Mycorrhizal Response Research Articles

Nitrogen cycling in microcosms of yellow birch exposed to elevated CO2: simultaneous positive and negative below‐ground feedbacks

This study investigated simultaneous plant and soil feedbacks on growth enhancement with elevated [CO2] within microcosms of yellow birch (Betula alleghaniensis Britt.) in the second year of growth. Understanding the integrated responses of model ecosystems may provide key insight into the potential net nutrient feedbacks on [CO2] growth enhancements in temperate forests. We measured the net biomass production, C:N ratios, root architecture, and mycorrhizal responses of yellow birch, in situ rates gross nitrogen mineralization and the partitioning of available NH4+ between yellow birch and soil microbes. Elevated atmospheric [CO2] resulted in significant alterations in the cycling of N within the microcosms. Plant C/N ratios were significantly increased, gross mineralization and NH4+ consumption rates were decreased, and relative microbial uptake of NH4+ was increased, representing a suite of N cycling negative feedbacks on N availability. However, increased C/N ratios may also be a mechanism which allows plants to maintain higher growth with a constant or reduced N supply. Total plant N content was increased with elevated [CO2], suggesting that yellow birch had successfully increased their ability to acquire nutrients during the first year of growth. However, plant uptake rates of NH4+ had decreased in the second year. This discrepancy implies that, in this study, nitrogen uptake showed a trend through ontogeny of decreasing enhancement under elevated [CO2]. The reduced N mineralization and relatively increased N immobilization are a potential feedback which may drive this ontogenetic trend. This study has demonstrated the importance of using an integrated approach to exploring potential nutrient‐cycling feedbacks in elevated [CO2].

Nutritional, growth, and reproductive responses of maize ( Zea mays L.) to arbuscular mycorrhizal inoculation during and after drought stress at tasselling

The effects of root colonization by the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith on nutritional, growth, and reproductive attributes of two tropical maize cultivars with different sensitivities to drought were studied. Freshly regenerated seeds of selection cycles 0 (cv. C0, drought-sensitive) and 8 (cv. C8, drought-resistant) of the lowland tropical maize population "Tuxpeno sequia" were used in this greenhouse experiment. Maize plants were subjected to drought stress for 3 weeks following tasselling (75–95 days after sowing) and rewatered for the subsequent 5 weeks until harvest. Mycorrhizal (M+) plants had significantly higher uptake of N, P, K, Mg, Mn, and Zn into grain than non-mycorrhizal (M–) plants under drought conditions. AM inoculation also produced significantly greater shoot masses in C0 and C8 regardless of the drought-stress treatment. In the sensitive cultivar C0, drought stress reduced the shoot mass and grain yield by 23% and 55%, respectively, when roots were not colonized, while the reductions were only 12% and 31%, respectively, with mycorrhizal association. In addition, the emergence of tassels and silks was earlier in M+ plants than in M– plants under drought conditions. Mycorrhizal response was more pronounced under both well-watered and drought conditions in C0 than in the C8 cultivar. The overall results suggest that AM inoculation affects host plant nutritional status and growth and thereby alters the reproductive behaviour of maize under drought conditions.

Relationship of soil properties and soil amendments to response of Glomus intraradices and soybeans

Knowledge of the role of soil properties on mycorrhizal relationships with host plants may contribute to the ecological understanding of the fungus and its effectiveness in indigenous or introduced soil conditions. This study examined the response of Glomus intraradices, an arbuscular mycorrhizal (AM) fungus, and soybean host in five benchmark soils. Glomus intraradices increased, with a few exceptions, shoot dry weight, root length, shoot phosphorus, and shoot zinc in all soils. Soil amendments (nitrogen, phosphorus, and pH adjustments) affected mycorrhizal hyphal length per plant and hyphal to root length ratio, as well as shoot Zn. Interactions among soil properties, soybean plant, and this mycorrhizal isolate suggested that soil properties, such as pH, pore diameter, and silt and organic matter, may have difficulties predicting arbuscular mycorrhizal fungal response. However, before soil amendment, regression models were accurate in predicting shoot dry weight, mycorrhizal colonization, and hyphal development of this mycorrhizal isolate on plants. When soil amendments were added to the soil property regression model, the predictive ability of the model was greatly reduced or nonexistent. The observations in this study indicate the effects of soil properties on root colonization and external hyphal production are important in examining AM fungal – plant interaction. An understanding of soil properties will be essential to understand mycorrhizal fungal ecology and to effectively use mycorrhiza in biological systems. Key words: mycorrhiza, soil properties, soybean growth, mycorrhizal hyphae, benchmark soils.

Mycorrhizal response in wheat cultivars: relationship to phosphorus

The effect of five mycorrhizal fungi on the growth of 10 wheat cultivars under three phosphorus regimes was assessed in a greenhouse study. Six of the cultivars responded positively, while four responded negatively or were nonresponsive to mycorrhizal inoculation. The responses of the individual cultivars were consistent regardless of inoculum source, suggesting that mycorrhizal responsiveness is an inherited trait rather than a response to individual fungi. Mycorrhizal responsiveness decreased with P fertilization for cultivars that were dependent on the symbiosis, but it was unaffected by P fertilization in cultivars that were negatively impacted by the mycorrhizae. Mycorrhizal and P responsiveness of each cultivar were highly correlated (r = 0.94), suggesting that P responsiveness may be a good predictor of the mycorrhizal dependence of selected wheat cultivars. The relationship between wheat biomass production and percentage root colonization was positive for cultivars, which responded favorably to the symbiosis, and negative for cultivars, which responded negatively or were nonresponsive to mycorrhizal inoculation. Amendment with P did not significantly affect these relationships. To determine whether differences in mycorrhizal responsiveness are related to nutrient uptake by the fungus,32P uptake of Turkey (responsive cultivar) and Newton (nonresponsive cultivar) was controlled by severing the mycorrhizal hyphae in a split-pot experiment. Plants with intact hyphae absorbed more32P than those with severed hyphae for both cultivars, and significantly more counts per minute of32P were evident in Newton than in Turkey, suggesting that mycorrhizal function is not impaired even in cultivars that do not display a biomass increase in response to mycorrhizal symbiosis. Keywords: vesicular–arbuscular mycorrhizae, pathogenesis, growth response, mycorrhizal symbiosis.

Increasing symbiotic potentials in white clover

Lines of white clover with higher and lower numbers of nodules were selected from a mutagenised Huia population, and seed was produced from a polycross of both lines. The progeny showed that the lines tended to have either more but smaller nodules, or fewer and larger nodules. In a further experiment, the line with fewer (larger average size) nodules, fixed more nitrogen (N) and grew bigger. Plants with fewer but larger nodules are a good model for efforts to increase the Nfixing capacity of white clover. An empirical screening method was used to obtain 15 genotypes that showed evidence of high mycotrophy. A second experiment confirmed that 5 of these genotypes gave abnormally high responses to mycorrhizal infection. Examination of the roots of these plants showed that the selections did not differ from their parent lines in terms of root length or root hair cylinder diameter. The results tend to confirm published work with spring wheat, which showed that land races and wild types respond more strongly to mycorrhizal infection than high yielding varieties do. Investigation of the mycorrhizal responses of clover ecotypes that are adapted to low phosphorus soils is a priority for future research. Keywords: mycorrhizas, nitrogen fixation, nodulation, symbioses, white clover

Duration of Herbivore Removal and Environmental Stress Affect the Ectomycorrhizae of Pinyon Pines

The mutualistic mycorrhizal symbionts of plants have shown mixed responses to herbivory; they either decrease, increase, or show no measurable change. We examined the ectomycorrhizal responses of pinyon pine (Pinus edulis) exposed to 1 yr of simulated herbivory in two environments, one stressful and one less so. We also compared levels of ectomycorrhizal colonization and conelet production in pinyons form which an important insect herbivore had been removed for either 1 or 10 yr. Pinyons that grew in more stressful cinder soils experienced significant reductions (19%) in ectomycorrhizal colonization following 1 yr of simulated herbivory while pinyons growing in less stressful sandy loam soils did not. These results indicate that the degree of environmental stress experienced by a plant could affect whether mycorrhizal reductions result from herbivory. In addition the reductions in ectomycorrhizal colonization that resulted from chronic herbivory remained for a full year following herbivore removal even though conelet production increased 250—fold in the same time period. Our findings regarding the role of environmental stress and duration of herbivory in affecting mycorrhizal responses may help explain the variable responses found in other systems.

Nitrogen and phosphorus requirements for raising mycorrhizal seedlings of Leucaena leucocephala in containers

A greenhouse study was undertaken to determine the nitrogen and phosphorus fertilization requirements for raising mycorrhizal seedlings in soil in containers. Seedlings of Leucaena leucocephala were grown for 40 days in dibble tubes containing fumigated or nonfumigated soil uninoculated or inoculated with Glomus aggregatum. The soil was fertilized with NH4NO3 solution to obtain 25–200 mg N kg-1 soil, and with a KH2PO4 solution to establish target soil solution P concentrations of 0.015–0.08 mg P l-1. At the end of 40 days, seedlings were transplanted into pots containing 5-kg portions of fumigated soil. Posttransplant vesicular arbuscular mycorrhizal fungal (VAMF) effectiveness, measured as pinnule P content, plant height, shoot dry weight and tissue N and P concentrations, was significantly increased by pretransplant VAMF colonization in both soils. The best posttransplant mycorrhizal colonization and mycorrhizal growth responses were observed if the nonfumigated pretransplant soil was amended with 50 mg N kg-1 soil and 0.04 mg P l-1 or if the fumigated pretransplant soil was amended with 100 mg N kg-1 soil and 0.04 mg P 1-1. There was no relationship between N∶P ratios of nutrients added to the pretransplant soil medium and shoot N∶P ratios observed after transplanting. Shoot N∶P ratio was also not correlated with root colonization level.

Chromosome location of mycorrhizal responsive genes in wheat

Symbiotic relationships between wheat plants and mycorrhizal fungi may play an important role in the growth and productivity of wheat as a crop. Wheat cultivars differ in their ability to form such relationships, but little is known concerning the genetic basis of such differences. A set of intervarietal substitution lines having individual chromosomes from 'Cheyenne' (nonresponsive to mycorrhizae) substituted into 'Chinese Spring' (nonresponsive to mycorrhizae) were tested for mycorrhizal response in greenhouse experiments. Chromosomes 1A, 5B, 6B, 7B, 5D, and 7D of 'Cheyenne' had positive effects on the trait, with homologous groups 5 and 7 in the B and D genomes having the largest effects. Chromosome 5B of 'Hope', a nonresponsive cultivar, also had a positive effect in a 'Chinese Spring' background. In addition, the mycorrhizal responsiveness of a range of other cultivars and ancestors was tested to expand information on the trait in wheat and its relatives. Data on chromosome location of mycorrhizal-response genes and genotypic responsiveness will permit more effective genetic manipulation of this trait. Key words: pathogenesis, growth response, vesicular–arbuscular mycorrhizae.

Relationships between Seed Reserves, Seedling Growth and Mycorrhizal Responses in 14 Related Shrubs (Rosidae) from a Low-Nutrient Environment

1. The influence of seed reserves on seedling growth and phosphorus acquisition of 14 vesicular-arbuscular mycorrhizal (VAM) shrub species belonging to the Rosidae and indigenous to the flora of the nutrient-poor soils of the Cape Fynbos, South Africa, was studied in the presence or absence of mycorrhizal inoculum. 2. It is expected that among potentially VAM plant species found in a low-nutrient, high-light environment those with small seed reserves will have a higher dependency on mycorrhizas for seedling establishment than larger-seeded species. 3. Growth of the seedlings when non-mycorrhizal was strongly controlled by seed reserves. Most non-mycorrhizal plants were unable to acquire soil phosphorus unless they possessed alternative nutrient-acquiring root modifications. 4. Mycorrhizal responses in terms of mass and phosphorus content of inoculated plants compared to uninoculated plants are negatively correlated with the logarithm of seed mass and phosphorus content. Thus, it is suggested that seed size among the Rosidae in a low-nutrient environment is a trade-off between dispersal distances and the ability to establish a competitive presence in the absence of mycorrhizal inoculum, with no single seed size conferring an absolute advantage to VAM plant species. 5. The simultaneous evolution of cluster roots and large seeds in low-nutrient environments is proposed as a mechanism allowing for the adoption of the non-mycorrhizal state in some members of the Rosidae

Influence of phosphorus supply and light intensity on mycorrhizal response inPisum-Rhizobium-Glomus symbiosis

The influence of mycorrhizal colonization withGlomus mosseae on parameters of N2 fixation and plant growth was studied in pot experiments with pea plants (Pisum sativum L.) infected withRhizobium leguminosarum and supplied with varied levels of phosphorus (P) and nitrogen (N). Reduced light intensities were used to evaluate the dependence of the microsymbionts on assimilate supply. In plants grown with low P supply, mycorrhization increased the concentration of P in shoots, and thus N2 fixation. Reduced light intensity significantly depressed mycorrhizal colonization and nodule growth in low-P plants. When P supply did not limit plant growth and N2 fixation, however, the percentage of mycorrhizal colonization was reduced due to the higher P status, and the microsymbionts were not impaired by low light intensities. To maximize carbohydrate supply, another experiment was carried out at high light intensity of 900 μmol m−2s−1 and with non-limiting P supply. Nitrogen fertilization, given as starter N, enhanced plant growth, but delayed nodule formation. Towards flowering, nodulation rapidly increased, but less so inGlomus inoculated plants. After 28 days mycorrhizal plants were lower in shoot dry weight, nodule dry weight and nitrogenase activity. The results suggest that under many, but not all, environmental conditions the host plant is able to restrict mycorrhizal colonization and, thus, to prevent impairment ofRhizobium symbiosis.

Mycorrhizal Dependence of Andropogon gerardii and Schizachyrium scoparium in Two Prairie Soils

-Previous research in tallgrass prairie in Kansas indicated that warm-season, C4, grasses are obligate mycotrophs and do not grow normally in the absence of mycorrhizal symbiosis. However, the degree to which such grasses depend on mycorrhizae in other prairie soils has not been examined. Growth and mycorrhizal colonization of roots of Andropogon gerardii and Schizachyrium scoparium were compared in soil collected from Konza Prairie Research Natural Area (KPRNA), Riley County, Kansas and from Sand Ridge State Forest (SRSF), Mason County, Illinois. Plants of both species were grown in the two soils and were inoculated with Glomus etunicatum spores originally collected from KPRNA or colonized root pieces from S. scoparium plants collected from SRSF. Glomus etunicatum inoculum resulted in significantly greater root colonization and biomass of both plant species in steamed KPRNA soil than did root piece inoculum. There was no benefit from inoculation in nonsterile soil which contained indigenous mycorrhizal fungi. In SRSF soil, there was no response to inoculation with mycorrhizal fungi from either source. The lack of mycorrhizal response in SRSF soil is attributed to the greater plant-available P level of this soil. For S. scoparium grown in SRSF soil, plants grown in steamed soil produced more biomass than plants grown in steamed soil amended with nonsterile soil sievings (containing soil organisms other than mycorrhizal fungi), or in nonsterile soil. These differences could be due to competition for inorganic nutrients between soil microbes and the plant or antagonistic relationships between the plant or the mycorrhizal association and the soil microbes. Thus, the mycorrhizal dependence of these plant species is related to both soil and inoculum type or species.

Mycorrhizal activity in warm- and cool-season grasses: variation in nutrient-uptake strategies

Because cool-season grasses display little or no mycorrhizal responsiveness in prairie soil, it is unclear whether the high levels of mycorrhizal activity observed previously in these grasses represent nutrient uptake by external hyphae or simply metabolism of stored fungal reserves in roots. To distinguish between these hypotheses, a warm-season grass, Andropogon gerardii, or a cool-season grass, Bromus inermis, were grown at two temperatures on one side of a pot divided by a 43-μm nylon root barrier. Mycorrhizal function was assessed by measuring the amount of 32P translocated from one side of the pot to plants on the other side. As a control, mycorrhizal hyphae crossing the barrier were severed manually. Approximately 100 times more 32P was observed in mycorrhizal B. inermis grown at 18 °C versus 29 °C, and in B. inermis with intact versus severed hyphae at the cooler temperature. In contrast, A. gerardii accumulated approximately 4 times more 32P at 29 °C than at 18 °C, and approximately 100 times more with intact versus severed hyphae at the warmer temperature. Thus, it appears that mycorrhizal hyphae are highly active in both plant species regardless of the host's mycorrhizal responsiveness. Furthermore, mycorrhizal activity is highest at the temperature that favors growth of each species. The considerable activity of mycorrhizae in B. inermis is enigmatic since it usually has no biomass response. To further clarify the relationship between nutrient uptake and biomass response, both plant species were fertilized with a range of P levels and grown at a neutral temperature that supported the growth of both species. Although the concentration of P in B. inermis plant tissue increased in response to fertilization, there was no corresponding increase in biomass. In contrast, for A. gerardii, there was a direct and positive relationship between P fertilization and biomass produced, but tissue P concentrations remained relatively stable. Mycorrhizal symbiosis had no overall effect on biomass of B. inermis but significantly improved the growth of A. gerardii. These experiments showed clear differences in the growth strategies used by these two plant species. It is unclear whether these are differences that can be attributed to warm- and cool-season grasses in general. Short-term biomass responses as a measure of a plant's reliance on the symbiosis may not entirely reflect the contribution of the symbiosis if plants store nutrients with subsequent and perhaps delayed effects on fecundity, offspring performance, or even biomass. However, if the stored nutrient merely represents luxury consumption, this could still affect competitive ability because luxury consumption preempts the availability of nutrients for competitors. Key words: mycorrhizal responsiveness, mycorrhizal dependence, big bluestem, smooth bromegrass.

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.

Mycorrhizal responses of barley cultivars differing in P efficiency

Mycorrhizal responses of barley cultivars differing in P efficiency

Evaluation of vesicular-arbuscular mycorrhizal fungi in diverse plants and soils

A regional study was made to identify vesicular-arbuscular mycorrhizal (VAM) fungi effective in promoting plant growth in diverse plant and soil systems. Eight cooperators in six states of the eastern United States evaluated six VAM fungal isolates on soybean ( Glycine max L. Merr.) and sorghum ( Sorghum bicolor L. Moench) in a shared soil and in at least one regional soil from each location. Plants were grown with high VAM inoculum densities (minimum of 20 VAM propagules ml −1) for 42–57 days in pasturized soils in greenhouses or growth chambers. Shoot and root dry masses, total and colonized root lengths and shoot-P concentrations were determined at harvest. Under the experimental conditions tested, the VAM fungal isolate was more important than the soil or host plant in determining effectiveness. In the shared soil, inoculation with two isolates of Glomus (GE329 and GENPI) resulted in the greatest shoot masses for soybeans, while the same two isolates and GE312 provided maximum response in sorghum. In the regional soils, GE329 and GENPI had the widest range of growth promotion with both soybean and sorghum; however, for both plant species the mycorrhizal response was greatest in soils with less than 10 mg extractable P kg −1. For soybeans, colonized root length was not related to VAM growth response. For sorghum, there was a positive correlation between colonized root length and plant growth response. We conclude that VAM isolates exist which are effective in promoting plant growth over a range of edaphic and host conditions.

Mycorrhizal dependence of modern wheat varieties, landraces, and ancestors

Using mycorrhizal fungi known to colonize wheat, the mycorrhizal dependence of various small grains including modem wheat varieties, primitive wheat lines, and wheat ancestors was studied. With the exception of the United States cultivar Newton and the German cultivars Apollo, Kanzler, and Sperber, dry weight of eight other modern wheats from the United States and Great Britain were increased by 29–100% following inoculation with mycorrhizal fungi. All landraces from Asian collections or early introduced American cultivars were also dependent on the symbiosis, with dry weight increases averaging 169 and 55%, respectively. All wheat ancestors of the AA and BB genomes (except Aegilops speltoides) benefitted significantly from the symbiosis, whereas no benefit was observed for ancestors of the DD genome, tetraploid wheats of the AABB or AAGG genomes, or in the hexaploid ancestor Triticum zhukovskyi (AAAAGG genome). These differences in mycorrhizal response of the ancestors, lines, and cultivars were highly correlated with root fibrousness ratings. When the fungi used as a combined inoculum in the previous experiment were inoculated individually onto selected plant species or cultivars, 6 of the 10 isolates stimulated growth of Andropogon gerardii, a highly dependent grass species, and 8 of the 10 stimulated the growth of 'Turkey' wheat. In contrast, none of the isolates positively affected growth of 'Newton' or 'Kanzler' wheat cultivars, and in fact several fungi decreased the biomass produced by these two cultivars. These studies have demonstrated a strong genetic basis for differences in mycorrhizal dependence among cultivars. A trend for greater reliance on the symbiosis in older cultivated wheats than iin wheat ancestors or modern wheats was also observed. The depression in growth associated with certain mycorrhizal fungi and wheat cultivars demonstrates that colonization of roots does not guarantee benefit from the symbiosis. Key words: root fibrousness, growth response, vesicular–arbuscular mycorrhizae.

Physiological Responses of 6 Wheatgrass Cultivars to Mycorrhizae

The physiological and morphological responses of 6 wheatgrass (Agropyron) cultivars to vesicular-arbuscular mycorrhizal inoculation were measured in the greenhouse. These included diploid, tetraploid, and hexaploid cultivars. Plants had up to 94% infection after 4 months. The 2 diploid cultivars (A. cristatum cv.'Fairway' and A. cristatum ssp. puberulum) formed infection most rapidly, and they also had significantly reduced root biomass and higher water use efficiency with infection. A hexaploid cultivar (A. cristatum from U.S.S.R.) produced significantly more tillers with inoculation, while the tetraploid A. desertorum cv. 'Nordan' had fewer tillers and wider leaves. Inoculation increased leaf phosphorus concentration in 4 of the 6 cultivars. Carbon dioxide gas exchange rate, transpiration rate, stomatal resistance, and N concentration were not significantly affected by mycorrhizal inoculation for any of the cultivars. The cultivar Nordan had the greatest number of physiological and morphological increases in response to mycorrhizal infection, while A. cristatum from Iran (hexaploid) performed most poorly in that it had reduced WUE and no apparent beneficial responses to infection. There was no relationship between ploidy level and mycorrhizal response.

Growth responses of mycorrhizal and non-mycorrhizal tropical forage species to different levels of soil phosphate

Three tropical forage legumes, Stylosanthes capitata, Pueraria phaseoloides and Centrosema macrocarpum, and one grass, Brachiaria dictyoneura, were grown in a sterile phosphate deficient soil amended with soluble or rock phosphate at rates ranging from 0 to 400 mg kg-1 soil. The effects of inoculation with Glomus manihotis on mycorrhizal infection and plant growth were assessed. Early growth and nodulation of P. phaseoloides in soil with and without rock phosphate fertilizer were also determined. In the legumes, mycorrhizal infection was high at all P levels and sources, except for a significant decrease of infection in S. capitata at high levels of superphosphate. Plant growth was significantly increased by phosphate fertilizer and mycorrhizal inoculation. Mycorrhizal responses were more pronounced at low P levels with both P sources. In B. dictyoneura mycorrhizal infection was decreased with increasing additions of P. No effects of mycorrhizal inoculation (except with no added P) were observed. Growth and nodulation of P. phaseoloides were greatly stimulated by mycorrhizal inoculation.

Réponses de souches de bouleau à la mycorhization et au sel

Two selected birch (Betulapendula) clones, isolated invitro, were studied for their responses to salt and mycorrhization. After 60 days of culture, when inoculated with Paxillusinvolutus, clone 4 showed the most growth stimulation, 120%, against 55% for clone 5. Clone 4 is also more tolerant to NaCl than clone 5, their respective decreases in biomass being 10 and 40% in medium containing 50 mM NaCl. Salt and mycorrhizal responses were linked to the osmotic and nutritional status of vitroplants. Translocation of ions towards the shoots thus appears to be involved in these cross-tolerances.

Suppression of vesicular–arbuscular mycorrhizal fungus spore germination by nonsterile soil

When the effect of nonsterile soil on vesicular–arbuscular mycorrhizal fungus spore germination was examined, significantly fewer Glomus etunicatum (22 vs. 63 and 64%) or Glomus mosseae (23 vs. 80 and 79%) spores germinated in nonsterile soil than in autoclaved or pasteurized soil, respectively. In some cases, addition of nonsterile soil sievings to autoclaved or pasteurized soil also reduced germination, as compared with germination in unamended pasteurized or autoclaved soil. Germination was reduced by as much as 45% in autoclaved or pasteurized soil if spores were surface sterilized. However, surface sterilization of G. etunicatum spores did not affect dry weight of inoculated big bluestem plants, but mycorrhizal root colonization was reduced when spore-associated microbes were removed from spores by surface sterilization. The detrimental effect of soil microbes on spore germination and mycorrhizal growth response may reflect microbial competition for nutrients since germination of G. etunicatum and G. mosseae was reduced when pasteurized soil was amended with 15, 30, or 60 ppm phosphorus and 60 ppm phosphorus, respectively. An optimum range of available phosphorus may exist, above or below which germination is suppressed.