Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Growth and nutrient status of citrus plants as influenced by mycorrhiza and phosphorus application

To test the hypothesis that high levels of soluble phosphate applied in combination with VAM fungi, to citrus plants, can cause growth depression even in the absence of other limiting factors, and also to test if rock phosphate, under these conditions, may be a satisfactory P source, a greenhouse experiment was conducted using sterilized soil with four levels of phosphate (0, 50, 100 and 200 ppm P) supplied either as soluble P or as rock phosphate. Citrus seedlings were either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus etunicatum or left uninoculated. Six months after the start of the experiment, the plants were harvested and shoot dry weight, P and K uptake, root colonization and the number of spores in 50 cm³ of soil were determined. Significant increases were found in dry matter yields and in P and K contents, due to VAM fungus inoculation, at the zero and 50 ppm soluble P levels and at all rock phosphate levels. At 100 ppm soluble P, the development of VAM plants was equilvalent to that of non-VAM plants, and at 200 ppm, growth was significantly less than that of non-VAM plants. Root colonization and sporulation were reduced at higher P levels. The absolute growth depression of VAM plants at the higher P level was likely due to P toxicity. In addition, high leaf P and K concentrations may have interfered with carbohydrate distribution and utilization in these symbioses. Rock phosphate may be used with VAM citrus to substitute for medium amounts of soluble phosphate.     

Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae

Summary Drought resistance of wheat (Triticum aestivum L.) as influenced by two vesiculararbuscular mycorrhizal (VAM) fungi,Glomus fasciculatum ¹⁰ andGlomus deserticola ¹⁹, was evaluated. Soil columns 0.15 m diam. by 1.20 m length were used to reduce the influence of limited rooting space. With initial soil water at 0.5 MPa (0.145 kg kg^–1), plants were subjected to low-level water stress throughout the experiment and severe water stress for 24 h at one (55 days after transplanting, Feekes scale 10.1) two (55 and 63 days, Feekes 10.1 and 10.2), or three (55, 63, and 70 days, Feekes 10.1, 10.1, and 10.2) periods. After each stress period, one set of plants was watered and grown to maturity without subsequent water stress. A second set of plants was harvested 1 week after stress.G. fasciculatum-inoculated plants harvested 7 days after stress at 55 days had greater leaf area and leaf, total plant, and root weight than non-VAM plants.G. deserticola-inoculated plants had greater leaf area and leaf weight than non-VAM plants. After stress at 55 and 63 days, leaf area, and leaf and total dry weight were again greater for VAM than for non-VAM plants. However, after stress at 55, 63, and 70 days, differences in aboveground biomass between VAM and non-VAM plants were not significant at P=0.05. Aboveground biomass was not affected by VAM species in plants stressed at 55 or 55 and 63 days, butG. fasciculatum-inoculated plants produced more tillers atter stress at 55 days. When grown to maturity, VAM plants which had undergone three stress periods had twice the biomass and grain yield as non-VAM plants subjected to the same stress. The three stress periods reduced number of heads and kernel numbers of weight of non-VAM plants compared to VAM plants.G. fasciculatum-inoculated plants consistently had increased root weight and rooting depth.Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebr. Agric. Exp. Stn., Univ. Nebr.-Lincoln, Lincoln, Nebr. Published as Paper No. 7571 Journal Series, Nebr. Agric. Exp. Stn.     

Micronutrient uptake and distribution in mycorrhizal or phosphorus-fertilized soybeans

Summary Soybean plants were grown in a soil very low in available P. Seedlings were inoculated with two vesicular-arbuscular mycorrhizal (VAM) fungi or were left non-inoculated and fertilized with P. Assimilation and allocation of micronutrients (Fe, Mn, Zn, and Cu) were determined during host development, and the uptake of trace elements in VAM plants was compared to P-fertilized, non-VAM plants of similar weight, growth stage, and P status. Copper and zinc concentrations were always higher in VAM plants, while iron and manganese concentrations were lower than in the equivalent P-fertilized soybeans. Differences in the micronutrient content of fully-mature soybean pods reflected differences in the leaves and roots. Thus, for trace elements, seed quality can be altered by VAM colonization in a fashion not duplicated by P fertilizer. Contribution from the Western Regional Research Center, USDA-ARS (CRIS No. 5325-20580-003).     

Effects of the root-lesion nematode Pratylenchus vulnus and the vesicular-arbuscular mycorrhizal fungus Glomus mosseae on the growth of three plum rootstocks

The effects of Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on growth of Myrobalan 605, Marianna 2624 and San Julian 655-2 plum rootstocks were measured under shadehouse conditions in the field for two growing seasons (1990–91). Shoot dry weights were higher in the majority of the vesicular-arbuscular mycorrhizal (VAM) alone inoculated plants after both growing seasons. Root weights of mycorrhizal Myrobalan and Marianna were higher than root weights of the same rootstocks lacking mycorrhizae, inoculated with P. vulnus, and VAM in combination with the nematode. Mycorrhizal Marianna inoculated with the nematode showed a considerably higher final nematode population in relation to non-inoculated VAM treatments. No correlation was found in the number of nematodes per gram of root between mycorrhizal and non-mycorrhizal treatments. P. vulnus adversely affected the mycorrhizal colonization in Marianna, but not in Myrobalan and San Julian. Marianna appears to be more mycorrhizal dependent than the two other rootstocks.     

Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake

Bethlenfalvay, G. J., Brown, M. S., Ames, R. N. and Thomas, R. S. 1988. Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake. - Physiol. Plant. 72: 565–571.
Soybean [ Glycine max (L.) Merr.] plants were grown in pot cultures and inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or provided with P fertilizer (non-VAM plants). After an initial growth period (21 days), plants were exposed to cycles of severe, moderate or no drought stress over a subsequent 28-day period by rewatering at soil water potentials of -1.0, -0.3 or -0.05 MPa. Dry weights of VAM plants were greater at severe stress and smaller at no stress than those of non-VAM plants. Phosphorus fertilization was applied to produce VAM and non-VAM plants of the same size at moderate stress. Root and leaf P concentrations were higher in non-VAM plants at all stress levels. All plants were stressed to permanent wilting prior to harvest. VAM plants had lower soil moisture content at harvest than non-VAM plants. Colonization of roots by G. mosseae did not vary with stress, but the biomass and length of the extraradical mycelium was greater in severely stressed than in non-stressed plants. Growth enhancement of VAM plants relative to P-fertilized non-VAM plants under severe stress was attributed to increased uptake of water as well as to more efficient P uptake. The ability of VAM plants to deplete soil water to a greater extent than non-VAM plants suggests lower permanent wilting potentials for the former.     

Root Respiration and Growth in Plantago major as Affected by Vesicular-Arbuscular Mycorrhizal Infection

Effects of vesicular-arbuscular mycorrhizal (VAM) infection and P on root respiration and dry matter allocation were studied in Plantago major L. ssp. pleiosperma (Pilger). By applying P, the relative growth rate of non-VAM controls and plants colonized by Glomus fasciculatum (Thaxt. sensu Gerdemann) Gerdemann and Trappe was increased to a similar extent (55-67%). However, leaf area ratio was increased more and net assimilation rate per unit leaf area was increased less by VAM infection than by P addition. The lower net assimilation rate could be related to a 20 to 30% higher root respiration rate per unit leaf area of VAM plants. Root respiration per unit dry matter and specific net uptake rates of N and P were increased more by VAM infection than by P addition. Neither the contribution of the alternative respiratory path nor the relative growth rate could account for the differences in root respiration rate between VAM and non-VAM plants. It was estimated that increased fungal respiration (87%) and ion uptake rate (13%) contributed to the higher respiratory activity of VAM roots of P. major.     

Host genotype dependency and growth enhancing ability of VA-mycorrhizal fungi for Eleusine coracana (finger millet)

Inoculation of finger millet (Eleusine coracana Gaertn.) plants with one of six different vesicular, arbuscular, mycorrhizal (VAM) fungi increased plant biomass, height, leaf area and absolute growth rate; however, effectiveness of the various VAM fungi varied significantly. Maximum root colonization and mycorrhizal efficacy was observed with plants inoculated with Glomus caledonicum. Among five host genotypes tested for mycorrhizal dependency against G. caledonicum, genotype HR-374 gave the highest plant biomass, mycorrhizal efficacy and root colonization, the inoculation resulting in increased mineral (phosphate, nitrogen, Zn²⁺ and Cu²⁺) content and uptake in shoots.     

Development of two endomycorrhizal symbioses on soybean and comparison with phosphorus fertilization

Summary Soybean (Glycine max L. Merr. cv. Amsoy 71) plants were grown in a greenhouse in a soil very low in plant-available P, and plants were harvested 5 times over a 21-week growth period. Soybeans were inoculated with one of two species of VAM fungi or received daily one of three nutrient solutions of different P concentrations (0.0, 0.2, or 1.0mMP). Until week 9, the dry weights, leaf areas and developmental stage of soybeans inoculated withG. fasciculatum orG. mosseae were similar to the 1.0 or 0.2mMP-treated plants, respectively. Phosphorus concentrations were significantly lower in VAM plants at weeks 6 and 9 as compared to non-VAM soybeans given 1.0mMP, suggesting P input in VAM plants was immediately used for new growth. Total P input for VAM plants was linear over 21 weeks, and the average rate of P uptake for these plants was 0.19mg P d⁻¹. Estimated specific P uptake rates (SPUR) for the mycorrhizae (VAM roots) were twice that of the control (0.0mMP) roots. The calculated SPURs forG. fasciculatum andG. mosseae hyphae were 95 and 120μg P g⁻¹ VAM d⁻¹ respectively, a 4 to 5 fold increase over non-inoculated roots, indicating more attention must be paid to P assimilation by VAM fungi in P-fixing substrates. Contribution from the Western Regional Research Center, USDA-ARS (CRIS No. 5325-20580-003).     

Receptivity of various wheat cultivars to infection by VA-mycorrhizal fungi as influenced by inoculum potential and the relation of VAM-effectiveness to succinic dehydrogenase activity of the mycelium in the roots

Wheat cultivars grown in pots in a greenhouse were inoculated either once or twice with the vesicular-arbuscular mycorrhizal fungus (VAMF) Glomus mosseae. If inoculum was only added to the soil once (before planting) the cultivars showed differences in subsequent VAM development. If additional inoculum was added ten days after planting, VAM development was much increased and cultivars which remained without VAM after only one inoculum application now became mycorrhizal.Succinate dehydrogenase (SDH) activity decreased throughout the experiment, but this decrease was less rapid in mycelium in twice-inoculated plants.No close relationship between SDH-activity and plant growth (VAM effectiveness) was found.     

Differential plant response to inoculation with two VA mycorrhizal fungi isolated from a low-pH soil

Sericea lespedaza and ladino clover were inoculated withAcaulospora laevis andGigaspora margarita VA mycorrhizal fungi, both isolated from a local soil having a pH of 4.4. Plants were grown in a greenhouse in fumigated (methyl bromide) soil with four rates of applied P. This soil had a pH of 5.1 for theG. margarita experiment and a 5.8 for theA. laevis experiment. Neither plant species responded to theG. margarita isolate in terms of mycorrhizal infection of roots, top growth, or elemental uptake. TheA. laevis isolate caused increased growth of Ladino clover at the lowest rate of P application and increased growth of sericea lespedeza at the two lowest P application rates. Shoot tissue concentrations of P for both plant species were greater at the two lowest rates of P application when inoculated withA. laevis. Inoculation withA. laevis also resulted in different VAM fungal root colonization patterns between the two plant species as a function of P application rate. Roots of both plants had high infection rates (near 70%) for the two lowest P application rates but sericea lespedeza declined to 40 and 6% at the next two P application rates, respectively, whereas infection in Ladino clover was 74 and 41% at the next two P application rates, respectively. The results of this study support the concept that plant host-VAM fungal endophyte interactions are extremely variable and that characterization of individual combinations must be done if types and magnitudes of responses are to be defined.     

Growth response ofTheobroma cacao L. seeldings to inoculation with vesicular-arbuscular mycorrhizal fungi

Summary The effects of vesicular-arbuscular mycorrhiza (VAM) on the growth and phosphorus uptake of cocoa seedlings (Theobroma cacao L.) grown for 100 days in polythene bags, were studied at five levels of phosphorus fertilization in both steamed and unsterile Bungor Series soil (a fine clayey, kaolinitic isohyperthermic Typic Paleudult). The cocoa seedlings responded well to phosphorus fertilization and mycorrhizal treatments. Plants inoculated with VAM fungi (Gigaspora spp.) gave the most vigorous growth and higher phosphorus in the leaf tissues in unsterile soil compared to plants grown in steamed soil. However, the mycorrhizal effect was significantly more pronounced (P<0.01) in plants grown in steamed than in unsterile soil. High levels of phosphorus application depressed mycorrhizal development. Phosphorus fertilizer applied at the rates of 250 and 500 ug g⁻¹ soil gave maximum root colonization and spore counts in both soil types used.     

Influence of temperature on colonization of spring barleys by vesicular arbuscular mycorrhizal fungi

The effects of three soil temperatures on growth of spring barleys (Hordeum vulgare L.) and on their root colonization by vesicular arbuscular mycorrhizal (VAM) fungi from agricultural soils in Montana (USA) or Syria at different inoculum concentrations were tested in soil incubators in the greenhouse. The number of mycorrhizal plants as well as the proportion and intensity of roots colonized increased with higher soil temperatures. VAM fungi from Montana, primarily Glomus macrocarpum, were cold tolerant at 11°C while those from Syria, primarily G. hoi, were heat tolerant at 26°C. Inoculum potential of Montana VAM fungi was higher than Syrian VAM fungi in cool soils. Harmal, selected from Syrian barley land races, had the highest colonization by mycorrhizal fungi of the cultivars tested.Journal Series Paper: J-2532 Montana Agricultural Experiment Station.     

Glycine-Glomus-Rhizobium Symbiosis: V. Effects of Mycorrhiza on Nodule Activity and Transpiration in Soybeans under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were nodulated (Bradyrhizobium japonicum) and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or left uncolonized. All plants were grown unstressed for 21 days initially. After this period, some VAM and non-VAM plants were exposed to four 8-day drought cycles while others were kept well watered. Drought cycles were terminated by rewatering when soil moisture potentials reached −1.2 megapascal. Nodule development and activity, transpiration, leaf conductance, leaf and root parameters including fresh and dry weight, and N and P nutrition of VAM plants and of non-VAM, P-fed plants grown under the same controlled conditions were compared. All parameters, except N content, were greater in VAM plants than in P-fed, non-VAM plants when under stress. The opposite was generally true in the unstressed comparisons. Transpiration and leaf conductance were significantly greater in stressed VAM than in non-VAM plants during the first half of the final stress cycle. Values for both VAM and non-VAM plants decreased linearly with time during the cycle and converged at a high level of stress (−1.2 megapascal). Effects of VAM fungi on the consequences of drought stress relative to P nutrition and leaf gas exchange are discussed in the light of these findings and those reported in the literature.     

Infection by the VA-mycorrhizal fungusGlomus caledonium inHedysarum coronarium as influenced by host plant and P content of soil

The low degree of infection ofHedysarum coronarium L. (sulla) exposed to inoculum of the VAM endophyteGlomus caledonium was investigated. Infection began after a prolonged lag phase and remained at very low levels even after three months’ growth. Neither very high rates of inoculum, nor very low P content of the soil raised the low infection level of the sulla plants. There appeared to be some differences in rate of infection among ten different ecotypes of sulla but the level of infection remained low in all cases. In all tested populations some plants remained uninfected. The low infection rate of sulla may therefore have a genetic basis. It was shown that the growth ofH. coronarium is hardly improved by phosphate fertilization. This may explain the poor response of this plant species, adapted to grow in nutrient-deficient soil, to VAM. Programmes aimed at increasing the productivity in marginal soils through the introduction of efficient VAM endophytes should take into account the fact that certain plant species growing in marginal soils may not always benefit from mycorrhizal inoculation, due to their inherently low mycorrhizal dependency.     

Field nursery inoculation of Hevea brasiliensis Muell. Arg. seedling rootstock with vesicular-arbuscular mycorrhizal (VAM) fungi

The growth response of Hevea brasiliensis to vesicular-arbuscular mycorrhizal (VAM) fungi inoculation was assessed in two field nursery sites containing indigenous mycorrhizal fungi (IMF). Seedling rootstocks were inoculated with mixed VAM-fungal species in a factorial combination with phosphorus (P) fertilizer application, and planted in randomised blocks on sandy (site 1) and clayey (site 2) soils. Plants were harvested after 26 weeks for measurements of shoot dry weight (DW), stem diameter, height, mycorrhizal root colonization and leaf nutrient contents. At site 1, VAM increased shoot DW, stem diameter and plant height only in treatments without P applied. Increases in shoot DW due to VAM were 70% greater than the uninoculated controls although this was reduced to 5% when P was applied. At site 2, VAM inoculation also increased shoot DW and stem diameter but the magnitude of the increases was smaller. Shoot DW response due to VAM was only 29%. At this second site, applying phosphate to uninoculated plants did not increase shoot yields further. Leaf concentrations of all nutrients were unaffected by VAM at both sites, except for copper (Cu) which was increased by VAM in treatments where P was not applied. However, leaf contents of P, potassium (K), magnesium (Mg) and Cu were increased by VAM at site 1, and of leaf nitrogen (N) and K at site 2. These experiments demonstrate that VAM-fungi could be introduced into field nursery sites to improve growth and P uptake by H. brasiliensis. The relevance of VAM-fungi to H. brasiliensis seedling rootstock development and the influence of IMF in determining field responses is discussed.     

Effects of belowground grazing by collembola on growth,mycorrhizal infection,and P uptake of Geranium robertianum

We hypothesized that the grazing of vesicular-arbuscular mycorrhizal (VAM) hyphae by soil animals could be responsible for the lack of a direct relationship between mycorrhizal infection intensity and nutrient uptake under field conditions. To test this hypothesis, we determined the effect of a range of densities of the collembola, Folsomia candida, on growth, VAM infection, and P uptake in Geranium robertianum, a common forest herb, under greenhouse conditions. Total and aboveground growth were greater at low collembola density than either at higher collembola density or without collembola. These differences were greater when the plants were grown in a high organic content soil mix than when grown in sand. Root mass was not affected by collembola density. In the soil mix, root length decreased with increasing collembola density, but not in the sand. The percent of root length infected with VAM was lower at any collembola density than when collembola were absent. Total infected root length decreased linearly with increasing collembola density. Few significant differences in P uptake or tissue concentration were found. Thus, plant growth (but not P uptake) may be stimulated at low collembola density and inhibited at high. We discuss mechanisms which may be responsible for this non-linear response, and the implications of the pattern of response to studies of plant competition, nutrient turnover, and revegetation.     

Dependency of cassava (Manihot esculanta Crantz) on vesicular-arbuscular mycorrhizal fungi

Cassava plants were started in the greenhouse either from small cuttings (2.0 mg P/cutting) or large cuttings (20.2 mg P/cutting) in a subsurface Oxisol not inoculated or inoculated with Glomus aggregatum at target soil solution P concentrations of 0.003–0.2 mg l^-1. Vesicular-arbuscular mycorrhizal (VAM) fungal colonization levels in excess of 60% were attained on cassava roots irrespective of the size of cutting material used or target soil solution P status. However, plants started from large cuttings grew faster and better than those started from smaller ones. Cassava was found to be very highly dependent on VAM fungi if grown from small cutting but only marginally dependent if grown from large cuttings. The lower dependence of cassava on VAM fungi when started from larger cuttings appears to be related to the high P reserve in these cuttings and hence the low requirement of the plants for soil P until the P reserve in the cuttings is significantly depleted.A contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3896     

Effect of vesicular-arbuscular mycorrhizal fungi and ofagrobacterium radiobacter on maize growth

An influence of dual inoculation with the rhizosphere bacteriumAgrobacterium radiobacter, and the VAM fungi,Glomus mosseae andGlomus sp., on maize growth and mycorrhizal infection was observed. Separate inoculations of bacteria or fungi showed significant positive effects on the shoot biomass production of pot-cultured plants only at the last of three consecutive harvests. Plant biomass production was enhanced substantially after a dual inoculation with bacteria and fungi. Synergistic interaction of fungal and bacterial inoculation and growth stimulation was evident at all three harvests compared to uninoculated plants and also compared to plants inoculated with fungi or bacteria only. The dual inoculation increased the shoot biomass of plants by approximately 30% as compared with control. No significant differences were found in mycorrhizal infection between plants uninoculated and inoculated with bacteria.Agrobacterium radiobacter seems to be compatible with mycorrhizal symbiosis and can act a synergistic partner of some VAM fungi.     

Effect of aromatic plant species on vesicular-arbuscular mycorrhizal establishment in Pistacia terebinthus

The inoculation of Pistacia terebinthus with vesicular-arbuscular mycorrhizal (VAM) fungi and the spread of the infection were studied using a mixed cropping system, under glasshouse conditions, with Salvia officinalis, Lavandula officinalis and Thymus vulgaris colonized by Glomus mosseae as an inoculation method. This method was compared with soil inoculum placed under the seed or distributed evenly in the soil. Indirect inoculation with all the aromatic plants tested significantly increased VAM root colonization of P. terebinthus compared with the use of soil inoculum, although the effect on plant growth was different for each one of the aromatic species used as inoculum source. Inoculation with L. officinalis and T. vulgaris were the best treatments resulting in high VAM colonization and growth enhancement of P. terebinthus.