of closer integration between molecular and ecological investigations of induced resistances that occur in the field.

In this paper we distinguish between invasive and noninvasive (finite) saprotrophic spread of the soil-borne fungal plant pathogen, Rhizoctonia solani amongst discrete sites of nutrient resource. Using simple concepts of percolation theory, we predict the critical threshold distance, associated with a threshold probability, between donor (colonized) and recipient (uncolonized) nutrient sites at which R. solani can spread invasively by mycelial growth through a population of nutrient sites on a lattice. The critical distance for invasive spread is estimated from colonization profiles derived from placement experiments that summarize the probability of colonization with distance between replicated pairs of colonized and uncolonized sites. Colonization profiles were highly nonlinear, decaying sigmoidally with distance. Thresholds for invasive spread were predicted at inter-site distances of 8.1 mm and 11.8 mm for sites of low and high nutrient agar, respectively. In population experiments with inter-site distances below the predicted thresholds, the spread of the fungus was invasive in all replicates. At large distances (>10 mm for low, and >14 mm for high nutrient sites) the spread of the fungus was always finite, with the proportion of finite replicates decreasing sharply close to the percolation threshold. Invasive spread did not depend on the furthest extent of growth of the fungus but on distances predicted by the percolation thresholds. Invasive spread of the fungus is also examined in a more natural and variable, nonsterile system involving the growth and colonization of a lattice of poppy seeds over sand. The system is characterized by a decay in the probability of colonization between older poppy seeds, which effectively ‘quenches’ saprotrophic spread. Hence in the population experiments with poppy seeds all growth was ultimately finite. The threshold distance, corresponding to the critical percolation probability for invasive growth changed from 18 mm to 4 mm over 21d leading to a switch from invasive to finite growth. We conclude that percolation theory can be used to link the growth of individual mycelial colonies to the formation of patches that result from the colonization of particulate organic matter. The nonlinearity of the colonization profiles combined with the presence of a percolation threshold means that small changes in the distance between nutrient sites can result in large differences in final patch size. The rapid decay of particulate organic matter in a more natural system can have a profound effect on the dynamics of colonization, restricting saprotrophic invasion of the soil. The consequences of invasion thresholds for colony growth of saprotrophic and parasitic fungi in dynamical systems are briefly discussed.

The non-reducing disaccharide trehalose is a major storage compound in most fungi. For a better understanding of carbon metabolism in the ectomycorrhizal-forming basidiomycete Amanita muscaria, trehalase activity was analysed from mycelium growing in liquid culture or on agar plates, and from mycorrhizas. Trehalase activity was found in both the culture medium and in mycelial extracts. The excreted trehalase was purified to apparent homogeneity by anion-exchange chromatography, gel filtration and preparative gel electrophoresis. The identified enzyme belongs to the group of acid trehalases. The apparent molecular mass of the native enzyme was estimated to be 165 kDa by gel filtration and 135 kDa by SDS–PAGE. Isoelectrofocusing indicated an isoelectric point (pI) of approx. 3.7, which is typical for acid trehalases. The enzyme is highly specific for its substrate trehalose, with an apparent Km of 0.38 mM. Validamycin and sucrose act as competitive inhibitors with Ki values of 45 nM and 15 mM, respectively. The activity of acid trehalase excreted into the growth medium is independent of the carbon source (glucose or trehalose), revealing that the enzyme is not regulated by its substrate trehalose. Nevertheless, fungal hyphae grown in the absence of an external carbon source showed enhanced enzyme excretion. The biochemical characteristics of the trehalase activity measured in the mycelium are in the same range as those determined for the excreted enzyme. The enzyme is localized in the cell wall and its activity is strongly decreased in ectomycorrhizas.

Using genetic crosses between single chs mutants of Ustilago maydis inoculated into maize (Zea mays) seedlings, two classes of double mutants affected in genes coding for chitin synthetases were isolated: chs3/chs4, and chs4/chs5. Analysis of the mutants showed almost no change in their phenotype compared with wild-type strains. Growth rate, effect of stress conditions, dimorphic transition and mating were not affected. The only salient differences were increased sensitivity to osmotics at acid pH, and decrease in chitin synthetase activity, especially when measured with CO2+, and in chitin content. Most significant was a decrease in virulence, although this appeared to be due a factor unrelated to CHS genes. These data can be taken as further evidence that multigenic control of chitin synthetase in fungi operates as a safety mechanism to guarantee fungal viability in changing and hostile environmental conditions.

Eucalyptus species are considered to have ectomycorrhizas (ECM), but many also have vesicular–arbuscular mycorrhizas (VAM) and their relative importance is unclear. Interactions between ECM and VAM fungi colonizing roots of Eucalyptus species were examined in a glasshouse experiment. This experiment investigated competition between these two types of fungi and compared benefits provided to Eucalyptus globulus and E. urophylla. Eucalyptus seedlings were inoculated with spores of the ECM fungus Laccaria lateritia and/or pot- culture soil for a VAM fungus (species of Glomus, Acaulospora or Scutellospora). Initial inoculum levels were important, as VAM fungi became established much more rapidly than Laccaria. In plants with both types of mycorrhizas, Laccaria mycorrhizas substantially increased after 2 or 3 months and the proportion of roots with VAM declined. However, the proportion of roots with VAM also decreased significantly in plants without ECM after 2 months. Thus, the relative susceptibility of eucalypt roots to these mycorrhizal associations changed. Substantial growth responses to mycorrhizal inoculation occurred when a low concentration of phosphorus fertilization was used (5 mg kg−1), but not at a higher concentration (10 mg kg−1). Treatments where Laccaria was applied, alone or in combination with a VAM fungus, resulted in the largest growth increases relative to nonmycorrhizal plants. An Acaulospora isolate was the most effective VAM fungus for E. urophylla although other VAM fungi also increased growth relative to the control. VAM fungi alone had little effect on E. globulus growth, but plants inoculated with both ECM and VAM fungi were larger than plants only inoculated with Laccaria. ECM fungi had a major impact on root system form by reducing the proportion of fine roots (specific root length) relative to nonmycorrhizal plants or those with VAM. Changes in root colonization patterns over time must be considered in studies of ECM/VAM interactions, as there can be substantial changes in the relative importance of the two associations.