Plant growth depressions in arbuscular mycorrhizal symbioses: not just caused by carbon drain?

* This study investigated effects of plant density and arbuscular mycorrhizal (AM) colonization on growth and phosphorus (P) nutrition of a cultivar of wheat (Triticum aestivum) that often shows early AM-induced growth depressions. * Two experiments were conducted. Expt 1 had three plant densities and one soil P concentration. Expt 2 had two plant densities and two P concentrations. Plants were grown in calcareous P-fixing soil, inoculated with Glomus intraradices or Gigaspora margarita, or noninoculated (nonmycorrhizal (NM)). Glomus intraradices colonized well and caused a growth depression only in Expt 1. Gigaspora margarita caused large growth depressions in both experiments even though it colonized poorly. * The results showed that growth depressions were mitigated by changes in relative competition for soil P by NM and AM plants, and probably by decreasing carbon costs of the fungi. * The different effects of the two fungi appear to be attributable to differences in the balance between P uptake by the fungal pathway and direct uptake via the roots. These differences may be important in other AM symbioses that result in growth depressions. The results show that mycorrhizal growth responses of plants grown singly may not apply at the population or community level.     

Fresh perspectives on the roles of arbuscular mycorrhizal fungi in plant nutrition and growth

Recent research on arbuscular mycorrhizas has demonstrated that AM fungi play a significant role in plant phosphorus (P) uptake, regardless of whether the plant responds positively to colonization in terms of growth or P content. Here we focus particularly on implications of this finding for consideration of the balance between organic carbon (C) use by the fungi and P delivery (i.e. the C-P trade between the symbionts). Positive growth responses to arbuscular mycorrhizal (AM) colonization are attributed frequently to increased P uptake via the fungus, which results in relief of P deficiency and increased growth. Zero AM responses, compared with non-mycorrhizal (NM) plants, have conventionally been attributed to failure of the fungi to deliver P to the plants. Negative responses, combined with excessive C use, have been attributed to this failure. The fungi were viewed as parasites. Demonstration that the AM pathway of P uptake operates in such plants indicates that direct P uptake by the roots is reduced and that the fungi are not parasites but mutualists because they deliver P as well as using C. We suggest that poor plant growth is the result of P deficiency because AM fungi lower the amount of P taken up directly by roots but the AM uptake of P does compensate for the reduction. The implications of interplay between direct root uptake and AM fungal uptake of P also include increased tolerance of AM plants to toxins such as arsenate and increased success when competing with NM plants. Finally we discuss the new information on C-P trade in the context of control of the symbiosis by the fungus or the plant, including new information (from NM plants) on sugar transport and on the role of sucrose in the signaling network involved in responses of plants to P deprivation.     

Influence of Arbuscular Mycorrhiza and Phosphorus Supply on Polyamine Content, Growth and Photosynthesis of Plantago lanceolata

A greenhouse pot experiment with different phosphorus supply was conducted to study growth, photosynthesis and free polyamine (PA) content in Plantago lanceolata L. plants in relation to arbuscular mycorrhizal (AM) colonization. Inoculum of Glomus fasciculatum (BEG 53) was used. Inoculated plants had high colonization intensities which were related to the P supply. Non-mycorrhizal (NM) plants showed a typical yield response curve for P availability. Dry masses of mycorrhizal (M) plants were higher at the lowest soil P content than those of NM plants, but the opposite was found at the highest P supply. P contents in M plants were always higher. There were no differences in chlorophyll (Chl) concentrations (except the lowest soil P content) and ratios of variable to maximum Chl fluorescence (Fv/Fm) values between M and NM plants, whereas M plants had higher ratios of leaf area to fresh mass (A/f.m.) at low soil P contents and they had significantly higher CO₂ fixation capacities per unit leaf area. Free putrescine (Put), spermidine (Spd) and spermine (Spm) contents in NM plants were usually highest at the lowest P supply. The ratios of Put/(Spd+Spm) were identical in M and NM leaves. They were significantly higher, however, in NM roots at the two low P doses. It is concluded, that a P nutritional status might exist, below which PA concentrations and ratio are increased drastically, possibly indicating P deficiency or a certain state of plant development with a higher demand for AM symbiosis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Arbuscular Mycorrhizal Fungi Enhance Zinc and Nickel Uptake from Contaminated Soil by Soybean and Lentil

Generally, soils in Pakistan are deficient in P and N. Due to intensive cropping and irrigation, Pakistani soils have also become deficient in micronutrients such as Zn, Fe, Cu, and Mn. Arbuscular mycorrhizal fungi, which form symbiotic associations with roots of most land plants, are known to enhance uptake of P and trace elements such as Cu, Ni, Pb, and Zn. The present study was conducted to investigate the role of arbuscular mycorrhizae (AM) in uptake of nickel (Ni) and zinc (Zn) by crops viz. soybean (Glycine max (L.) Merrill) and lentil (Lens culinaris Medic). Zn and Ni were applied as ZnSO₄ 7H₂O and NiCl₂ respectively, in four concentrations (0.0, 1.0, 3.0, and 5.0 g kg^-1 soil). AM inoculum consisted of sand containing sporocarps, spores, and AMF infected root pieces from a pot culture of Glomus mosseae. Control plants received pot culture filtrate containing soil microflora minus AM fungal propagules. A significant difference (p < 0.05) was observed in the dry weights of roots and shoots of the mycorrhizal (M) and nonmycorrhizal (NM) cereal plants. The sievate-amended treatments did not stimulate plant growth to the same extent as the AM fungal amended treatments. Trace metals inhibited the extent of mycorrhizal colonization of the cereal roots. The concentrations of the trace metals in the plant tissues of 12-week old cereal plants were found significantly (p < 0.05) higher in M than NM plants. These results indicate that mycorrhize can be used as effective tools to supply sufficient Zn in generally Zn-deficient Pakistani soils and to ameliorate the toxicity of trace metals in polluted soils. The contents of Ni in mycorrhizal soybean plant tissues were higher than those in the mycorrhizal lentil plant tissues. The implications of these results in mycorrhizo remediation of agricultural soils are discussed.     

Evidence that soil nutrient stoichiometry controls the competitive abilities of arbuscular mycorrhizal vs. root-borne non-mycorrhizal fungi

A majority of plant species has roots that are colonized by both arbuscular mycorrhizal (AM) and non-mycorrhizal (NM) fungi. The latter group may include plant mutualists, commensals, parasites and pathogens. The co-occurrence of these two broad groups may translate into competition for root volume as well as for plant-derived carbon (C). Here we provide evidence that the relative availability of soil nitrogen (N) and phosphorus (P) (i.e., soil nutrient stoichiometry) controls the competitive balance between these two fungal guilds. A decrease in the soil available N:P ratio resulted in a lower abundance of AM fungi and a corresponding increase in NM fungi. However, when the same fertilization treatments were applied in a soil in which AM fungi were absent, lowering the soil available N:P ratio did not affect NM fungal abundance. Taken collectively, our results suggest that the increase in NM fungal abundance was not a direct response to soil nutrient stoichiometry, but rather a competitive release from AM fungi responding negatively to higher soil P. We briefly discuss the mechanisms that may be responsible for this competitive release.     

Effectiveness of arbuscular mycorrhizal fungi in phytoremediation of lead- contaminated soil by vetiver grass

A greenhouse experiment was conducted to evaluate the effectiveness of arbuscular mycorrhizal (AM) fungi in phytoremediation of lead (Pb)-contaminated soil by vetiver grass. Experiment was a factorial arranged in a completely randomized design. Factors included four Pb levels (50, 200, 400, and 800 mg kg^?1) as Pb (NO₃)₂, AM fungi at three levels (non mycorrhizal (NM) control, Rhizophagus intraradices, Glomus versiforme). Shoot and root dry weights (SDW and RDW) decreased as Pb levels increased. Mycorrhizal inoculation increased SDW and RDW compared to NM control. With mycorrhizal inoculation and increasing Pb levels, Pb uptake of shoot and root increased compared to those of NM control. Root colonization increased with mycorrhizal inoculation but decreased as Pb levels increased. Phosphorus concentration and uptake in shoot of plants inoculated with AM fungi was significantly higher than NM control at 200 and 800 mg Pb kg^?1. The Fe concentration, Fe and Mn uptake of shoot in plants inoculated with Rhizophagus intraradices in all levels of Pb were significantly higher than NM control. Mycorrhizal inoculation increased Pb extraction, uptake and translocation efficiencies. Lead translocation factor decreased as Pb levels increased; however inoculation with AM fungi increased Pb translocation.     

Wheat Responses to Arbuscular Mycorrhizal Fungi in a Highly Calcareous Soil Differ from those of Clover, and Change with Plant Development and P supply

We evaluated the roles of arbuscular mycorrhizal (AM) fungi in growth and phosphorus (P) nutrition of wheat (Triticum aestivum L.) in a highly calcareous soil and compared the responses of wheat with those of clover (Trifolium subterraneum L). In the first experiment wheat (cv. Brookton) was harvested at 6 wk. Colonisation by four AM fungi was low (<20%). Clover was harvested at 8 wk. Colonisation varied with different fungi, with the highest value (52%) obtained with Glomus intraradices. Although suffering from P deficiency, non-mycorrhizal (NM) wheat grew relatively well with no added P (P0) and application of P at 100 mg kg⁻¹ (P100) increased the dry weight (DW). Shoot P concentrations increased with P application and there were positive effects of all AM fungi at P100. In contrast, NM clover grew very poorly at P0 and did not respond to P application. Clover responded positively to all AM fungi at both P levels, associated with increases in P uptake. In the second experiment colonisation by a single AM fungus (G. intraradices) of two wheat cultivars (Brookton and Krichauff) was well established at 6 wk (~50% in P0 plants) and continued to increase up to maturity (~70%), but decreased greatly at both harvests as P supply was increased (up to 150 mg P kg⁻¹: P150). Addition of P significantly increased plant growth, grain yield and P uptake irrespective of cultivar and harvest time, and the optimum soil P for grain yield was P100. In both cultivars, a growth depression in AM plants occurred at 6 wk at all P levels, but disappeared at 19 wk with added P. At P0, AM plants also produced lower grain yield (weight) per plant, but with higher P, AM plants produced higher grain yields than NM plants. There was a significant positive effect of AM on grain P concentration at P0, but not at other P levels. Brookton was somewhat more P efficient than Krichauff, and the latter responded more to AM fungi. This study showed that responses of wheat to AM inoculation and P supply were quite different from those of clover, and changed during development. Results are discussed in relation to the underlying soil properties.     

丛枝菌根真菌参与下植物-土壤系统的养分交流及调控

近几年随着有机农业的发展,丛枝菌根的作用受到特别关注。丛枝菌根是由植物根系与丛枝菌根真菌(AMF)形成的一种共生体。在植物-AMF-土壤系统中,AMF为植物提供N、P等营养的同时从根系得到所需的C。概述了植物-AMF-土壤系统中C、N、P等营养物质的交流以及AMF与土壤微生物的互作关系。丛枝菌根的形成可显著提高植物对P的吸收,且在高P条件下多余的P可储存于AMF中。AMF对土壤N循环的影响相当复杂,可能参与调控N循环的多个过程,如硝化作用、反硝化作用和氨氧化作用等。在有机质丰富的土壤中AMF菌丝可快速扩增并吸收其中的N,主要供菌丝自身所需,只有一小部分传递给植物。AMF对土壤C库的影响尚存争议,可能存在时间尺度的差异。短期内可活化土壤C,而在长期尺度上可能有利于土壤C的储存。AMF能够通过改变土壤微生物群落结构而影响植物-土壤体系的物质交流。AMF与解磷菌、根瘤菌和放线菌的协同增效作用可促进土壤有机质的降解或增强其固氮能力;AMF对氨氧化菌的抑制作用可降低氨的氧化减少N2O的释放。AMF与外生共生真菌EMF共存时,表现出协同增效作用,但EMF的优先定殖会限制AMF的侵染。AMF不同类群之间则主要表现为竞争和拮抗关系。AMF与土壤微生物之间的互作关系受土壤无机环境的影响,在养分亏缺条件下微生物之间往往表现为竞争关系。因植物、AMF与土壤微生物之间存在复杂的互作关系,为此AMF并不总是表现出其对植物营养的促进作用。目前关于AMF的作用机理仍以假说为主,需要进一步的实验验证。在植物-AMF-土壤系统中N与C的交流和P与C的交流并未表现出一致性,对N、P循环相互关系的进一步探讨有助于深入理解植物-土壤体系中的养分循环。植物、AMF和土壤微生物的养分来源及其对养分的相对需求强度和吸收效率尚未可知,因此无法深入理解AMF在植物-土壤体系中养分交流和转化的作用。在方法上,传统的土壤学方法在养分动态研究中存在局限性,现代分子生物学手段和化学计量学的结合值得尝试。     

Influence of Arbuscular Mycorrhizal (AM) Symbiosis on Phosphorus Leaching through Soil Cores

Two experiments with soil cores were carried out to investigate the effects of arbuscular mycorrhizal (AM) fungal colonization on mobility of phosphorus (P) during leaching of repacked columns of a soil with a loamy sand texture. Trifolium subterraneum plants inoculated with an AM fungus or not inoculated were grown in cores with low or high P concentrations for 8 or 10 weeks in the glasshouse. Cores were then irrigated with 2500 mL water and the leachate collected. Plant growth and the amounts of P removed by plants, remaining in soil as available P and removed dissolved in leachate were measured. Mycorrhizal fungal colonization and development of external hyphae were also determined. Inoculation and/or P application significantly increased plant growth and plant P removal and decreased P leaching. In low P soils AM fungal colonization significantly increased plant P uptake and decreased soil available P and total dissolved P in leachates. Lower P leaching from cores with AM plants under low P conditions was related to enhancement of plant growth and to scavenging and removal of P from the soil by roots and/or external hyphae. When P was applied AM effects were not observed and available P remaining in the soil after leaching was much higher, regardless of AM fungal colonization.     

Influence of arbuscular mycorrhizal fungi on growth and selenium uptake by garlic plants

The aim of this study is to investigate the effects of arbuscular mycorrhizal fungi (AMF) on garlic plants growth and the uptake of selenium (Se). Garlic plants were grown in the pots inoculated with Glomus fasciculatum and G. mosseae and maintained in a greenhouse. Three weeks after planting, the pots had received different concentrations of Se (5, 10, 15, 20, 25 mg kg^?1 of soil) in the form of selenium dioxide (SeO₂) at 3 weeks intervals up to 12 weeks. For physiological and biochemical analysis, the samples were randomly collected from five plants of each experiment. Maximum AM infection, spore population and plant biomass were observed in the roots of mycorrhizal-mediated plants without Se, and they were gradually declined in both mycorrhizal and non-mycorrhizal (NM) plants with increasing concentrations of Se. Among the two Glomus species tested, G. fasciculatum-mediated plants showed higher AM infection, spore population and plant biomass than G. mosseae. No differences were observed for the uptake of Se in mycorrhizal plants and NM plants. However, NM plants uptake more Se than mycorrhizal plants. Higher contents of total chlorophyll and sugars were observed in plants inoculated with G. fasciculatum without Se and they were decreased in the presence of Se. In contrast, increased amount of glutathione peroxidase was observed at increasing concentrations of Se up to 20 mg kg^?1. High-performance liquid chromatography data revealed that SeO₂ converted to organic form of Se as γ-glutamyl-Se-methylselenocysteine. These results are basis for further investigations on the role of AMF on plant growth and uptake of Se in crop plants.     

Plant performance in stressful environments: interpreting new and established knowledge of the roles of arbuscular mycorrhizas

Arbuscular mycorrhizal (AM) symbioses are formed by approximately 80% of vascular plant species in all major terrestrial biomes. In consequence an understanding of their functions is critical in any study of sustainable agricultural or natural ecosystems. Here we discuss the implications of recent results and ideas on AM symbioses that are likely to be of particular significance for plants dealing with abiotic stresses such as nutrient deficiency and especially water stress. In order to ensure balanced coverage, we also include brief consideration of the ways in which AM fungi may influence soil structure, carbon deposition in soil and interactions with the soil microbial and animal populations, as well as plant-plant competition. These interlinked outcomes of AM symbioses go well beyond effects in increasing nutrient uptake that are commonly discussed and all require to be taken into consideration in future work designed to understand the complex and multifaceted responses of plants to abiotic and biotic stresses in agricultural and natural environments.     

Litter N retention over winter for a low and a high phenolic species in the alpine tundra

Mycorrhizal fungus colonization of roots may modify plant metal acquisition and tolerance. In the present study, the contribution of the extraradical mycelium of an arbuscular mycorrhizal (AM) fungus, Glomus mosseae (BEG 107), to the uptake of metal cations (Cu, Zn, Cd and Ni) by cucumber (Cucumis sativus) plants was determined. The influence of the amount of P supplied to the hyphae on the acquisition and partitioning of metal cations in the mycorrhizal plants was also investigated. Pots with three compartments were used to separate root and root-free hyphal growing zones. The shoot concentration of Cd and Ni was decreased in mycorrhizal plants compared to non-mycorrhizal plants. In contrast, shoot Zn and Cu concentrations were increased in mycorrhizal plants. High P supply to hyphae resulted in decreased root Cu concentrations and shoot Cd and Ni concentrations in mycorrhizal plants. These results confirm that some elements required for plant growth (P, Zn, Cu) are taken up by mycorrhizal hyphae and are then transported to the plants. Conversely, Cd and Ni were transported in much smaller amounts by hyphae to the plant, so that arbuscular mycorrhizal fungus colonization could partly protect plants from toxic effects of these elements. Selective uptake and transport of plant essential elements over non-essential elements by AM hyphae, increased growth of mycorrhizal plants, and metal accumulation in the root may all contribute to the successful growth of mycorrhizal plants on metal-rich substrates. These effects are stimulated when hyphae can access sufficient P in soil.     

Arbuscular mycorrhizal contribution to heavy metal uptake by maize (Zea mays L.) in pot culture with contaminated soil

In two pot-culture experiments with maize in a silty loam (P2 soil) contaminated by atmospheric deposition from a metal smelter, root colonization with indigenous or introduced arbuscular mycorrhizal (AM) fungi and their influence on plant metal uptake (Cd, Zn, Cu, Pb, Mn) were investigated. Soil was -irradiated for the nonmycorrhizal control. In experiment 1, nonirradiated soil provided the mycorrhizal treatment, whereas in experiment 2 the irradiated soil was inoculated with spores of a fungal culture from P2 soil or a laboratory reference culture, Glomus mosseae. Light intensity was considerably higher in experiment 2 and resulted in a fourfold higher shoot and tenfold higher root biomass. Under the conditions of experiment 1, biomass was significantly higher and Cd, Cu, Zn and Mn concentrations significantly lower in the mycorrhizal plants than in the nonmycorrhizal plants, suggesting a protection against metal toxicity. In contrast, in experiment 2, biomass did not differ between treatments and only Cu root concentration was decreased with G. mosseae-inoculated plants, whereas Cu shoot concentration was significantly increased with the indigenous P2 fungal culture. The latter achieved a significantly higher root colonization than G. mosseae (31.7 and 19.1%, respectively) suggesting its higher metal tolerance. Zn shoot concentration was higher in both mycorrhizal treatments and Pb concentrations, particularly in the roots, also tended to increase with mycorrhizal colonization. Cd concentrations were not altered between treatments. Cu and Zn, but not Pb and Cd root-shoot translocation increased with mycorrhizal colonization. The results show that the influence of AM on plant metal uptake depends on plant growth conditions, on the fungal partner and on the metal, and cannot be generalized. It is suggested that metal-tolerant mycorrhizal inoculants might be considered for soil reclamation, since under adverse conditions AM may be more important for plant metal resistance. Under the optimized conditions of normal agricultural practice, however, AM colonization even may increase plant metal absorption from polluted soils.     

Ecto- and arbuscular mycorrhizal symbiosis can induce tolerance to toxic pulses of phosphorus in jarrah (Eucalyptus marginata) seedlings

In common with many plants native to low P soils, jarrah (Eucalyptus marginata) develops toxicity symptoms upon exposure to elevated phosphorus (P). Jarrah plants can establish arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations, along with a non-colonizing symbiosis described recently. AM colonization is known to influence the pattern of expression of genes required for P uptake of host plants and our aim was to investigate this phenomenon in relation to P sensitivity. Therefore, we examined the effect on hosts of the presence of AM and ECM fungi in combination with toxic pulses of P and assessed possible correlations between the induced tolerance and the shoot P concentration. The P transport dynamics of AM (Rhizophagus irregularis and Scutellospora calospora), ECM (Scleroderma sp.), non-colonizing symbiosis (Austroboletus occidentalis), dual mycorrhizal (R. irregularis and Scleroderma sp.), and non-mycorrhizal (NM) seedlings were monitored following two pulses of P. The ECM and A. occidentalis associations significantly enhanced the shoot P content of jarrah plants growing under P-deficient conditions. In addition, S. calospora, A. occidentalis, and Scleroderma sp. all stimulated plant growth significantly. All inoculated plants had significantly lower phytotoxicity symptoms compared to NM controls 7 days after addition of an elevated P dose (30 mg P kg^?1 soil). Following exposure to toxicity-inducing levels of P, the shoot P concentration was significantly lower in R. irregularis-inoculated and dually inoculated plants compared to NM controls. Although all inoculated plants had reduced toxicity symptoms and there was a positive linear relationship between rank and shoot P concentration, the protective effect was not necessarily explained by the type of fungal association or the extent of mycorrhizal colonization.     

Influence of the arbuscular mycorrhizal fungus Glomus mosseae on uptake of arsenate by the As hyperaccumulator fern Pteris vittata L.

We report for the first time some effects of colonization by an arbuscular mycorrhizal (AM) fungus (Glomus mosseae) on the biomass and arsenate uptake of an As hyperaccumulator, Pteris vittata. Two arsenic levels (0 and 300 mg As kg^–1) were applied to an already contaminated soil in pots with two compartments for plant and hyphal growth in a glasshouse experiment. Arsenic application had little or no effect on mycorrhizal colonization, which was about 50% of root length. Mycorrhizal colonization increased frond dry matter yield, lowered the root/frond weight ratio, and decreased frond As concentration by 33–38%. Nevertheless, transfer of As to fronds showed a 43% increase with mycorrhizal colonization at the higher soil As level. Frond As concentrations reached about 1.6 g kg^–1 (dry matter basis) in non-mycorrhizal plants in the As-amended soil. Mycorrhizal colonization elevated root P concentration at both soil As levels and mycorrhizal plants had higher P/As ratios in both fronds and roots than did non-mycorrhizal controls.     

Seed phosphorus (P) content affects growth,and P uptake of wheat plants and their association with arbuscular mycorrhizal (AM) fungi

Two experiments were carried out in a growth chamber and a naturally lit glasshouse to investigate the influence of seed phosphorus (P) reserves on growth and P uptake by wheat plants (Triticum aestivum cv Krichauff), and their association with arbuscular mycorrhizal (AM) fungi. Increased seed P reserves improved plant growth at a range of P supply up to over 100 mg P kg^–1 soil. Plants grown from seeds with high P reserves tended to accumulate more P from soil, which was mainly attributed to better root system development. Mycorrhizal colonisation did not significantly affect P uptake of plants grown with low irradiance (in growth chamber). However, in the naturally lit glasshouse, mycorrhizal plants had significantly higher P concentrations than non-mycorrhizal plants. Furthermore, mycorrhizal plants grown from seeds low in P accumulated similar amounts of P compared with those grown from seeds with high P, indicating that mycorrhizal colonisation may overcome the disadvantage of having low seed P reserves in the field.     

Studies of iron transport by arbuscular mycorrhizal hyphae from soil to peanut and sorghum plants

 The influence of an arbuscular mycorrhizal (AM) fungus on phosphorus (P) and iron (Fe) uptake of peanut (Arachis hypogea L.) and sorghum (Sorghum bicolor L.) plants was studied in a pot experiment under controlled environmental conditions. The plants were grown for 10 weeks in pots containing sterilised calcareous soil with two levels of Fe supply. The soil was inoculated with rhizosphere microorganisms only or with rhizosphere microorganisms together with an AM fungus (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe). An additional small soil compartment accessible to hyphae but not roots was added to each pot after 6 weeks of plant growth. Radiolabelled P and Fe were supplied to the hyphae compartment 2 weeks after addition of this compartment. After a further 2 weeks, plants were harvested and shoots were analysed for radiolabelled elements. In both plant species, P uptake from the labelled soil increased significantly more in shoots of mycorrhizal plants than non-mycorrhizal plants, thus confirming the well-known activity of the fungus in P uptake. Mycorrhizal inoculation had no significant influence on the concentration of labelled Fe in shoots of peanut plants. In contrast, ⁵⁹Fe increased in shoots of mycorrhizal sorghum plants. The uptake of Fe from labelled soil by sorghum was particularly high under conditions producing a low Fe nutritional status of the plants. These results are preliminary evidence that hyphae of an arbuscular mycorrhizal fungus can mobilise and/or take up Fe from soil and translocate it to the plant. Accepted: 6 March 1998     

纳米氧化锌和接种丛枝菌根真菌对大豆生长及营养吸收的影响

纳米氧化锌是应用最广的人工纳米颗粒(nanoparticles,NPs)之一,具有一定生物毒性。丛枝菌根(arbuscular mycorrhizal,AM)真菌能与陆地上80%以上的高等植物形成丛枝菌根共生体,并能改善宿主植物矿质营养,提高其抗逆性。然而纳米ZnO与丛枝菌根的关系尚不清楚。通过温室沙培盆栽试验,研究了施加不同水平纳米ZnO(0、500、1000、2000、3000 mg/kg)和接种AM真菌Acaulospora mellea对大豆生长及营养状况的影响。结果表明,3000 mg/kg的纳米ZnO显著抑制大豆植株生长,表现出植物毒性,在其他水平时没有显著影响。纳米ZnO在施加水平500、1000 mg/kg时没有抑制AM真菌对大豆根系的侵染,但是高施加水平(2000 mg/kg)时对AM真菌产生毒害,几乎完全抑制大豆根系菌根侵染。接种AM真菌仅在500 mg/kg纳米ZnO时显著促进大豆生长,增加大豆植株对P、K、N的吸收,降低根系Zn含量。纳米ZnO可能会持续释放锌离子,并抑制大豆根系对矿质营养元素的吸收,从而产生生物毒性,而AM真菌与大豆根系的共生可起到有益作用。     

The mobilisation and fate of soil and rock phosphate in the rhizosphere of ectomycorrhizal Pinus radiata seedlings in an Allophanic soil

A pot experiment was conducted to investigate the uptake of Zn from experimentally contaminated calcareous soil of low nutrient status by maize inoculated with the arbuscular mycorrhizal (AM) fungus Glomus caledonium. EDTA was applied to the soil to mobilize Zn and thus maximize plant Zn uptake. The highest plant dry matter (DM) yields were obtained with a moderate Zn addition level of 300 mg kg^?1. Plant growth was enhanced by mycorrhizal colonization when no Zn was added and under the highest Zn addition level of 600 mg kg^?1, while application of EDTA to the soil generally inhibited plant growth. EDTA application also increased plant Zn concentration, and Zn accumulation in the roots increased with increasing EDTA addition level. The effects of inoculation with G. caledonium on plant Zn uptake varied with Zn addition level. When no Zn was added, Zn translocation from roots to shoots was enhanced by mycorrhizal colonization. In contrast, when Zn was added to the soil, mycorrhizal colonization resulted in lower shoot Zn concentrations in mycorrhizal plants. The P nutrition of the maize was greatly affected by AM inoculation, with mycorrhizal plants showing higher P concentrations and P uptake. The results indicate that application of EDTA mobilized soil Zn, leading to increased Zn accumulation by the roots and subsequent plant toxicity and growth inhibition. Mycorrhizal colonization alleviated both Zn deficiency and Zn contamination, and also increased host plant growth by influencing mineral nutrition. However, neither EDTA application nor arbuscular mycorrhiza stimulated Zn translocation from roots to shoots or metal phytoextraction under the experimental conditions. The results are discussed in relation to the environmental risk associated with chelate-enhanced phytoextraction and the potential role of arbuscular mycorrhiza in soil remediation.