Competition among strains of Bradyrhizobium and vesicular-arbuscular mycorrhizae for groundnut (Arachis hypogaea L.) root infection and their effect on plant growth and yield

Summary Strains of Bradyrhizobium influenced root colonization by a species of vesicular-arbuscular mycorrhizae (VAM), and species of VAM influenced root nodulation by strains of Bradyrhizobium in pot experiments. In a field experiment, the effects of VAM on competition amongst inoculated bradyrhizobia were less evident, but inoculation with Bradyrhizobium strains increased root colonization by VAM. Certain VAM/Bradyrhizobium inoculum strain combinations produced higher nodule numbers. Plants grown without Bradyrhizobium and VAM, but supplied with ammonium nitrate (300 g ml^–1) and potassium phosphate (16 g ml^–1), produced higher dry-matter yields than those inoculated with both symbionts in the pot experiment. Inoculation with either symbiont in the field did not result in higher pod and haulm yields at harvest.ICRISAT Journal Article No. 886     

Possible influence of Rhizobium on VA mycorrhiza metabolic activity in double symbiosis of alfalfa plants (Medicago sativa L.) grown in a pot experiment

Summary Alfalfa (Medicago sativa, L. cv Aragón) plants were grown under greenhouse conditions in pots of inert sand and vermiculite. The plants were inoculated with Rhizobium meliloti strain 102F28, with Glomus fasciculatus or with a mixture of both microorganisms. Plants inoculated with both Rhizobium and Glomus had the highest shoot dry weight and the lowest root-to-shoot ratio. Roots from dually inoculated plants also had a higher oxygen uptake and nodule nitrogenase activity than those from plants inoculated with either of the two microsymbionts alone. However, the dry weight of the roots from only VAM-infected plants was higher than those from Rhizobium or from Rhizobium plus Glomus-inoculated ones. These differences did not correlate with succinate dehydrogenase activity, which was similar between treatments. Nutrient element concentrations were increased in dually infected plants in comparison with those of plants inoculated with only Rhizobium or Glomus. These data suggest that Rhizobium may affect fungal metabolism and that the effect is not achieved via the tricarboxylic acid pathway.     

The influence of inorganic nutrient fertilization on the growth,nutrient composition and vesicular-arbuscular mycorrhizal colonization of pretransplant rice (Oryza sativa L.) plants

Summary The effects of P, N and Ca+Mg fertilization on biomass production, leaf area, root length, vesiculararbuscular mycorrhizal (VAM) colonization, and shoot and root nutrient concentrations of pretransplant rice (Oryza sativa L.) plants were investigated. Mycorrhizal plants generally had a higher biomass and P, N, K, Ca, Mn, Fe, Cu, Na, B, Zn, Al, Mg, and S shoot-tissue nutrient concentrations than non-mycorrhizal plants. Although mycorrhizal plants always had higher root-tissue nutrient concentrations than non-mycorrhizal plants, they were not significantly different, except for Mn. N fertilization stimulated colonization of the root system (colonized root length), and increased biomass production and nutrient concentrations of mycorrhizal plants. Biomass increases due to N were larger when the plants were not fertilized with additional P. P fertilization reduced the colonized root length and biomass production of mycorrhizal plants. The base treatment (Ca+Mg) did not significantly affect biomass production but increased the colonized root length. These results stress the importance of evaluating the VAM rice symbiosis under various fertilization regimes. The results of this study suggest that pretransplant mycorrhizal rice plants may have a potential for better field establishment than non-mycorrhizal plants.     

Selection of efficient vesicular-arbuscular mycorrhizal fungi for wetland rice (Oryza sativa L.)

Summary We tested the response of the wetland rice cultivar Prakash to inoculation with ten vescular-arbucular mycorrhizal (VAM) fungi (three selected from the first screening and seven isolated from local paddy fields) in a pot experiment under flooded conditions in order to select the most efficient mycorrhizal fungi to inoculate the rice nursery. A sandy clay loam soil was used as the substrate, fertilized with the recommended N and K levels (100 kg N ha^–1 as ammonium sulphate and 50 kg K ha^–1 as muriate of potash) and half the recommended level of P (25 kg ha^–1 as super phosphate). The inoculation was made into dry nursery beds and the beds were flooded when the seedlings were about 25 cm high, in 15 days. Twenty-eight-day old seedlings were transferred to pots filled with well puddled soil flooded with 5 cm of standing water. Based on the increase in grain yield and total biomass, Glomus intraradices and Acaulospora sp. were considered efficient and suitable for inoculation into rice nurseries.     

Dual inoculation of pretransplant stage Oryza sativa L. plants with indigenous vesicular-arbuscular mycorrhizal fungi and fluorescent Pseudomonas spp.

Summary This study examined the response of rice (Oryza sativa L.) plants at the pretransplant/nursery stage to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and fluorescent Pseudomonas spp., singly or in combination. The VAM fungi and fluorescent Pseudomonas spp. were isolated from the rhizosphere of rice plants. In the plants grown in soil inoculated with fluorescent Pseudomonas spp. alone, I found increases in shoot growth, and in root length and fine roots, and decreases in root growth, and P and N concentrations. In contrast, in the plants colonized by VAM fungi alone, the results were the reverse of those of the pseudomonad treatment. Dual inoculation of soil with VAM fungi and fluorescent Pseudomonas spp. yielded plants with the highest biomass and nutrient acquisition. In contrast, the plants of the control treatment had the lowest biomass and nutrient levels. The dual-inoculated plants had intermediate root and specific root lengths. The precentages of mycorrhizal colonization and colonized root lengths were significantly lower in the dual-inoculated treatment than the VAM fungal treatment. Inoculation of plants with fluorescent Pseudomonas spp. suppressed VAM fungal colonization and apparently reduced photosynthate loss to the mycorrhizal associates, which led to greater biomass and nutrient levels in dual-inoculated plants compared with plants inoculated with VAM fungi alone. Dual inoculation of seedlings with fluorescent Pseudomonas spp. and VAM fungi may be preferable to inoculation with VAM alone and may contribute to the successful establishment of these plants in the field.     

The effect of suppression treatments on the uptake of 15N by intercropped corn from labeled alfalfa (Medicago sativa)

In greenhouse experiments, we examined the N transferred to intercropped corn from ¹⁵N-labeled alfalfa shoot residue and intact roots in an undisturbed soil system in response to two different suppression treatments and complete killing of alfalfa. The alfalfa treatments included complete killing (glyphosate only), glyphosate injury + cutting, and cutting only, with alfalfa shoot residue returned to the soil surface in all three treatments. Corn was planted in each pot following application of the treatments. When alfalfa was suppressed by glyphosate injury + cutting, corn had recovered 12% of the alfalfa N by 8 weeks of growth, but with cutting only, N recovery by corn was reduced to 4.0%. The completekill treatment resulted in 8% recovery by corn of alfalfa N. In all treatments, most of the alfalfa-N remained in the soil organic pool. A second experiment tested a cutting only treatment with ¹⁵N-labeled alfalfa residue returned to the soil surface. The ¹⁵N-labeled alfalfa residue contributed 4.1% of N to corn during the 8-week growth cycle. Twice as much ¹⁵N was found in the active microbial biomass pool in the two treatments with live intereropped plants compared to the monoculture treatments with complete killing (non-intercropped) and the control treatment of alfalfa regrowth only. An analysis of the change in the ¹⁵N content of the undisturbed alfalfa roots from just before the suppression until 8 weeks later suggested that approximately 80% of the root ¹⁵N was lost from the plant suppressed by cutting. This corresponds to 28% of the total N released from the alfalfa. The results suggest that the degree of legume suppression was a key factor in the availability of legume N to the second crop. When the two species were intercropped, more of the N available from legume residues went to plant uptake and microbial biomass and was not stabilized as quickly in the soil organic pool. Appropriate management schemes must be designed to increase N availability to the second crop without yield reduction. These studies suggest severe suppression is necessary; if successful, more of the N can be maintained in active pools.     

Influence of the earthworm Aporrectodea trapezoides (Lumbricidae) on the colonization of alfalfa (Medicago sativa L.) roots by Rhizobium meliloti L5-30R and the survival of R. meliloti L5-30R in soil

A greenhouse study was performed to examine the ability of the earthworms Microscolex dubius and Aporrectodea trapezoides to transport Rhizobium meliloti L5-30R through soil. When R. meliloti L5-30R was inoculated into either ezi-mulch (a pelleted formulation of cereal-pea straw), oat hay, pea hay, or sheep dung and placed on the soil surface together with either A. trapezoides or M. dubius, >10⁴ colony-forming units (CFU) of R. meliloti L5-30R g^-1 soil were detected at 90 mm soil depth after 18 days. In the absence of earthworms, R. meliloti L5-30R was not detected at 90 mm soil depth after this time. In a second experiment using ezi-mulch as the inoculant material and in the presence of A. trapezoides (in a number equivalent to 471 or 785 m^-2), ca. 10³ CFU of R. meliloti L5-30R per 10 mm of alfalfa root were detected at 0–30, 30–60, and 60–90 mm soil depth after 18 days, while <3 CFU were detected per 10 mm of root in the absence of A. trapezoides. In a third experiment in which R. meliloti L5-30R was distributed evenly through soil at the start of the experiment, A. trapezoides (in a number equivalent to 157, 471, or 785 m^-2) significantly decreased the survival of L5-30R in soil after 40 days of incubation at 15°C, but not after 20 days. The decrease in survival of R. meliloti L5-30R was correlated with the density of A. trapezoides. These results demonstrate that A. trapezoides can increase root colonization of alfalfa by R. meliloti L5-30R, but may also reduce the ability of R. meliloti L5-30R to survive in soil.     

Effect of fertilizer and endomycorrhizal inoculum on growth and nutrient uptake of cocoa (Theobroma cacao L.) seedlings

Summary A greenhouse experiment was carried out to evaluate the influence of vesicular-arbuscular mycorrhiza (VAM) on growth and nutrient uptake of cocoa seedlings treated with five levels of palm oil mill effluent, in an unsterilized Oxisol and an Ultisol, either with or without addition of the VAM fungus Scutellospora calospora (Nicol. & Gred.) Walker and Sanders. Inoculation with the VAM fungi significantly increased nutrient uptake and plant growth in both soils. The dry matter yield, and the tissue N and K concentration in the plant tops increased significantly with increasing levels of palm oil mill effluent applied to both the Oxisol and the Ultisol. The maximum tissue P concentration, however, was obtained from plants grown in the Ultisol that was given 50.0 g palm oil mill effluent per kg while the maximum P recovery of 26% was obtained from plants given only 16.7 g effluent per kg. Overall, the percentage of P recovery decreased with the addition of increasing levels of palm oil mill effluent. In the Oxisol, the tissue P concentration increased with the addition of increasing levels of palm oil mill effluent, but the maximum recovery of P was recorded from plants given only 0.3 g effluent per kg. The percentage P recovery decreased with subsequent additions of the effluent.     

Effect of vesicular-arbuscular mycorrhizae and zinc application on dry matter and zinc uptake of greengram (Vigna radiata L. Wilczek)

Summary In a greenhouse study we examined the effects of vesicular-arbuscular mycorrhizae (VAM) inoculation, using Glomus macrocarpum and of Zn application on dry matter production and Zn uptake by greengram in two mollisols. The VAM inoculation significantly increased the dry weight of different plant parts and the Zn uptake in both soils. Inoculated plants showed a greater response to the application of Zn at 2.5 and 5.0 mg kg^-1 soil in a Zn deficient clay loam soil. The inoculated plants also absorbed — more water than the uninoculated plants. Mass flow and diffusion were the principal processes by which Zn reached the plant roots; mass flow was particularly important in the absence of VAM in a sandy soil fertilized with higher Zn doses (5 and 10 mg kg^-1 soil). The greater supply of Zn to inoculated roots was attributed to an apparent diffusion process rather than to mass flow of Zn.     

Migration,colonization and seedling growth of rhizobia with matrine treatment in alfalfa (Medicago sativa L.)

Purpose: The purposes of this study were to determine the effect of matrine on the migration and the colonization dynamics of the two fluorescent-tagged rhizobia in Gannong No.5 alfalfa (Medicago sativa L. Gannong No. 5) tissues, and also to determine the effect of the combination treatments on alfalfa seedlings’ growth.

Materials and methods: 0, 100, 200, 300 and 400 mg L^?1 matrine levels were added into two cyan fluorescent protein (CFP)-tagged rhizobia; Ensifer meliloti LZgn5f (gn5f) and Ensifer meliloti 12531f (12531f), respectively; and drenched the alfalfa root with the inoculants. Then the migration and colonization of the two rhizobia in alfalfa on D7, D14, D21 and D28, and subsequently seedling growth were investigated.

Results: The results showed that the optimum matrine level enhanced the colonization of both fluorescent-tagged rhizobia in alfalfa roots and the highest colonization densities of log 6.31 cfu g^?1 and log 5.87 cfu g^?1 were achieved by adding 300 mg L^?1 matrine into 12531f and adding 100 mg L^?1 matrine into gn5f, respectively. They could migrate to the aerial tissues and most colonize stems through the application of adding 300 mg L^?1 matrine into 12531f and 100 mg L^?1 matrine into gn5f, respectively. No fluorescent-tagged rhizobia were detected in the control treatment. Alfalfa seedling growth parameters like leaf chlorophyll content, seedling growth rate, root length, seedling biomass and total N percentage also increased the most when 300 mg L^?1 matrine was added into 12531f and 100 mg L^?1 matrine added into gn5f treatments.

Conclusions: Our results suggest that 300 mg L^?1 matrine added into 12531f and 100 mg L^?1 matrine added into gn5f might be exploited to promote the colonization of rhizobia in alfalfa tissue and positively impact growth and yield, indicating possible benefits for plant cultivation.     

Comparative effects of vesicular-arbuscular mycorrhizal inoculation and phosphorus fertilization on growth and phosphorus uptake of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) plants under drought-stressed conditions

Maize (Zea mays L.) and sorghum (Sorghum bicolor L.) Moench (local variety called Masakwat) plants were grown in a sterilized low-P soil in the greenhouse for 12 weeks. Each plant species was either mycorrhizal with vesicular-arbuscular mycorrhizal (VAM) fungi, non-mycorrhizal but minimally fertilized with soluble P, or non-mycorrhizal but highly fertilized with soluble P. Drought stress was imposed after 4 weeks at weekly intervals. Under unstressed conditions, leaf area, shoot dry weights, xylem pressure, and soil water potentials were similar for VAM and the two non-mycorrhizal P-fertilized treatments but each of the VAM-infected species had a greater total root length. Total P uptake was similar for the maize treatments but higher for VAM than non-mycorrhizal P-fertilized sorghum treatments. Under drought-stressed conditions, the growth parameters and soil water potential were similar for all maize treatments but they were reduced by mycorrhizal inoculation in sorghum. Greater water extraction occurred in drought-stressed mycorrhizal sorghum. In both plant species, total P uptake and P uptake per unit root length (including unstressed species) were significantly enhanced in non-mycorrhizal P-fertilized treatments compared with the mycorrhizal treatment. Except for the root dry weight of sorghum plants, there were no differences in the growth parameters and P uptake between minimally and highly P-fertilized non-mycorrhizal treatments for either maize or sorghum. The increased total root length in drought-stressed mycorrhizal sorghum plants and the similar infected root lengths in unstressed and drought-stressed sorghum plants may have caused high C partitioning to drought-stressed mycorrhizal roots and therefore caused the reduced growth parameters in mycorrhizal plants compared to the non-mycorrhizal P-fertilized counterparts. The results indicate that P fertilization in addition to mycorrhizal inoculation may improve the drought tolerance of maize and sorghum plants.     

Growth response of field-grown Siratro (Macroptilium atropurpureum Urb.) and Aeschynomene american L. to inoculation with selected vesicular-arbuscular mycorrhizal fungi

Summary The effect of inoculation with a selected isolate of Glomus etunicatum Becker and Gerdemann and one of G. intraradices Schenck and Smith on the growth and nutrient content of Macroptilium atropurpureum Urb. cv. Siratro and Aeschynomene americana L., at applied P levels of 10, 30, 60, and 120 kg ha^-1, was studied under field conditions. At all P levels and for all harvests, the shoot dry mass of Siratro and A. americana were greater for the plants inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungi than the control plants. Differences between the VAM fungus-inoculated and the control plants were most marked between 30 and 90 kg ha^-1 of applied P and diminished at 120 kg ha^-1. At the first harvest of Siratro, the plants inoculated with G. etunicatum had a greater shoot dry mass than those inoculated with G. intraradices, for all levels of applied P. However, for subsequent harvest of Siratro and for the one harvest of A. americana the response of shoot dry mass to the two VAM fungi was equivocal. Fungal inoculation gave at least a 30% saving in the amount of P fertilizer required (40 kg ha^-1) for the maximum yield. The plants inoculated with VAM fungi had a greater tissue concentration and total content of P and N than the control plants at low and intermediate levels of applied P. The percentage of root colonized by VAM fungi for the inoculated plants of the two legumes increased linearly with P additions up to 60 kg ha^-1. The conclusion is that under amended (limed and fertilized) soil conditions, inoculation with selected VAM fungi can improve the establishement and growth of forage legumes in fields that contain ineffective populations of native VAM fungi.     

Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N2 fixation (15N) inMedicago sativa at four salinity levels

The rose of an isolate of the arbuscular mycorrhizal (AM) fungusGlomus mosseae in the protection ofMedicago sativa (+Rhizobium meliloti) against salt stress induced by the addition of increasing levels of soluble salts was studied. The interactions between soluble P in soil (four levels), mycorrhizal inoculum and degree of salinity in relation to plant growth, nutrition and infective parameters were evaluated. Salt stress was induced by sequential irrigation with saline water having four concentrations of three salts (NaCl, CaCl₂, and MgCl₂).¹⁵N-labelled ammonium sulphate was added to provide a quantitative estimate of N₂ fixation under moderate to high salinity levels. N and P concentration and nodule formation increased with the amount of plant-available P or mycorrhizal inoculum in the soil and generally declined as the salinity in the solution culture increased from a moderate to a high level. The mycorrhizal inoculation protected the plants from salt stress more efficiently than any amount of plant-available P in soil, particularly at the highest salinity level applied (43.5 dS m^–1). Mycorrhizal inoculation matched the effect on dry matter and nutrition of the addition in the soil of 150 mg P kg^–1. Nevertheless the highest saline solution assayed (43.5 dS m^–1) affected more severely plants supplemented with phosphorus than those with the addition of mycorrhizal inoculum. Such a saline-depressing effect was 1.5 (biomass), 1.4 (N) and 1.5 (P) times higher in plants supplied with soluble phosphate than with AM inoculum. Mechanisms beyond those mediated by P must be involved in the AM-protectioe effect against salinity. The¹⁵N methodology used allowed the determination of N₂ fixation as influenced by different P applications compared to mycorrhizal inoculation. A lack of correlation between nodule formation and function (N₂ fixation) was evidenced in mycorrhizal-inoculated plants. In spite of the reduced activity per nodule in mycorrhizal-inoculated In spite of the reduced activity per nodule in mycorrhizal-inoculated plants, the N contents determined indicated the highest acquisition of N occurred in plants with the symbiotic status. Moreover, N and P uptake increased while Ca and Mg decreased in AM-inoculated plants. Thus P/Ca ratios and cation/anion balance in general were altered in mycorrhizal treatments. This study therefore confirms previous findings that AM-colonized plants have optional and alternative mechanisms available to satisfy their nutritive requirements and to maintain their physiological status in stress situations and in disturbed ecosystems.     

苜蓿植株及根际土壤中主要酚酸和香豆素物质含量测定

通过高效液相色谱(HPLC)法测定不同试验地间18个苜蓿(Medicago sativa L.)品种植株及根际土壤中主要酚酸和香豆素物质的含量,分析其在不同品种苜蓿植株中的分布特征及不同试验地间差异。经过ASE 350型加速溶剂萃取仪萃取苜蓿植株及根际土壤中酚酸类和香豆素物质,萃取液存放于2oC冰箱,并用0.45μm有机滤膜过滤后通过HPLC测定自毒物质含量。结果显示,18个苜蓿品种中香豆素、阿魏酸、绿原酸、咖啡酸的含量存在差异,其中香豆素、绿原酸含量显著高于阿魏酸、咖啡酸含量。各苜蓿品种间单一自毒物质含量差异显著(P0.05),香豆素、阿魏酸、绿原酸和咖啡酸的总含量差异明显。武威试验地酚酸和香豆素物质平均含量比会宁试验地低4.01%。研究表明:不同苜蓿品种间香豆素、阿魏酸、绿原酸、咖啡酸物质总含量差异显著;苜蓿植株中香豆素、阿魏酸、绿原酸、咖啡酸物质总含量与根际土壤中含量差异极显著。     

苜蓿对旱地土壤有机碳氮变化的驱动作用

根据旱地长期定位试验和分离土壤有机质中微粒有机质及矿物结合有机质的方法,研究了长期生长苜蓿对土壤有机碳、全氮变化的驱动作用。结果表明:不施肥条件下,苜蓿地土壤有机碳、全氮的含量比裸地土壤中的含量明显增加,土壤有机碳和氮库中的微粒有机碳、氮的含量没有显著增加,而增加的有机碳、氮主要分布在矿物结合有机碳、氮组分中。在施用有机肥及氮、磷化肥条件下,苜蓿地土壤有机碳和全氮的含量显著地提高,增加的有机碳、氮几乎平均地分布在微粒有机碳、氮和矿物结合有机碳、氮组分中。由此表明,保持适宜的苜蓿生产力并采用合理的施肥措施,长期生长苜蓿能够有效地驱动土壤固定有机碳和氮素,从而保持和提高土壤肥力。     

Influence of vesicular-arbuscular mycorrhizae on phosphate fertilizer efficiency in two tropical acid soils planted with micropropagated oil palm (Elaeis guineensis jacq.)

Summary The influence of vesicular-arbuscular mycorrhizae on the efficiency of triple superphosphate and rock phosphate fertilizers was compared in two tropical, acid, P-fixing soils (Ivory Coast) in which the available P was labelled with ³²PO _inf4 ^sup3- . Both soils were planted with micropropagated oil palms. The growth reponses to the fertilizer applications were low unless accompanied by VAM inoculation, but both fertilizers were equally available to plants. Isotopic-dilution kinetics analyses indicated that the rock phosphate was solubilized in both soils and there was an enrichment of the labile pool of plant-available P, similar to that with superphosphate. The specific activity and the fraction of P derived from either fertilizer was similar in both mycorrhizal and non-mycorrhizal plants, showing that both absorbed P from the same labile pool of P in the fertilized soils. However, VAM inoculation increased the fertilizer utilization coefficient of plants 2.7- to 5.6-fold, depending on the soil and fertilizer. We conclude that VAM inoculation increases fertilizer efficiency, as much of rock phosphate as of superphosphate, for plants growing in acid, P-fixing soils, and the processes involved are not different for the two fertilizers.     

Physiological enhancement of early growth of rice seedlings (Oryza sativa L.) by production of phytohormone of N2-fixing methylotrophic isolates

Three plant-growth promoting, N₂-fixing methylotrophic strains isolated from rice cultivars (Oryza sativa L.), viz, Methylobacterium sp. CBMB20, Enterobacter sp. CBMB30, Burkholderia sp. CBMB40, were selected, and their activities in promoting the early growth of rice were studied. Seeds treated with the methylotrophic strains improved seed germination, seedling vigor index (SVI), and biomass of rice seedlings. The methylotrophic population in the treated seedlings increased in the vegetative stages when compared to seeding stages. Treated seedlings showed a higher accumulation of plant hormones viz trans-zeatin riboside, isopentenyladenosine, and indole-3-acetic acid than untreated seedlings. Plant hormones were detected immunologically using the phytodetek kit. Conformational evidence suggested that cytokinins were produced by the epiphytic bacteria colonizing the plants rather than by the plants themselves. In addition, the inoculated early stage rice seedlings also exhibited a wide range of acetylene reduction activity. The results suggest the potential use of these bacteria to stimulate germination, SVI, and biomass production, which is mediated by production of plant hormone accumulation and nitrogen fixation.     

Synergistic effects of vesicular-arbuscular mycorrhizal fungi and diazotrophic bacteria on nutrition and growth of sweet potato (Ipomoea batatas)

Summary Sweet potatoes were micropropagated and then transplanted from axnic conditions to fumigated soil in pots in the greenhouse. Spores of Glomus clarum were obtained from Brachiaria decumbens or from sweet potatoes grown in soil infected with this fungus and with an enrichment culture of Acetobacter diazotrophicus. Three experiments were carried out to measure the beneficial effects of vesicular-arbuscular mycorrhizal (VAM) fungi-diazotroph interactions on growth, nutrition, and infection of sweet potato by A. diazotrophicus and other diazotrophs obtained from sweet potato roots. In two of these experiments the soils had been mixed with ¹⁵N-containing organic matter. The greatest effects of mycorrhizal inoculation were observed with co-inoculation of A. diazotrophicus and/or mixed cultures of diazotrophs containing A. diazotrophicus and Klebsiella sp. The tuber production was dependent on mycorrhization, and total N and P accumulation were increased when diazotrophs and G. clarum were applied together with VAM fungal spores. A. diazotrophicus infected aerial plant parts only when inoculated together with VAM fungi or when present within G. clarum spores. More pronounced effects on root colonization and intraradical sporulation of G. clarum were observed when A. diazotrophicus was co-inoculated. In non-fumigated soil, dual inoculation effects, however, were of lower magnitude. ¹⁵N analysis of the aerial parts and roots and tubers at the early growth stage (70 days) showed no statistical differences between treatments except for the VAM+Klebsiella sp. treatment. This indicates that the effects of A. diazotrophicus and other diazotrophs on sweet potato growth were caused by enhanced mycorrhization and, consequently, a more efficient assimilation of nutrients from the soil than by N₂ fixation. The possible interactions between these effects are discussed.     

Effect of fungicides on three vesicular-arbuscular mycorrhizal fungi associated with winter wheat (Triticum aestivum L.)

Summary We studied the effects of field application rates of three fungicides on spore germination and root infection in three species of Glomus. Bavistin (carbendazim) prevented germination of spores of G. monosporum and G. mosseae whereas Calixin (tridemorph) had no effect. Tilt Turbo (propiconazole) inhibited spore germination less than Bavistin. Spores of G. geosporum germinated in the presence of all three fungicides. Foliar applications of Bavistin alone and in a triple-spray programme significantly reduced infection of all three Glomus species in wheat roots grown in conditions of low P availability (2 mg P 1^–1). Two fungicides with triazole derivatives, Sportak and Tilt Turbo, also decreased infection and the former significantly reduced spore production of all three Glomus species. Calixin-treated plants, however, showed increased infection levels, resulting in greater yields and greater foliar-P concentrations than control plants. Milgo (ethirimol) was not detrimental to the vesicular-arbuscular mycorrhizal (VAM) symbiosis when applied as a foliar spray.