Functional diversity of arbuscular mycorrhizal fungal isolates in relation to extraradical mycelial networks

We investigated the functional significance of extraradical mycorrhizal networks produced by geographically different isolates of the arbuscular mycorrhizal fungal (AMF) species Glomus mosseae and Glomus intraradices. A two-dimensional experimental system was used to visualize and quantify intact extraradical mycelium (ERM) spreading from Medicago sativa roots. Growth, phosphorus (P) and nitrogen (N) nutrition were assessed in M. sativa plants grown in microcosms. The AMF isolates were characterized by differences in extent and interconnectedness of ERM. Phenotypic fungal variables, such as total hyphal length, hyphal density, hyphal length per mm of total or colonized root length, were positively correlated with M. sativa growth response variables, such as total shoot biomass and plant P content. The utilization of an experimental system in which size, growth rate, viability and interconnectedness of ERM extending from mycorrhizal roots are easily quantified under realistic conditions allows the simultaneous evaluation of different isolates and provides data with a predictive value for selection of efficient AMF.     

Effect of diazotrophic bacteria isolated from a mycelium of arbuscular mycorrhizal fungi on colonization of maize roots by Glomus fistulosum

The inoculation of mycorrhizal maize plants with three isolates of microaerophilic diazotrophic bacteria obtained from the mycelium of arbuscular mycorrhizal fungi associated with three grasses (Arrhenatherum elatius - bacterial isolate ARR, Agropyrum repens - isolate AGR and Poa annua - isolate POA) caused no increase in nitrogen content in plant biomass. The inoculation with bacterial isolate ARR resulted in the decreased plant growth. Bacterial isolate AGR decreased the percentage of the root length colonized by arbuscular mycorrhizal fungus Glomus fistulosum. The inoculation with both mycorrhizal fungus and isolate POA increased significantly the concentration of phosphorus in plant shoots compared to uninoculated control. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth, respiration and nutrient acquisition by the arbuscular mycorrhizal fungus Glomus mosseae and its host plant Plantago lanceolata in cooled soil

Although plant phosphate uptake is reduced by low soil temperature, arbuscular mycorrhizal (AM) fungi are responsible for P uptake in many plants. We investigated growth and carbon allocation of the AM fungus Glomus mosseae and a host plant (Plantago lanceolata) under reduced soil temperature. Plants were grown in compartmented microcosm units to determine the impact on both fungus and roots of a constant 2.7 °C reduction in soil temperature for 16 d. C allocation was measured using two (13)CO(2) pulse labels. Although root growth was reduced by cooling, AM colonization, growth and respiration of the extraradical mycelium (ERM) and allocation of assimilated (13)C to the ERM were all unaffected; the frequency of arbuscules increased. In contrast, root respiration and (13)C content and plant P and Zn content were all reduced by cooling. Cooling had less effect on N and K, and none on Ca and Mg content. The AM fungus G. mosseae was more able to sustain activity in cooled soil than were the roots of P. lanceolata, and so enhanced plant P content under a realistic degree of soil cooling that reduced plant growth. AM fungi may therefore be an effective means to promote plant nutrition under low soil temperatures.     

Influence of a Bacillus sp. on physiological activities of two arbuscular mycorrhizal fungi and on plant responses to PEG-induced drought stress

The effects of bacterial inoculation (Bacillus sp.) on the development and physiology of the symbiosis between lettuce and the arbuscular mycorrhizal (AM) fungi Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and Glomus intraradices (Schenck and Smith) were investigated. Plant growth, mineral nutrition and gas-exchange values in response to bacterial inoculation after PEG-induced drought stress were also evaluated. In AM plants, inoculation with Bacillus sp. enhanced fungal development and metabolism, measured as succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities, more than plant growth. Under non-stressed conditions, G. intraradices colonization increased all plant physiological values to a higher extent when in dual inoculation with the bacterium. Under stress conditions, the bacterium had an important stimulatory effect on G. intraradices development. Under such conditions, the effects of the bacterium on photosynthetic rate, water use efficiency (WUE) and stomatal conductance of lettuce plants differed with the fungus species. Plant-gas exchange was enhanced in G. intraradices- and reduced in G. mosseae-colonized plants when co-inoculated with Bacillus sp. Thus, the effects of each fungus on plant physiology were modulated by the bacterium. Stress was detrimental, particularly in G. intraradices-colonized plants without the bacterium, reducing intra and extraradical mycelium growth and vitality (SDH), as well as plant-gas exchange. Nevertheless, Bacillus sp. inoculation improved all these plant and fungal parameters to the same level as in non-stressed plants. The highest amount of alive and active AM mycelium for both fungi was obtained after co-inoculation with Bacillus sp. These results suggest that selected free-living bacteria and AM fungi should be co-inoculated to optimize the formation and functioning of the AM symbiosis in both normal and adverse environments.     

The mobilisation and fate of soil and rock phosphate in the rhizosphere of ectomycorrhizal Pinus radiata seedlings in an Allophanic soil

A pot experiment was conducted to investigate the uptake of Zn from experimentally contaminated calcareous soil of low nutrient status by maize inoculated with the arbuscular mycorrhizal (AM) fungus Glomus caledonium. EDTA was applied to the soil to mobilize Zn and thus maximize plant Zn uptake. The highest plant dry matter (DM) yields were obtained with a moderate Zn addition level of 300 mg kg^?1. Plant growth was enhanced by mycorrhizal colonization when no Zn was added and under the highest Zn addition level of 600 mg kg^?1, while application of EDTA to the soil generally inhibited plant growth. EDTA application also increased plant Zn concentration, and Zn accumulation in the roots increased with increasing EDTA addition level. The effects of inoculation with G. caledonium on plant Zn uptake varied with Zn addition level. When no Zn was added, Zn translocation from roots to shoots was enhanced by mycorrhizal colonization. In contrast, when Zn was added to the soil, mycorrhizal colonization resulted in lower shoot Zn concentrations in mycorrhizal plants. The P nutrition of the maize was greatly affected by AM inoculation, with mycorrhizal plants showing higher P concentrations and P uptake. The results indicate that application of EDTA mobilized soil Zn, leading to increased Zn accumulation by the roots and subsequent plant toxicity and growth inhibition. Mycorrhizal colonization alleviated both Zn deficiency and Zn contamination, and also increased host plant growth by influencing mineral nutrition. However, neither EDTA application nor arbuscular mycorrhiza stimulated Zn translocation from roots to shoots or metal phytoextraction under the experimental conditions. The results are discussed in relation to the environmental risk associated with chelate-enhanced phytoextraction and the potential role of arbuscular mycorrhiza in soil remediation.     

Interactions between the root-knot nematode Meloidogyne incognita and Glomus mosseae in banana

The effects of the interaction between the arbuscular mycorrhizal fungus Glomus mosseae and the root-knot nematode Meloidogyne incognita on growth and nutrition of micropropagated ;Grand Naine banana (Musa AAA) cultivar was studied under greenhouse conditions. Inoculation with two G. mosseae isolates significantly increased growth of plants in relation to non-mycorrhizal plants. Response to mycorrhizae was as effective as with an optimum P fertilization in promoting plant development for most growth parameters. Meloidogyne incognita had no apparent effect on the percentage of root colonization in mycorrhizal plants. In contrast, G. mosseae suppressed root galling and nematode buildup in the roots. The percentage of mycorrhizal colonization was high (over 80%) in low P fertilized plants, but optimum P rates for bananas (four times higher than low P) significantly reduced mycorrhizal colonization. Most elements were within sufficiency levels for banana with exception of N which was low for all treatments. Mycorrhizal plants fertilized with a low P rate showed higher N, P, K, Ca, and Mg contents as compared to non-mycorrhizal plants low in P with or without the nematode. Inoculation with G. mosseae favours growth of banana plants by enhancing plant nutrition and by suppressing nematode reproduction and galling during the early stages of plant development.     

Drought Tolerance and Antioxidant Activities in Lavender Plants Colonized by Native Drought-tolerant or Drought-sensitive <Emphasis Type="Italic">Glomus</Emphasis> Species

This study compared the effectiveness of four arbuscular mycorrhizal (AM) fungal isolates (two autochthonous presumably drought-tolerant Glomus sp and two allochthonous presumably drought-sensitive strains) on a drought-adapted plant (Lavandula spica) growing under drought conditions. The autochthonous AM fungal strains produced a higher lavender biomass, specially root biomass, and a more efficient N and K absorption than with the inoculation of similar allochthonous strains under drought conditions. The autochthonous strains of Glomus intraradices and Glomus mosseae increased root growth by 35% and 100%, respectively, when compared to similar allochthonous strains. These effects were concomitant with an increase in water content and a decline in antioxidant compounds: 25% glutathione, 7% ascorbate and 15% H₂O₂ by G. intraradices, and 108% glutathione, 26% ascorbate and 43% H₂O₂ by G. mosseae. Glutathione and ascorbate have an important role in plant protection and metabolic function under water deficit; the low cell accumulation of these compounds in plants colonized by autochthonous AM fungal strains is an indication of high drought tolerance. Non-significant differences between antioxidant activities such as glutathione reductase (GR), catalase (CAT) and superoxide dismutase (SOD) in colonized plants were found. Thus, these results do not allow the generalization that GR, CAT and SOD were correlated with the symbiotic efficiency of these AM fungi on lavender drought tolerance. Plants colonized by allochthonous G. mosseae (the less efficient strain under drought conditions) had less N and K content than those colonized by similar autochthonous strain. These ions play a key role in osmoregulation. The AM symbiosis by autochthonous adapted strains also produced the highest intraradical and arbuscular development and extraradical mycelial having the greatest fungal SDH and ALP-ase activities in the root systems. Inoculation of autochthonous drought tolerant fungal strains is an important strategy that assured the greatest tolerance water stress contributing to the best lavender growth under drought.     

Effects of Arbuscular-Mycorrhizal Glomus Species on Drought Tolerance: Physiological and Nutritional Plant Responses

The tolerance of lettuce plants (Lactuca sativa L. cv. Romana) to drought stress differed with the arbuscular-mycorrhizal fungal isolate with which the plants were associated. Seven fungal species belonging to the genus Glomus were studied for their ability to enhance the drought tolerance of lettuce plants. These fungi had different traits that affected the drought resistance of host plants. The ranking of arbuscular-mycorrhizal fungal effects on drought tolerance, based on the relative decreases in shoot dry weight, was as follows: Glomus deserticola > Glomus fasciculatum > Glomus mosseae > Glomus etunicatum > Glomus intraradices > Glomus caledonium > Glomus occultum. In this comparative study specific mycorrhizal fungi had consistent effects on plant growth, mineral uptake, the CO(inf2) exchange rate, water use efficiency, transpiration, stomatal conductance, photosynthetic phosphorus use efficiency, and proline accumulation under either well-watered or drought-stressed conditions. The ability of the isolates to maintain plant growth effectively under water stress conditions was related to higher transpiration rates, levels of leaf conductance, and proline, N, and P contents. Differences in proline accumulation in leaves among the fungal symbioses suggested that the fungi were able to induce different degrees of osmotic adjustment. The detrimental effects of drought were not related to decreases in photosynthesis or water use efficiency. Neither of these parameters was related to P nutrition. The differences in P and K acquisition, transpiration, and stomatal conductance were related to the mycorrhizal efficiencies of the different fungi. Our observations revealed the propensities of different Glomus species to assert their protective effects during plant water stress. The greater effectiveness of G. deserticola in improving water deficit tolerance was associated with the lowest level of growth reduction (9%) under stress conditions. The growth of plants colonized by G. occultum was reduced by 70% after a progressive drought stress period. In general, the different protective effects of the mycorrhizal isolates were not associated with colonizing ability. Nevertheless, G. deserticola was the most efficient fungus and exhibited the highest levels of mycorrhizal colonization, as well as the greatest stimulation of physiological parameters.     

Application of Aspergillus niger-treated agrowaste residue and Glomus mosseae for improving growth and nutrition of Trifolium repens in a Cd-contaminated soil

The microbial transformation of sugar beet (SB) agrowaste with or without rock-phosphate (RP) has utility for the improvement of plant growth in a Cd (5 microg g-1) artificially contaminated soil, particularly when the soil is co-inoculated with arbuscular mycorrhizal (AM) fungus Glomus mosseae isolated from a Cd-polluted area. Under such Cd-polluted conditions, the limited growth, mineral nutrition, symbiotic developments (nodulation and AM-colonization) and soil enzymatic activities were stimulated using SB or SB+RP as soil amendments and G. mosseae as inoculant. G. mosseae enhanced plant establishment in a higher extent in amended soil; it is probably due to the interactive effect increasing the potential fertility of such compounds and its ability for decreasing Cd transfer from soil to plant. The amount of Cd transferred from soil solution to biomass of AM-colonized plants ranged from 0.09 microg Cd g-1 (in SB+RP-amended soil) to 0.6 microg Cd g-1 (in non-amended soil). Nodule formation was more sensitive to Cd than AM-colonization, and both symbioses were stimulated in amended soils. Not only AM-colonization but also amendments were critical for plant growth and nutrition in Cd-polluted soil. The high effectiveness of AM inoculum increasing nutrients and decreasing Cd in amended soil indicated the positive interaction of these treatments in increasing plant tolerance to Cd contamination.     

Arbuscular mycorrhizal fungal application to improve growth and tolerance of wheat (Triticum aestivum L.) plants grown in saline soil

A pot experiment was conducted to examine the effects of three different arbuscular mycorrhizal fungi, Glomus mosseae, G. deserticola and Gigaspora gergaria, on growth and nutrition of wheat (Triticum aestivium L. cv. Henta) plants grown in saline soil. Under saline condition, mycorrhizal inoculation significantly increased growth responses, nutrient contents, acid and alkaline phosphatases, proline and total soluble protein of wheat plants compared to non-mycorrhizal ones. Those stimulations were related to the metabolic activity of the each mycorrhizal fungus. The localization of succinate dehydrogenase “SDH” (as a vital stain for the metabolically active fungus) in the arbuscular mycorrhizal fungi was variable. In general, mycorrhizal shoot plant tissues had significantly higher concentrations of P, N, K and Mg but lower Na concentration than those of non-mycorrhizal plants. In saline soil, growth and nutrition of wheat plants showed a high degree of dependency on mycorrhizal fungi (especially G. mosseae). The use of the nitroblue tetrazolium chloride method as a vital stain for SDH activity showed that all the structures of mycorrhizal infections in the wheat plant estimated by the trypan blue staining (non-vital stain) were not metabolically active. Interestingly, the reduction in Na uptake along with associated increases in P, N and Mg absorption and high proline, phosphatase activities and chlorophyll content in the mycorrhizal plants could be important for salt alleviation in plants growing in saline soils.     

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.     

13C incorporation into signature fatty acids as an assay for carbon allocation in arbuscular mycorrhiza

The ubiquitous arbuscular mycorrhizal fungi consume significant amounts of plant assimilated C, but this C flow has been difficult to quantify. The neutral lipid fatty acid 16:1omega5 is a quantitative signature for most arbuscular mycorrhizal fungi in roots and soil. We measured carbon transfer from four plant species to the arbuscular mycorrhizal fungus Glomus intraradices by estimating (13)C enrichment of 16:1omega5 and compared it with (13)C enrichment of total root and mycelial C. Carbon allocation to mycelia was detected within 1 day in monoxenic arbuscular mycorrhizal root cultures labeled with [(13)C]glucose. The (13)C enrichment of neutral lipid fatty acid 16:1omega5 extracted from roots increased from 0.14% 1 day after labeling to 2.2% 7 days after labeling. The colonized roots usually were more enriched for (13)C in the arbuscular mycorrhizal fungal neutral lipid fatty acid 16:1omega5 than for the root specific neutral lipid fatty acid 18:2omega6,9. We labeled plant assimilates by using (13)CO(2) in whole-plant experiments. The extraradical mycelium often was more enriched for (13)C than was the intraradical mycelium, suggesting rapid translocation of carbon to and more active growth by the extraradical mycelium. Since there was a good correlation between (13)C enrichment in neutral lipid fatty acid 16:1omega5 and total (13)C in extraradical mycelia in different systems (r(2) = 0.94), we propose that the total amount of labeled C in intraradical and extraradical mycelium can be calculated from the (13)C enrichment of 16:1omega5. The method described enables evaluation of C flow from plants to arbuscular mycorrhizal fungi to be made without extraction, purification and identification of fungal mycelia.     

Underground resource allocation between individual networks of mycorrhizal fungi

Fusions between individual mycelia of arbuscular mycorrhizal (AM) fungi have been observed in two-dimensional systems but never in soil systems. Here, phosphorus ((32)P) labelling was used to demonstrate nutrient transfer between individual mycelia and to investigate the possible role of anastomosis. Trifolium subterraneum colonized by Glomus mosseae were grown in root-retaining mesh bags, which were placed 20 cm apart. The mycelium of one plant, the donor, had access to (32)P-labelled soil placed adjacent to the mesh bag. Transfer of (32)P from the donor mycelium to the receiver plant was measured at three harvests. In a second-harvest control treatment the receiver was colonized by Glomus caledonium in order to determine whether transfer occurred by other means than hyphal fusions. Significant amounts of P were transferred to the receiver plant at the last harvests when the two mycelia of G. mosseae overlapped. The transfer probably occurred via anastomoses between the mycelia as no transfer of (32)P was detected between the mycelia of different fungi at the second harvest. The indicated ability of AM fungal mycelia to anastomose in soil has implications for the formation of large plant-interlinking functional networks, long-distance nutrient transport and retention of nutrients in readily plant-available pools.     

Interactions between Glomus mosseae and arbuscular mycorrhizal sporocarp-associated saprophytic fungi

The saprophytic fungi Wardomyces inflatus (Marchal) Hennebert, Paecilomyces farinosus (Holm & Gray) A. H. S. Brown & G. Sm., Gliocladium roseum Bain., sterile dark mycelium (SDM-54), Trichoderma pseudokoningii Rifai and Trichoderma harzianum Rifai were isolated from sporocarps of Glomus mosseae. The effect of saprophytic fungi on G. mosseae spore germination was tested on water agar. Wardomyces inflatus decreased the percent germination of G. mosseae spores; G. roseum, T. pseudokoningii and T. harzianum had no effect on germination; and P. farinosus and SDM-54 increased the percentage of spore germination of G. mosseae after 4 d. Wardomyces inflatus significantly decreased hyphal length of spores which germinated, but no other saprophytic fungi affected hyphal growth. Trichoderma pseudokoningii, T. harzianum, P. farinosus and SDM-54 increased the number of auxiliary cells formed by G. mosseae. The effect of saprophytic fungi on arbuscular mycorrhizal colonization of soybean was studied in a greenhouse trial. The percentage of soybean root length colonized was decreased by W. inflatus, unaffected by SDM-54 and T. harzianum, and increased by P. farinosus. Gliocladium roseum decreased root length colonized when plants were 12 wk old, and T. pseudokoningii increased colonization of roots when plants were 4 wk old. Antagonistic, synergistic and neutral actions of G. mosseae upon the saprophytic fungi were observed. The population of T. harzianum decreased and the populations of T. pseudokoningii and SDM-54 increased in the presence of G. mosseae. Our results indicate a complex interaction between G. mosseae and associated saprophytic fungi.     

Interaction between Glomus mosseae and soil yeasts on growth and nutrition of cowpea

A glass house experiment was conducted to study the interaction between the mycorrhizal fungus, Glomus mosseae and six soil yeasts (Rhodotorula mucilaginosa, Metschnikowia pulcherrima, Trichosporon cutaneum var. cutaneum, Saccharomyces cerevisiae, Cryptococcus laurentii, Debaryomyces occidentalis var. occidentalis), and their effect on growth and nutrition of cowpea. All the yeasts had a synergistic interaction with the mycorrhizal fungus and dual inoculation improved plant growth compared to single inoculation with G. mosseae alone. Nitrogen and phosphorus uptake of plants was also enhanced significantly in G. mosseae and soil yeasts combinations. Growth, N, P, chlorophyll and phenol content and yield of cowpea were highest in plants treated with G. mosseae+R. mucilaginosa. Mycorrhizal root colonization, spore numbers and population of yeasts in the root zone soil were also highest in the treatment G. mosseae+R. mucilaginosa and least in the uninoculated plants.     

培养容器容积对AM真菌生长发育的影响

研究宿主植物栽培容器对丛枝菌根(Arbuscularmycorrhizae,AM)真菌Glomusmosseae生长发育的影响。结果表明:小容积容器的根系密度相对较大,在菌根共生体建立初期,菌根真菌繁殖体与根接触的机会增大,对于菌根真菌的迅速侵染及共生体的迅速建立非常有利,同时还增大了根外菌丝二次侵染的机会,从而使菌根真菌生长发育形成了一个良性循环,最终有利于根外孢子的形成。容器对共生体的影响决不是简单的盆的体积问题,而与其面积和体积之比有关,也和种植密度有密切关系。     

Contribution of six arbuscular mycorrhizal fungal isolates to water uptake by Lactuca sativa plants under drought stress

It is currently accepted that, along with nutrients, arbuscular mycorrhizal (AM) fungi also transport water to their host plant. However, the quantity of water supplied and its significance for plant water relations remain controversial. The objective of this work was to evaluate and compare the ability of six AM fungi to alter rates of root water uptake under drought stress conditions. Soil drying rates of uninoculated control plants of comparable size and nutritional status and mycorrhizal plants were recorded daily. Lactuca sativa plants colonized by Glomus coronatum , G. intraradices , G. claroideum and G. mosseae depleted soil water to a higher extent than comparably sized uninoculated control plants or plants colonized by G. constrictum or G. geosporum . The differences ranged from 0.6% volumetric soil moisture for G. mosseae -colonized plants to 0.95% volumetric soil moisture for G. intraradices -colonized plants. These differences in soil moisture were equivalent to 3–4.75 ml plant⁻¹ day⁻¹, respectively, and could not be ascribed to differences in plant size, but to the activity of AM fungi. The AM fungi tested in this study differed in their effectiveness to enhance plant water uptake from soil. This ability seems to be related to the amount of external mycelium produced by each AM fungus and to the frequency of root colonization in terms of live and active fungal structures.     

Mycorrhizas improve nitrogen nutrition of Trifolium repens after 8 yr of selection under elevated atmospheric CO2 partial pressure

Altered environmental conditions may change populations of arbuscular mycorrhizal fungi and thereby affect mycorrhizal functioning. We investigated whether 8 yr of free-air CO2 enrichment has selected fungi that differently influence the nutrition and growth of host plants. In a controlled pot experiment, two sets of seven randomly picked single spore isolates, originating from field plots of elevated (60 Pa) or ambient CO2 partial pressure (pCO2), were inoculated on nodulated Trifolium repens (white clover) plants. Fungal isolates belonged to the Glomus claroideum or Glomus intraradices species complex, and host plants were clonal micropropagates derived from nine genets. Total nitrogen (N) concentration was increased in leaves of plants inoculated with fungal isolates from elevated-pCO2 plots. These isolates took up nearly twice as much N from the soil as isolates from ambient-pCO2 plots and showed much greater stimulation of biological N2 fixation. The morpho-species identity of isolates had a more pronounced effect on N2 fixation and on root length colonized than isolate identity. We conclude that rising atmospheric pCO2 may select for fungal strains that will help their host plants to meet increased N demands.     

Relating foliar dehydration tolerance of mycorrhizal Phaseolus vulgaris to soil and root colonization by hyphae

Mycorrhizal symbiosis can modify plant response to drying soil, but little is known about the relative contribution of soil vs. root hyphal colonization to drought resistance of mycorrhizal plants. Foliar dehydration tolerance, characterized as leaf and soil water potential at the end of a lethal drying episode, was measured in bean plants (Phaseolus vulgaris) colonized by Glomus intraradices or by a mix of arbuscular mycorrhizal fungi collected from a semi-arid grassland. Path analysis modeling was used to evaluate how colonization rates and other variables affected these lethal values. Of several plant and soil characteristics tested, variation in dehydration tolerance was best explained by soil hyphal density. Soil hyphal colonization had larger direct and total effects on both lethal leaf water potential and soil water potential than did root hyphal colonization, root density, soil aggregation, soil glomalin concentration, leaf phosphorus concentration or leaf osmotic potential. Plants colonized by the semi-arid mix of mycorrhizal fungi had lower lethal leaf water potential and soil water potential than plants colonized by G. intraradices. Our findings support the assertion that external, soil hyphae may play an important role in mycorrhizal influence on the water relations of host plants.     

Superoxide dismutase and total peroxidase activities in relation to drought recovery performance of mycorrhizal shrub seedlings grown in an amended semiarid soil

We studied the effect of inoculation with a mixture of three arbuscular mycorrhizal (AM) fungi (Glomus intraradices Schenck & Smith, Glomus deserticola (Trappe, Bloss. & Menge) and Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe) and addition of a composted organic residue on plant growth, nutrient uptake, mycorrhizal colonisation and superoxide dismutase (SOD, EC 1.15.1.1) and total peroxidase (POX, EC 1.11.1.7) activities in shoots of Juniperus oxycedrus seedlings after well-watered, drought and recovery periods. The mycorrhizal inoculation and composted residue addition significantly increased the growth, foliar nutrients (N, P, K) and shoot water content of the plants, independent of the water regime. POX activity in control plants increased during drought (about 250% higher than under well-watered conditions) and returned to initial levels after re-watering. The seedlings inoculated with AM fungi showed the highest values of POX activity, followed by the plants grown in the amended soil, which varied little during the drought and recovery periods. Drought decreased the SOD activity in shoots of both J. oxycedrus seedlings inoculated with AM fungi and those grown with composted residue, but did not affect that of control plants. After re-watering, the SOD activity in mycorrhizal or residue-amended plants increased, showing values similar to control plants.