Glomus Macrocarpum Tul Research Articles

Attenuation of cadmium toxicity in mycorrhizal celery (Apium graveolens L.)

A pot-culture experiment was carried out to investigate the effect of arbuscular mycorrhizal (AM) fungus (Glomus macrocarpum Tul. and Tul.) on plant growth and Cd2+uptake by Apium graveolens L. in soil with different levels of Cd2+. Mycorrhizal (M) and non-mycorrhizal (NM) plants were grown in soil with 0, 5, 10, 40 and 80 Cd2+ mg kg−1soil. The infectivity of the fungus was not affected by the presence of Cd2+ in the soil. M plants showed better growth and less Cd2+ toxicity symptoms. Cd2+ root : shoot ratio was higher in M plants than in NM plants. These differences were more evident at highest Cd2+ level (80 mg kg−1 soil). Chlorophyll a and chlorophyll b concentrations were significantly higher in AM-inoculated celery leaves. The dilution effect due to increased biomass, immobilization of Cd2+ in root and enhanced P-uptake in M plants may be related to attenuation of Cd2+toxicity in celery.

Suppression of root-knot nematode by mycosymbionts on pyrethrum (<i>Chrysanthemum cinerariefolium</i> Vis.)

Five arbuscular mycorrhiza fungi (AMF) obtained from pyrethrum fields were screened for efficacy in improving pyrethrum ( Chrysanthemum cinerariefolium Vis.) growth and in suppressing a root-knot nematode, Meloidogyne hapla Chitwood in the greenhouse. The fungi screened were Glomus macrocarpum Tul and Tul, G. constrictum Trappe, G. monosporum Gerd. & Trappe, Scutellospora calospora (Nicol. Gerd.) Walker & Sanders and Gigaspora gigantea (Nicol. Gerd.) Gerd. & Trappe. A mixed fungal inoculum was incorporated into sterilized sand-soil mixture before transplanting 6-week-old pyrethrum seedlings. The inoculum consisted of the growth medium, spores, external mycelia and infected root segments. The plants were inoculated with 6000 M. hapla second stage juvenile (J-2) 3 months after fungal inoculation. Dry shoot weights, fresh root weights, percent root colonization by the fungi, nematode gall indices, number of eggs and females in the root system and number of J-2 in the soil were determined at the end of the experiment, 2 months after nematode inoculation. Glomus macrocarpum and S. calospora significantly improved top biomasses of fungus-treated and fungusnematode- treated plants. Glomus macrocarpum was the most effective, causing a 35% top biomass increase. Glomus constrictum, G. monosporum and G. gigantea improved top biomasses of fungusnematode- treated plants. Scutellospora calospora and G. constrictum significantly increased fresh root weights of pyrethrum by 46% and 53%, respectively. Except G. constrictum and G. monosporum , all the other fungi suppressed M. hapla disease severity and egg production. Glomus macrocarpum was the most effective, causing 87% nematode suppression. Scutellospora calospora and Gigaspora gigantea suppressed disease severity and egg production by up to 33% and 37%, respectively. All the fungi significantly reduced the number of females and J-2 in pyrethrum roots. Nematodes did not affect root colonization by the fungi except in G. constrictum and G. monosporum -colonized plants. Key Words: Gigaspora gigantea, Glomus macrocarpum, G. constrictum, G. monosporum, Meloidogyne hapla , pyrethrum, Scutellospora calospora , suppression J. Trop. Microbiol Vol.1 2002: 64-73

Influence of lead additions on arbuscular mycorrhiza and Rhizobium symbioses under soybean plants

The influence of the arbuscular mycorrhizal fungus (AMF) Glomus macrocarpum Tul. and Tul. on Bradyrhizobium-inoculated soybean ( Glycine max L. Merrill var. IAC-14) growth, nutrients and Pb uptake was studied in soils with different levels of Pb. Mycorrhizal and non-mycorrhizal plants were grown in a soil amended with 0, 150, 300 and 600 mg dm −3 of Pb, using Pb(NO 3) 2, and plants were harvested at two different growth stages: flowering and maturity. Soybean shoot dry weights were not affected by increasing doses of lead in the soil, but the number of pods decreased significantly. Nodule dry weights of mycorrhizal roots were reduced by soil Pb additions, although the mycorrhizae stimulated plant nodulation significantly. The inoculation of AMF in soybeans provided higher rates of nutrients uptake, mainly P, inducing greater mycorrhizal-soybean growth. Thus, mycorrhizae improved Pb uptake, produced shoots with Pb concentrations 30% lower than those of non-mycorrhizal plants, at the highest Pb concentration added to the soil. AM fungus was more susceptible to the higher Pb doses added to the soil than the soybean plants, decreasing both root AM colonisation and spore production. This work indicated that a concentration of 600 mg dm −3 of Pb in the soil interfered with the establishment of double symbioses between AMF and Bradyrizhobium, and with the fungus perpetuation in the soil.

Respostas de Acacia mangium Willd e Sclerolobium paniculatum Vogel a fungos micorrízicos arbusculares nativos provenientes de áreas degradadas pela mineração de bauxita na Amazônia

A resposta de Acacia mangium Willd (mangium) e Sclerolobium paniculatum Vogel (tachi) à inoculação de fungos micorrízicos arbusculares (FMA), oriundos de áreas em recuperação após a extração de bauxita, foi avaliada em experimento com delineamento inteiramente casualizado, com 14 tratamentos (duas espécies leguminosas e sete tipos de solo) e três repetições. Avaliou-se o número de esporos no solo, a colonização micorrízica, a matéria seca total, o P acumulado, a dependência micorrízica das mudas, e a abundância e a freqüência de espécies. O número de propágulos infectivos (NPI) foi estudado em delineamento em blocos casualizados, com oito diluições de solo inóculo, cinco repetições e uma planta isca (Brachiaria decumbens Stapf). Utilizou-se substrato da mistura de um Planossolo mais areia lavada e fosfato de rocha araxá (0,60 g/kg). O número de esporos aumentou em função do tempo de cobertura das leguminosas. A colonização micorrízica foi mais intensa no tachi. Os valores de matéria seca dessa espécie foram inferiores aos de mangium, que por sua vez extraiu em torno de seis vezes mais P do substrato. Em geral, mangium, ao contrário do tachi, foi facultativa à presença dos FMA, sugerindo sua utilização na recuperação de áreas degradadas sem inoculação prévia. Dentre as 39 espécies de FMA identificadas, Glomus macrocarpum Tul. & Tul. apresentou maior índice de abundância e freqüência (IAF) e maior NPI, destacando-se entre as espécies pioneiras, ao passo que outras apareceram apenas em estádios sucessionais mais avançados das áreas em recuperação.

Arbuscular mycorrhizal fungi associated with Festuca species in the Canadian High Arctic

Root and soil samples from 13 collecting sites located in the Canadian High Arctic were harvested between July 18 and 29, 1991, and surveyed for root colonization and spore populations of arbuscular mycorrhizal fungi. Grasses of the genus Festuca (Festuca brachyphylla Schult. & Schult., Festuca baffinensis Polunin, and Festuca hyperborea Holmen ex Frederiksen) served as target plants. Of the 197 plant-root systems and soil rhizospheres examined, 28% were associated with arbuscular mycorrhizae. Roots of both F. brachyphylla andF. baffinensis were colonized, while none were detected in F. hyperborea root specimens. Five arbuscular mycorrhizal fungal species were extracted from indigenous soils. The most frequent and abundant species was Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe, which was isolated from 8 of the 13 sites sampled. Other Glomales species in Arctic soils were Glomus macrocarpum Tul & Tul., Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. & Trappe emend. Walker & Koske, and two other unidentified Glomus species. Trap cultures of the indigenous soil with leek (Allium porrum L.) plants confirmed the identity of the species previously identified from original soil and allowed the detection of an additional species, Glomus aggregatum Schenck. & Smith. The novelty of these observations and the relationship between plant mycorrhizal status, fungal species, and soil disturbance are discussed.Key words: mycorrhizae, Arctic, biodiversity, Glomus, Festuca.

Fungos micorrízicos arbusculares (Glomales, Zygomycota) na Ilha dos Eucaliptos, Represa do Guarapiranga, São Paulo, SP

Com o objetivo de se conhecer a micota de fungos micorrízicos arbusculares (FMA, Glomales, Zygomycota), coletou-se amostras de solo rizosférico de plantas instaladas em três áreas (plantio de eucaliptos, mata e transição entre ambas) em três épocas (março/1996, época chuvosa e final de verão; junho/1996, época de inverno; janeiro/1997, início das chuvas e do verão) na Ilha dos Eucaliptos (cerca de 323.500m²), na Represa do Guarapiranga, São Paulo, SP. O solo foi processado para a separação dos esporos de FMA segundo a técnica do peneiramento e decantação em via úmida, seguindo-se centrifugação em solução de sacarose 50% (V:V). Os esporos isolados foram preparados em lâminas permanentes com resina PVL. Foram constatados 21 táxons de FMA: Acaulospora excávala Ingleby, Walker & Mason, Acaulospora foveata Trappe & Janos, Acaulospora longula Spain & Schenck, Acaulospora mellea Spain & Schenck, Acaulospora morrowiae Spain & Schenck, Acaulospora spinosa Walker & Trappe, Acaulospora sp. 1, Acaulospora sp. 2, Acaulospora sp. 3, Acaulospora sp. 4, Entrophospora colombiana Spain & Schenck, Gigaspora gigantea Nicol. & Gerd., Gigaspora ramisporopfwra Spain, Sieverding & Schenck, Glomus aggregatum (Schenck & Smith) Koske, Glomus etunicatum Becker & Gerd., Glomus geosporum (Nicol. & Gerd.) Walker, Glomus invennayum Hall, Glomus macrocarpum Tul. & Tui., Glomus multisubstensum Mukerjii, Bhattacharjee & Tewari, Scutellospora aff. biornata Spain, Sieverding & Toro e Scutellospora erythropa Koske & Walker. A aplicação do índice de similaridade de Sõrensen entre populações de FMA nas diferentes áreas e as épocas de coletas mostrou valores de 64 a 90%, sugerindo não haver seleção dos FMA mediada por esses fatores.

A mycorrhizal pathogen ( Glomus macrocarpum Tul. & Tul.) of tobacco: effects of long- and short-term cropping on the mycorrhizal fungal community and stunt disease

A crop rotation experiment was conducted on two adjacent tracts of land differing in long-term croppin history (30 year in tall fescue pasture or 3 year in sorghum-sudangrass hybrid (SSGH).. Short-term crops were 2 years in tobacco or low-endophyte ( Acremonium coenophialum Morgan-Jones & Gams) tall fescue. Tobacco was grown on all plots in Year 3, and data relating mycorrhizal stunt disease of tobacco and populations of the stunt pathogen, Glomus macrocarpum Tul. & Tul., and of other members of the mycorrhizal fungal community, were taken. Disease incidence was highest with SSGH-tobacco and lowest with fescue-fescue, the other two combinations being intermediate. Mycorrhizal colonization was related to disease occurence. At the beginning of the season, populations of G. macrocarpum were equally high in land with a long-term history of SSGH, regardless of its short-term history; but at the end of the season, populations of G. macrocarpum among the four treatments were proportional to the incidence of disease. Populations of four mycorrhizal fungal species which were high in land with a short-term history of fescue were depressed by production of tobacco. Monocropping of tobacco appeared to narrow the diversity of the mycorrhizal fungal community and increased the proportion which is pathogenic, the overall result being lower productivity of soil for tobacco. Crop rotation of tobacco with fescue decreased the proportion of the mycorrhizal fungal community which is pathogenic and maintained productivity of soil for tobacco.

Mycorrhizal fungi in relation to growth and mineral nutrition of apple seedlings

The effects of two mycorrhizal fungi ( Glomus epigaeum Daniels & Trappe and Glomus macrocarpum Tul. & Tul.) on the growth and mineral composition of shoots and roots of apple seedlings ( Malus micromalus Makino) were studied. Variables were air temperature and sterilization of soil. In sterilized soil, the two species differed in their effects on seedlings grown at different temperatures. G. epigaeum was mutualistic at low and high temperature, but G. macrocarpum was ineffective at low temperature. In unsterilized soil, neither species was mutualistic at low temperature, but both increased plant growth at high temperature. In sterilized soil, mutualism by G. macrocarpum was related to colonization. Enhanced growth of seedlings was accompanied by improved uptake of immobile nutrient elements, mainly Cu, P, Zn. Apple has a high dependency on mycorrhizal fungi, and mycorrhizal isolates selected for effectiveness may be useful for producing seedlings under Chinese production conditions.

Control of Pathogenic Mycorrhizal Fungi in Maintenance of Soil Productivity by Crop Rotation

Pathogenic mycorrhizal fungi may be a major reason crops must be rotated to maintain soil productivity. We studied the role such fungi may have in the maintenance of productivity of soil for tobacco (Nicotiana tabacum L.) by rotation with tall fescue (Festuca arundinaceae Schreb). Tall fescue Kentucky 31' and continuous tobacco plots were established on a central Kentucky field previously shown to be infested with the mycoorhizal fungus Glomus macrocarpum Tul and Tul., the cause of a stunt disease of tobacco. Fumigation with 67% methyl bromide-33% chloropicrin (MBC, trichloronitromethane) covered with plastic was used as a control (...)

Effect of Vesicular-Arbuscular Mycorrhizae on Tissue Culture-derived Plantlets of Strawberry

The responses of four micropropagated strawberry (Fragaria × ananassa Duch.) cultivars (`Douglas', `Tioga', `Aiko', and `Pajaro') to colonization by three vesicular-arbuscular mycorrhizal (VAM) fungi were determined under nursery conditions. Species of VAM endophytes were Glomus sp. CPH-23, Glomus macrocarpum Tul. & Tul., and Glomus versiforme Berth & Trappe. Yield in VAM plants tended to exceed that of nonVAM plants during the latter part of the harvest, but VAM effects differed widely with host-endophyte combinations. Cultivar-endophyte combinations producing the best yield were `Douglas'-Glomus sp. CPH-23, `Tioga'- G. macrocarpum, and `Aiko'- G. versiforme. The number of strawberries per plant differed significantly (P < 0.01) for `Tioga', depending on the cndophytes used. Root colonization by the endophytes varied from 25% to 75%. Yield was not related to colonization.

Root temperature affects mycorrhizal efficacy in Fraxinus pennsylvanica Marsh

SUMMARYGrowth parameters and carbohydrate contents were determined for Fraxinus pennsylvanica Marsh, seedlings inoculated with Glomus macrocarpum Tul. and Tul. or Glomus fasciculatum (Thaxt.) Gerd. and Trappe, and for uninoculated controls that were grown in a greenhouse for 16 weeks at root temperatures of 15, 25 or 35 °C. Seedling growth responded rapidly to mycorrhizal inoculation when root temperatures were 25 or 35 °C. Response to inoculation at 15 °C was delayed but seedlings at this temperature finally attained the size of those grown at 25 and 35 °C. Growth of noninoculated plants was best at 15 and 25 °C. Inoculation with mycorrhizal fungi moderated the adverse effect of higher root temperature. Infection of roots by both fungal species was greater at 25 °C than at 15 or 35 °C. Sugar content increased as root temperature increased whereas starch content was inversely related. Both soluble sugar and starch content were less in mycorrhizal than in nonmycorrhizal seedlings.

Mycorrhiza development in onion in response to inoculation with chitindecomposing actinomycetes

SUMMARYTwelve actinomycetes, originally isolated from spores of Glomus macrocarpum Tul. & Tul., were inoculated into nonsterilized soil containing G. macrocarpum and G. mosseae (Nicol. & Gerd.) Gerd. & Trappe. The chitin‐decomposing actinomycetes were strongly antagonistic toward fungi, bacteria or actinomycetes, or combinations of these based on prior studies. Eight Streptomyces, two Nocardia, and two unidentified species were tested for their effects on mycorrhiza development in onion (Allium cepa L.), plant growth and nutrient content, soil hyphal density, and soil populations of bacteria and actinomycetes. None of the actinomycetes adversely affected plant growth or nutrient content. The lower foliar P content of two of the treatments could be explained as a dilution effect brought about by the significantly larger shoot growth. With few exceptions, the soil microbial populations and saprophytic fungal hyphal densities were not significantly different from the controls. Seven of the twelve actinomycetes significantly increased the percent mycorrhizal root colonization and four significantly increased the density of hyphae > 5μm in diameter, predominantly those of vesicular‐arbuscular mycorrhizal (VAM) fungi. This is the first study to demonstrate that actinomycetes isolated from spores of a VAM fungus can be beneficial to mycorrhiza development and growth of onion in a nonsterile soil system.

Chitin‐decomposing actinomycetes associated with a vesicular–arbuscular mycorrhizal fungus from a calcareous soil

SUMMARYSpores of Glomus macrocarpum Tul. & Tul. were sieved from a calcareous, silty clay loam soil, washed in sterile water, and plated on chit in agar for the isolation of actinomycetes. Of the 190 spores examined, 100 were colonized by one or more chitin‐decomposing microorganisms: 82% were colonized by actinomycetes, 17% by bacteria, and 1 % by fungi. Fifty‐one actinomycete isolates were classified on the basis of morphological, cultural and biochemical criteria. Tentative identifications to genera and numbers of different isolates are as follows: Streptomyces (29); Nocardia (3); Streptosporangium (1); Streptoverticillium (1); Intrasporangium (1); Nocardiodes (1); and unidentified (15). This is the first study to report the prevalence and diversity of chitin‐decomposing actinomycetes associated with spores of a vesicular‐arbuscular mycorrhizal fungus in field soil.

Spread of endomycorrhizal colonization and effects on growth of apple seedlings

SUMMARYAn experiment to determine the short‐term characteristics of root colonization by two vesicular‐arbuscular (VA) mycorrhizal fungi with apple (Malus × domestica Borkh.) was initiated concurrently with an experiment to determine longer‐term effects of colonization by these fungi on the growth of apple seedlings. Short‐term characteristics of colonization were determined by sequential harvesting of a cuvette system which also allowed monitoring of hyphal spread from inoculated ‘spreader’ plants, through a root‐free soil region, to non‐inoculated ‘receiver’ plants. Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe exceeded Glomus macrocarpum Tul. & Tul. in rate of initial colonization, amount of maximum colonization, and persistence of arbuscules and external hyphae. Colonization by either fungus reduced shoot fresh weight 11 days after inoculation but increased shoot and total fresh weight by day 38. Hyphae of G. mosseae had traversed through 2 and 3 cm of root‐free soil to colonize non‐inoculated ‘receiver’ roots by 29 days, and hyphae of G. macrocarpum by 38 days. G. mosseae colonized ‘receiver’ roots more intensely than did G. macrocarpum at 2 and 3 cm, and 3 and 4 cm distance, at subsequent harvests. The fungi were effective, singly and in combination, in increasing apple plant biomass, as compared to non‐mycorrhizal controls. Relative height growth rate, and colonization at harvest, of G. macrocarpum‐inoculated plants were less than those of G. mosseae‐inoculated or G. mosseae + G. macrocarpum‐ inoculated plants, but greater than those of the non‐mycorrhizal control trees.

Determining Viability of Endogonaceous Spores with a Vital Stain

Spores of endogonaceous fungi are often isolated and counted in studying the ecology of these fungi in natural soils or in standardizing inoculum for experiments. The value of these counts is limited because the viability of spores is not known. Spores killed by soil fumigation and known to be nonviable, as determined by Most Probable Number bioassay, may persist in a field soil and appear normal upon isolation for at least several months (McGraw and Hendrix, 1986). The current methods for determining viability, assessing germinability and ability to infect plants, are labor-intensive, time-consuming, and imprecise. Germination assays may be inaccurate because of dormancy of spores (Tommerup, 1983), and research on dormancy is hampered by lack of an independent measure of viability. Vital stains have been little used in determining the viability of endogonaceous spores. Daniels Hetrick (cited as a personal communication by Menge and Timmer, 1982) tested triphenyl tetrazolium chloride and rose bengal and found it difficult to differentiate living from dead spores. Living spores placed in 1% aqueous thionin for 30 min stained deep blue, whereas dead spores accumulated stain only in the walls. This method has had limited use because some experience was required to use it effectively (Menge and Timmer, 1982). The stain used in the present experiments was 3-(4,5-dimethylthiazol-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT), a compound used successfully to determine viability of oospores of Phytophthora (Cohen, 1984; Sutherland and Cohen, 1983). The chemistry of tetrazolium salts was reviewed by Altman (1976). Briefly, the salt is reduced by cellular dehydrogenase(s) to form a colored formazan product (Gahan and Kalina, 1968; Kalina and Palmer, 1968). Spores were isolated by wet-sieving and sucrose centrifugation (Fang et al., 1983). Spores were either freshly isolated from a pot culture or st red as dry soil inoculum at 3 C [5 years for Glomus macrocarpum Tul. & Tul. or 3 years for Glom s mosseae (Nicol. & Gerd.) Gerdemann & Trappe]. Spores killed with 70% ethanol were treated for 30 min. Spores killed by heat were autoclaved for 15 min.

THE RESPONSE OFACACIA SCLEROXYLATUSS. TO MYCORRHIZAL INOCULATION

SUMMARY Seeds of Acacia scleroxyla Tuss., collected from trees in the Subtropical Dry Forest near Mao, Dominican Republic, were germinated and seedlings inoculated with six species of mycorrhizal fungi (Glomus macrocarpum Tul. and Tul., Glomus fasciculatum (Thaxt.) Gerd, and Trappe, Glomus mosseae Nicol, and Gerd., Glomus etunicatum Becker and Gerd., Glomus epigaeum Daniels and Trappe, or Gigaspora margarita Becker and Hall). Seedlings were grown in a greenhouse for 32 weeks and various growth parameters and percent infection of the roots measured. Mycorrhizal infection significantly improved the growth of Acacia scleroxyla seedlings compared to that of nonmycorrhizal seedlings. Growth varied in response to the different fungal species and could have been due to differences in efficacy of the fungal species, differences in percent infection of the root, or a combination of these factors.

RATES OF INFECTION AND EFFECTS OF FIVE VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI ON APPLE

Apple plants, cultivar Novole, were either uninoculated (control) or inoculated with one of the following mycorrhizal fungi: Glomus macrocarpum Tul. and Tul., Glomus intraradices Schenk and Smith, Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe, Glomus mosseae (Nicol. and Gerd.) Ger. and Trappe, or Glomus epigaeum Daniels and Trappe. Groups of plants were harvested of three successive intervals. Glomus intraradices and G. epigaeum caused most rapid and highest levels of root infection, and increased P uptake compared with plants which were inoculated with the other fungi, or were uninoculated. Plants inoculated with G. intraradices and G. epigaeum had greatest dry weights.Key words: Glomus macrocarpum, Glomus intraradices, Glomus fasciculatum, Glomus mosseae, Glomus epigaeum, phosphorus

Influence of soil compaction and vesicular–arbuscular mycorrhizae on root growth of yellow poplar and sweet gum seedlings

A greenhouse study was conducted to determine the influence of soil compaction on root growth of yellow poplar (Liriodendrontulipifera L.) and sweet gum (Liquidambarstyraciflua L.) seedlings grown in association with the mycorrhizal fungi Glomusmacrocarpum Tul. and Tul. or G. fasciculatum (Thaxt) Gerd. and Trappe. Seedlings were transplanted into pots that contained silt loam compacted to bulk densities of 1.25, 1.40, or 1.55 Mg m−3. Fungal chlamydospores or control filtrates were used to inoculate seedlings. Weight and length of yellow poplar roots were significantly greater at the lower bulk densities than at the highest bulk density, but fibrosity of the root system was unaffected by increasing bulk density. Weight, length, and fibrosity of the sweetgum root system decreased significantly with each increase in bulk density. Inoculated yellow poplar seedlings had greater root weight at each bulk density than noninoculated seedlings, but root length was not influenced by mycorrhizal treatments at higher bulk densities. Fibrosity of yellow poplar roots varied by mycorrhizal treatment at each bulk density. Results indicate that for yellow poplar, compaction effects may outweigh mycorrhizal benefits at higher bulk densities. At each bulk density, sweet gum seedlings inoculated with G. fasciculatum showed the greatest root growth, suggesting that effects of compaction can be alleviated for sweet gum by inoculation with this mycorrhizal fungus.

LOCALIZED INCREASE IN NODULE ACTIVITY BUT NO COMPETITIVE INTERACTION OF COWPEA RHIZOBIA DUE TO PRE‐ESTABLISHMENT OF VESICULAR‐ARBUSCULAR MYCORRHIZA

SUMMARYCowpea [Vigna unguiculata (L.) Walp.] plants were grown in a split‐root system using a calcined montmorillonite clay (Turface) as the growth medium. At transplanting, either the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus macrocarpum Tul. & Tul. and 100 mg hydroxyapatite (HAP) or 100, 200 or 400 mg HAP kg−1 without the VAM fungus were mixed into side 1 of the split‐root system. Side 2 received only 100 mg HAP kg−1. After 30 d, a combined inoculum of four Rhizobium isolates (TAL169, TAL173, TAL658 and IRC256) was applied to the surface of all pots. At harvest (60 d), root systems were separated from shoots, and nodule activity (acetylene reduction), plant dry weight, nitrogen (N) and phosphate (P) content and the percentage of VAM colonization were determined. There was no significant effect of treatment on either the N or P content of leaves or roots. The VAM fungus, however, significantly (P < 0.01) increased shoot dry weight. Root dry weight and nodule activity were significantly increased only on the VAM side of the split‐root system. Rhizobia from 30 nodules per half‐root system were identified using fluorescent antibody techniques. Strain TAL 173 was detected from 95% of the nodules and neither the VAM fungus nor P application affected competitive interaction by rhizobia for nodulation sites. Our work suggests the existence of a localized, non‐systemic, non‐P‐mediated influence of a VAM fungus on root dry weight and nodule activity in cowpea.

Aggregation of a Silty Clay Loam Soil by Mycorrhizal Onion Roots

AbstractOnion plants (Allium cepa L.) inoculated in the root zone with a vesicular‐arbuscular mycorrhizal (VAM) fungus, or left uninoculated, were grown in potted soil for 230 d to determine the influence of the VAM fungus (Glomus macrocarpum Tul. and Tul.) on soil structure. The silty clay loam soil was maintained at a moisture content between 25 and 30%. Paired inoculated (+M) and uninoculated (−M) plants were harvested (20 pairs over 150 d) beginning 80 d after planting. Relationships between plant, fungal, and soil parameters and changes with time were evaluated by regression analysis. Root colonization by the VAM fungus in +M plants ranged from 49% to 60% over the sampling period, while no mycorrhizae were detected in the ‐M plants. Total dry mass of VAM plants was five to six times that of the non‐VAM plants. Soil from the +M treatment was significantly better aggregated, more porous, and had greater water permeability than ‐M soil. Root dry mass and VAM hyphal density in the +M soil were both significantly correlated with the relative abundance of water‐stable soil macroaggregates. Correlation of root mass with aggregate abundance was stronger, however, suggesting that soil changes were mainly mediated by direct root effects of a host plant whose growth was stimulated dramatically by its VAM fungal endophyte.