The propagules of vesicular-arbuscular mycorrhizal (VAM) fungi capable of initiating VAM infection after topsoil disturbance

Summary The removal and storage of topsoil decreases the infectivity of vesicular-arbuscular mycorrhizal (VAM) fungi. The propagules of VAM fungi include spores, root fragments containing hyphae and vesicles, and soil hyphae. The viability of each type of propagule after disturbance will determine the initiation of VAM associations with plants recolonizing the disturbed site. This study aimed to examine which of the propagules of VAM fungi are capable of initiating VAM infection after soil disturbance. Soil from an open woodland site of low soil fertility, in southeastern Australia was wetsieved through a tier of three sieves (1 mm, 250 m and 106 m), and the following fractions were extracted: (i) root fragments, (ii) fungal hyphae, and (iii) VAM spores. Each fraction was tested to determine its potential to initiate VAM. Hyphae of VAM fungi grew from root fragments within 14 days. The VAM spore fraction initiated VAM infection after 28 days. VAM hyphal fragments did not produce any VAM infection even after 42 days.     

Arbuscular mycorrhizal fungal propagules in a salt marsh

The tolerance of indigenous arbuscular mycorrhizal fungi (AMF) to stressful soil conditions and the relative contribution of spores of these fungi to plant colonization were examined in a Portuguese salt marsh. Glomus geosporum is dominant in this salt marsh. Using tetrazolium as a vital stain, a high proportion of field-collected spores were found to be metabolically active at all sampling dates. Spore germination tests showed that salt marsh spores were not affected by increasing levels of salinity, in contrast to two non-marsh spore isolates, and had a significantly higher ability to germinate under increased levels of salinity (20) than in the absence of or at low salinity (10). Germination of salt marsh spores was not affected by soil water levels above field capacity, in contrast to one of the two non-marsh spore isolates. For the evaluation of infectivity, a bioassay was established with undisturbed soil cores (containing all types of AM fungal propagules) and soil cores containing only spores as AM fungal propagules. Different types of propagules were able to initiate and to expand the root colonization of a native plant species, but spores were slower than mycelium and/or root fragments in colonizing host roots. The AM fungal adaptation shown by this study may explain the maintenance of AMF in salt marshes.     

The impact of wildfire on vesicular-arbuscular mycorrhizal fungi and their potential to influence the re-establishment of post-fire plant communities

Wildfires are a typical event in many Australian plant communities. Vesicular-arbuscular mycorrhizal (VAM) fungi are important for plant growth in many communities, especially on infertile soils, yet few studies have examined the impact of wildfire on the infectivity of VAM fungi. This study took the opportunity offered by a wildfire to compare the infectivity and abundance of spores of VAM fungi from: (i) pre-fire and post-fire sites, and (ii) post-fire burned and unburned sites. Pre-fire samples had been taken in May 1990 and mid-December 1990 as part of another study. A wildfire of moderate intensity burned the site in late December 1990. Post-fire samples were taken from burned and unburned areas immediately after the fire and 6 months after the fire. A bioassay was used to examine the infectivity of VAM fungi. The post-fire soil produced significantly less VAM infection than the pre-fire soil. However, no difference was observed between colonization of plant roots by VAM fungi in soil taken from post-fire burned and adjacent unburned plots. Soil samples taken 6 months after the fire produced significantly more VAM than corresponding soil samples taken one year earlier. Spore numbers were quantified be wet-sieving and decanting of 100-g, air-dried soil subsamples and microscopic examination. For the most abundant spore type, spore numbers were significantly lower immediately post-fire. However, no significant difference in spore numbers was observed between post-fire burned and unburned plots. Six months after the fire, spore numbers were the same as the corresponding samples taken 1 year earlier. All plants appearing in the burned site resprouted from underground organs. All post-fire plant species recorded to have mycorrhizal associations before the fire had the same associations after the fire, except for species of Conospermum (Proteaceae), which lacked internal vesicles in cortical cells in the post-fire samples.     

Quantification and correlation of vesicular-arbuscular mycorrhizal propagules with soil properties of some mollisols of northern India

The populations of vesicular-arbuscular mycorrhizae (VAM) propagules by the most probable number method in some mollisols and their correlations with some important soil properties were determined. On average, the six soils, Phoolbagh clay loam, Beni silty clay loam, Haldi loam, Nagla loam, Khamia sandy loam and Patherchatta sandy loam contained 4.9, 4.0, 7.9, 7.9, 3.3 and 13.0 propagules/g soil, respectively, i.e. none of the soils was found to be high in VAM. The size of the VAM population was compared to soil properties such as pH, organic carbon, sand content, available phosphorus and available potassium, cation-exchange capacity, silt and clay contents. A significant positive correlation (r=0.586) was only found with available soil phosphorus (P<0.05) and a significant negative correlation (r=-0.555) with soil clay content (P<0.05).Directorate research paper series No. 7862     

Seasonal variation of vesicular-arbuscular mycorrhizae in eroded soils from southern Spain

A survey was carried out of the seasonal variations in the number of spores in the soil and of the percentage of root infection. The stage of development of the host plants, environmental variations and physicochemical characteristics of the soil were taken into account. Fifteen plants valid as forage and adaptable to semi-arid conditions and poor soils were selected. In general, the maximum spore density was reached in the fruit-bearing period of the plants. It remained high during autumn, fell to a minimum in winter and tended to increase in spring. Root infection was at a maximum when the plants flowered, after which it decreased to a minimum in summer.     

The influence of host plant,nitrogen fertilization and fungicide application on the abundance and seasonal dynamics of vesicular-arbuscular mycorrhizal fungi in arable soils of northern Germany

The present investigation examines whether the crop plant, increased nitrogen (N) fertilization and fungicide application influence the pattern of vesicular-arbuscular mycorrhizal fungi (VAM) populations. For this purpose, two arable field locations in Lower Saxony (Hotteln and Langreder) were chosen and the formation of mycorrhiza, spore density, number of infectious propagules (MPN) and frequency of spore types within VAM populations were investigated. The influence of crop plants was examined over two cultivation periods (1986/1987 and 1987/1988) in Hotteln, comparing winter wheat, winter barley and sugar beet. The effects of increased N fertilization and fungicide application were investigated on winter wheat in Langreder in the cultivation period 1988 only. Both the frequency of mycorrhizal infection and the spore dynamics in soil differed with the crops grown. Spore density and MPN increased until harvesting when host plants (winter wheat, winter barley) were cultivated, whereas both diminished with a non-host plant (sugar beet). Different spore types increased or decreased, according to the plant species grown, but the predominating types of the location remained constant. Increased N fertilization caused marginal inhibition of mycorrhizal infection and sporulation on winter wheat, whereas both leaf and base application of fungicides resulted in minor increases in mycorrhizal colonization of roots and sporulation in soil. Both increased N fertilization and fungicide application distinctly decreased the sporulation of one type in May, but the characteristic compositions of the VAM populations remained unchanged.     

Soil disturbance and infection of Trifolium repens roots by vesicular-arbuscular mycorrhizal fungi

Removal and storage of the surface layers of soil is known to decrease the infectivity of vesicular-arbuscular mycorrhizal (VAM) fungi. Previous studies have mostly examined the effects of profound soil disturbance on the infectivity of VAM fungi. This study examined the effects of increasing degrees of topsoil disturbance on the infectivity of VAM fungi in two sites on sandstone soils in southeastern Australia. Intact soil blocks (20×20×15 cm) were taken from each of the two sites. Increasing degrees of topsoil disturbance were achieved by cutting the blocks longitudinally into four (dist. 1), nine (dist. 2), and 25 (dist. 3) equal portions. Seeds of Trifolium repens L. were sown into the blocks and harvested 14, 21, 28, 35 and 42 days after sowing. At each sampling date, total root length, root length colonised by VAM fungi and shoot dry mass were measured. VAM colonisation had commenced by 14 days in the roots of seedlings grown in intact, dist. 1, and dist. 2 soil blocks. The initiation of VAM colonisation was delayed by up to 6 weeks for seedlings grown in the dist. 3 soil blocks. The low (i.e. dist. 1) and intermediate (i.e. dist. 2) degrees of soil disturbance did not cause a delay in the initiation of VAM, bud did significantly reduce the proportion of root length colonised by VAM fungi after 21 days. After 21 days, shoot dry mass was significantly less in the seedlings grown in the dist. 3 soil blocks though not in the low and intermediate disturbance treatments. It is concluded that the most severe experimental disturbance probably disturbed the external hyphal network and root fragments (containing hyphae and vesicles), which in turn temporarily reduced the infective potential of the fungus to zero. The observed delay in the initiation of VAM in the most disturbed blocks can, therefore, be explained by the time required for hyphae to grow from other propagules in the soil which survived the disturbance event.     

Effect of vesicular-arbuscular mycorrhizal fungi on verticillium wilt of cotton

The development of vesicular-arbuscular mycorrhizal fungi (VAMF): Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe, Glomus versiforme (Karsten) Berch, Sclerocystis sinuosa Gerdemann and Bakhi and Verticillium dahliae and the effects of the VAMF on the verticillium wilt of cotton (Gossypium hirsutum L. and Gossypium barbadense L.) were studied with paper pots, black plastic tubes and clay pots under natural growth conditions. All of the tested VAMF were able to infect all the cotton varieties used in the present experiment and typical vesicles and arbuscules were formed in the cortical cells of the cotton roots after inoculation. The cap cells, meristem, differentiating and elongating zones of the root tip were found to be colonized by the VAMF. In the case of most V. dahliae infection, the colonization occurred mostly from the root tip up to 2 cm. VAMF and V. dahliae mutually reduced their percentage of infection when inoculated simultaneously. VAMF inoculation reduced the numbers of germinable microsclerotia in the soil of the mycorrhizosphere, while the quantity of VAM fungal spores in the soil was not influenced by infection of with V. dahliae. The % of arbuscule colonization in roots was negatively correlated with the disease grades, while the numbers of vesicles in roots were not. These results suggest that certain vital competition and antagonistic reactions exist between VAMF and V. dahliae. VAMF reduced the incidence and disease indices of verticillium wilt of cotton during the whole growth phase. It is evident that cotton seedling growth was promoted, flowering was advanced, the numbers of flowers and bolls were increased, and this resulted in an increase in the yield of seed cotton. Among the VAMF species, Glomus versiforme was the most effective, and Sclerocystis sinuosa was inferior. So far as the author is aware, such an effect of VAMF on the increase of cotton wilt tolerance/resistance is reported here far the first time.     

Modifications to clearing methods used in combination with vital staining of roots colonized with vesicular-arbuscular mycorrhizal fungi

Leek, maize, and pigmented soybean roots colonized by vesicular-arbuscular mycorrhizal (VAM) fungi were assessed for succinate dehydrogenase (SDH) activity using the nitro blue tetrazolium chloride (NBT)-succinate method. NBT-succinate-reacted roots, cleared in a 55° C drying oven in 5% (w/v) KOH for 24 h or longer and observed as whole mounts, revealed signs of intraradical VAM fungus colonization more clearly than roots cleared by the standard 20% (w/v) boiling chloral hydrate method. Combined clearing of NBT-succinate-reacted roots using boiling chloral hydrate followed by clearing in 5% KOH at 55° C for prolonged periods also improved the visualization of intraradical fungal structures. Bleaching of NBT-succinate-reacted roots using the standard NH₃-H₂O₂ method removed pigmentation from roots and did not alter the viability indicator, formazan. Pigmented, field-collected soybean roots were successfully cleared and bleached to reveal signs of viable and nonviable intraradical fungal structures. Counterstaining of NBT-succinate-reacted roots with acid fuchsin clearly revealed both viable and nonviable intraradical fungal structures. The NBT-succinate solution infiltrated all intraradical fungal structures after 24 h; formazan products were observed at similar concentrations in viable structures after 24, 36, and 48 h.     

Multiple mycorrhizal associations of individual calcicole host plants in the alpine grass-heath zone

Summary Mycorrhizal colonization of the fibrous roots of alpine grasses and perennial herbs in microhabitats on rendzina soil were examined. Various host plants were associated simultaneously with more than one species of vesicular-arbuscular mycorrhizal fungi. All dominant graminoids had a high degree of endomycorrhizal infection. Septate endophytes (Phialophora sp., Rhizoctonia sp.) often occurred together with Acaulospora sp., Glomus tenue, G. tortuosum and Scutelispora calospora on individual hosts.     

The management of populations of vesicular-arbuscular mycorrhizal fungi in acid-infertile soils of a savanna ecosystem

Topsoil stockpiled for 4 years resulted in an accumulation of NH₄-N at depths of 1m or more in mound, as measured by an ammonia gas-sensing electrode. When leached with water these soils were also found to contain high concentrations of dissolved organic C below 1m. Both NH₄-N and DOC were products of microbial mineralisation of soil organic matter that accumulated under anaerobic conditions. When these soils were restored a flush of decomposition took place, fuelled by labile organic matter and soluble nitrogen.Stockpiled soil which underwent an ammonium-rich perfusion regime in the laboratory indicated that in-mound soils rapidly attained greater nitrification potential than surface mound soils and also had greater potential for further mineralisation of organic matter to NH₄-N. This further production was seen as a contribution from the bacterial flush, stimulated by the large labile-C pool already present.As the bulk of stored soil was anaerobic, restored soils were seen as potentially wasteful of their N-reserves; the fate of nitrogen and soluble carbon compounds in restored soils is discussed.     

Effect of manganese on vesicular-arbuscular mycorrhizal development in red clover plants and on soil Mn-oxidizing bacteria

Summary The effect of manganese on the development of vesicular-arbuscular (VA) mycorrhizae and on the population of Mn-oxidizing soil bacteria was studied using red clover as host plant and Glomus mosseae or G. aggregatum as VA-colonizing fungi. The addition of Mn to the substrate in which plants grew had a detrimental effect on root colonization, G. aggregatum being more susceptible than G. mosseae. Mn uptake was lower in mycorrhizal than in non-mycorrhizal plants, especially when colonized by G. mosseae. The development of mycorrhizae seems to favour the Mn-oxidizing bacteria population.     

Dual axenic culture of sheared-root inocula of vesicular-arbuscular mycorrhizal fungi associated with tomato roots

Surface-sterilized sheared-root inocula of two vesicular-arbuscular mycorrhizal (VAM) fungi (Glomus intraradices and G. versiforme) from pot cultures associated with excised tomato roots showed significant sporulation and the production of an extensive hyphal biomass. As many as 10²–10³ axenic mature spores were recovered in Petri dishes during 3 months incubation in the dark. Propagules of both species were able to complete their vegetative life cycle in vitro and efficiently colonize Acacia albida roots after 1 month under greenhouse conditions. The effectiveness of 0.5 cm pieces of VAM roots as starter inocula indicates the high inoculum potential of intravesicle propagules.     

Assessing diversity of arbuscular mycorrhizal fungi in a local community: role of sampling effort and spatial heterogeneity

Diversity of arbuscular mycorrhizal fungi (AMF) was assessed in two 9.2 × 9.2-m plots planted with landscape trees and shrubs at an experimental site in Phoenix, AZ, USA. Twenty-five soil samples were collected in a regular grid pattern from each plot, and AMF species were identified using trap cultures. A total of 12 species were detected, with 7 species detected in one plot and 11 in the other. We found that sampling effort had a major impact on assessing species richness and composition in this local community. Fifteen samples would be necessary to detect 70–80% of species present in each plot. A limited number of additional undetected species are likely to be present in both plots, based on the sampling effort curves and jackknife estimates. Only two species, Glomus eburneum and Glomus microaggregatum, were detected in over 50% of the samples from both plots, and rank–frequency plots revealed a lognormal species distribution. Despite the patchiness of plants in the plots, the number of species detected per point exhibited spatial structuring only at the smallest sampling scale in a single plot, and only a single species in each plot was not randomly distributed. These results indicate that sampling effort and strategy can affect perceptions of AMF community structure.     

Molecular biodiversity of arbuscular mycorrhizal fungi in trace metal-polluted soils

We assessed the indigenous arbuscular mycorrhizal fungi (AMF) community structure from the roots and associated soil of Plantago major (plantain) plants growing on sites polluted with trace metals (TM) and on unpolluted sites. Uncontaminated and TM-contaminated sites containing As, Cd, Cu, Pb, Sn and Zn were selected based on a survey of the TM concentration in soils of community gardens in the City of Montréal. Total genomic DNA was extracted directly from these samples. PCR followed by denaturing gradient gel electrophoresis (PCR-DGGE), augmented by cloning and sequencing, as well as direct sequencing techniques, was all used to investigate AMF community structure. We found a decreased diversity of native AMF (assessed by the number of AMF ribotypes) in soils and plant roots harvested from TM-polluted soils compared with unpolluted soils. We also found that community structure was modified by TM contamination. Various species of Glomus, Scutellospora aurigloba and S. calospora were the most abundant ribotypes detected in unpolluted soil; ribotypes of G. etunicatum, G. irregulare/G. intraradices and G. viscosum were found in both polluted and unpolluted soils, while ribotypes of G. mosseae and Glomus spp. (B9 and B13) were dominant in TM-polluted soils. The predominance of G. mosseae in metal-polluted sites suggests the tolerance of this species to TM stress, as well as its potential use for phytoremediation. These data are relevant for our understanding of how AMF microbial communities respond to natural environments that contain a broad variety of toxic inorganic compounds and will substantially expand our knowledge of AMF ecology and biodiversity.     

The influence of aqueous extracts of burnt or heated soil on the activity of vesicular-arbuscular mycorrhizal fungi propagules

Summary Aqueous extracts of burnt soil, unburnt soil and oven-heated unburnt soil were tested as to their effects on vesicular-arbuscular mycorrhizal (VAM) fungi (spore germination, mycelial propagule activity and root colonization). The extracts of burnt or heated soil inhibited VAM spore germination and extrarrhizal mycelium activity.