Background and Aims Mycorrhizal fungi play a vital role in providing a carbon subsidy to support ... more Background and Aims Mycorrhizal fungi play a vital role in providing a carbon subsidy to support the germination and establishment of orchids from tiny seeds, but their roles in adult orchids have not been adequately characterized. Recent evidence that carbon is supplied by Goodyera repens to its fungal partner in return for nitrogen has established the mutualistic nature of the symbiosis in this orchid. In this paper the role of the fungus in the capture and transfer of inorganic phosphorus (P) to the orchid is unequivocally demonstrated for the first time. † Methods Mycorrhiza-mediated uptake of phosphorus in G. repens was investigated using spatially separated, twodimensional agar-based microcosms. † Results External mycelium growing from this green orchid is shown to be effective in assimilating and transporting the radiotracer 33 P orthophosphate into the plant. After 7 d of exposure, over 10 % of the P supplied was transported over a diffusion barrier by the fungus and to the plants, more than half of this to the shoots. † Conclusions Goodyera repens can obtain significant amounts of P from its mycorrhizal partner. These results provide further support for the view that mycorrhizal associations in some adult green orchids are mutualistic.
Semi-natural calcareous and acidic grasslands are known to be sensitive to increased atmospheric ... more Semi-natural calcareous and acidic grasslands are known to be sensitive to increased atmospheric N deposition. However, the fate of pollutant N within these systems is unknown. This paper reports on the first studies to determine the fate of added N within a calcareous and an acidic grassland subject to long-term simulated enhanced N deposition. Intact soil/turf cores were removed from field plots treated for six years with enhanced N deposition (ambient +0, +35 and +140 kg N ha −1 year −1). Cores were inserted into lysimeters and output fluxes of N were monitored in detail. Complete N budgets-calculated from the N flux data-showed considerable accumulation of N within the treated grasslands, up to 76% and 38% of pollutant N in the calcareous and acidic grasslands respectively. In the second study, the short-term (21 day) fate of pollutant N was determined by tracing 15 N labelled ammonium nitrate (+35 kg N ha −1 year −1) though the acidic and calcareous lysimeters into plant, soil and leachate pools. Up to 91% and 59% of 15 N was recovered in soils and vegetation of the calcareous and acidic grasslands respectively, with negligible amounts recovered in soil extractable ammonium and nitrate (<0.3%) and in leachate (<0.02%). This rapid short-term immobilisation of pollutant N supports the long-term accumulation of the element calculated from the N flux study.
ABSTRACT The movement of carbon from plants into their natural communities of arbuscular-mycorrhi... more ABSTRACT The movement of carbon from plants into their natural communities of arbuscular-mycorrhizal (AM) fungi was investigated. Mesh-bound cores, which allowed in-growth of AM mycelium to be controlled but excluded roots, were inserted into turf monoliths removed from an upland grassland and were exposed to 14CO2. Flux of 14C-labelled carbon from plants to hyphae of AM fungi for 70h post-labelling was measured by (a) trapping CO2 released from soil cores containing AM hyphae linked to the plants compared to cores from which AM hyphal connections to the plant roots had been severed, and (b) quantification of the total amount of 14C in the cores. Release of 14CO2 from the cores colonised by active AM mycelium was highest for 0–28h from the onset of labelling and declined rapidly thereafter. The amount of 14C allocated into mycorrhizal mycelium 0–70h after labelling accounted for 3.4% of the 14C initially fixed by the plants. The results confirm the rapidity of photosynthate allocation to AM mycelium and demonstrate the importance of the short-term dynamics of C fluxes in undisturbed grasslands.
Annual carbon flux through soil respiration is ten times greater than fossil fuel combustion, but... more Annual carbon flux through soil respiration is ten times greater than fossil fuel combustion, but its component parts are poorly understood because they are the product of complex multitrophic interactions between soil organisms. A major component of carbon flux from plants to soil occurs through networks of symbiotic arbuscular mycorrhizal fungi. Here, using 13 CO 2 pulse labeling, we show that natural densities of the numerically dominant fungal feeding invertebrate Protaphorura armata (order Collembola) reduces 13 C enrichment of mycorrhizosphere respiration by 32%. Our findings emphasize the importance of multitrophic interactions in regulating respiration of recent plant photosynthate from soil.
We have studied the effects of factorial combinations of lime and N additions on soil microbial b... more We have studied the effects of factorial combinations of lime and N additions on soil microbial biomass, respiration rates and phosphatase activity of an upland grassland. We also used an Agrostis capillaris seedling bioassay to assess the effect of the treatments on the activity of arbuscular-mycorrhizal (AM) fungi and root surface phosphatase enzymes and the concentrations of N and P
Summary The flux of pulse‐derived 13C from upland pasture plants to the external mycelium of the... more Summary The flux of pulse‐derived 13C from upland pasture plants to the external mycelium of their arbuscular mycorrhizal (AM) symbionts was traced and quantified over a 7‐d post‐labelling period. Mesh cores, which allowed in‐growth of native AM mycelium but were impenetrable to roots, were inserted into unlimed and limed plots and the surrounding vegetation was exposed to 13CO2 at ambient CO2 concentrations. Release of 13CO2 from cores colonized by AM mycelium peaked 9–14 h after labelling and declined within 24 h after severance of mycelial connections to roots. Between 5 and 8% of carbon lost by plants was respired by AM mycelium over the first 21 h after labelling. Liming increased the amount of carbon fixed by plants and subsequently allocated to fine roots and AM mycelium. The results demonstrate for the first time under field conditions that AM mycelia provide a rapid and important pathway of carbon flux from plants to the soil and atmosphere.
ABSTRACT Summary • A novel in-growth core system, enabling functional studies of natural communit... more ABSTRACT Summary • A novel in-growth core system, enabling functional studies of natural communities of arbuscular mycorrhizal (AM) mycelia in soil is described and tested. • The cores have windows covered with nylon mesh of 35 µm pore size that prevent in-growth of roots but permit penetration of AM hyphae. They were inserted into grassland turf and contained either sterilized sand and a ‘bait’ seedling of Trifolium repens or nonsterile natural soil without bait plants. The impacts of hyphal severance, achieved by periodic rotation of some of the cores, upon AM colonization of bait plants (experiment 1) and transfer of 33P from soil to plants outside the cores (experiment 2) were examined. • Severance of AM hyphae reduced both AM colonization of bait plants and their shoot P concentrations. The shoot 33P concentrations of plants with mycelial access to 33PO4-labelled cores were 10-fold greater than those which had no mycelial access. • It is concluded that this novel approach enables the functioning of mycorrhizal mycelial networks to be evaluated under conditions closely simulating those occurring in nature.
The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (... more The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (N) to adult orchids are poorly understood. Here, we employed isotopically labelled sources of C and N to investigate these processes in the green forest orchid, Goodyera repens. Fungus-to-orchid transfers of C and N were measured using mass spectrometry after supplying extraradical mycelial systems with double-labelled [13C-15N]glycine. Orchid-to-fungus C transfer was revealed and quantified by radioisotope imaging and liquid scintillation counting of extraradical mycelium following 14CO2 fixation by shoots. Both 13C and 15N were assimilated by the fungus and transferred to the roots and shoots of the orchid. Contrary to previous reports, considerable quantities (2.6% over 72 h) of fixed C were shown to be allocated to the extraradical mycelium of the fungus. This study demonstrates, for the first time, mutualism in orchid mycorrhiza, bidirectional transfer of C between a green orchid and its fungal symbiont, and a fungus-dependent pathway for organic N acquisition by an orchid.
The processes of symbiotic germination and seedling development were analysed in the myco‐heterot... more The processes of symbiotic germination and seedling development were analysed in the myco‐heterotrophic orchid Corallorhiza trifida, seeds of which were buried in ‘packets’ either adjacent to or at varying distances from adult plants in defined communities of ectomycorrhizal tree species. Germination occurred within eight months of burial under Betula–Alnus and within seven months under Salix repens. It was always associated with penetration of the suspensor by a clamp‐forming mycorrhizal fungus. Four distinct developmental stages were defined and the rates of transition through these stages were plotted. There was no evidence of a relationship between extent of germination or rate of development and the presence of naturally distributed plants of C. trifida at the spatial scale of 1 m. The best germination and the most rapid rate of development of C. trifida seedlings occurred in a Salix repens community located at a considerable distance from any extant C. trifida population. Dete...
• The patterns of nitrogen (N) utilization and of N isotope fractionation were determined when tw... more • The patterns of nitrogen (N) utilization and of N isotope fractionation were determined when two ecto-(ECM) and an ericoid (ERM) mycorrhizal fungus were grown with inorganic (ammonium or nitrate) or organic (glutamic acid or glycine) N sources of predetermined N isotope composition. • All N sources were readily utilized by each of the fungi but substantial differences in the pattern of N isotope fractionation were observed both between the fungi and the N sources. • Whereas several of the ECM-N source combinations exhibited significant net fractionation in favour of 15 N, no such effect was seen in the ERM fungus, where, on ammonium, there was preferential assimilation of 14 N. • It is concluded that isotopic fractionation during N uptake and metabolism can cause significant shifts in the 15 N abundance of mycorrhizal fungi and that, as a result, any attempt to use the tissue 15 N abundance as a means of identifying the substrates being exploited by mycorrhizal fungi, or their plant partners, in nature, are likely to be unrealistic.
summary The effect of pH on the production and specific activity of the extracellular proteinase ... more summary The effect of pH on the production and specific activity of the extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. The proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endophyte of the calcicolous alpine shrub Rhodothamnus chamaecistus (L.) Reichenb. growing in soil of pH 6.5. The fungi were grown in liquid culture at pH values ranging from 3.0 to 8.0 with pure protein, bovine serum albumin, as sole source of N. Both fungi yielded an extracellular acid proteinase with pH optimum for activity between 20 and 30. The production and activity of these enzymes was strongly affected by pH of the culture medium. Maximum enzyme production during exponential growth occurred in both fungi at a culture pH of 4.0–5.0, whereas higher pH treatments severely inhibited enzyme production. The acid proteinase of H. ericae was tolerant of extreme acidity and retained near-optimal activity in solutions of pH 2.0. In contrast, the activity of the enzyme from the Rhodothamnus endophyte was almost completely inhibited at this pH. However, proteinase from the Rhodothamnus endophyte retained activity at much higher pH values than did the proteinase from H. ericae. Unlike H. ericae, the isolated endophyte of Rhodothamnus was able to grow and use protein as sole source of N at pH 7.0 and 8.0. The effects of pH on enzyme production and upon growth of the fungi are discussed in relation to the characteristics of the environments of their host plants.
• The effects of 7 yr enhanced nitrogen (N) deposition (3.5-14 g N m − 2 yr − 1) in combination w... more • The effects of 7 yr enhanced nitrogen (N) deposition (3.5-14 g N m − 2 yr − 1) in combination with phosphorus (P) additions, on growth, shoot N and P content, and root-surface phosphomonoesterase (PME) activities were determined along with mycorrhizal infection rates in seedlings of a sedge (Carex flacca), grass (Koeleria macrantha) and forb (Leontodon hispidus). • Seedlings were grown for 14-28 d in mesh-walled cores inserted into turfs from treated field plots enabling complete root recovery. • After 14 d, root-surface PME activity was typically more than doubled by 3.5 and 14 g N m − 2 yr − 1 , and by 28 d the N treatments consistently gave dose-dependent effects. PME activity was reduced by P additions in the sedge and grass by 55 and 65%, respectively, and correlated with soil and shoot N and P concentrations, again most strongly in the sedge and grass. Mycorrhizal infection was least in the sedge (1%) and greater in the grass (49%) and forb (76%). • Long-term N enrichment of calcareous grassland stimulates root-surface PME in representatives of the three major higher-plant functional types. PME response to P additions was greatest in least mycorrhizal-dependant species with roots more adapted for direct P uptake.
summaryThe effect of pH on the production and specific activity of the extracellular proteinase e... more summaryThe effect of pH on the production and specific activity of the extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. The proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endophyte of the calcicolous alpine shrub Rhodothamnus chamaecistus (L.) Reichenb. growing in soil of pH 6.5. The fungi were grown in liquid culture at pH values ranging from 3.0 to 8.0 with pure protein, bovine serum albumin, as sole source of N.Both fungi yielded an extracellular acid proteinase with pH optimum for activity between 20 and 30. The production and activity of these enzymes was strongly affected by pH of the culture medium. Maximum enzyme production during exponential growth occurred in both fungi at a culture pH of 4.0–5.0, whereas higher pH treatments severely inhibited enzyme production.The acid proteinase of H. erica...
SummaryThe effects of the metals, iron and aluminium, on acid phosphatase (P‐NPPase) activity of ... more SummaryThe effects of the metals, iron and aluminium, on acid phosphatase (P‐NPPase) activity of the ericoid endophyte, Hymenoseyphus ericae (Read) Korf and Kernan were determined. The endophyte was grown in pure culture and then treated with Fe2+ and Al2−over a range of concentrations (0–1000mgl−1), and at two concentrations of inorganic phosphorus (20μ and 200 μM p). Activities of p‐NPPase were determined. Enzyme activity was greatly inhibited at the highest concentration of external P even in the absence of the metal ions. The presence of the metals in the low P treatment had effects determined by their concentration. At the highest concentration used, they precipitated products of acid phosphatase activity (p‐NP), and so invalidated the use of the assay at these metal levels. However, the assay could reliably be used over the concentration range 0–200 mg 1−1 Al2+ and 1–100 mg 1 −1 Fe2+, the upper concentrations in both cases being considerably above those normally found in soil ...
The role of mycorrhizal infection in the calcium nutrition of Callliuia vuilgaris (L.) Hull is in... more The role of mycorrhizal infection in the calcium nutrition of Callliuia vuilgaris (L.) Hull is investigated. Callutna plants were grown in the infected (M) and uninfected (NI\') condition in sand supplied with nutrient solutions containing one of the three ecologically important calcium salts: CaSOV, CaCO3 and CaClI, each supplied over a range of concentrations from zero to 1000 /ug ml-'. The influence of infection upon growth and Ca content of whole plants, shoots and roots were determined. Yields of MVJ plants were significantly larger than those of NI\' on each of the Ca sources. These differences were particularly marked at the lowest concentrations of applied Ca. Root growth of M plants was strongly enhanced in all Ca treatments, so that the root/shoot ratios of these plants were consistently greater than in those which were NM. M plants also had consistently higher Ca concentrations in their shoots than their NM counterparts. Shoot concentrations were 32, 60, and 23 O' higher in M than NM plants grown on CaCl2, CaCO3 and CaSO4 respectively. Similarly, mean total Ca contents were 29, 77 and 87 0O higher in M than NM plants on CaCl2, CaCO3, and CaSO4 respectively. Improved growth of infected plants at high concentrations of applied CaCO3 appeared to be due to alleviation of CO3 toxicity. The complex interactions involving the direct nutritional effects of Ca ions, the indirect effects of its associated anions, and of other ionic species, notably H+, Al3` and Fe2+ are discussed. It is concluded that the principal effect of infection at low exogenous Ca concentrations is to increase the effciencv of Ca capture whereas at high concentrations it is to alleviate toxicity of some auxiliary anions.
• The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limeston... more • The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limestone grassland soil supporting different synthesized vascular plant assemblages that had developed for 3 yr. • The experimental treatments comprised: bare soil; monocultures of the nonmycotrophic sedge Carex flacca ; monocultures of the mycotrophic grass Festuca ovina ; and a species-rich mixture of four forbs, four grasses and four sedges. The diversity of AM fungi was analysed in roots of Plantago lanceolata bioassay seedlings using terminal-restriction fragment length polymorphism (T-RFLP). The extent of AM colonization, shoot biomass and nitrogen and phosphorus concentrations were also measured. • The AM diversity was affected significantly by the floristic composition of the microcosms and shoot phosphorus concentration was positively correlated with AM diversity. The diversity of AM fungi in P. lanceolata decreased in the order: bare soil > C. flacca > 12 species > F. ovina. • The unexpectedly high diversity in the bare soil and sedge monoculture likely reflects differences in the modes of colonization and sources of inoculum in these treatments compared with the assemblages containing established AM-compatible plants.
Background and Aims Mycorrhizal fungi play a vital role in providing a carbon subsidy to support ... more Background and Aims Mycorrhizal fungi play a vital role in providing a carbon subsidy to support the germination and establishment of orchids from tiny seeds, but their roles in adult orchids have not been adequately characterized. Recent evidence that carbon is supplied by Goodyera repens to its fungal partner in return for nitrogen has established the mutualistic nature of the symbiosis in this orchid. In this paper the role of the fungus in the capture and transfer of inorganic phosphorus (P) to the orchid is unequivocally demonstrated for the first time. † Methods Mycorrhiza-mediated uptake of phosphorus in G. repens was investigated using spatially separated, twodimensional agar-based microcosms. † Results External mycelium growing from this green orchid is shown to be effective in assimilating and transporting the radiotracer 33 P orthophosphate into the plant. After 7 d of exposure, over 10 % of the P supplied was transported over a diffusion barrier by the fungus and to the plants, more than half of this to the shoots. † Conclusions Goodyera repens can obtain significant amounts of P from its mycorrhizal partner. These results provide further support for the view that mycorrhizal associations in some adult green orchids are mutualistic.
Semi-natural calcareous and acidic grasslands are known to be sensitive to increased atmospheric ... more Semi-natural calcareous and acidic grasslands are known to be sensitive to increased atmospheric N deposition. However, the fate of pollutant N within these systems is unknown. This paper reports on the first studies to determine the fate of added N within a calcareous and an acidic grassland subject to long-term simulated enhanced N deposition. Intact soil/turf cores were removed from field plots treated for six years with enhanced N deposition (ambient +0, +35 and +140 kg N ha −1 year −1). Cores were inserted into lysimeters and output fluxes of N were monitored in detail. Complete N budgets-calculated from the N flux data-showed considerable accumulation of N within the treated grasslands, up to 76% and 38% of pollutant N in the calcareous and acidic grasslands respectively. In the second study, the short-term (21 day) fate of pollutant N was determined by tracing 15 N labelled ammonium nitrate (+35 kg N ha −1 year −1) though the acidic and calcareous lysimeters into plant, soil and leachate pools. Up to 91% and 59% of 15 N was recovered in soils and vegetation of the calcareous and acidic grasslands respectively, with negligible amounts recovered in soil extractable ammonium and nitrate (<0.3%) and in leachate (<0.02%). This rapid short-term immobilisation of pollutant N supports the long-term accumulation of the element calculated from the N flux study.
ABSTRACT The movement of carbon from plants into their natural communities of arbuscular-mycorrhi... more ABSTRACT The movement of carbon from plants into their natural communities of arbuscular-mycorrhizal (AM) fungi was investigated. Mesh-bound cores, which allowed in-growth of AM mycelium to be controlled but excluded roots, were inserted into turf monoliths removed from an upland grassland and were exposed to 14CO2. Flux of 14C-labelled carbon from plants to hyphae of AM fungi for 70h post-labelling was measured by (a) trapping CO2 released from soil cores containing AM hyphae linked to the plants compared to cores from which AM hyphal connections to the plant roots had been severed, and (b) quantification of the total amount of 14C in the cores. Release of 14CO2 from the cores colonised by active AM mycelium was highest for 0–28h from the onset of labelling and declined rapidly thereafter. The amount of 14C allocated into mycorrhizal mycelium 0–70h after labelling accounted for 3.4% of the 14C initially fixed by the plants. The results confirm the rapidity of photosynthate allocation to AM mycelium and demonstrate the importance of the short-term dynamics of C fluxes in undisturbed grasslands.
Annual carbon flux through soil respiration is ten times greater than fossil fuel combustion, but... more Annual carbon flux through soil respiration is ten times greater than fossil fuel combustion, but its component parts are poorly understood because they are the product of complex multitrophic interactions between soil organisms. A major component of carbon flux from plants to soil occurs through networks of symbiotic arbuscular mycorrhizal fungi. Here, using 13 CO 2 pulse labeling, we show that natural densities of the numerically dominant fungal feeding invertebrate Protaphorura armata (order Collembola) reduces 13 C enrichment of mycorrhizosphere respiration by 32%. Our findings emphasize the importance of multitrophic interactions in regulating respiration of recent plant photosynthate from soil.
We have studied the effects of factorial combinations of lime and N additions on soil microbial b... more We have studied the effects of factorial combinations of lime and N additions on soil microbial biomass, respiration rates and phosphatase activity of an upland grassland. We also used an Agrostis capillaris seedling bioassay to assess the effect of the treatments on the activity of arbuscular-mycorrhizal (AM) fungi and root surface phosphatase enzymes and the concentrations of N and P
Summary The flux of pulse‐derived 13C from upland pasture plants to the external mycelium of the... more Summary The flux of pulse‐derived 13C from upland pasture plants to the external mycelium of their arbuscular mycorrhizal (AM) symbionts was traced and quantified over a 7‐d post‐labelling period. Mesh cores, which allowed in‐growth of native AM mycelium but were impenetrable to roots, were inserted into unlimed and limed plots and the surrounding vegetation was exposed to 13CO2 at ambient CO2 concentrations. Release of 13CO2 from cores colonized by AM mycelium peaked 9–14 h after labelling and declined within 24 h after severance of mycelial connections to roots. Between 5 and 8% of carbon lost by plants was respired by AM mycelium over the first 21 h after labelling. Liming increased the amount of carbon fixed by plants and subsequently allocated to fine roots and AM mycelium. The results demonstrate for the first time under field conditions that AM mycelia provide a rapid and important pathway of carbon flux from plants to the soil and atmosphere.
ABSTRACT Summary • A novel in-growth core system, enabling functional studies of natural communit... more ABSTRACT Summary • A novel in-growth core system, enabling functional studies of natural communities of arbuscular mycorrhizal (AM) mycelia in soil is described and tested. • The cores have windows covered with nylon mesh of 35 µm pore size that prevent in-growth of roots but permit penetration of AM hyphae. They were inserted into grassland turf and contained either sterilized sand and a ‘bait’ seedling of Trifolium repens or nonsterile natural soil without bait plants. The impacts of hyphal severance, achieved by periodic rotation of some of the cores, upon AM colonization of bait plants (experiment 1) and transfer of 33P from soil to plants outside the cores (experiment 2) were examined. • Severance of AM hyphae reduced both AM colonization of bait plants and their shoot P concentrations. The shoot 33P concentrations of plants with mycelial access to 33PO4-labelled cores were 10-fold greater than those which had no mycelial access. • It is concluded that this novel approach enables the functioning of mycorrhizal mycelial networks to be evaluated under conditions closely simulating those occurring in nature.
The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (... more The roles of mycorrhiza in facilitating the acquisition and transfer of carbon (C) and nitrogen (N) to adult orchids are poorly understood. Here, we employed isotopically labelled sources of C and N to investigate these processes in the green forest orchid, Goodyera repens. Fungus-to-orchid transfers of C and N were measured using mass spectrometry after supplying extraradical mycelial systems with double-labelled [13C-15N]glycine. Orchid-to-fungus C transfer was revealed and quantified by radioisotope imaging and liquid scintillation counting of extraradical mycelium following 14CO2 fixation by shoots. Both 13C and 15N were assimilated by the fungus and transferred to the roots and shoots of the orchid. Contrary to previous reports, considerable quantities (2.6% over 72 h) of fixed C were shown to be allocated to the extraradical mycelium of the fungus. This study demonstrates, for the first time, mutualism in orchid mycorrhiza, bidirectional transfer of C between a green orchid and its fungal symbiont, and a fungus-dependent pathway for organic N acquisition by an orchid.
The processes of symbiotic germination and seedling development were analysed in the myco‐heterot... more The processes of symbiotic germination and seedling development were analysed in the myco‐heterotrophic orchid Corallorhiza trifida, seeds of which were buried in ‘packets’ either adjacent to or at varying distances from adult plants in defined communities of ectomycorrhizal tree species. Germination occurred within eight months of burial under Betula–Alnus and within seven months under Salix repens. It was always associated with penetration of the suspensor by a clamp‐forming mycorrhizal fungus. Four distinct developmental stages were defined and the rates of transition through these stages were plotted. There was no evidence of a relationship between extent of germination or rate of development and the presence of naturally distributed plants of C. trifida at the spatial scale of 1 m. The best germination and the most rapid rate of development of C. trifida seedlings occurred in a Salix repens community located at a considerable distance from any extant C. trifida population. Dete...
• The patterns of nitrogen (N) utilization and of N isotope fractionation were determined when tw... more • The patterns of nitrogen (N) utilization and of N isotope fractionation were determined when two ecto-(ECM) and an ericoid (ERM) mycorrhizal fungus were grown with inorganic (ammonium or nitrate) or organic (glutamic acid or glycine) N sources of predetermined N isotope composition. • All N sources were readily utilized by each of the fungi but substantial differences in the pattern of N isotope fractionation were observed both between the fungi and the N sources. • Whereas several of the ECM-N source combinations exhibited significant net fractionation in favour of 15 N, no such effect was seen in the ERM fungus, where, on ammonium, there was preferential assimilation of 14 N. • It is concluded that isotopic fractionation during N uptake and metabolism can cause significant shifts in the 15 N abundance of mycorrhizal fungi and that, as a result, any attempt to use the tissue 15 N abundance as a means of identifying the substrates being exploited by mycorrhizal fungi, or their plant partners, in nature, are likely to be unrealistic.
summary The effect of pH on the production and specific activity of the extracellular proteinase ... more summary The effect of pH on the production and specific activity of the extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. The proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endophyte of the calcicolous alpine shrub Rhodothamnus chamaecistus (L.) Reichenb. growing in soil of pH 6.5. The fungi were grown in liquid culture at pH values ranging from 3.0 to 8.0 with pure protein, bovine serum albumin, as sole source of N. Both fungi yielded an extracellular acid proteinase with pH optimum for activity between 20 and 30. The production and activity of these enzymes was strongly affected by pH of the culture medium. Maximum enzyme production during exponential growth occurred in both fungi at a culture pH of 4.0–5.0, whereas higher pH treatments severely inhibited enzyme production. The acid proteinase of H. ericae was tolerant of extreme acidity and retained near-optimal activity in solutions of pH 2.0. In contrast, the activity of the enzyme from the Rhodothamnus endophyte was almost completely inhibited at this pH. However, proteinase from the Rhodothamnus endophyte retained activity at much higher pH values than did the proteinase from H. ericae. Unlike H. ericae, the isolated endophyte of Rhodothamnus was able to grow and use protein as sole source of N at pH 7.0 and 8.0. The effects of pH on enzyme production and upon growth of the fungi are discussed in relation to the characteristics of the environments of their host plants.
• The effects of 7 yr enhanced nitrogen (N) deposition (3.5-14 g N m − 2 yr − 1) in combination w... more • The effects of 7 yr enhanced nitrogen (N) deposition (3.5-14 g N m − 2 yr − 1) in combination with phosphorus (P) additions, on growth, shoot N and P content, and root-surface phosphomonoesterase (PME) activities were determined along with mycorrhizal infection rates in seedlings of a sedge (Carex flacca), grass (Koeleria macrantha) and forb (Leontodon hispidus). • Seedlings were grown for 14-28 d in mesh-walled cores inserted into turfs from treated field plots enabling complete root recovery. • After 14 d, root-surface PME activity was typically more than doubled by 3.5 and 14 g N m − 2 yr − 1 , and by 28 d the N treatments consistently gave dose-dependent effects. PME activity was reduced by P additions in the sedge and grass by 55 and 65%, respectively, and correlated with soil and shoot N and P concentrations, again most strongly in the sedge and grass. Mycorrhizal infection was least in the sedge (1%) and greater in the grass (49%) and forb (76%). • Long-term N enrichment of calcareous grassland stimulates root-surface PME in representatives of the three major higher-plant functional types. PME response to P additions was greatest in least mycorrhizal-dependant species with roots more adapted for direct P uptake.
summaryThe effect of pH on the production and specific activity of the extracellular proteinase e... more summaryThe effect of pH on the production and specific activity of the extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. The proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endophyte of the calcicolous alpine shrub Rhodothamnus chamaecistus (L.) Reichenb. growing in soil of pH 6.5. The fungi were grown in liquid culture at pH values ranging from 3.0 to 8.0 with pure protein, bovine serum albumin, as sole source of N.Both fungi yielded an extracellular acid proteinase with pH optimum for activity between 20 and 30. The production and activity of these enzymes was strongly affected by pH of the culture medium. Maximum enzyme production during exponential growth occurred in both fungi at a culture pH of 4.0–5.0, whereas higher pH treatments severely inhibited enzyme production.The acid proteinase of H. erica...
SummaryThe effects of the metals, iron and aluminium, on acid phosphatase (P‐NPPase) activity of ... more SummaryThe effects of the metals, iron and aluminium, on acid phosphatase (P‐NPPase) activity of the ericoid endophyte, Hymenoseyphus ericae (Read) Korf and Kernan were determined. The endophyte was grown in pure culture and then treated with Fe2+ and Al2−over a range of concentrations (0–1000mgl−1), and at two concentrations of inorganic phosphorus (20μ and 200 μM p). Activities of p‐NPPase were determined. Enzyme activity was greatly inhibited at the highest concentration of external P even in the absence of the metal ions. The presence of the metals in the low P treatment had effects determined by their concentration. At the highest concentration used, they precipitated products of acid phosphatase activity (p‐NP), and so invalidated the use of the assay at these metal levels. However, the assay could reliably be used over the concentration range 0–200 mg 1−1 Al2+ and 1–100 mg 1 −1 Fe2+, the upper concentrations in both cases being considerably above those normally found in soil ...
The role of mycorrhizal infection in the calcium nutrition of Callliuia vuilgaris (L.) Hull is in... more The role of mycorrhizal infection in the calcium nutrition of Callliuia vuilgaris (L.) Hull is investigated. Callutna plants were grown in the infected (M) and uninfected (NI\') condition in sand supplied with nutrient solutions containing one of the three ecologically important calcium salts: CaSOV, CaCO3 and CaClI, each supplied over a range of concentrations from zero to 1000 /ug ml-'. The influence of infection upon growth and Ca content of whole plants, shoots and roots were determined. Yields of MVJ plants were significantly larger than those of NI\' on each of the Ca sources. These differences were particularly marked at the lowest concentrations of applied Ca. Root growth of M plants was strongly enhanced in all Ca treatments, so that the root/shoot ratios of these plants were consistently greater than in those which were NM. M plants also had consistently higher Ca concentrations in their shoots than their NM counterparts. Shoot concentrations were 32, 60, and 23 O' higher in M than NM plants grown on CaCl2, CaCO3 and CaSO4 respectively. Similarly, mean total Ca contents were 29, 77 and 87 0O higher in M than NM plants on CaCl2, CaCO3, and CaSO4 respectively. Improved growth of infected plants at high concentrations of applied CaCO3 appeared to be due to alleviation of CO3 toxicity. The complex interactions involving the direct nutritional effects of Ca ions, the indirect effects of its associated anions, and of other ionic species, notably H+, Al3` and Fe2+ are discussed. It is concluded that the principal effect of infection at low exogenous Ca concentrations is to increase the effciencv of Ca capture whereas at high concentrations it is to alleviate toxicity of some auxiliary anions.
• The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limeston... more • The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limestone grassland soil supporting different synthesized vascular plant assemblages that had developed for 3 yr. • The experimental treatments comprised: bare soil; monocultures of the nonmycotrophic sedge Carex flacca ; monocultures of the mycotrophic grass Festuca ovina ; and a species-rich mixture of four forbs, four grasses and four sedges. The diversity of AM fungi was analysed in roots of Plantago lanceolata bioassay seedlings using terminal-restriction fragment length polymorphism (T-RFLP). The extent of AM colonization, shoot biomass and nitrogen and phosphorus concentrations were also measured. • The AM diversity was affected significantly by the floristic composition of the microcosms and shoot phosphorus concentration was positively correlated with AM diversity. The diversity of AM fungi in P. lanceolata decreased in the order: bare soil > C. flacca > 12 species > F. ovina. • The unexpectedly high diversity in the bare soil and sedge monoculture likely reflects differences in the modes of colonization and sources of inoculum in these treatments compared with the assemblages containing established AM-compatible plants.
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