Vesicular-arbuscular Mycorrhizal Research Articles

Seasonality of vesicular-arbuscular mycorrhizae in sedges in a semi-arid tropical grassland

Abstract Vesicular-arbuscular mycorrhizal (VAM) colonization and spore numbers in the rhizosphere of Cyperus iria L. and C . rotundus L., growing in a semi-arid tropical grassland, was studied during the 1993 and 1994 monsoons. In addition, climatic and chemical properties of the soils were determined in order to investigate their influence on mycorrhizal variables. VAM fungal association in the sedges was confirmed by plant- and root-trap culture techniques. The soil nutrients exhibited seasonal variations, but were highly variable between years. Intercellular hyphae and vesicles with occasional intraradical spores characterized mycorrhizal association in sedges. Dark septate fungi also colonized roots of sedges. Temporal variations in mycorrhizal colonization and spore numbers occurred, indicating seasonality. However, the patterns of mycorrhizal colonization and spore numbers were different during both the years. The VAM fungal structures observed were intercellular hyphae and vesicles. Changes in the proportion of root length with VAM structures, total colonization levels and spore numbers were related to climatic and edaphic factors. However, the intensity of influence of climatic and soil factors on VAM tended to vary with sedge species.

Briza media L.

A loosely tufted perennial with short rhizomes from which arise vegetative shoots. Culms 15-60 (-100) cm high, slender, erect, 2-3-noded, solitary, smooth. Leaves with dull glaucous to mid-green adaxial and abaxial surfaces, hairless; blades usually 4-20 (-28) cm long, 2-4 (-7) mm wide, with minute acroscopic teeth on the margins, with a slender boat-shaped tip; ribs numerous, closely packed, less obvious on the abaxial surface; sheaths smooth, entire, soon splitting, dark reddish-brown at base; ligules up to 1.5 mm long, membranous, blunt with irregular margins. Culm leaves 3 or 4, uppermost with long sheaths. Panicles 410 (-18) cm long, lax, roughly pyramidal, with up to 20 branches, and up to 60 spikelets, sparsely divided, spreading, with scabrous hair-like pedicels 5-20 (-50) mm long and slightly thickened below the spikelets. Spikelets 4-7 mm long and wide, loosely scattered and drooping, broadly elliptic to broadly ovate, laterally compressed, generally purplish-brown, usually breaking up at maturity beneath the lemmas but many spikelets remaining undispersed on the branches and shed from the panicle intact, 4-9 (-12)-flowered. Glumes slightly unequal, deeply concave, hooded at the apex, boat-shaped, 2.5-3.5 mm long, with 3-5 veins, firmly membranous, dull-purple with white and shiny margins. Lemmas 3.7-4.1 mm, strongly cordate at the base, bluntly keeled in the lower half, closely overlapping, rounded on the back, with 7-9 veins, variegated purple and green, with hyaline margin. Paleas flat, thin, almost as long as the lemma, with two keels, narrowly winged. Lodicules 2, linear-lanceolate, acute. Grain enclosed within the papery lemma and palea, c. 2 mm long, rounded on the back, flattened on the front; mean oven-dry weight 0.53 mg. The root system is a dense mat of whitish fibrous roots. The primary roots are up to 1 mm in diameter and are largely unbranched, but there is a dense network of branching secondary roots. The root mass is fairly vertical with an average root length of c. 40 cm. The species produces pale brown rhizomes up to 13 cm long and 1.5 mm in diameter. Native. Subspecies media occurs throughout the range; subspecies elatior (Sibth. & Sm.) Rohlena occurs in south-east Europe.

Effect of two vesicular-arbuscular mycorrhizal fungi on the growth of micropropagated potato plantlets and on the extent of disease caused by Rhizoctonia solani.

Two micropropagated potato cultivars, Goldrush and LP89221, were inoculated into sowing trays with either Glomus etunicatum or G. intraradices in a greenhouse. After 2 weeks, plantlets were transplanted into pots and roots were challenged 7 days later with Rhizoctonia solani. At different times after R. solani infection, disease severity, mortality rate, root colonization levels, various growth parameters, and shoot mineral content were evaluated. In Goldrush, only inoculation with G. etunicatum led to a significant reduction in disease severity, ranging between 60.2% and 71.2%, on both shoot and crown. This decrease was not observed in LP89221. Compared with the control plantlets, inoculation of Goldrush with G. etunicatum or G. intraradices reduced significantly the mortality rate by 77% and 26%, respectively, whereas vesicular-arbuscular mycorrhizal (VAM) fungi did not significantly influence the mortality rate in LP89221. In Goldrush, inoculation with G. etunicatum significantly increased shoot fresh weight, root dry weight and the number of tubers produced per plant, whereas G. intraradices only significantly increased the number of tubers. Tuber and root fresh weights of both potato cultivars were significantly reduced by R. solani infection. However, R. solani-infected plantlets of both Goldrush and LP89221, inoculated with G. etunicatum, produced significantly greater tuber fresh weight than non-VAM plantlets. In R. solani-infected plantlets of Goldrush but not LP89221, G. etunicatum and G. intraradices increased root fresh weight by approximately 140.3% and 76.5%, respectively, compared with non-VAM plants. The potato cultivars Goldrush and LP89221 responded differently to VAM fungal inoculation and to R. solani infection in terms of shoot mineral content.

Mycorrhization of coriander (Coriandrum sativum L) to enhance the concentration and quality of essential oil

AbstractThe effect of association of two vesicular arbuscular mycorrhizal (VAM) fungi, Glomus macrocarpum and G fasciculatum, on the concentration and composition of essential oil in coriander (Coriandrum sativum) was studied. VAM inoculation increased the essential oil concentration in fruits by as much as 43%. Although significant variation in effectiveness of the two fungal species was observed, the quality of essential oil was significantly enhanced on mycorrhization. GC characterisation of essential oil showed increased concentration of geraniol and linalool in plants inoculated with G macrocarpum and G fasciculatum respectively.© 2002 Society of Chemical Industry

Network Establishment of Vesicular-Arbuscular Mycorrhizal Hyphae in the Rhizospheres between Trifoliate Orange and Some Plants.

The distributions of vesicular-arbuscular mycorrhizal (VAM) hyphae in the rhizospheres of trifoliate orange (Poncirus trifoliata Raf.) and some plants grown under greenhouse conditions were explored. Furthermore, the effect of the 25% MeOH eluates of bahiagrass (Paspalum notatum Flugge.) and millet (Pennisetum glaucum L. R. Br.) roots fractionated by flash chromatography on the network establishment of VAM hyphae was examined. Special acrylic root boxes with three compartments separated by nylon screens were used for the experiment. Seedlings of trifoliate orange were transplanted into one outer compartment in all boxes, and seedlings of bahiagrass, millet, cockscomb (Celosia cristata L. cv. Parade), radish (Raphanus sativus L. cv. Natsumino), and tomato (Lycopersicon esculentum Mill. cv. Fukuju) were transplanted into the other outer compartment. A VAM fungus, Gigaspora margarita Becker and Hall, was inoculated in the center compartment. In plots with only trifoliate orange seedlings, 25% MeOH eluates of bahiagrass root extract (BRE) and millet root extract (MRE) were applied into the other outer compartment once a week. A plot with only trifoliate orange seedlings and without BRE and MRE applications was also prepared as a control. The density of hyphae, percentage of infection, and number of spores in the bahiagrass and millet compartments were higher than those in the compartments with trifoliate orange. These parameters in the cockscomb, radish and tomato compartments, however, were markedly low as compared with the trifoliate orange compartments. Although few VAM hyphae and spores were observed in the control (no plant compartment), the density of hyphae in the compartments treated with BRE and MRE increased to around 18%, and a few spores were found in these compartments. Root exudates of trifoliate orange, bahiagrass, and millet stimulated the hyphal growth of G. margarita in vitro. Particularly, the stimulation by bahiagrass and millet was significantly greater than that of the control (agar only). But the hyphal length of cockscomb, radish, and tomato treatments was shorter than that of the control. These results suggest that under field conditions, root exudates will affect the behaviour of VAM fungi in the soil. Moreover, some compounds in BRE and MRE may act as chemotropic signals for attracting VAM hyphae.

Effect of the vesicular–arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum on the essential oil yield related characters and nutrient acquisition in the crops of different cultivars of menthol mint ( Mentha arvensis) under field conditions

The effects of inoculation with vesicular–arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum on the root colonization, growth, essential oil yield and nutrient acquisition of three cultivars of menthol mint ( Mentha arvensis); Kalka, Shivalik and Gomti, were studied under field conditions. The VAM inoculation significantly increased the root colonization, plant height, fresh herbage and dry matter yield, oil content and oil yield as compared to non-inoculated cultivars. The effect of VAM inoculation on the root colonization, growth and yield of mint was more pronounced with the cv Shivalik than the cvs Kalka and Gomati, indicating Shivalik as a highly mycorrhizal dependent genotype. VAM inoculation significantly increased the uptake of N, P and K by shoot tissues of mint, but most markedly increased the uptake of P. The VAM-inoculated mint plants depleted the available N, P and K in the rhizosphere soil as compared to non-inoculated control plants, however the extent of nutrient depletion was greater for P than N and K. We conclude that the VAM inoculation could significantly increase the root colonization, growth, essential oil yield and nutrient acquisition of mint for obtaining economic production under field conditions.

Vesicular-arbuscular mycorrhizal infection of Quercus rubra seedlings.

• The influence of vesicular-arbuscular mycorrhizal (VAM) host plants is reported on VAM infection of Quercus rubra seedlings, a predominately ectomycorrhizal species. • In a field experiment, Q. rubra seedlings were planted near Quercus montana and Acer rubrum (a VAM host) stump sprouts and near dead Quercus spp. stumps. In a subsequent glasshouse study, seedlings of Q. rubra were grown with or without VAM inoculum (±V) and with or without Sorghum bicolor (±S), a VAM host. • In the field experiment, seedlings planted near A. rubrum had greater VAM infection than other seedlings; ectomycorrhizal infection was greatest on seedlings near Q. montana. In the glasshouse experiment, VAM infection was increased in both extent (proportion of root length infected in roots of infected seedlings) and frequency (number of seedlings with infection) in the presence of S. bicolor. Seedling mass and N concentration were reduced in the+V treatment; however, these variables were not correlated with extent of VAM infection. • VAM infection of Q. rubra is greatly enhanced in the presence of VAM hosts in both field and glasshouse conditions. No evidence for positive effects of VAM infection on Q. rubra seedlings was found.

Stimulation of Growth and Nutrient Uptake by VAM Fungi in Brassica Oleracea Var. Capitata

Cabbage (Brassica oleracea, var. capitata, cv. Hercules) seedlings were inoculated with vesicular-arbuscular mycorrhizal (VAM) fungi Glomus fasciculatum, G. aggregatum, and G. mosseae. Differential efficiency in mycorrhizal colonization and the specificity of fungal symbiont to stimulate the growth and nutrient uptake of the host were observed. In addition, there was an increase in phenol, protein, reducing sugar contents, and peroxidase activity in the VAM inoculated seedlings. Since these compounds are known to confer resistance against fungal pathogens, the use of VAM as a biological control agent to protect cabbage against several root diseases is suggested.

Effects of Light Environment on Vesicular‐Arbuscular Mycorrhiza Development in Inga leiocalycina, a Tropical Wet Forest Tree1

ABSTRACTResults of a shadehouse experiment and a field study with seedlings and saplings of Inga leiocalycina Willd. (Legum‐inoseae), a canopy tree of American lowland tropical wet forests, indicated that plant light environment affects both (1) the degree of root colonization by vesicular‐arbuscular mycorrhizal (VAM) fungi and (2) the development of VAM structures in the root. Among shadehouse‐grown seedlings, VAM colonization increased with increasing light intensity, from a mean of 20 percent (21.5 cm colonized root) at a light level approximating forest understory environments to 47 percent (233.1 cm) at a light level found in centers of treefall gap environments. In contrast, VAM colonization levels were similar between gap and understory habitats among field‐collected sapling roots; however, the anatomy of the symbiosis differed between light environments. Roots of light gap saplings had significantly more intracellular fungal coils than understory sapling roots. At subsaturating levels for photosynthesis, light availability appeared to be a limiting resource for this symbiosis, restricting the extent of VAM fungal colonization and development of symbiotic structures. Despite this light‐mediated limitation, woody seedlings showed evidence of VAM fungi in 20 to 30 percent of their roots at light levels typical of the tropical wet forest ecosystem in general. I consider these results in the context of forest dynamics, and I suggest that, for woody seedlings, maintaining moderate levels of VAM in understory environments is of long‐term benefit because it enables them to respond rapidly to the increased nutrient demand imposed by their increased growth rate when canopy openings occur.ABSTRACTLos resultados de un experimento en cámara oscura y un estudio de campo con plántulas y arbolillos de Inga leiocalycina Willd. (Leguminoseae), un árbol del dosel de los bosques tropicales húmedos bajos de América, indicaron que el ambiente luminoso de la planta afecta tanto (1) el grado de colonización de la raíz por las micorrizas vesículo‐arbusculares (MVA) como (2) el desarrollo de sus estructuras en la raíz. Entre las plántulas de la cámara oscura, la colonizacion de MVA aumentó junto con la intensidad luminosa; de un promedio de 20 por ciento (21.5 cm de raíz colonizada), a un nivel de iluminación similar al del sotobosque, hasta 47 por ciento (233.1 cm), a un nivel similar al del centra de los claros de bosque. En contraste, los niveles de colonización de MVA fueron similares entre las raices de los arbolillos colectados en el campo, tanto en ambientes del sotobosque como en los claros de bosque, aunque la anatomía de la simbiosis fue diferente en los dos ambientes luminosos. Las raices de los arbolillos del claro del bosque tuvieron más ovillos micoticos que aquellas del sotobosque. A niveles subsaturados de fotosintesis, la disponibilidad de luz parece ser un recurso limitante para esta simbiosis, pues restringe la extensión de la colonización de MVA y el desarrollo de las estructuras simbióticas. A pesar de esta limitación, las plántulas leñosas muestran evidencia de MVA en 20–30 por ciento de sus raices a niveles de iluminación típicos de la mayoría de los bosques tropicales húmedos. Considerando estos resultados dentro del contexto de la dínamica forestal, puedo sugerir que mantener niveles moderados de MVA representa un beneficio a largo plazo para las plántulas leñosas del sotobosque, ya que les permite responder rápidamente a la demanda de nutrimentos que impone el incremento de su tasa de crecimiento cuando ocurren aperturas en el dosel.

Responses of sugarcane, maize, and soybean to phosphorus and vesicular-arbuscular mycorrhizal fungi

The presence of vesicular-arbuscular mycorrhizal (VAM) fungi in long-term cane-growing fields associated with yield decline led to the supposition that VAM fungi may be responsible for the poor yields. A glasshouse trial was established to test the effectiveness of a species of VAM fungi, Glomus clarum, extracted from one of these North Queensland fields on the growth of sugarcane (Saccharum interspecific hybrid), maize (Zea mays), and soybean (Glycine max) for 6 phosphorus (P) rates (0, 2.7, 8.2, 25, 74, 222 mg/kg). For maize and soybean plants that received VAM (+VAM), root colonisation was associated with enhanced P uptake, improved dry weight (DW) production, and higher index tissue-P concentrations than those without VAM (–VAM). By comparing DW responses of maize and soybean for different P rates, savings in fertiliser P of up to 160 and 213 kg/ha, respectively, were realised. Sugarcane plants were generally less responsive. Apart from a 30% DW increase with VAM when 2.7 mg P/kg was added, DW of +VAM plants was equivalent to, or worse than in the case of 222 mg P/kg, DW of –VAM plants. For all 3 host species, colonisation was least at the highest P application, presumably from excessive P within the plant tissue. Critical P concentrations for the 3 host species were below those reported elsewhere, and for soybean and sugarcane, the critical concentration for +VAM plants was lower than that of –VAM plants. There are 3 implications that arise from this study. First, VAM fungi present in cane-growing soils can promote the growth of maize and soybean, which are potential rotation crops, over a range of P levels. Second, the mycorrhizal strain taken from this site did not generally contribute to a yield decline in sugarcane plants. Third, application of P fertiliser is not necessary for sugarcane when acid-extractable P is <30 mg/kg if sufficient VAM propagules are present, or mp;lt;47 mg/kg if a mycorrhizal response is not anticipated.

Preinoculation of lettuce and onion with VA mycorrhizal fungi reduces deleterious effects of soil salinity

The hypothesis that inoculation of transplants with vesicular-arbuscular mycorrhizal (VAM) fungi before planting into saline soils alleviates salt effects on growth and yield was tested on lettuce (Lactuca sativa L.) and onion (Allium cepa L.). A second hypothesis was that fungi isolated from saline soil are more effective in counteracting salt effects than those from nonsaline soil. VAM fungi from high- and low-salt soils were trap-cultured, their propagules quantified and adjusted to a like number, and added to a pasteurized soil mix in which seedlings were grown for 3–4 weeks. Once the seedlings were colonized by VAM fungi, they were transplanted into salinized (NaCl) soil. Preinoculated lettuce transplants grown for 11 weeks in the saline soils had greater shoot mass compared with nonVAM plants at all salt levels [2 (control), 4, 8 and 12 dS m−1] tested. Leaves of VAM lettuce at the highest salt level were significantly greener (more chlorophyll) than those of the nonVAM lettuce. NonVAM onions were stunted due to P deficiency in the soil, but inoculation with VAM fungi alleviated P deficiency and salinity effects; VAM onions were significantly larger at all salt levels than nonVAM onions. In a separate experiment, addition of P to salinized soil reduced the salt stress effect on nonVAM onions but to a lesser extent than by VAM inoculation. VAM fungi from the saline soil were not more effective in reducing growth inhibition by salt than those from the nonsaline site. Colonization of roots and length of soil hyphae produced by the VAM fungi decreased with increasing soil salt concentration. Results indicate that preinoculation of transplants with VAM fungi can help alleviate deleterious effects of saline soils on crop yield.

Vesicular-Arbuscular Mycorrhiza and Plant Growth Response to Soil Amendment with Composted Grape Pomace or Its Water Extract

Composted materials with high humic and microbial content, and their water extracts, are increasingly used in the nursery industry as potting mix components or as liquid amendments for the purposes of enhancing plant growth. Common speculation is that such materials either contain beneficial microbes or stimulate those in or added to the medium, such as vesicular-arbuscular mycorrhizal (VAM) fungi, known to have growth-stimulating effects on plants. Experiments were conducted to determine if one such compost enhanced plant growth by stimulating VAM fungi or other growth-enhancing microbes, by simply providing limiting nutrients [phosphorus (P)], or a combination of the two. Highly mycorrhiza-responsive onion (Allium cepa) `White Lisbon' was used to evaluate the interactions of composted grape pomace (CGP), the VAM fungus Glomus intraradices, and preplant soil heat treatment on onion growth under P-limiting conditions. CGP and its water extract stimulated onion growth under P-limiting conditions in the absence of VAM; the extract was more effective than the granular CGP. Growth was enhanced further by addition of G. intraradices, and the extract enhanced its colonization of roots. Heat pretreatment of the soil inconsistently affected growth-enhancement by CGP or its extract. Thus, inoculating plant roots with mycorrhizal fungi in combination with this composted organic amendment or its extract was beneficial. The effect could have been due to the CGP providing a source of P to overcome the P-limiting conditions, and to the mycorrhizal fungus enhancing P uptake by its extraradical hyphae and thereby increasing nutrient-use efficiency.

Effect of manganese on endomycorrhizal sugar maple seedlings

Manganese (Mn) toxicity may play an important role in the poor survival of seedlings in declining sugar maple (Acer saccharum Marsh.) stands in northern Pennsylvania. To determine the effect of Mn on the growth of sugar maple seedlings, 1‐year‐old seedlings inoculated with vesicular‐arbuscular mycorrhizal (VAM) fungi and growing in sand‐vermiculite‐peat moss medium were irrigated for 7 weeks with nutrient solution (pH 5) containing 0.1 (control), 1, 2, 4, 8, or 16 mg L−1 Mn. Total seedling dry weight was negatively correlated with Mn, becoming significantly different than the control at 2 mg L−1 Mn. Stem and root dry weight were reduced by lower Mn levels than leaf dry weight. Manganese had no effect on the root/shoot ratio. The concentration of Mn in roots and leaves increased as the level of Mn in the nutrient solution increased, with the concentration in the leaves 2.2‐ to 3.7‐fold greater than the concentration in the roots. Except for a reduction of P in the roots, Mn had little effect on the concentration of nutrient elements in the roots or leaves. Colonization of the roots by VAM fungi was increased by Mn, with a maximum percentage at 4 mg L−1 Mn. Manganese toxicity symptoms in the leaves, small discrete chlorotic spots, began to appear at 1 mg L−1 Mn. The sensitivity of sugar maple seedlings to Mn found in this study supports the hypothesis that Mn may affect regeneration in declining sugar maple stands. However, evaluation of the effects of Mn on seedlings in native soils under field conditions will be necessary before the role of Mn in sugar maple regeneration can be understood.

Vesicular-Arbuscular Mycorrhizal Fungi Associated with Ocimum spp.

ABSTRACT Ocimum basilicum, Ocimum canum, Ocimum gratissimum, Ocimum sanctum, and Ocimum kilmandscharicum are important essential oil bearing plants of India, important in the perfumery, cosmetic and pharmaceutical industries. In screening trials at the Central Institute of Medicinal and Aromatic Plants, Lucknow, India, for symbiotic association of the Ocimum spp. with vesicular-arbuscular mycorrhizal (VAM) fungi, Glomus aggregatum, Glomus fasciculatum, Glomus geosporum, Gomus mosseae, Gigaspora spp., Sclerocystis sinuosa, and Acaulospora spp. were identified and quantified. Glomus aggregatum and Glomus fasciculatum were predominantly present and associated with all the Ocimum species. Root colonization studies revealed a maximum colonization (62.8%) in Ocimum basilicum (Indian basil) and a minimum colonization (35.2%) in Ocimum gratissimum. The application of VAM fungi may improve the growth and productivity of Ocimum species.

Assessment of VA mycorrhizal inoculum potential in relation to the establishment of cactus seedlings under mesquite nurse-trees in the Sonoran Desert

A commonly observed preferential association was quantified between mature native mesquite ( Prosopis articulata) trees and the seedlings of six cactus species ( Pachycereus pringlei, Opuntia cholla, Lophocereus schottii, Machaerocereus gummosus, Lemaireocereus thurberi, Mammilaria sp.) in a previously-disturbed area of the Sonoran Desert of Baja California, Mexico. We hypothesized that, in addition to more favorable edaphic factors, the inoculum potential of beneficial vesicular–arbuscular mycorrhizal (VAM) fungi was higher, and therefore, more favorable for cactus seedling establishment under the mesquite tree canopy (UC) compared to adjacent barren areas (BAs) away from the trees. In the greenhouse inoculum potential assays, VAM fungi were detected in onion ( Allium cepa) trap plants from all soil samples regardless of collection site, but cardon cactus ( P. pringlei) trap seedlings formed no VAM even after 6.5 months. Test soils were further used to preinoculate new onion seedlings transplanted into pots, to serve as nurse plants to inoculate adjacent cardon seedlings by vegetative transfer. After 15 months, cardon seedlings did develop slight VAM colonization, confined exclusively to the outermost cortical layers. Examination of test soils for spores or root fragments revealed very few to none, and spore production on onion trap plant roots was also sparse even though colonization was high. Analysis of UC and BA soils revealed that the water holding capacity, nutrient content, cation exchange capacity, total carbon, and total nitrogen contents of the UC soils were all higher than those of the BA soils. Since the VAM inoculum density in this study was not different between sites under and away from the mesquite tree canopy, we concluded that VAM inoculum density is not the primary factor for the establishment of cactus seedlings and that edaphic factors probably play a more important role. Our results suggest, however, that VAM inoculum potential in these hot desert soils, although relatively low, is probably maintained in the upper layers by means of hyphal fragments rather than spores.

Mineralization of nitrogen from crop residues and N recovery by maize inoculated with vesicular-arbuscular mycorrhizal fungi

Legume crop residues serve as a source of nitrogen (N) for succeeding crops in low-input production systems, and characterizing the release of this N supports efforts to develop sound economic and environmental management practices. Nitrogen mineralization of 15N-labelled field crop residues was monitored in a Greenville sandy loam during a 140-day laboratory incubation at 25°C. Residue type strongly influenced the rate of N mineralization; decomposition rate constants were 0.283, 0.083, 0.00047 and 0.0014 day-1 for alfalfa (Medicago sativa L.) stover (above-ground plant parts), alfalfa roots, maize (Zea mays L.) stover (above-ground plant parts excluding cob and kernels) and maize roots, respectively. At the end of the incubation, 50% of alfalfa stover and 25% of alfalfa root residues N were mineralized, whereas these proportions were 8% for maize stover and 12% for root residues. Mineralization of 15N from alfalfa stover residues was also monitored in a greenhouse experiment to determine N availability during the growth of maize inoculated or not with vesicular-arbuscular mycorrhizal (VAM) fungi, and to evaluate the effects of the presence of plants on the decomposition of the residues. Stover and root dry matter yields were greatest for maize inoculated with VAM fungi and grown in residue-amended soil. At the final harvest, maize grown in residue-amended soils had accumulated 44% more dry matter and 40% more N than maize grown in unamended soils. Enhanced VAM colonization of roots inoculated with a mixture of three Glomus spp. increased the residue N accumulation in maize roots at 5 weeks after silking and at the final harvest. Alfalfa stover decomposed rapidly both in the presence and absence of maize plants, but the amount of 15N mineralized at the end of the experiment was influenced by the presence of living roots; 23% of the 15N in alfalfa stover residues was mineralized in soil without plants compared to about 38% when maize plants were present. These results suggest that N mineralization is enhanced by the presence of living roots.

Effect of Citrus Juice Pomace Extracts on in vitro Hyphal Growth of Vesicular-Arbuscular Mycorrhizal Fungi and their in vivo Infections of Citrus Roots.

The effect of citrus juice pomace extracts on in vitro hyphal growth of vesicular-arbuscular mycorrhizal (VAM) fungi and the mycorrhizal formation on trifoliate orange (Poncirus trifoliata Raf.) seedlings and Citrus iyo Tanaka trees was investigated. One hundred μl [equivalent to 6.5 g dry weight (DW) of satsuma mandarin (Citrus unshiu Marc.) and 4.3 gDW of Citrus hassaku Tanaka] of the 25% MeOH eluates of citrus juice pomace extracts significantly increased in vitro hyphal growth of Gigaspora ramisporophora Spain, but a 100% MeOH eluates inhibited it. In a plot where the solution consisting of 25% MeOH eluates of satsuma mandarin juice pomace extracts (equivalent to 10 gDW) and liquid fertilizers was applied to the soil inoculated with the spores of Gigaspora ramisporophora, the percentage of VAM infection in trifoliate orange roots was six times higher than that in the control (water only) after one week. When the above mixed solution was administered to a Citrus iyo orchard via a sprinkler irrigation system, VAM development was markedly stimulated as compared to the untreated plots (liquid fertilizer only) and the control (solid fertilizer only). These results show that 25% MeOH eluates of citrus juice pomace extracts contain stimulatory substances for VAM fungi. Furthermore, the application of the 25% MeOH eluates to the soil is very effective in stimulating VAM development.

Influence of Inoculation with Vesicular-Arbuscular Mycorrhizae on Posttransplant Growth of Prairie Forb Seedlings

The objectives of this study were to compare the growth of prairie forb seedlings inoculated with vesicular-arbuscular mycorrhizal (VAM) fungi to noninoculated seedlings transplanted to a highway right-of-way and to evaluate the effect of different VAM fungal species or combinations on posttransplant seedling growth. Four species of prairie forbs: pale-purple coneflower (Echinacea pallida Nutt.), prairie blazingstar (Liatris pycnostachya Michx.), prairie phlox (Phlox pilosa L.), and gray-headed coneflower [Ratibida pinnata (Venten.) Barnh.], were grown in greenhouse mix and inoculated with Gigaspora margarita Becker and Hall, or Glomus interadicies Schenk and Smith, or with a native Indiana prairie soil inoculum, or with a mix of all three. They were transplanted to a highway site in June, 1994. Only gray-headed coneflower exhibited a positive growth response to VAM inoculation. Inoculation of gray-headed coneflower with G. margarita produced the largest growth response by the end of the experiment.

Distribution of Vesicular-Arbuscular Mycorrhizal Hyphae in the Rhizospheres of Trifoliate Orange and Bahia Grass Seedlings under an Intercropping System.

An experiment was conducted under green house conditions to observe the distribution of vesicular-arbuscular mycorrhizal (VAM) hyphae between plants. Special acrylic root boxes with three compartments were constructed. The two outer compartments, into which the trifoliate orange (Poncirus trifoliata Raf.) seedlings and Bahia grass (Paspalum notatum Flugge.) were transplanted, were separated from the center compartment by a barrier made of a nylon screen of 37μm mesh. The inoculation with a VAM fungus, Gigaspora margarita Becker and Hall, was done in the center compartment. The density of hyphae, percentage of infection, and the number of spores were higher in the Bahia grass compartment than those in the compartment with trifoliate orange. A petri dish experiment showed that although the Bahia grass and trifoliate orange root exudates stimulated the hyphal growth of G. margarita, the length of hyphae in the Bahia grass treatment was longer than that of the trifoliate orange. In this experiment it was possible to observe in detail the distribution of hyphae in the rhizosphere of two plants under intercropping system conditions. These hyphae would be related to the interconnection between plants to promote a network system ; the formation and development of this network system is influenced by some root exudates.

Growth Stimulants for Vesicular-Arbuscular Mycorrhizal Fungi in Satsuma Mandarin Pomace.

The isolation and identification of growth stimulants for vesicular-arbuscular mycorrhizal (VAM) fungi in the 25% methanol (MeOH) extracts of satsuma mandarin (Citrus unshiu Marc.) pomace extracts were carried out. Three compounds were isolated from the eluates by using a high pressure liquid chromatograph (HPLC). When the effect of the three compounds was tested on the in vitro hyphal growth of a VAM fungus, Gigaspora margarita, two compounds had a stimulatory effect. The two stimulants were identified as hesperidin (hesperetin-7-β-rutinoside) and narirutin (naringenin-7-β-rutinoside). Narirutin at 1 to 30 ppm and hesperidin at 1 to 10 ppm significantly stimulated the hyphal growth of G. margarita.