J. A. Fortin

Growth responses of several plant species to mycorrhizae in a soil of moderate P-fertility

SummaryThe growth of twenty plant species was compared under field conditions in a methyl bromide fumigated and non-fumigated soil. The non-fumigated soil had a wild endomycorrhizal flora and contained 100 μg/g of available phosphorus. No phosphorus was added to the soil but both fumigated and non-fumigated plots received a basal fertilization of 100 kg/ha N−NH4NO3 and 100 kg/ha K−KCl. Based on plant growth responses, three groups of plants were distinguishable. Plants from group I were mycorrhizal and had better growth in non-fumigated than in the fumigated soil. This group was the most important, including sixteen plant species. Stunting of plants from group I following soil fumigation was mainly attribuable to the destruction of mycorrhizae. Plants from group II (oat and wheat) grew equally well in non-fumigated and fumigated soils. For these plants which were mycorrhizal in the non-fumigated plots, the P-content of the soil was sufficient for growth and therefore no stunting was observed in the absence of mycorrhizae. Plants from group III (cabbage and garden beet) grew better in fumigated than in non-fumigated soil. Their better growth in fumigated soil was tentatively attributed to the destruction of soil-borne pathogens. They did not form mycorrhizae in non-fumigated soil.A new method of calculating mycorrhizal dependency is proposed, and the value calculated was named relative field mycorrhizal dependency (RFMD) index. It is also proposed that the acronym RFMD receive a superscript representing in μg/g the quantity of available P in the soil. Carrot with its characteristic root systems had the highest RFMD100 index (99.2%), but other plants with high phosphorus requirements for normal growth had a wide range of RFMD100 index values.

Enhanced hyphal growth and spore production of the arbuscular mycorrhizal fungus Glomus intraradices in an in vitro system in the absence of host roots

Arbuscular mycorrhizal (AM) fungi are ecologically important for most vascular plants because they benefit plant growth and survival. The obligate biotrophic nature of AM fungi imposes limitations in inoculum production which could be used in the management of the symbiosis in field crops. Glomus intraradices was grown on genetically transformed Daucus carota roots in a two-compartment in vitro system. The growth of mycorrhizal roots was restricted to one compartment (proximal) containing a complete growth medium. Only the endosymbiont was permitted to grow on to the second compartment (distal) containing the same medium lacking sugar. Colonization of the distal compartment by the mycelium took place between six and eight weeks after subculturing the mycorrhizal roots in the proximal compartment. Hyphal- and spore-densities were significantly higher in the distal compartment. Up to 34000 spores with a mean of 15 000 mostly viable spores per plate were counted in the distal compartment. This opens the possibility of producing aseptic spores, not only for research purposes but also for large-scale inoculum production. The possible factors involved in the enhancement of hyphal- and spore-densities and their ecological role are discussed.

Managing arbuscular mycorrhizal fungi in cropping systems

Market globalization, demographic pressure, and environmental degradation have led us to reconsider many of our current agricultural systems. The heavy use of chemical inputs, including fertilizers and pesticides, has resulted in pollution, decreased biodiversity in intensively-farmed regions, degradation of fragile agro-ecosystems, and prohibitive costs for many farmers. Low input sustainable cropping systems should replace conventional agriculture, but this requires a more comprehensive understanding of the biological interactions within agro-ecosystems. Mycorrhizal fungi appear to be the most important telluric organisms to consider. Mycorrhizae, which result from a symbiosis between these fungi and plant roots, are directly involved in plant mineral nutrition, the control of plant pathogens, and drought tolerance. Most horticultural and crop plants are symbiotic with arbuscular mycorrhizal fungi. Mycorrhizal literature is abundant, showing that stimulation of plant growth can be mainly attributed to i...

Altered growth of Fusarium oxysporum f. sp. chrysanthemi in an in vitro dual culture system with the vesicular arbuscular mycorrhizal fungus Glomus intraradices growing on Daucus carota transformed roots

Vesicular arbuscular mycorrhizal fungi can reduce plant disease symptoms and populations of pathogens through mechanisms that are not well understood. Glomus intraradices was grown on Daucus carota transformed roots in a two-compartment in vitro system. One compartment contained mycorrhizal roots on a complete growth medium, while the other contained a medium lacking sugar on which only mycelial growth was allowed. The direct interaction between G. intraradices and Fusarium oxysporum f. sp. chrysanthemi was studied in the compartment lacking sugar during a 5-day period. G. intraradices hyphal density and spore number were estimated along with F. o. chrysanthemi conidial germination, mycelial growth and sporulation. Five hours after inoculation, germination of F. o. chrysanthemi conidia doubled in the presence of G. intraradices. Radial growth of F. o. chrysanthemi colonies was always slightly but significantly enhanced in the presence of G. intraradices. No correlation was obtained between G. intraradices hyphae or spore densities and F. o. chrysanthemi hyphal growth. Overall sporulation of the 5-day-old F. o. chrysanthemi colonies was not influenced by the presence of G. intraradices. However, significant negative correlations were found between F. o. chrysanthemi conidia production and G. intraradices hyphae or spore concentrations. G. intraradices increased F. o. chrysanthemi conidial germination and slightly stimulated its hyphal growth in dual culture without any root influences. No antibiosis was observed between the fungi. The significance of the results and their potential implication for rhizosphere biology are discussed.

Growth responses of several plant species to mycorrhizae in a soil of moderate P-fertility: II. Soil fumigation induced stunting of plants corrected by reintroduction of the wild endomycorrhizal flora

SummaryA greenhouse experiment was carried out comparing the growth of various plant species in non-fumigated, fumigated, and fumigated-inoculated soils. The soil used contained numerous pieces of root of Broom-Corn Millet (Panicum miliaceum L.) that were found intensely colonized by indigenous endomycorrhizal fungi. The soil was fumigated with methyl bromide and the inoculum used was a mixture of VA mycorrhizal root fragment from plants grown in the field from which the soil was collected. Plants used were cabbage (Brassica oleracea L. var Copenhagen Market), carrot (Daucus carota L. var. Nantaise), leek (Allium porrum L. var. American Flag), marigold (Tagetes patulus L. var. Golden Boy), tomato (Lycopersicum esculentum Mill. var. Michigan Ohio), sweet corn (Zea mays L. var. Span Cross) and wheat (Triticum aestivum L. var. Glenlea). No phosphorus was added to the soil which contained 93 μg/g of available P (bray II). All plants tested formed mycorrhizae except cabbage. Generally, values of the root endomycorrhizal colonization (REC) index were higher in fumigated-inoculated soil than in non-fumigated soil. Cabbage grew equally well in fumigated and fumigated-inoculated soil, but better than in non-fumigated soil. Cabbage did not form VA mycorrhizae and its better growth in fumigated soil was tentatively attributed to the destruction of soil-borne pathogens and the absence of competition. Wheat grew equally well in the three treatments, because 93 μg/g of available P is sufficient for wheat growth and thus the mycorrhizae were not efficient. The five other plant species used were severely stunted in fumigated soil and the inoculation permitted the reestablishment of normal growth as in non-fumigated soil. Growth stimulation is attributed to the efficiency of VA mycorrhizae since these plants were mycorrhizal in non-fumigated soil and in fumigated-inoculated soil. Stunting of these plants in fumigated soil was due to the destruction of VA mycorrhizae since results show that this stunting cannot be attributed to methylbromide residues in the soil. Moreover soil pH and nutrient content were not markedly changed after fumigation.

Enhanced removal of trapped non-aqueous phase liquids from saturated soil using surfactant solutions

Abstract Column experiments were performed under water-saturated conditions to evaluate the effectiveness of low-concentration surfactant solution at removing a trapped light non-aqueous phase liquid (LNAPL) o -xylene and a dense non-aqueous phase liquid (DNAPL) o -dichlorobenzene from soil. Sorption of the ethoxylated alcohol (Witconol SN90) surfactant on the porous material was evaluated using both batch and column flow experiments. The batch sorption results showed that the sorption of the surfactant on sand reached a minimum value near the surfactant concentration selected for the experiments. Negligible sorption of the surfactant was observed in the column sorption experiment at the 90 cm day −1 flow rate used in our study, implying that the residence time was too short for equilibrium sorption to be reached. The NAPLs were removed from the columns by solubilization, emulsification and mobilization. Macroemulsions were formed in both batch and column experiments, suggesting that the NAPL was mostly removed by immiscible displacement. The direction of flow was an important parameter in LNAPL removal, influencing both the location of the chemical in the column and the removal efficiency. The percentage of DNAPL removed from the column was successfully described using a dissolving sphere model with increased apparent solubility. However, the model failed to describe the NAPL distribution in the column adequately, suggesting that a more complex process than the solubility enhancement accounted for by the model is occurring.

Influence of substrate on the interaction ofGlomus intraradices andFusarium oxysporum f.sp.radicis-lycopersici on tomatoes

SummaryThe influence of five substrates on the interaction betweenGlomus intraradices andFusarium oxysporum f.sp.radicis-lycopersici and its effect on tomato plants development was investigated. The presence ofG. intraradices decreased root necrosis in all substrates and affected the Fusarium population with different intensity depending on the substrate used. Substrates were found to influence disease development, Fusarium population in the substrate, root colonization by the endomycorrhizal fungus and growth of the host plant. In addition to providing good experimental conditions, the use of calcined montmorillonite clay also facilitated washing, recuperation, necrosis evaluation and staining of roots. Its use is proposed as a standard medium for experimental work on the interactions between endomycorrhizal fungi, root pathogens and host plants.

GERMINATION AND COMPARATIVE MORPHOLOGY OF BASIDIOSPORES OF PISOLITHUS ARHIZUS

Eleven specimens of basidiospores of Pisolithus arhizus of various provenances were assayed for their ability to germinate. Three incubation amendments were tested for their basidiospore germination induction value. Two amendments were made in combination, using different seedlings on differently amended media. An additional assay was made using two germination activator compounds. Basid? iospores of two specimens, one from South Africa and one from Australia germinated on some ofthe combination plates of seedlings and amended media. No germination of basidiospores occurred with the activator compounds (1-nonanol and abietic acid). Intrastrain single-spore colonies were mated to form heterokaryotic colonies and a heterothallic, tetrapolar mating system was determined. Homokaryotic colonies of the four mating types of the two isolates were paired and were found not to be compatible. Basidiospores of the eleven specimens were grouped according to basidiospore spine morphology into three distinct groups. From these results the existence of biological species in P. arhizus is suggested.

Taxonomy and population structure of E-strain mycorrhizal fungi inferred from ribosomal and mitochondrial DNA polymorphisms

E-strain fungi form ecto- and ectendomycorrhizal associations with many trees, and are important mycorrhizal symbionts in disturbed forest habitats and conifer nurseries. Unlike the majority of mycorrhizal fungi, which are basidiomycetes, E-strain fungi are ascomycetes belonging to the genus Wilcoxina (Pezizales). We analysed variation in the nuclear and mitochondrial ribosomal RNA genes to elucidate species concepts among E-strain fungi and to examine their population structure. We found that most E-strain isolates can be assigned to two taxa, W. mikolae and W. rehmii, that have different habitat preferences. Wilcoxina mikolae is the predominant taxon in disturbed forest habitats on soils that are often low in organic matter, such as burned sites, while W. rehmii prefers peaty soils. Analysis of mitochondrial DNA also revealed that within each species, isolates could be differentiated based upon host preference. This is the first report of population subdivision based upon host in E-strain fungi.

Production of glomus intraradices propagules, an arbuscular mycorrhizal fungus, in an airlift bioreactor

This work addresses the symbiotic culture of the arbuscular mycorrhizal (AM) fungus Glomus intraradices with Daucus carota hairy roots transformed by Agrobacterium rhizogenes, in two submerged culture systems: Petri dish and airlift bioreactor. AM fungi play an active role in plant nutrition and protection against plant pathogens. These fungi are obligate biotrophs as they depend on a host plant for their needs in carbohydrates. The effect of the mycorrhizal roots inoculum-to-medium volume ratio on the growth of both symbionts was studied. A critical inoculating condition was observed at approximately 0.6 g dry biomass (DW). L-1 medium, above which root growth was significantly reduced when using a low-salt minimal (M) liquid medium previously developed for hairy root-AM fungi co-culture. Below critical inoculum conditions the maximum specific root growth and specific G. intraradices spore production rates of 0.021 and 0.035 d-1, respectively, were observed for Petri dish cultures. Maximum spore production in the airlift bioreactor was ten times lower than that of Petri dish cultures and obtained with the lowest inoculum assessed (0.13 g DW. L-1 medium) with 1.82 x 10(5) +/- 4.05 x 10(4) (SEM) spores (g DW inoculum)-1 (L medium)-1 in 107 d. This work proposes a second-generation bioprocess for AM fungi propagule production in bioreactors. Copyright 1999 John Wiley & Sons, Inc.