Effects Of Mycorrhizal Colonization Research Articles

The effects of mycorrhizal colonization on phytophagous insects and their natural enemies in soybean fields.

The use of belowground microorganisms in agriculture, with the aim to stimulate plant growth and improve crop yields, has recently gained interest. However, few studies have examined the effects of microorganism inoculation on higher trophic levels in natural conditions. We examined how the diversity of phytophagous insects and their natural enemies responded to the field-inoculation of soybean with a model arbuscular mycorrhizal fungus (AMF), Rhizophagus irregularis, combined with a nitrogen-fixing bacterium, Bradyrhizobium japonicum, and a plant growth-promoting bacterium, Bacillus pumilus. We also investigate if the absence or presence of potassium fertilizer can affect this interaction. We found an increase in the abundance of piercing-sucking insects with the triple inoculant irrespective of potassium treatment, whereas there were no differences among treatments for other insect groups. A decrease in the abundance of the soybean aphid, Aphis glycines, with the double inoculant Rhizophagus + Bradyrhizobium was observed in potassium enriched plots and in the abundance of Empoasca spp. with potassium treatment independent of inoculation type. Although it was not possible to discriminate the mycorrhization realized by inoculum from that of the indigenous AMF in the field, we confirmed global negative effects of overall mycorrhizal colonization on the abundance of phytophagous piercing-sucking insects, phytophagous chewing insects, and the alpha diversity of phytophagous insects. In perspective, the use of AMF/Rhizobacteria inoculants in the field should focus on the identity and performance of strains to better understand their impact on insects.

Effects of mycorrhizal colonization and medicine quality of Paris polyphylla var. yunnanensis inoculated by different foreign AM fungi species

After 28 foreign species of AM fungi were inoculated in sterilized soil, the effects of the AM mycorrhizal colonization and the medicine quality of Paris polyphylla var. yunnanensis were observed by combination of inoculation test in pot at room temperature and instrumental analysis. The results showed that, compared with control group (CK), the inoculation of foreign AM fungi in the soil influenced the spore density, mycorrhizal infection rate, and colonization intensity of AM fungi in root system of P. polyphylla var. yunnanensis. The inoculation of foreign AM fungi enhanced the mycorrhiza viability of P. polyphylla var. yunnanensis by increasing the activity of succinic dehydrogenase (SDH) and alkaline phosphatase (ALP) in intraradical hyphae. The content of single steroid saponin in rhizome of P. polyphylla var. yunnanensis showed variation after P. polyphylla var. yunnanensis was inoculated by different foreign species of AM fungi, which was beneficial for increasing the medicine quality; however, the kinds of steroid saponin showed no difference. In a degree, there was a selectivity of symbiosis between P. polyphylla var. yunnanensis and foreign AM fungi. And we found that the Claroideoglomus claroideum and Racocetra coralloidea were best foreign AM fungi species for cultivating P. polyphylla var. yunnanensis under field condition.

Effect of Mycorrhizal Colonization and Light Limitation on Growth and Reproduction of Lima Bean (Phaseolus lunatus L.)

Plants can respond with sink stimulation of photosynthesis when colonized with fungal or bacterial root symbionts, compensating costs of carbohydrate allocation to the microbes. However, constraints may arise under light limitation when plants cannot extensively increase photosynthesis. We hypothesize that under such conditions the costs for maintaining the symbiosis outweigh the benefits, ultimately turning the mutualist microbes into parasites, resulting in reduced plant growth and reproduction. Using lima bean ( Phaseolus lunatus ) as experimental plant, we applied two levels of light (full light, 75% shading) and microbial inoculation (sterile soil, mycorrhizal fungi) and quantified both vegetative and generative plant traits. As expected, shaded plants produced less vegetative biomass and seeds than non-shaded plants. However, individual seeds were significantly heavier in shaded plants and required less time for germination. While under both light conditions mycorrhizal plants showed a significantly reduced belowground biomass, mycorrhizal fungi neither enhanced overall plants performance in terms of total biomass and seed production nor resulted in measurable costs in either light condition. Our study suggest that mycorrhizal colonization neither provided benefits to lima bean plants grown under full light, nor created costs when photosynthesis was limited.

Host plant—ectomycorrhizal fungus combination drives resource allocation in willow: Evidence for complex species interaction from a simple experiment

Root-associated mycorrhizal fungi affect plant growth and resource allocation. Our major aim was to explore the plant-internal mechanisms behind the effects of mycorrhizal colonization on leaf chemistry of willow (Salix spp.). Combinations of 2 willow varieties (Loden, Tora) and 2 ectomycorrhizal fungal species (Hebeloma fastibile, Tricholoma cingulatum) were grown under controlled conditions. Host plant variety and fungal species effects on host resource allocation (biomass and leaf chemistry) varied in a complex way. Shoot biomass growth and allocation was mostly affected by host plant variety, whereas leaf and root biomass allocation were strongly affected by mycorrhizal treatment. Leaf biomass production was affected by willow variety, mycorrhizal treatment, and the interaction between them. The results indicate a strong effect of mycorrhizal colonization on host plant biomass allocation, which can mediate mycorrhizal effects on leaf chemistry. For example, leaf biomass allocation was inversely correlated with foliar concentrations of salicylic acid, suggesting a functional link between the two. However, the apparent complexity of the host plant variety-fungal species interaction makes any prediction of their possible outcome very difficult, especially in an ecological context. Still, fungal species identity seems to be more important for the responses of Salix to mycorrhizal colonization than the amount of fungal biomass.

丛枝菌根真菌对枳根净离子流及锌污染下枳苗矿质营养的影响

盆栽实验研究了不同施Zn水平(0、300 mg/kg和600 mg/kg)下,接种丛枝菌根真菌<em>Glomus intraradices</em>对枳苗生长、Zn、Cu、P、K、Ca、Mg分布的影响,并采用非损伤微测技术测定分析了菌根化与非菌根化枳根净Ca²⁺、H⁺、NO₃^-离子流动态。结果表明:(1)在不同施Zn水平下,接种菌根真菌显著提高了枳苗地上部及根部鲜重;随着施Zn水平的提高,菌根侵染率呈降低趋势,枳苗地上部与根部Zn含量呈增加趋势,且接种株根部Zn含量显著高于未接种株。(2)接种株未施Zn处理的地上部Cu、P、K、Mg和根部Cu含量、施600 mg/kg Zn处理的根部Cu及施300 mg/kg Zn处理的根部P含量均显著高于对照,而菌根真菌侵染对枳苗Ca含量并无显著性影响。(3)接种株未施Zn处理的根部距根尖端0 μm和600 μm处净Ca²⁺流出速率、600 μm处净H⁺流入速率、2400 μm处净NO₃^-流入速率均显著高于未接种株。;We investigated the effects of arbuscular mycorrhizal fungus inoculation on net ion flux in the roots of trifoliate orange (<em>Poncirus trifoliata</em> (L.) Raf.) and mineral nutrition in seedlings under zinc contamination. A pot experiment was conducted to study plant growth as well as the distribution of zinc, copper, phosphorus, potassium, calcium, and magnesium in trifoliate orange seedlings inoculated by the arbuscular mycorrhizal fungus, <em>Glomus intraradices</em>, under greenhouse conditions. Plants were grown in yellow soil:quartz sand (9∶1, v/v) medium and were irrigated with solutions containing three different zinc concentrations (0, 300 and 600 mg/kg), which corresponded to control, medium and high zinc contamination treatments, respectively. Meanwhile, a non-invasive micro-test technique was used to determine the net fluxes of Ca²⁺, H⁺ and NO₃^- along mycorrhizal and non-mycorrhizal roots of trifoliate orange seedlings grown in medium with no zinc added. Arbuscular mycorrhizal colonization significantly increased the fresh weight of shoots and roots exposed to different zinc levels. With increasing zinc concentrations, the mycorrhizal colonization percentage decreased, while the zinc levels in the shoots and roots progressively increased. Zinc concentrations in the roots were significantly higher in arbuscular mycorrhizal seedlings than in non-arbuscular mycorrhizal seedlings. The shoot/root ratios of the zinc translocation coefficient gradually decreased in arbuscular mycorrhizal seedlings with increasing zinc concentrations. This indicated that at medium or high levels of zinc contamination, a lot of zinc was immobilized in roots through the establishment of mycorrhizae. Therefore, phytotoxicity might be alleviated. Copper, phosphorus, potassium, and magnesium concentrations in shoots of plants grown in medium with no added zinc were significantly higher in arbuscular mycorrhizal seedlings than those in non-arbuscular mycorrhizal seedlings. Similarly, copper concentrations in the roots of plants in medium with 600 mg/kg of added zinc and phosphorus concentrations in roots of plants in medium with 300 mg/kg added zinc were higher in arbuscular mycorrhizal seedlings. Arbuscular mycorrhizal colonization had no significant effects on calcium concentrations in seedlings. Copper and phosphorus concentrations gradually decreased in both arbuscular mycorrhizal and non-arbuscular mycorrhizal seedlings with increasing zinc levels, which demonstrated that zinc levels in seedlings are negatively correlated with copper or phosphorus. With no added zinc, phosphorus, potassium, magnesium and copper uptake was promoted by arbuscular mycorrhizal fungus infection. Under zinc contamination, phosphorus, and copper uptake was still accelerated by mycorrhizal colonization. Thus, the effects of mycorrhizal colonization were not only related to the degree of zinc pollution, but were also correlated with the species of fungi and host plants. Additionally, net Ca²⁺ efflux at 0 μm and 600 μm, net H⁺ influx at 600 μm, and net NO₃^- influx at 2400 μm from the root tip of arbuscular mycorrhizal seedlings in medium with no added zinc were significantly higher than those of non-arbuscular mycorrhizal seedlings. These results suggest that mycorrhizal symbiosis activates Ca²⁺-ATPase, Ca²⁺/H⁺ antiporters and NO₃^-/H⁺ symporters in root plasma membranes. Nutrient uptake and stimulation of growth are mediated by arbuscular mycorrhizal fungi. Furthermore, the variations detected in arbuscular mycorrhizal roots for Ca²⁺, H⁺ and NO₃^- fluxes point to a significant involvement of the fungus.

The Significance of Ectomycorrhizas in Chemical Quality of Silver Birch Foliage and Above-Ground Insect Herbivore Performance

We tested whether the ectomycorrhizal (ECM) infection level of roots of silver birch (Betula pendula) affects performance of above-ground insect herbivores by increasing available plant biomass, by enhancing availability of nutrients, or by modifying concentration of defense compounds, i.e., phenolics, in birch foliage. Insect performance was determined for a phloem-feeding generalist (Lygus rugulipennis, the European tarnished plant bug), a phloem-feeding specialist (Calaphis flava, the birch aphid), and a chewing generalist (Epirrita autumnata, the autumnal moth larva). Silver birch plantlets had either natural ECM infection level (on average 24% of short roots with ECM), reduced ECM infection level with fungicide (F-, 9% ECM), or enhanced ECM infection level after inoculation with the fungus Paxillus involutus (PI+, 45% ECM) or Leccinum versipelle (LV+, 42% ECM). In general, the most pronounced effect of ECM was observed on growth of plantlets, i.e., stem growth decreased. In PI+-treated plants, leaf biomass also decreased. The effect of mycorrhizal colonization on the host plant's nitrogen (N) and phosphorous (P) concentration was dependent on the mycorrhizal species and experiment. Fungicide treatment did not cause a consistent decrease in nutrients. Finally, defense of birch against herbivory, expressed as foliar phenolic concentration in plantlets, was not modified by ECM. However, E. autumnata had a significantly higher relative growth rate on PI+ plantlets with high leaf N concentration than on LV+ plantlets with low leaf N concentration. The birch aphid C. flava produced significantly less nymphs on birches with enhanced ECM infection levels (PI+ and LV+ plantlets) than on controls. In summary, our data show that the ECM infection level mainly affects the growth parameters of plantlets, whereas effects on leaf chemical quality are minor. Our data show that effects of ECM infection of birch roots on aboveground herbivores are multifaceted and depend on the fungal species forming ectomycorrhiza and also on the degree of specialization and feeding guild of insects.

Arbuscular mycorrhizal colonisation increases copper binding capacity of root cell walls of Oryza sativa L. and reduces copper uptake

There is evidence that colonisation by mycorrhizal fungi can protect host plants from toxic concentrations of heavy metals. The mechanism by which protection is provided by the fungus for any particular metal is poorly understood. Rice ( Oryza sativa L.) plants were inoculated with Glomus mosseae and grown for 4 weeks to ensure strong colonisation. The plants were then exposed to low to toxic concentrations of copper (Cu) and the uptake and distribution were examined. The effect of mycorrhizal colonisation on the cell wall composition and Cu binding capacity of roots was also investigated. Mycorrhizal plants showed moderate reductions in Cu concentrations in roots but large reductions in shoots. In roots, mycorrhizal plants accumulated more Cu in cell walls but much less in the symplasm compared to non-mycorrhizal plants. The differences in cell wall binding of Cu could be partly explained by changes in the composition of the cell wall. The mechanistic basis for the reduced Cu accumulation and the potential beneficial consequences of mycorrhizal associations on plant growth in Cu toxic soil are discussed.

Ectomycorrhizal colonization and intraspecific variation in growth responses of lodgepole pine

Across different host plant species, the effects of mycorrhizal colonization on host growth parameters can vary, but intraspecific variation in this relationship has rarely been measured. We tested the direction and consistency of the relationship between ectomycorrhizal colonization level and growth responses across seed families of Pinus contorta var. latifolia. Root tips of seedlings from eight full sib seed families varied in levels of ectomycorrhizal fungal colonization from 39% to 100%. We observed positive, negative, or neutral relationships between colonization level and shoot mass, depending on plant family. For the majority of seed families no relationship was observed between colonization level and root mass; however, two seed families showed negative relationships. Shoot height differed only by seed family. Results from our study indicate that the relationship between colonization level and host growth depends on host genotype. We suggest that models of plant intraspecific interactions should consider ectomycorrhizal associations when assessing phenotypic variability.

Effects of Mycorrhizal Colonization and Fertilization on Growth and Photosynthesis of Sweet Basil Under Salt Stress

ABSTRACT A greenhouse experiment was conducted to investigate the effects of inoculation with an arbuscular mycorrhizal fungus (AMF) and of different levels of fertilization on growth, photosynthetic efficiency, and ion uptake by sweet basil (Ocimum basilicum L.) in conditions of saline stress. Eight different treatments were compared, deriving from the factorial combination of two levels of fertilization, two levels of salinity of the irrigation water and the presence or absence of mycorrhizal colonization, performed with a crude inoculum of Glomus intraradices Schenck and Smith. Salinity significantly reduced growth and fluorescence level of plants, and higher sodium (Na) and chloride (Cl) contents in both roots and shoots were associated with these phenomena, while potassium (K) was slightly reduced. Both mycorrhizal inoculation and high fertilization acted in the direction of reducing the symptoms of salinity stress by increasing growth and fluorescence. Mycorrhizal colonization showed generally more pronounced effects than a high application rate of fertilizer. The simultaneous application of colonization and high fertilization provided higher tolerance to salinity than one single factor, but the combined effect was less than proportional to the sum of the separate effects. In particular, the differences in growth and photochemical efficiency between inoculated and uninoculated plants under salt stress were significantly reduced by the contemporary presence of additional fertilization. The rate of root colonization by the mycorrhizal fungus was significantly reduced by both saline irrigation and high fertilization.

Effects of Mycorrhizal Colonization on Growth Parameters of Onion under Different Irrigation and Soil Conditions

The effects of three Arbuscular Mycorrhizal Fungi (AMF), Glomus versiforme, G. intraradices and G. etonicatum) and three irrigation intervals (7, 9 and 11 days) on growth of onion (Allium cepa L.) cv. Red Azar Shahr were studied under two soil conditions (sterilized and non-sterilized). The results indicated that, AMF colonization improved plant height, Leaf Area Index (LAI), total biomass, bulb dry mass and diameter, Harvest Index (HI) and chlorophyll content (p < 0.001). Bulbing occurred 10-15 days earlier in mycorrhizal plants. Irrigation interval decreased biomass, LAI, Leaf Area Ratio (LAR), bulb diameter and dry mass and chlorophyll content (b and total) at 11 day irrigation interval. In term of interaction, G. versiforme at 9 day and non-mycorrhizal plants at 11 day produced the greatest and the lowest LAI (8.56 vs. 1.57), respectively. Mycorrhizal onions in contrary to non-mycorrhizal ones produced more LAI and biomass in sterilized soil and inoculation with G. etonicatum and the non-mycorrhizal onions in sterilized soil had the highest and the lowest biomass, respectively.

Changes in root morphology accompanying mycorrhizal alleviation of phosphorus deficiency in micropropagated Annona cherimola Mill. plants

The effect of the mycorrhizal inoculation on survival rate, growth, nutrient uptake and root morphology during the acclimatization period and plant establishment of micropropagated juvenile or adult cherimoya plants ( Annona cherimola Mill.) was determined. Although mycorrhizal colonization did not improve the survival rate of plants, which was already high in non-inoculated plants, it had a positive effect on plant development (shoot length, leaf number, leaf area and fresh and dry weights). Mycorrhizal juvenile plants tripled the macronutrients and increased by four the micronutrient uptake, and mycorrhizal adult plants increased by two phosphorus and all micronutrients, with copper uptake increased five times. Moreover, mycorrhizal colonization changed the root morphology of adult plants increasing three-fold the total number of roots, doubling the number of first-order laterals and increasing second-order laterals by four. Total root length was also increased three-fold, adventitious root length was almost doubled, first-order laterals tripled and second-order roots length increased four-fold. The effect of mycorrhizal colonization seems to be stronger or different in juvenile than in adult plants, suggesting that ontogenic stage is an important factor determining mycorrhizal effect and the plant performance during the acclimatization period.

Effects of mycorrhizal colonisation on Thymus polytrichus from heavy-metal-contaminated sites in northern England.

A study was performed to establish whether colonisation with arbuscular mycorrhizal (AM) fungi is beneficial to wild thyme [ Thymus polytrichus A. Kerner ex Borbás ssp. britannicus (Ronn.) Kerguelen (Lamiaceae)] growing in the heavy-metal-contaminated soils along the River South Tyne, United Kingdom. T. polytrichus plants of the same genotype were grown under controlled conditions with and without Zn contamination, and differences between AM-colonised and -uncolonised plants in mean shoot and root growth (dry weight) and Zn concentration were assessed. When grown in the heavy-metal-contaminated, low-P soil from one of the South Tyne sites, AM-colonised plants grew significantly larger than uncolonised plants; however, there was no significant difference in growth between AM and non-AM plants grown in an artificial substrate with a larger available P concentration, with or without Zn contamination. Mycorrhizal colonisation increased tissue Zn concentrations during the experiments. It is concluded that AM fungi are beneficial, if not essential, to T. polytrichus growing in the low-nutrient soils along the River South Tyne, because of their role in enhancing plant uptake of P (and possibly other nutrients). There was no evidence from this study that the fungi reduce plant uptake of heavy metals at these sites, but rather increase Zn uptake. However, the resulting tissue metal concentrations do not appear to be large enough to be detrimental to plant growth.

Changes in bacterial community structure induced by mycorrhizal colonisation in split-root maize

Maize plants were grown in an autoclaved quartz sand–soil mix to which the bacterial communities of the soil and the mycorrhizal inocula were reintroduced. The root systems of the plants were divided with the two halves growing in separate pots. There were five different treatments: plants with both root halves non-mycorrhizal either at high or low P availability (nm-nm HP and nm-nm LP) or mycorrhizal plants grown at low P availability with one side of the root system non-mycorrhizal while the other side was inoculated with Glomus intraradices (GI-nm) or G. mosseae (GM-nm) or plants with both sides mycorrhizal but each side inoculated with a different fungus (GI–GM). The plants were harvested after 3 or 6 weeks. Shoot dry weight and shoot P concentration of the nm-nm HP plants were always higher than the plants grown at low P supply. Acid phosphatase activity in the rhizosphere was similar in all treatments and did not change over time. However, after 6 weeks alkaline phosphatase activity was higher in the rhizosphere of both root halves in the mycorrhizal plants as compared to the non-mycorrhizal plants. Mycorrhizal colonisation increased from 15–34% after 3 weeks to 78–87% after 6 weeks with no significant difference between GI and GM. The bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE), changed over time and was specific for each of the three compartments, non-rhizosphere soil, rhizosphere soil and root surface. While the two P levels in the non-mycorrhizal treatments had no significant effect on the bacterial communities, mycorrhizal colonisation changed the bacterial community structure on the root surface and in the non-rhizosphere soil. The bacterial communities of the GI–GM plants differed more from the non-mycorrhizal plants than those of the plants with only one half of the root system mycorrhizal. The bacterial communities of both root halves of the GI-nm plants did not differ from each other and were very similar to those of the non-mycorrhizal plants. After 3 weeks, the bacterial communities of the two sides of the GM-nm plants differed: the mycorrhizal side of the resembled that of the GI–GM plants, while the non-mycorrhizal side of the root system was similar to that of the nm-nm plants. However after 6 weeks, the bacterial community structures of the two sides of the root system of the GM-nm plants were similar and differed from those of the nm-nm plants. It is concluded that the effect of mycorrhizal colonisation on the bacterial community structure in the rhizosphere may, at least in part, be plant-mediated.

The responses of seven co‐occurring woodland herbaceous perennials to localized nutrient‐rich patches

Summary 1 Nutrient‐rich patches can occur in soils at a wide range of spatial and temporal scales. Differences in the root proliferation response between species may be due to differing abilities of root systems to locate and recognize patches of differing size and nutrient concentration. 2 We investigated the root proliferation responses of seven co‐occurring herbaceous perennial British woodland species (Ajuga reptans, Glechoma hederacea, Oxalis acetosella, Silene dioica, Stachys sylvatica, Veronica montana and Viola riviniana) and the effect of mycorrhizal colonization on any response. 3 Plants were planted in nutrient‐poor sand in the centre of rectangular pots, with a nutrient‐rich patch to one side and a control (sand) patch on the other. Size and nutrient concentration of the patches were varied between treatments. 4 Species differed in the size of their root systems and in their ability to respond to localized nutrient‐rich patches. Oxalis acetosella and Viola riviniana, which produced the smallest root systems, showed similar root growth in nutrient‐rich and control patches. All other species responded to the presence of a nutrient‐rich patch by various combinations of root proliferation, changes in root branching pattern and by an increase in specific root length. 5 In some species the response was affected by patch attributes: Silene dioica and Veronica montana were sensitive to the nutrient concentration of the patch, and Glechoma hederacea did not respond to the smallest sized patch. 6 Mycorrhizal colonization had little effect on root proliferation. Only one species (Oxalis acetosella) could be shown to benefit from colonization by increased phosphate uptake. 7 The soil is a heterogeneous environment in terms of nutrient availability; differences between species in the ability to exploit this heterogeneity may affect their distribution, and could be a mechanism that reduces interspecific root competition.

Plasma membrane ATPase and H+ transport activities in microsomal membranes from mycorrhizal tomato roots

plants, including many agricultural important crop species (Barea et al., 1993). The fungi biotrophically colonize the ATPase activity, ATP-dependent H+ transport and the cortex of the root to obtain carbon compounds from the amount of antigenic tomato plasma membrane H+- host plant, while assisting the plant with the supply of ATPase have been analysed in membrane vesicles isol- phosphate and other mineral nutrients that the external ated from Glomus mosseae -o rGlomus intraradices- fungal mycelium takes up from the soil (Harley and colonized roots and from non-mycorrhizal tomato Smith, 1983). The transfer mechanisms between symroots. Microsomal protein content was higher in mycor- bionts are of considerable interest and are likely to involve rhizal than in control roots. The specific activity of the specific alterations in membrane transport processes. plasma membrane H+-ATPase was not affected by Woolhouse was the first to suggest that the controlled mycorrhizal colonization, although this activity in- transfer of nutrients required for a functional mycorrhizal creased in membranes isolated from mycorrhizal roots symbiosis is carried out by means of active transport when expressed on a fresh weight basis. Western blot processes ( Woolhouse, 1975). analysis of microsomal proteins using antibodies raised In many cellular systems, transport of solutes into and against the Arabidopsis thaliana plasma membrane H+- out of the cell involves processes driven by the H+ ATPase showed that mycorrhizal colonization did not electrochemical gradient generated by the plasma memchange the relative amount of tomato plasma mem- brane H+-ATPase, which pumps protons out of the cell brane ATPase in the microsomes. However, on a fresh (Serrano, 1989). Therefore, the hypothesis of an active weight basis, there was a greater amount of this protein transport in mycorrhizal symbioses requires the presence in roots of mycorrhizal plants. In addition, mycorrhizal of a H+ pumping activity. This hypothesis has been membranes showed a higher specific activity of the supported by cytochemical, electrophysiological and biovanadate-sensitive ATP-dependent H+ transport than chemical data. In particular, cytochemical studies (Marx membranes isolated from control roots. These results et al., 1982; Smith and Smith, 1990; Gianinazzi-Pearson suggest that mycorrhiza might regulate the plasma et al., 1991) have shown that in arbuscular mycorrhizas membrane ATPase by increasing the coupling effici- ATPase activity is associated with both plant and fungal ency between H+ transport and ATP hydrolysis. The membranes of the arbuscular interface, which is consistent observed effects of mycorrhizal colonization on plasma with active transport processes occurring at this location. membrane H+-ATPase were independent of the AM ATPases represent proton pumps that establish proton motive forces and drive the uptake of P i or hexose against

Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (380 ± 50 µmol mol-1) and elevated (600 ± 150 Émol mol-1) atmospheric CO2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO2 stimulated plant growth to the same extent in mycorrhizal and non-mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO2, and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, of plants to elevated CO2. 13C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the the plants as dark-respiration. The need for detailed studies in this area is emphasized.

Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (380±50 μmol mol -1 ) and elevated (600 ± 150 μmol mol -1 ) atmospheric CO 2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO 2 stimulated plant growth to the same extent in mycorrhizal and non-mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO 2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO 2 , and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, of plants to elevated CO 2 . 13 C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO 2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO 2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the the plants as dark-respiration. The need for detailed studies in this area is emphasized.

Can arbuscular mycorrhiza change the effect of root competition between conspecific plants of different ages?

Seedlings and adults of Hypericum perforatum L., common St. Johnis wort, were grown together in an experiment of factorial design, where the presence of root competition and arbuscular mycorrhizal inoculation were used as treatments. There was moderate shoot competition, the intensity of which was not manipulated. To check the response of plants to arbuscular mycorrhiza in noncompetitive conditions, adults and seedlings were also grown singly in pots. Single individuals of seedlings and adults responded positively to mycorrhizal inoculation, with the response of seedlings significantly greater. In the competition experiment the positive effect of mycorrhizal colonization on seedling growth vanished, since with root competition, both mycorrhizal and nonmycorrhizal seedlings were of the same size. Without root competition, the shoots of mycorrhizal adults were much larger than of nonmycorrhizal adults, but under root competition the shoot weight did not differ. Arbuscular mycorrhiza increased the biomass differences between competing seedlings and adults. We did not confirm our hypothesis that mycorrhiza makes competition between seedlings and adults more balanced because of the stronger positive response of seedlings to inoculation. The positive effect of arbuscular mycorrhizae on growth vanishes in more crowded conditions. It was concluded that if mycorrhizal inoculation has age-specific positive effect of seedlings in field conditions, it is more probably due to higher tolerance to abiotic stress than due to higher competitive ability of seedlings.

Effects of mycorrhizal colonization by Paxillus involutus on uptake of Ca and P by Picea abies and Betula pendula grown in unlimed and limed peat

summaryUptake of P(32P) and Ca(45Ca) by seedlings of Picea abies (L.) Karst and Betula pendula Roth, non‐mycorrhizal or mycorrhizal with Paxillus involutus (Batsch: Fr) Fr. was studied. Seedlings were grown in unamended peat (pH 4.0) or in peat limed (CaO) to pH 5.1 or 6.1. A double‐labelled (32P and 45Ca) complete nutrient solution was added to the peat 7 wk after planting. An 8 d period was allowed for uptake of the isotopes before the seedlings were harvested.Mycorrhizal colonization clearly increased the uptake of P(32P) in the unlimed substrate and in the substrate limed to a pH of 5.1. At the highest lime rate, the uptake of P(32P) was greatly reduced in both mycorrhizal and non‐mycorrhizal seedlings. The difference in P uptake between mycorrhizal and non‐mycorrhizal seedlings was small at this lime rate. The mycorrhizal colonization of the roots was not affected by liming. How‐ever, it is possible that the mycelial growth into the substrate was inhibited. The reduction in uptake could thus be an effect of a lower availability of P in combination with a decreased fungal uptake surface at the highest lime rate.The mycorrhizal effect on uptake of Ca was much smaller than its effect on uptake of P. Mycorrhizal colonization increased the Ca(45Ca) uptake in the unlimed treatment, where the Ca content in the substrate was very small. In the limed substrates the uptake of Ca was as high or higher in the non‐mycorrhizal than in the mycorrhizal seedlings.

Endomycorrhizal fungi in nitrogen transfer from soybean to maize

Using 15N as a tracer, interspecific N-transfer was studied during the course of plant development. The use of barriers of differing permeabilities between donor and receiver plants allowed separation of the effect of mycorrhizal colonization, root or hyphal contact and interplant hyphal bridging, on 15N-transfer from soybean (Glycine max (L.) Merrill) to maize (Zea mays L.). More transfer was measured between mycorrhizal plants, but transport of 15N from the labelled host plant to Glomus versiforme (Karsten) Berch did not seem to occur at the symbiotic interface, suggesting that the fungus is independent of its host for its N-nutrition, and that the role of hyphal bridges in N-transfer between plants, is not significant. Uptake by the receiver plant of the N excreted by the donor plant root system appears to be the mechanism of N-transfer between plants. The factor most affecting 15N-transfer between plants was found to be the extent of the contact between plant root systems. The presence of the endomycorrhizal fungus in plant roots reduced 15N-loss from soybean, but at the same time, its extensive hyphal network improved the efficiency of the maize root system for the recovery of the 15N excreted by soybeans. The net result was a better conservation of the N resource within the plant system. The transfer of N between mycorrhizal plants was particularly enhanced by the death of the soybean.