Improved zinc tolerance in Eucalyptus globulus inoculated with Glomus deserticola and Trametes versicolor or Coriolopsis rigida

The potential of interactions between saprophytic and arbuscular mycorrhizal (AM) fungi to improve Eucalyptus globulus grown in soil contaminated with Zn were investigated. The presence of 100 mg kg ⁻¹ Zn decreased the shoot and root dry weight of E. globulus colonized with Glomus deserticola less than in plants not colonized with AM. Zn also decreased the extent of root length colonization by AM and the AM fungus metabolic activity, measured as succinate dehydrogenase (SDH) activity of the fungal mycelium inside the E. globulus root. The saprophytic fungi Trametes versicolor and Coriolopsis rigida increased the shoot dry weight and the tolerance of E. globulus to Zn when these plants were AM-colonized. Both saprophytic fungi increased the percentage of AM root length colonization and elevated G. deserticola SDH activity in the presence of all Zn concentrations applied to the soil. In the presence of 500 and 1000 mg kg⁻¹ Zn, there were higher metal concentrations in roots and shoots of AM than in non-AM plants; furthermore, both saprophytic fungi increased Zn uptake by E. globulus colonized by G. deserticola. The higher root to shoot metal ratio observed in mycorrhizal E. globulus plants indicates that G. deserticola enhanced Zn uptake and accumulation in the root system, playing a filtering/sequestering role in the presence of Zn. However, saprophytic fungi did not increase the root to shoot Zn ratio in mycorrhizal E. globulus plants. The effect of the saprophytic fungi on the tolerance and the accumulation of Zn in E. globulus was mediated by its effect on the colonization and metabolic activity of the AM fungi.     

Hydrolytic enzyme activities in maize (Zea mays) and sorghum (Sorghum bicolor) roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices

Two strains of Gluconacetobacter diazotrophicus (Pal 5, UAP5541) and the arbuscular mycorrhizal fungus Glomus intraradices increased both the shoot and root dry weight of sorghum 45 days after inoculation, whereas they had no effect on the shoot and root dry weight of maize. Co-inoculation (Gluconacetobacter diazotrophicus plus Glomus mosseae) did not increase the shoot and root dry weight of either plant. There was a synergistic effect of Gluconacetobacter diazotrophicus on root colonization of maize by Glomus intraradices, whereas an antagonistic interaction was observed in the sorghum root where the number of Gluconacetobacter diazotrophicus and the colonization by Glomus intraradices were reduced. Plant roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices, either separately or together, significantly increased root endoglucanase, endopolymethylgalacturonase and endoxyloglucanase activities. The increase varied according to the plant. For example, in comparison with non-inoculated plants, there were higher endoglucanase (+328%), endopolymethylgalacturonase (+180%) and endoxyloglucanase (+125%) activities in 45-day old co-inoculated maize, but not in 45-day old sorghum. The possibility is discussed that hydrolytic enzyme activities were increased as a result of inoculation with Gluconacetobacter diazotrophicus, considering this to be one of the mechanisms by which these bacteria may increase root colonization by AM fungi.     

The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae

Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.     

Accumulation of specific flavonoids in soybean (Glycine max (L.) Merr.) as a function of the early tripartite symbiosis with arbuscular mycorrhizal fungi and Bradyrhizobium japonicum (Kirchner) Jordan

This study is the first report assessing the effect of soil inoculation on the signalling interaction of Bradyrhizobium japonicum, arbuscular mycorrhizal fungi (AMF) and soybean plants throughout the early stages of colonisation that lead to the tripartite symbiosis. In a study using soil disturbance to produce contrasting indigenous AMF treatments, the flavonoids daidzein, genistein and coumestrol were identified as possible signals for regulating the establishment of the tripartite symbiosis. However, it was unclear whether soil disturbance induced changes in flavonoid root accumulation other than through changing the potential for AMF colonization. In this study, soil treatments comprising all possible combinations of AMF and B. japonicum were established to test whether (1) modifications in root flavonoid accumulation depend on the potential for AMF colonization, and (2) synthesis and accumulation of flavonoids in the roots change over time as a function of the early plant-microbial interactions that lead to the tripartite symbiosis. The study was comprised of two phases. First, maize was grown over 3-week periods to promote the development of the AM fungus Glomus clarum. Second, the interaction between soybean, G. clarum and B. japonicum was evaluated at 6, 10, 14 and 40 days after plant emergence. Root colonization by G. clarum had a positive effect on nodulation 14 days after emergence, producing, 30% more nodules which were 40% heavier than those on roots solely inoculated with B. japonicum. The tripartite symbiosis resulted in 23% more N₂ being fixed than did the simpler symbiosis between soybean and B. japonicum. The presence of both symbionts changed accumulation of flavonoids in roots. Daidzein and coumestrol increased with plant growth. However, development of the tripartite symbiosis caused a decrease in coumestrol; accumulation of daidzein, the most abundant flavonoid, was reduced in the presence of AMF.     

Soil temperature,mycorrhizal infection and nodulation of Medicago truncatula and Trifolium subterraneum

Establishment of vesicular-arbuscular mycorrhizal fungi in plant roots involves a pre-infection phase of propagule germination, hyphal growth and appressorium formation, followed by growth of the fungus within the root. The effect of soil temperature on the pre-infection stage was examined by counting the numbers of fungal “entry-points” on the main roots of Medicago truncatula and Trifolium subterraneum, grown at soil temperatures of 12°, 16°, 20° and 25°C for periods up to 12 days. Increased root temperature was positively associated with increased numbers of “entry-points”. This effect was more marked between 12° and 16°C than at higher temperatures, as shown by comparing plants at the same stage of development (emergence of spade leaf) and by calculating the results as entry points per cm root.The first root nodules appeared sooner at higher temperatures (20° and 25°), but subsequent development of nodules (measured as nodule number and aggregate volume of nodules per plant, up to 21 days) was best at 16°C for both host Rhizobium combinations in non-sterile and autoclaved soil. There was no evidence that competition between mycorrhizal fungi and Rhizobium for infection sites occurred.A method of obtaining numbers of infective propagules of vesicular-arbuscular mycorrhizal fungi in soil is described.     

Colonisation of seminal roots of wheat and barley by egg-parasitic nematophagous fungi and their effects on Gaeumannomyces graminis var. tritici and development of root-rot

This study provides evidence that egg-parasitic nematophagous fungi, Pochonia chlamydosporia, Pochonia rubescens and Lecanicillium lecanii, can also reduce root colonisation and root damage by a fungal pathogen. Interactions of nematophagous fungi with the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt), and their influence on severity of the root disease it causes were studied in laboratory and pot experiments. In Petri dish experiments the three nematophagous fungi reduced colonisation of barley roots by Ggt and also reduced necrotic symptoms. On the contrary, root colonisation by nematophagous fungi was unaffected by Ggt. In growth tube experiments, the three nematophagous fungi again reduced Ggt root colonisation and increased effective root length of barley seedlings. This was true for both simultaneous and sequential inoculation of nematophagous fungi versus Ggt. In the pot experiments the inoculum of the tested fungi in soil was applied in the same pot, as a mixture or in layers, or in coupled pots used for wheat grown with a split-root system. The nematophagous fungi P. chlamydosporia (isolate 4624) and L. lecanii (isolate 4629), mixed with Ggt or in split root systems with the pathogen, promoted growth of wheat (i.e. increased shoot weight), although no disease reduction was found. In split root systems, lower levels of peroxidase activity were found in seedlings inoculated with Ggt in combination with the nematophagous isolates 4624 and 4629 than when the take-all fungus was applied alone.Our results show that nematophagous fungi reduce root colonisation by Ggt, root damage and stress induced senescence in Ggt-inoculated plants.     

The impact of arbuscular mycorrhizal fungi on strawberry tolerance to root damage and drought stress

Individually, arbuscular mycorrhizal fungi (AMF), drought stress, and root damage can alter terrestrial plant performance but the joint effects of these three factors have not been explored. Because AMF can improve water relations, colonization by these root symbionts may increase the host’s tolerance of drought especially when roots have been compromised by herbivory. This full factorial study examined effects of AMF, water deficit, and artificial root herbivory in three genotypes of wild strawberry, Fragaria virginiana Duchesne that originated from the same restored tallgrass prairie as the AMF inoculum. Drought stress and root damage altered allocation to roots vs. shoots but the effects were not additive and the interaction did not depend on AMF treatment. Effects of AMF were absent with one exception: root damage significantly reduced belowground mass only in plants inoculated with AMF. Although drought stress did not interact with the AMF treatment, both drought stress and root damage reduced the abundance of arbuscules, and especially vesicles, and colonization varied among genotypes. Failure to detect strong effects of AMF on host growth could be due to variable responses of individual AMF species summing to no net effects. Functionally, AMF were primarily commensals of strawberry in this study.     

Effects of nutrient supply and below-ground herbivory by Diaprepes abbreviatus L. (Coleoptera: Curculionidae) on citrus growth and mineral content

Three-year-old citrus trees were grown in the greenhouse to study the effects of fertilizer concentration and root herbivory on plant growth and mineral concentration. In separate experiments, sour orange (Citrus aurantium L.) and Swingle citrumelo (C. paradisi Macf. × Poncirus trifoliate L.) plants were treated with a complete fertilizer diluted to provide 25, 100, 200, or 400 ppm N and grown for 7 weeks with or without Diaprepes abbreviatus L. larvae. Increased fertilizer concentration increased the shoot mass and the shoot:root ratio of both sour orange and Swingle citrumelo. Root herbivory also increased the shoot:root ratio by depressing root growth more than shoot growth. Effects of root herbivory on growth were consistent across the four levels of fertilizer concentration, indicating that tolerance is not a function of nutrient status. For both rootstocks, concentrations of nitrogen in roots and leaves increased with fertilizer concentration, and C:N ratios decreased. In sour orange, root herbivory most strongly affected the concentration of carbon in roots, whereas in Swingle citrumelo, root herbivory most strongly affected leaf nitrogen. In general, herbivory reduced mineral concentrations of roots but the strength, and sometimes the direction, of herbivore effects varied significantly among fertilizer treatments. This research indicates that application of excess, balanced fertilizer is unlikely to offset growth reductions due to root herbivory by D. abbreviatus, and suggests that supplementation of specific nutrients may be of value.     

Effects of soil application of olive mill wastewaters on the structure and function of the community of arbuscular mycorrhizal fungi

Recycling of olive mill wastewaters (OMW) into agricultural soils is a controversial issue since benefits to soil fertility should counterbalance potential short-term toxicity effects. We investigated the short-term effects of OMW on the soil-plant system, regarding the diversity, structure and root colonization capacity of arbuscular mycorrhizal (AM) fungi and the respective growth response of Vicia faba L, commonly used as green manure in olive-tree plantations. A compartmentalized pot system was used that allowed the establishment of an AM fungal community in one compartment (feeder) and the application of three OMW dose levels in an adjacent second compartment (receiver). At 0, 10, and 30 days after OMW treatment (DAT), V. faba pre-germinated seeds were seeded in the receiver compartment. At harvest, shoot and root dry weights, AM fungal root colonization, soil hyphal length and P availability were recorded in the receiver compartment. In addition, OMW effects on AM fungal diversity in plant roots were studied by DGGE. A transient effect of OMW application was observed; plant growth and AM fungal colonization were initially inhibited, whereas soil hyphal length was stimulated, but in most cases differences were absent when seeding was performed 30 DAT. Similarly, changes induced in the structure of the root AM fungal community were of transient nature. Cloning and sequencing of all the major DGGE bands showed that roots were colonized by Glomus spp. The transient effects of OMW on the structure and function of AM fungi could be attributed to OMW-derived phytoxicity to V. faba plants or to an indirect effect via alteration of soil nutritional status. The high OMW dose significantly increased soil P availability in the presence of AM fungi, suggesting efficient involvement of AM fungi in organic-P minerilization. Overall our results indicate that soil application of OMW would cause transient changes in the AM fungal colonization of V. faba plants, which, would not impair their long-term plant growth promoting ability.     

Fraxinus-Glomus-Pisolithus symbiosis: Plant growth and soil aggregation effects

It has been established that arbuscular mycorrhizal (AM) fungi are involved in the conservation of soil structure. However, the effect of ectomycorrhizal (EM) fungi alone or in interaction with AM fungi in soil structure has been much less studied. This experiment evaluated EM and AM fungi effects on soil aggregation and plant growth. Ash plants (Fraxinus uhdei) were grown in pots, and were inoculated with Glomus intraradices and Pisolithus tinctorius separately but also in combination. Our results showed that F. uhdei established a symbiotic association with EM and AM fungi, and that these organisms, when interacting, showed synergistic and additive effects on plant growth compared to singly inoculated treatments. EM and AM fungi prompted changes in root morphology and increased water-stable aggregates. AM fungi affect mainly small-sized macroaggregates, while EM and EM-AM fungi interaction mainly affected aggregates bigger than 0.5 mm diameter. These results suggest that ectomyccorrhizal as well as arbuscular mycorrhizal fungi should be considered in restoration programs with Fraxinus plants.     

Glycosidation of apigenin results in a loss of its activity on different growth parameters of arbuscular mycorrhizal fungi from the genus Glomus and Gigaspora

The effect of different concentrations (0.5, 2 and 8 μM) of apigenin and its glycosidated form 5,7,4′-hydroxy flavone glycoside on arbuscular mycorrhizal (AM) fungal spore germination, hyphal growth, hyphal branching, the formation of entry points and root colonization of Gigaspora. rosea, Gi. margarita, Glomus mosseae and G. intraradices was tested. The lowest apigenin concentration (0.5 μM) nearly doubled hyphal branching, the formation of entry points and root colonization of all four tested fungi, whereas higher concentrations (2 and 8 μM) nearly doubled the hyphal growth of Gi. margarita, G. mosseae and G. intraradices. In none of the treatments with the apigenin-glycoside any effect on AM fungi could be observed. Our data show that apigenin exhibits an AM fungal genus and even species activity and we provide strong evidence that glycosidation results in a loss of its activity towards AM fungi.     

Grazing by nematodes on rhizosphere bacteria enhances nitrate and phosphorus availability to Pinus pinaster seedlings

The microbial loop is thought to play a major role in the mineralization of nutrients such as nitrogen (N) and phosphorus (P) in terrestrial ecosystems. This microbial loop is based on the grazing of bacteria by predators such as bacterial-feeding nematodes. However, little is known about the impact of grazing by nematodes on the mineral nutrition of woody plants. This study was undertaken to quantify the effect of nematode grazing on bacteria in the rhizosphere on the root architecture, growth and mineral nutrition (N and P) of a woody species (Pinus pinaster). Young P. pinaster seedlings were cultivated for 35 days in a simplified sterile experimental system with bacteria (Bacillus subtilis) and bacterivorous nematodes (Rhabditis sp.) isolated from soil samples collected from a 15-year old stand of maritime pine. To check the hypothesis that bacteria could be a source of nutrients, especially N, two N sources were supplied in the medium: (i) bacterial N labeled with ¹⁵N and (ii) nitrate. Phosphorus was supplied as insoluble inorganic tri-calcium phosphate (TCP). The results showed that the ¹⁵N flow from the bacteria to the plant shoots was only significant when nematodes were present, with an average accumulation of 14 ± 5 μg plant⁻¹ of ¹⁵N. Plants cultivated with nematodes also accumulated significantly more total N in their shoots than sterile ones or inoculated with bacteria, resulting in a net average increase in N of 700 μg plant⁻¹. The same result was observed for the total P accumulation in the shoots, as plants with nematodes accumulated an average of 300 μg plant⁻¹ more P than sterile ones or inoculated with bacteria. However, the presence of bacteria, whether alone or with nematodes, did not modify the root architecture. These results demonstrated that the presence of bacterial-feeding nematodes significantly enhanced N and P availability to P. pinaster seedlings, probably by improving plant use of nitrate and insoluble P supplied in the medium.     

Organic acid exudation by mycorrhizal Andropogon virginicus L. (broomsedge) roots in response to aluminum

Elevated aluminum (Al) availability limits plant growth on acidic soils. Although this element is found naturally in soils, acidic conditions create an environment where Al solubility increases and toxic forms of Al impact plant function. Plant resistance to Al is often attributed to organic acid exudation from plant roots and the chelation of cationic Al in the rhizosphere. The association of arbuscular mycorrhizal (AM) fungi with the roots of plants may alleviate Al toxicity by altering soil Al availability or plant exposure through the binding of Al to fungal structures or through the influence of fungi on exudation from roots. Diverse communities of AM fungi are found in soil ecosystems and research suggests that AM fungi exhibit functional diversity that may influence plant performance under varying edaphic environments. In the present study, we evaluated acidic isolates of six AM species in their responses to Al. Andropogon virginicus (broomsedge), a warm-season grass that commonly grows in a range of stressful environments including acidic soils, was used as a plant host for Acaulospora morrowiae, Glomus claroideum, Glomus clarum, Glomus etunicatum, Paraglomus brasilianum, and Scutellospora heterogama. Fungal spores were germinated and exposed to 0 or 100 μM Al on filter paper in sand culture or were grown and exposed to Al in sand culture in association with A. virginicus. Short- and long-term responses to Al were evaluated using direct measurements of fungal spore germination, hyphal elongation, and measurements of A. virginicus colonization and plant growth as a phytometer of AM function in symbio. Spore germination and hyphal elongation varied among AM species in response to Al, but patterns were not consistent with the influences of these AM species on A. virginicus under Al exposure. Exposure to Al did not influence colonization of roots, although large differences existed in colonization among fungal species. Plants colonized by G. clarum and S. heterogama exhibited the least reduction in growth when exposed to Al, produced the highest concentrations of Al-chelating organic acids, and had the lowest concentrations of free Al in their root zones. This pattern provides evidence that variation among AM fungi in Al resistance conferred to their plant hosts is associated with the exudation of Al-binding organic acids from roots and highlights the role that AM fungal diversity may play in plant performance in acidic soil environments.     

Herbivory of an invasive slug in a model grassland community can be affected by earthworms and mycorrhizal fungi

Invasion of non-native species is among the top threats for the biodiversity and functioning of native and agricultural ecosystems worldwide. We investigated whether the herbivory of the slug Arion vulgaris (formerly Arion lusitanicus; Gastropoda), that is listed among the 100 worst alien species in Europe, is affected by soil organisms commonly present in terrestrial ecosystems (i.e. earthworms—Annelida: Lumbricidae and arbuscular mycorrhizal fungi—AMF, Glomerales). We hypothesized that slug herbivory would be affected by soil organisms via altered plant nutrient availability and plant quality. In a greenhouse experiment, we created a simple plant community consisting of a grass, a forb, and a legume species and inoculated these systems with either two earthworm species and/or four AMF taxa. Slugs were introduced after plants were established. Earthworms significantly reduced total slug herbivory in AMF-inoculated plant communities (P?=?0.013). Across plant species, earthworms increased leaf total N and secondary metabolites, AMF decreased leaf thickness. Mycorrhizae induced a shift in slug feeding preference from non-legumes to legumes; the grass was generally avoided by slugs. AMF effects on legume herbivory can partly be explained by the AMF-induced increase in total N and decrease in C/N ratio; earthworm effects are less clear as no worm-induced alterations of legume plant chemistry were observed. The presence of earthworms increased average AMF colonization of plant roots by 140 % (P?<?0.001). Total shoot mass was significantly increased by AMF (P?<?0.001). These data suggest that the feeding behavior of this invasive slug is altered by a belowground control of plant chemical quality and community structure.     

The mycorrhizal fungus Glomus intraradices and rock phosphate amendment influence plant growth and microbial activity in the rhizosphere of Acacia holosericea

Plants inoculated with arbuscular mycorrhizal (AM) fungi utilize more soluble phosphorus from soil mineral phosphate than non-inoculated plants. However, there is no information on the response of soil microflora to mineral phosphate weathering by AM fungi and, in particular, on the catabolic diversity of soil microbial communities.The AM fungus, Glomus intraradices was examined for (i) its effect on the growth of Acacia holosericea, (ii) plant-available phosphate and (iii) soil microbial activity with and without added rock phosphate.After 4-months culture, AM fungal inoculation significantly increased the plant biomasses (by 1.78× and 2.23× for shoot and root biomasses, respectively), while mineral phosphate amendment had no effect in a sterilized soil. After 12-months culture, the biomasses of A. holosericea plants growing in a non-sterilized soil amended with mineral phosphate were significantly higher than those recorded in the control treatment (by 2.5× and 5× for shoot and root biomasses, respectively). The fungal inoculation also significantly stimulated plant growth, which was significantly higher than that measured in the mineral phosphate treatment. When G. intraradices and mineral phosphate were added together to the soil, shoot growth were significantly stimulated over the single treatments (inoculation or amendment) (1.45×). The P leaf mineral content was also higher in the G. intraradices+mineral phosphate treatment than in G. intraradices or rock phosphate amendment. Moreover, the number of fluorescent pseudomonads has been significantly increased when G. intraradices and/or mineral phosphate were added to the soil. By using a specific type of multivariate analysis (co-inertia analysis), it has been shown that plant growth was positively correlated to the metabolization of ketoglutaric acid, and negatively linked to the metabolisation of phenylalanine and other substrates, which shows that microbial activity is also affected.G. intraradices inoculation is highly beneficial to the growth of A. holosericea plants in controlled conditions. This AM symbiosis optimises the P solubilization from the mineral phosphate and affects microbial activity in the hyphosphere of A. holosericea plants.     

Mycorrhizal alleviation of acid soil stress in the sweet potato (Ipomoea batatas)

It is not known why sweet potato (Ipomoea batatas) cultivated in tropical regions tolerates acid soil. Here, we report the involvement of mycorrhizal symbiosis in this tolerance. Plants were grown in root-boxes filled with either acidic soil (pH 4.2) or the same soil amended with lime (pH 5.2) for 30 d in a growth chamber. In the inoculated treatments, the percentage of root length colonized by Gigaspora margarita was not affected by soil pH (23±9% at pH 4.2 vs. 30±12% at pH 5.2). The root and shoot dry weights of the non-mycorrhizal plants at pH 4.2 were 27 and 35%, respectively, of those at pH 5.2. The root and shoot dry weights of the mycorrhizal plants at pH 4.2 were 70 and 51% of those at pH 5.2. Growth promotion in mycorrhizal plants was significant only at pH 4.2 (2-fold increase in whole plant dry weight), but not at pH 5.2. As a result, no significant difference was detected in whole plant dry weight between the mycorrhizal plants at pH 4.2 and non-mycorrhizal plants at pH 5.2. The mycorrhizal plants at pH 4.2 showed reduced toxic symptoms of Mn (brown specks on mature leaves) and Al (poor root growth) compared to non-mycorrhizal ones, but tissue concentrations of P, K and Ca did not increase in mycorrhizal plants. We assume that the mycorrhizal colonization can reduce toxic effects of those elements while the exact mechanisms should be further investigated.     

The influence of arbuscular mycorrhizal colonization and environment on root development in soil

The production of fine roots is one of the principal means by which carbon, fixed during photosynthesis, enters the soil, and quantifying the production for particular combinations of environmental and biotic factors is important for predicting the sequestration of carbon in the soils of grassland ecosystems. Arbuscular mycorrhizal fungi (AMF) can have a major effect on the production of roots, and we studied how colonization by AMF affects the lifespan of roots. Twenty per cent of control roots of Trifolium repens survived for longer than 42 days whereas 37% survived that long in AMF‐colonized plants. The overall survival of the roots of Lolium perenne was less than in T. repens: around 10% of roots survived beyond 42 days and this was not affected by AMF colonization. Previous studies have shown that lifespans of roots can be affected by temperature. We tested the hypothesis that these observations are linked to a change in the morphology of the root system caused by temperature and also by AMF. We found that inoculation with AMF in a microcosm study using Plantago lanceolata grown at various temperatures, with and without AMF, showed no clear effect of AMF on branching patterns. Temperature had a significant effect on total lengths, numbers and branching rates of some higher orders of roots. Total lengths of both secondary and tertiary roots grown at 27°C were about double those of plants grown at 15°C. Colonization by AMF tended to reduce this effect. Evidently the effect of colonization by AMF on root lifespan depends on the species. Increased branching, and thus a greater proportion of ephemeral roots, was responsible for shortening the lives of the roots at increased temperature, which suggests a strong link between lifespan and morphology.     

Belowground responses by AM fungi and animal trophic groups to repeated defoliation in an experimental grassland community

We tested a hypothesis that the effects of defoliation on plants and soil organisms vary with the number of successive defoliations. We established a 23-week greenhouse experiment using replicated grassland microcosms that were composed of three plant species, Trifolium repens, Plantago lanceolata and Phleum pratense, growing together in grassland soil with a diverse soil community. The experiment consisted of two treatment factors-defoliation and harvest time-in a fully factorial design. The defoliation treatment had two levels, i.e. no trimming and trimming of plants every 2 weeks, and the harvest time five levels, i.e. harvests after 1-3, 5 and 7 trimmings. Shoot production (trimmed plus harvested shoot mass), harvested shoot and root mass and root N and C concentrations increased with time but were reduced by defoliation. Colonization rates of arbuscular mycorrhizal (AM) fungi decreased with time in T. repens roots but were enhanced by defoliation, whereas AM colonization rates in P. pratense roots were not affected by harvest time or defoliation. The abundance of bacterivorous and fungivorous nematodes decreased and that of herbivorous and predatory nematodes increased with time, while the abundance of omnivorous nematodes and detritivorous enchytraeids varied in time without a linear trend. Defoliation had no effect on fungivores and predators but increased the abundance of bacterivores. Defoliation also increased the abundance of herbivores, omnivores and detritivores after 2 trimmings and that of omnivores and detritivores after 5 trimmings, but had a negative effect on omnivores after 3 trimmings and on herbivores after 7 trimmings. Among nematode genera, some deviation from the trophic group responses existed: for instance, defoliation reduced the abundance of bacterivorous Acrobeloides spp. and did not affect the abundance of herbivorous Filenchus spp. and Paratylenchus spp. Our results show that the effects of defoliation on plants, AM fungi and some soil animal trophic groups may remain constant all the way through several defoliations, whereas other animal trophic groups may have different and even opposite responses to defoliation depending on the length of the defoliation period before monitoring. This shows how separate studies with defoliation periods of different length can produce contradictory results of the effects of defoliation on the abundance of soil animals.     

Defensive strategies of soil fungi to prevent grazing by Folsomia candida (Collembola)

Fungi represent a major part of the living biomass in the upper soil horizon and serve as an important food source for many soil organisms. We hypothesized that certain mycelial characteristics may serve to protect fungi from grazing. Specifically, this study focused on the influence of poisonous or other repellent metabolites and crystalline structures at the hyphal surface on the feeding preference of the soil microarthropod Folsomia candida Willem. The formation of crystalline structures was studied microscopically and the content of certain metabolites such as amanitin and muscarin was investigated using analytical methods. The feeding preference of F. candida was studied in different in-vitro food choice experiments. Additionally, the palatability of the fungal isolates was estimated by the amount of egg clusters laid by F. candida and by analysing the carbon and nitrogen content of the mycelia. F. candida was repelled by fungal species with toxic metabolites or crystals on their hyphal surface, which indicates that these traits serve as feeding protection. F. candida preferred dark-pigmented fungi. Total number of egg clusters and feeding preference were not correlated. However, insects that fed on fungi without repellent characteristics laid the most eggs. The amount of carbon and nitrogen in the mycelium had no influence on feeding behaviour. We conclude that the content of repellent metabolites and crystalline structures at the hyphal surface are defensive strategies of soil fungi and strongly influence feeding preference of F. candida. Other traits such as palatability were less important. Our results help to explain collembolan feeding behaviour and interactions between soil fungi and Collembola.     

Mycorrhizal colonization and nitrogen uptake by maize: combined effect of tropical earthworms and velvetbean mulch

Earthworms and mulch can have positive or negative effects on mycorrhizae (fungus-roots) and N uptake by plants. In the present experiment, maize plants were grown under greenhouse conditions with or without tropical earthworms (Balanteodrilus pearsei) and mulch of velvetbean (Mucuna pruriens var. utilis). The formation of vesicles and hyphae of arbuscular-mycorrhizal (AM) fungi in roots and N uptake by maize plants was measured at harvest. The addition of earthworms and velvetbean reduced AM root colonization. Earthworms had no effect on plant root or shoot biomass. In the absence of velvetbean, earthworms reduced AM colonization, but when velvetbean was present, this effect disappeared. The addition of velvetbean mulch, on the other hand, had an effect on plant biomass (above- and belowground) and a positive effect on AM fungal colonization of roots in presence of worms, but a negative effect when worms were absent. When both M. pruriens and B. pearsei were added, shoot and root biomass and N concentrations increased. Vesicle formation was related to velvetbean mulch decomposition as well as the higher N concentration in maize roots. Management of mulch–earthworm interactions may be of value, particularly in low-input and organic agricultural systems, and deserves further investigation.