Different native arbuscular mycorrhizal fungi influence the coexistence of two plant species in a highly alkaline anthropogenic sediment

Different species of arbuscular mycorrhizal fungi (AMF) can produce different amounts of extraradical mycelium (ERM) with differing architectures. They also have different efficiencies in gathering phosphate from the soil. These differences in phosphate uptake and ERM length or architecture may contribute to differential growth responses of plants and this may be an important contributor to plant species coexistence. The effects of the development of the ERM of AMF on the coexistence of two co-occurring plant species were investigated in root-free hyphal chambers in a rhizobox experimental unit. The dominant shrub (Salix atrocinerea Brot.) and herbaceous (Conyza bilbaoana J. Rémy) plant species found in a highly alkaline anthropogenic sediment were studied in symbiosis with four native AMF species (Glomus intraradices BEG163, Glomus mosseae BEG198, Glomus geosporum BEG199 and Glomus claroideum BEG210) that were the most abundant members of the AMF community found in the sediment. Different AMF species did not influence total plant productivity (sum of the biomass of C. bilbaoana and S. atrocinerea), but had a great impact on the individual biomass of each plant species. The AMF species with greater extracted ERM lengths (G. mosseae BEG198, G. claroideum BEG210 and the four mixed AMF) preferentially benefited the plant species with a high mycorrhizal dependency (C. bilbaoana), while the AMF species with the smallest ERM length (G. geosporum BEG199) benefited the plant species with a low mycorrhizal dependency (S. atrocinerea). Seed production of C. bilbaoana was only observed in plants inoculated with G. mosseae BEG198, G. claroideum BEG210 or the mixture of the four AMF. Our results show that AMF play an important role in the reproduction of C. bilbaoana coexisting with S. atrocinerea in the alkaline sediment and have the potential to stimulate or completely inhibit seed production. The community composition of native AMF and the length of the mycelium they produce spreading from roots into the surrounding soil can be determinant of the coexistence of naturally co-occurring plant species.     

Detection of arbuscular mycorrhizal fungi (Glomales) in roots by nested PCR and SSCP (Single Stranded Conformation Polymorphism)

PCR amplification of a region of the large subunit ribosomal DNA sequence with Glomus specific primers was used to detect arbuscular mycorrhizal fungi in root tissue of four plant species. The primers were specific to Glomus mosseae, Glomus caledonium, Glomus geosporum, Glomus coronatum, Glomus fragilistratum and Glomus constrictum, and did not recognise sequences from Glomus claroideum. Sequence differences between isolates were detected by Single Stranded Conformation Polymorphisms (SSCPs) in polyacrylamide gels under non-denaturing conditions. Isolates of G. mosseae, G. caledonium and G. coronatum could be separated by their SSCP patterns, while three isolates of G. geosporumshowed no variation. Specific SSCP patterns from isolates of G. mosseae and G. caledonium allowed detection of both fungi in the same root segment. Sequence differences leading to variations in SSCP patterns were confirmed by direct sequencing. This revised version was published online in June 2006 with corrections to the Cover Date.     

Does the sequence of plant dominants affect mycorrhiza development in simulated succession on spoil banks?

The aim of this field study was to examine how the development of arbuscular mycorrhizal fungi (AMF) on coal mine spoil banks is affected by the presence of plants with different mycorrhizal status. A 3-year trial was conducted on the freshly created spoil bank Vršany, North-Bohemian coal basin, the Czech Republic. Three plant species – non-mycotrophic annual Atriplex sagittata, highly mycotrophic annual Tripleurospermum inodorum (both dominants of early stages of succession) and facultatively mycotrophic Arrhenatherum elatius (a perennial grass species of the later stage of succession) – were planted on 1 m² plots over 3 years in different sequences that simulated the progress of succession on spoil banks. The development of AMF populations was monitored by evaluation of mycorrhizal colonization of plant roots and by measurement of the mycorrhizal inoculation potential (MIP) of soil. These two parameters were compared between plots inoculated with the mixture of three AMF isolates – Glomus mosseae BEG95, G. claroideum BEG96 and G. intraradices BEG140 – (“inoculated plots”) and plots exposed only to natural dispersal of AMF propagules (“uninoculated plots”). Highly colonized roots of plants together with a high MIP of soil in uninoculated plots were already found at the end of the first season, indicating rapid natural dispersal of AMF propagules. Root colonization of facultatively mycotrophic and non-mycotrophic plants in later years was affected by the mycorrhizal status of the previous plant species. The MIP of soil continuously increased throughout the experiment; in uninoculated plots, the MIP was temporarily decreased if plant species of higher mycotrophy were replaced by species of lower mycotrophy. The results lead to the conclusion that AMF colonize freshly formed sites very quickly and reproduce or accumulate in the soil, which leads to increasing MIP values. However, this infective potential can be decreased if non-mycotrophic plants predominate on the site.     

Diversity and infectivity of arbuscular mycorrhizal fungi in agricultural soils of the Sichuan Province of mainland China

Knowledge about the presence and diversity of arbuscular mycorrhizal fungi (AMF) in a specific area is an essential first step for utilizing these fungi in any application. The community composition of AMF in intensively managed agricultural soil in the Sichuan Province of southwest China currently is unknown. In one set of samples, AMF were trapped in pot cultures from 40 fields growing legumes in the Panxi region, southeast Sichuan. In a second set of samples, the MPN method with four-fold dilutions and maize as host was used to estimate infective propagules in soil from another 50 agricultural sites throughout the province. Soil types were heterogeneous and were classified as purple, yellow, paddy and red. Crops at each site were either maize, wheat or sweet orange. From this set of soil, AMF spores were also extracted and identified. Including all ninety soils, thirty glomeromycotan species in Glomus (20 species), Acaulospora (four species), Scutellospora (three species), Ambispora (one species), Archaeospora (one species) and Paraglomus (one species) were identified. Yellow, red and purple soils yielded similar numbers of AMF species, while AMF species diversity was clearly lower in paddy soil. In trap culture soils, the most frequent species were Glomus aggregatum or Glomus intraradices, Glomus claroideum and Glomus etunicatum. The species Acaulospora capsicula, Acaulospora delicata, G. aggregatum (or intraradices), G. claroideum, Glomus epigaeum, G. etunicatum, Glomus luteum, Glomus monosporum, Glomus mosseae and Glomus proliferum were successfully cultured as single-species pot cultures in Plantago lanceolata. The three most frequent species in field soils were G. mosseae, Glomus caledonium and Glomus constrictum. MPN values varied between 17 and 3334 propagules 100 g soil⁻¹ among the fifty field sites sampled. Regression analysis, including host&soil, log(P) and pH as explanatory variables explained 59% of the variation in log(MPN). The highest MPN estimates were found in purple soil cropped with maize and citrus, 324 and 278 propagules 100 g soil⁻¹, respectively. The lowest MPN value, 54 propagules 100 g soil⁻¹, was measured in wheat in purple and yellow soil. Despite intensive agricultural management that can include often repeated tillage, our examination of 90 agricultural sites revealed that soils of the Sichuan region have moderate to high numbers of infective AMF propagules as well as a high AMF species diversity. This opens possibilities for further studies and utilization of AMF in agriculture and horticulture in the Sichuan province, People’s Republic of China.     

Phosphorus Acquisition Strategies within Arbuscular Mycorrhizal Fungal Community of a Single Field Site

Diversity in phosphorus (P) acquisition strategies was assessed among eight isolates of arbuscular mycorrhizal fungi (AMF) belonging to three Glomus species, all obtained from the same field site. Maize (Zea mays L. cv. Corso) was used as a test plant. Compartmented cultivation containers coupled with ³³P radioisotope labeling of soil P were employed to estimate (1) the distance from the roots that AMF were able to acquire soil P from, (2) the rate of soil colonization, (3) the efficiency of uptake of soil P by AMF, (4) benefits provided to maize in terms of P acquisition and growth. Glomus mosseae and G. intraradices took up P 10 cm from roots, whereas G. claroideum only up to 6 cm from the roots. G. mosseae most rapidly colonized the available soil volume and transported significant amounts of P to maize from a distance, but provided no net P uptake benefit to the plants. On the other hand, both G. intraradices and three out of four G. claroideum isolates significantly improved net P uptake by maize. These effects seem to be related to variability between and to a limited extent also within AMF species, in mycelium development, efficiency of hyphal P uptake and effects on plant P acquisition via the root pathway. In spite of absence of maize growth responses to inoculation with any of the AMF isolates, this study indicates remarkable functional diversity in the underground component of the studied field site.     

Effects of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal fungi in symbiosis with cucumber

 The effect of Pseudomonas fluorescens DF57 on growth and P uptake of two arbuscular mycorrhizal (AM) fungi in symbiosis with cucumber plants was studied in compartmentalised growth systems. Hyphae of Glomus intraradices Schenck & Smith (BEG87) or G. caledonium (Nicol. & Gerd.) Trappe & Gerdeman (BEG15) grew into lateral root-free compartments. Non-mycorrhizal plants served as control. The soil in half of the growth units of each mycorrhizal treatment was inoculated with P. fluorescens DF57. P. fluorescens DF57 enhanced hyphal length density of one of the AM fungi, G. caledonium, but this was not reflected in a higher hyphal transport of P from the root-free soil to the plant. The total P content was higher in plants grown in symbiosis with G. intraradices than in plants in the other treatments. G. caledonium and P. fluorescens DF57 had a synergistic effect in that total P content in plants inoculated with G. caledonium was higher in the presence than in the absence of P. fluorescens DF57. Accepted: 7 January 1999     

Extraction of extraradical arbuscular mycorrhizal mycelium from compartments filled with soil and glass beads

This study presents a novel method for the extraction and quantification of extraradical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF) from a substrate that simulates soil better than previously used artificial growth media. Fungal compartments were constructed from small net pots with a latticed wall and filled with a mixture of glass beads and 40 m wet sieved soil. The net pots were surrounded by a 30-m mesh membrane through which hyphae but not roots could grow. They were inserted into soil where a Glomus intraradices (BEG 110) colonized potato plant was growing. The ERM that had grown out from roots through the membrane was successfully collected and quantified after harvest by washing out the soil/glass bead mixture through a sieve with a mesh width of 40 m. Concentrations of P, Zn, Cu and Mn in the AMF ERM were analysed.     

Studies on the diversity of arbuscular mycorrhizal fungi and the efficacy of two native isolates in a highly alkaline anthropogenic sediment

A field survey of the arbuscular mycorrhizal status of herbaceous plant species was conducted in a highly alkaline anthropogenic sediment resulting from the disposal of waste from an acetylene and polyvinyl chloride factory. Most plant species found at the site were mycorrhizal and the dominant mycotrophic plant species was Conyza bilbaoana. Fungal species richness was assessed by identification of spores extracted from the sediment and from continuously propagated trap pot cultures. All of the six species of arbuscular mycorrhizal fungi (AMF) found were from the genus Glomus. Glomus intraradices and G. mosseae were found in field-collected sediment samples and also occurred most frequently in trap cultures. To test the symbiotic effectiveness of these two fungi, seedlings of C. bilbaoana were inoculated with either native G. intraradices BEG163 or G. mosseae BEG198 and non-native G. intraradices BEG75 or G. mosseae BEG25 isolates in sterile and non-sterile sediment collected from the study site. All four isolates were able to colonise C. bilbaoana. However, AMF native to the target sediments were generally more effective than the non-native fungi in promoting plant establishment and growth under highly alkaline conditions. The non-native G. intraradices was, however, more effective than the non-native G. mosseae. The results of this study suggest the use of adapted AMF as inoculants for phytorestoration of alkaline anthropogenic-stressed sediments.     

Tryptophan dimer produced by water-stressed bahia grass is an attractant for <Emphasis Type="Italic">Gigaspora margarita</Emphasis> and <Emphasis Type="Italic">Glomus caledonium</Emphasis>

We isolated and elucidated the structure of several stimulants for arbuscular mycorrhizal fungi (AMF) in water-stressed bahia grass roots. We could isolate some compounds that promoted the growth of Gigaspora margarita Becker and Hall and Glomus caledonium (Nicol. and Gerd.) Trappe and Gerd. In these compounds, tryptophan dimer (Trp–Trp) was elucidated the structure. Trp–Trp was abundantly produced in water-stressed bahia grass roots and exuded to the soil, although it was scarcely detected in non-stressed root exudates. Interestingly, this peptide strongly attracted the hyphae of Gi. margarita and G. caledonium and promoted their hyphal growth in vitro (1.8 × longer than the control). Tryptophan, however, had no effect on hyphal growth and attraction. Thus, Trp–Trp exuded from water-stressed roots would play an important role as a major signal for AMF. An erratum to this article can be found at     

The effect of agricultural practices on the development of indigenous arbuscular mycorrhizal fungi. II. Studies in experimental microcosms

Two glasshouse experiments were performed to assess the development and metabolic activity of mycorrhizas formed by isolates of arbuscular mycorrhizal fungi (AMF) from three different genera, Acaulospora, Gigaspora and Glomus on Desmodium ovalifolium L. plants. In the first experiment the effect of disturbance of a pre-established extra-radical mycelium (ERM) was studied. In the second experiment the effect of phosphate addition as either organic matter (OM) or fertiliser was studied. Disturbance of a pre-established ERM reduced the formation of mycorrhizas by Gigaspora rosea (BEG111) and increased that by Glomus manihotis (BEG112) on D. ovalifolium plants. Acaulospora tuberculata (BEG41) failed to form mycorrhizas in the experiment. Either Gi. rosea (BEG111) or G. manihotis (BEG112) appeared to be the major component of the colonisation resulting from treatments with combinations of two or three of the AMF and determined the sensitivity of these treatments to disturbance of a pre-established ERM. The addition of phosphate fertiliser (10 mg P kg^-1) reduced mycorrhiza formation by each species of AMF compared with the addition of OM (10 mg P kg^-1). This work indicates that AMF from different genera respond differently to management by agricultural practices when in association with a tropical legume. Clearly, there is potential to alter the formation of mycorrhizas of AMF from different genera, through the use of agricultural practices. The significance of the development and metabolic activity of mycorrhizas formed by AMF from different genera for plant growth is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of arbuscular mycorrhizal fungi and simulated acid rain on the growth and coexistence of the grasses Calamagrostis villosa and Deschampsia flexuosa

Two grass species — Calamagrostis villosa (Chaix) J.F. Gmelin and Deschampsia flexuosa (L.) Trin. — are expanding in mountain Norway spruce (Picea abies L. Karst.) forests of Central Europe damaged by anthropogenic pollution constituted particularly of acid rain. This invasion of grasses may be caused by the higher irradiance reaching the forest floor after the pollution-induced tree defoliation. The relative abundance of the two grass species is changing during the process of forest decline. Our study investigated the effects of arbuscular mycorrhizal fungi (AMF) on the growth and coexistence of both species under simulated acid rain (SAR) and two levels of irradiance. Three microcosm experiments were conducted to investigate how both grasses are influenced by the AMF when grown separately or together interacting via extraradical mycelium (ERM). A positive growth response to inoculation with Glomus mosseae BEG 25 was found for both grass species when cultivated separately and the mycorrhizal dependence and the growth benefit for D. flexuosa was greater than for C. villosa. However, when both grass species were grown together in the rhizoboxes with separated root and hyphal compartments, the growth effect of the AMF was the opposite, i.e. C. villosa benefited more. The plants did not benefit from the AMF inoculation under the SAR treatment compared with dH2O treatment. The SAR also negatively influenced root length colonised by AMF, length of the ERM, alkaline phosphatase and NADH diaphorase activities of the ERM. The role of the ERM in transporting phosphorus between these grasses was verified by applying the radioisotope ³²P. There was a greater transport of isotopic ³²P between inoculated plants C. villosa and D. flexuosa grown in separated root compartments, as compared to non-inoculated plants. The amount of transported ³²P was low: a maximum of 3% of applied ³²P was detected in the shoots of receiver plants. Mechanical disturbance of the ERM significantly decreased the ³² P transport between plants. The ³²P transport between mycorrhizal plants was higher in the D. flexuosa to C. villosa direction than in the opposite one. Neither the SAR nor the low level of irradiance influenced the amount of transported ³²P. We discuss the role of ERM links between root systems in the coexistence of both grass species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inoculation of grass and tree seedlings used for reclaiming eroded areas in Iceland with mycorrhizal fungi

The objective of this study was to investigate the response of plant species used for reclamation of eroded areas in Iceland to inoculation with mycorrhizal fungi. In a greenhouse trial,Leymus arenarius andDeschampsia beringensis were grown in pots with volcanic ash collected from a site near the Mt. Hekla volcano in Iceland and were inoculated with arbuscular mycorrhizal fungi (AMF) isolatesGlomus mosseae BEG25 orGlomus intraradices BEG75. In two field experiments conducted on volcanic tephra fields near Mt. Hekla, a native soil inoculum or commercial inocula TerraVital-D and Terra Vital-G Ecto Mix were compared for efficacy onL. arenarius andBetula pubescens. After four months of growth, the presence of AMF in the pot experiment significantly increased the capacity of grass root systems to bind soil particles. In the field, inoculation significantly increased the number ofL. arenarius plants, which emerged from seed and their subsequent survival and growth. Seedlings ofB. pubescens grew best following inoculation with ectomycorrhizal fungal (ECMF) inoculum and a subsequent application of inorganic NP-fertilizer. The addition of native soil inoculum had almost no effect on growth of either grass or trees. Our results indicate that reclamation of eroded areas in Iceland could benefit from the use of a ppropriate mycorrhizal fungi, which might improve plant establishment and growth and increase soil aggregation and stability.     

Arbuscular mycorrhizal fungi can induce the production of phytochemicals in sweet basil irrespective of phosphorus nutrition

The potential of three arbuscular mycorrhizal fungi (AMF) to enhance the production of antioxidants (rosmarinic and caffeic acids, RA and CA) was investigated in sweet basil (Ocimum basilicum). After adjusting phosphorus (P) nutrition so that P concentrations and yield were matched in AM and non-mycorrhizal (NM) plants we demonstrated that Glomus caledonium increased RA and CA production in the shoots. Glomus mosseae also increased shoot CA concentration in basil under similar conditions. Although higher P amendments to NM plants increased RA and CA concentrations, there was higher production of RA and CA in the shoots of AM plants, which was not solely due to better P nutrition. Therefore, AMF potentially represent an alternative way of promoting growth of this important medicinal herb, as natural ways of growing such crops are currently highly sought after in the herbal industry.     

Improvement by soil yeasts of arbuscular mycorrhizal symbiosis of soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) colonized by<Emphasis Type="Italic"> Glomus mosseae</Emphasis>

The effects of the soil yeasts Rhodotorula mucilaginosa, Cryptococcus laurentii and Saccharomyces kunashirensis on the arbuscular mycorrhizal (AM) fungus Glomus mosseae (BEG 12) was studied in vitro and in greenhouse trials. The presence of yeasts or their soluble and volatile exudates stimulated the percentage spore germination and hyphal growth of G. mosseae. Percentage root length colonized by G. mosseae and plant dry matter of soybean (Glycine max L. Merill) were increased only when the soil yeasts were inoculated prior to the AM fungus. Higher beneficial effects on AM colonization and plant dry matter were found when the soil yeasts were inoculated as an aqueous solution rather than as a thin agar slice. Although soluble and volatile exudates of yeasts benefited the AM symbiosis, their modes of action were different.This revised version was published online in May 2004 with corrections to the section of the article.     

Different levels of hyphal self-incompatibility modulate interconnectedness of mycorrhizal networks in three arbuscular mycorrhizal fungi within the Glomeraceae

Arbuscular mycorrhizal fungi (AMF) live in symbiosis with most plant species and produce underground extraradical hyphal networks functional in the uptake and translocation of mineral nutrients from the soil to host plants. This work investigated whether fungal genotype can affect patterns of interconnections and structural traits of extraradical mycelium (ERM), by comparing three Glomeraceae species growing in symbiosis with five plant hosts. An isolate of Funneliformis coronatus consistently showed low ability to form interconnected ERM and self-incompatibility that represented up to 21 % of hyphal contacts. The frequency of post-fusion self-incompatible interactions, never detected before in AMF extraradical networks, was 8.9 %. In F. coronatus ERM, the percentage of hyphal contacts leading to perfect hyphal fusions was 1.2–7.7, while it ranged from 25.8–48 to 35.6–53.6 in Rhizophagus intraradices and Funneliformis mosseae, respectively. Low interconnectedness of F. coronatus ERM resulted also from a very high number of non-interacting contacts (83.2 %). Such findings show that AMF genotypes in Glomeraceae can differ significantly in anastomosis behaviour and that ERM interconnectedness is modulated by the fungal symbiont, as F. coronatus consistently formed poorly interconnected networks when growing in symbiosis with five different host plants and in the asymbiotic stage. Structural traits, such as extent, density and hyphal self-compatibility/incompatibility, may represent key factors for the differential performance of AMF, by affecting fungal absorbing surface and foraging ability and thus nutrient flow from soil to host roots.     

Flavonoids in roots of white clover: interaction of arbuscular mycorrhizal fungi and a pathogenic fungus

The effects of two arbuscular mycorrhizal fungi (AMF) (Glomus mosseae and G. claroideum) and a pathogenic fungus (Pythium ultimum) on the production of eight flavonoids in roots of two white clover (Trifolium repens L.) cultivars were evaluated. Quantification of AM and pathogenic fungi in the roots showed that the AM symbiosis significantly reduced P. ultimum biomass and in some cases prevented infection. The flavonoid productions in clover roots varied depending on the presence of beneficial and/or pathogenic fungi, fungal isolate or plant cultivar. Only plants colonized with G. claroideum showed detectable concentrations of either coumestrol or kaempferol (cultivar-dependant). In addition, inoculation with G. claroideum resulted in significantly higher concentrations of coumestrol in cv. Sonja and medicarpin in cv. Milo. A low production of coumestrol and kaempferol in mycorrhizal plants may be G. mosseae-specific. Only the concentrations of formononetin and daidzein increased in clover roots in response to infection with P. ultimum. These flavonoids are supposedly stress metabolites, synthesized or produced from glycosides in response to pathogen infection. However, the presence of one or both AMF significantly lowered the formononetin and daidzein concentrations, and overruled the inductive effect of P. ultimum. Therefore the antagonistic action of AM against the pathogen must take place through another mechanism.     

A method for assessing arbuscular mycorrhizal fungi group distribution in tree roots by intergenic transcribed sequence variation

We identified five taxonomic groups of arbuscular mycorrhizal fungi (AMF) inside roots of young trees of six species of legumes and six species of non-legumes from a field site in southern Costa Rica using an AMF group-specific PCR assay of the intergenic transcribed sequence and 18S rRNA gene fragment. Assay specificity was verified by cloning and sequencing representatives from four of the five AMF groups. We found no difference in overall AMF diversity levels between legumes and non-legumes or between plant species. Some groups of AMF may associate more frequently with legumes than others, as Glomus Group A (Glomus mosseae/intradices group) representatives were detected more frequently in legumes than non-legumes relative to Glomus Group B (Glomus etunicatum/claroideum) representatives.     

Functional diversity in arbuscular mycorrhizas: exploitation of soil patches with different phosphate enrichment differs among fungal species

Most terrestrial plant species form associations with arbuscular mycorrhizal fungi (AMF) that transfer soil P to the plant via their external hyphae. The distribution of nutrients in soils is typically patchy (heterogeneous) but little is known about the ability of AMF to exploit P patches in soil. This was studied by growing symbioses of Linum usitatissimum and three AMF (Glomus intraradices, G. mosseae and Gigaspora margarita) in pots with two side-arms, which were accessible to hyphae, but not to roots. Soil in one side-arm was either unamended (P0) or enriched with P; simultaneous labelling of this soil with ³²P revealed that G. intraradices responded to P enrichment both in terms of hyphal proliferation and P uptake, whereas the other AMF did not. Labelling with ³³P of P0 soil in the other side arm revealed that the increased P uptake by G. intraradices from the P-enriched patch was paralleled by decreased P uptake by other parts of the mycelium. This is the first demonstration of variation in growth and nutrient uptake by an AMF as influenced by a localized P enrichment of the soil. The results are discussed in the context of functional diversity of AMF.     

Traits related to differences in function among three arbuscular mycorrhizal fungi

Diversity in phosphorus (P) acquisition strategies was assessed among three species of arbuscular mycorrhizal fungi (AMF) isolated from a single field in Switzerland. Medicago truncatula was used as a test plant. It was grown in a compartmented system with root and root-free zones separated by a fine mesh. Dual radioisotope labeling (³²P and ³³P) was employed in the root-free zone as follows: ³³P labeling determined hyphal P uptake from different distances from roots over the entire growth period, whereas ³²P labeling investigated hyphal P uptake close to the roots over the 48 hours immediately prior to harvest. Glomus intraradices, Glomus claroideum and Gigaspora margarita were able to take up and deliver P to the plants from maximal distances of 10, 6 and 1 cm from the roots, respectively. Glomus intraradices most rapidly colonized the available substrate and transported significant amounts of P towards the roots, but provided the same growth benefit as compared to Glomus claroideum, whose mycelium was less efficient in soil exploration and in P uptake and delivery to the roots. These differences are probably related to different carbon requirements by these different Glomus species. Gigaspora margarita provided low P benefits to the plants and formed dense mycelium networks close to the roots where P was probably transiently immobilized. Numerical modeling identified possible mechanisms underlying the observed differences in patterns of mycelium growth. High external hyphal production at the root-fungus interface together with rapid hyphal turnover were pointed out as important factors governing hyphal network development by Gigaspora, whereas nonlinearity in apical branching and hyphal anastomoses were key features for G. intraradices and G. claroideum, respectively.     

Status of arbuscular mycorrhizal fungi (AMF) in the Sundarbans of India in relation to tidal inundation and chemical properties of soil

The arbuscular mycorrhizal status of fifteen mangroves and one mangrove associate was investigated from 27 sites of three inundation types namely, diurnal, usual springtide and summer springtide. Roots and rhizospheric soil samples were analysed for spore density, frequency of mycorrhizal colonization and some chemical characteristics of soil. Relative abundance, frequency and spore richness of AMF were assessed at each inundation type. All the plant species except Avicennia alba exhibited mycorrhizal colonization. The study demonstrated that mycorrhizal colonization and spore density were more influenced by host plant species than tidal inundation. Forty four AMF species belonging to six genera, namely Acaulospora, Entrophospora, Gigaspora, Glomus, Sclerocystis and Scutellospora, were recorded. Glomus mosseae exhibited highest frequency at all the inundation types; Glomus fistulosum, Sclerocystis coremioides and Glomus mosseae showed highest relative abundance at sites inundated by usual springtides, summer springtides and diurnal tides, respectively. Spore richness of AMF was of the order usual springtide > diurnal > summer springtide inundated sites. The mean spore richness was 3.27. Diurnally inundated sites had the lowest concentrations of salinity, available phosphorus, exchangeable potassium, sodium and magnesium. Statistical analyses indicated that mycorrhizal frequency and AMF spore richness were significantly negatively correlated to soil salinity. Spore richness was also significantly negatively correlated to available phosphorus. The soil parameters of the usual springtide inundated sites appeared to be favourable for the existence of maximum number of AMF. Glomus mosseae was the predominant species in terms of frequency in the soils of the Sundarbans.