外源NO对缺镁胁迫下玉米幼苗生长和离子平衡的影响

研究了在缺镁胁迫下,外源NO对缺镁玉米幼苗生长、根系活力和离子含量的影响。结果表明,缺镁胁迫使玉米幼苗株高、根长和干鲜重下降,根系活力降低,N元素在地上部和根部分配失调,新叶和老叶中Mg2＋、Cu2＋、Fe3＋、Mn2＋等离子含量下降,Ca2＋、K＋、Zn2＋等离子含量上升。根中Mg2＋离子含量下降,Ca2＋、K＋、Zn2＋、Cu2＋、Fe3＋、Mn2＋等离子含量上升。用100μmol·L-1一氧化氮供体硝普钠（SNP）处理后,玉米幼苗株高、根长、干重和鲜重均提高,根系活力增强,改善了N代谢,新叶中Ca2＋、K＋和Zn2＋等离子含量下降,Mg2＋、Cu2＋、Fe3＋和Mn2＋等离子含量提高,老叶中Mg2＋、Ca2＋、K＋和Zn2＋等离子含量下降,Cu2＋、Fe3＋和Mn2＋等离子含量提高,根中Mg2＋、Ca2＋、K＋、Cu2＋、Zn2＋、Fe3＋和Mn2＋离子含量均下降。实验结果表明,NO保护玉米幼苗免受缺镁胁迫的影响。     

Effect of spermine on proliferation of hyphae ofGlomus fistulosum, an arbuscular mycorrhizal fungus, in maize roots

Effects of two oligoamines, putrescine and spermine, on proliferation of intraradical hyphae in surface disinfected root segments were studied under axenic conditions in vitro. No significant effects of putrescine were observed. Spermine significantly stimulated hyphal growth at a concentration of about 1.5 mumol/L. High concentration (> 150 mumol/L) caused a strong inhibition of hyphal growth and of the percentage of root segments bearing proliferating hyphae. DL-alpha-difluoromethylornithine, a metabolic inhibitor of polyamine synthesis, caused a significant inhibition of proliferation of the hyphae only in the presence of 2 mumol/L spermine.     

镁、锰、活性炭和石灰及其交互作用对小麦镉吸收的影响

采用盆栽试验,研究了在镉污染土壤上施用石灰、硫酸镁、硫酸锰和活性炭不同用量以及交互作用对小麦生长和吸收重金属镉的影响.研究结果表明,在试验条件下施用适量的硫酸镁、硫酸锰或与石灰配合能明显提高小麦籽粒产量,单施石灰或与活性炭配合施用降低了小麦籽粒产量;与对照(CK)相比,所有处理秸秆产量均下降.施用硫酸镁能显著降低小麦籽粒和秸秆中Cd浓度,且随用量的增加两增大.低量硫酸锰能有效降低小麦籽粒和秸秆中Cd浓度,高量反而增加小麦对Cd的吸收.石灰、活性炭单独施用或配合施用都能明显减少小麦对Cd的吸收,但籽/杆中Cd比却随石灰用量的增加呈明显的上升趋势.叶面喷施硫酸镁对降低小麦吸收镉的效果与土施相当,但叶面喷施硫酸锰却比土施硫酸锰显著降低了小麦籽粒中的镉浓度与吸收量.硫酸镁与硫酸锰,或石灰、硫酸镁和硫酸锰3种物质配合施用,对小麦籽粒镉浓度和吸收量的降低表现出明显的正交互作用,对抑制小麦体内镉从秸秆向籽粒的转移具有显著效果.     

Increasing phosphorus concentration in the extraradical hyphae of <Emphasis Type="Italic">Rhizophagus irregularis</Emphasis> DAOM 197198 leads to a concomitant increase in metal minerals

Plants associated with arbuscular mycorrhizal fungi (AMF) acquire phosphorus via roots and extraradical hyphae. How soil P level affects P accumulation within hyphae and how P in hyphae influences the accumulation of metal minerals remains little explored. A bi-compartmented in vitro cultivation system separating a root compartment (RC), containing a Ri T-DNA transformed carrot root associated to the AMF Rhizophagus irregularis DAOM 197198, from a hyphal compartment (HC), containing only the extraradical hyphae, was used. The HC contained a liquid growth medium (i.e., the modified Strullu-Romand medium containing P in the form of KH₂PO₄) without (0 μM) or adjusted to 35, 100, and 700 μM of KH₂PO₄. The accumulation of P and metal minerals (Ca, Mg, K, Na, Fe, Cu, Mn) within extraradical hyphae and AMF-colonized roots, and the expression of the phosphate transporter gene GintPT were assessed. The expression of GintPT in the extraradical hyphae did not differ in absence of KH₂PO₄ or in presence of 35 and 100 μM KH₂PO₄ in the HC but was markedly reduced in presence of 700 μM KH₂PO₄. Hyphal P concentration was significantly lowest in absence of KH₂PO₄, intermediate at 35 and 100 μM KH₂PO₄ and significantly highest in presence of 700 μM KH₂PO₄ in the HC. The concentrations of K, Mg, and Na were positively associated with the concentration of P in the extraradical hyphae developing in the HC. Similarly, P concentration in extraradical hyphae in the HC was related to P concentration in the growth medium and influenced the concentration of K, Mg, and Na. The accumulation of the metal mineral K, Mg, and Na in the extraradical hyphae developing in the HC was possibly related to their function in neutralizing the negative charges of PolyP accumulated in the hyphae.     

Variations in manganese effect on 6-phosphogluconate dehydrogenases from the thyroid and liver of the mouse and an evidence for non-interchangeability of manganese and magnesium

Both manganese (Mn) and magnesium (Mg) are known to activate 6-phosphogluconate dehydrogenase (6-PGDH). Yet in the present study none of the metal ions stimulated 6-PGDH from the thyroid of mice, but Mn alone stimulated the enzyme from the liver. Such a stimulatory effect of Mn on the liver enzyme was seen regardless of the presence of Mg, This suggests that the reaction sites for the two ions must be apart from one another or that the affinity of Mn is stronger than that of Mg. The enzyme preparations from livers of those mice which had been administered intraperitoneally with excess of Mn for 1 - 14 days were greatly stimulated by further in vitro addition of Mn, whereas the enzyme sources from the thyroids thus treated were almost insensitive to Mn in vitro. The results indicate that cellular Mn hardly reaches the levels for maximal stimulation of the enzyme in the liver. In addition, some sex difference was observed in the Mn effect on 6-PGDH from the liver.     

赤松不定根和愈伤组织与松口蘑外生菌根菌的相互作用

本实验报道了以简单的离体培养方式来诱导赤松不定根和愈伤组织与松口蘑的菌根反应。不定根和愈伤组织均起源于无菌苗的下胚轴,接种2周后,菌丝体开始包围不定根。接种3周后,菌丝体出现在不定根皮层细胞间,哈蒂氏网型的形成也同时被确认。在愈伤组织培养物中,细胞间也能观察到菌丝体及拟-哈蒂氏网结构。这是第一个离体条件下成功地诱导赤松培养组织与松口蘑形成外生菌根的报道。     

Interactions between rye (Secale cereale) root border cells (RBCs) and pathogenic and nonpathogenic rhizosphere strains of Fusarium culmorum

Interactions of rye (Secale cereale) root border cells (RBCs), generated during plant growth and surrounding the root cap, with nonpathogenic rhizosphere Fusarium culmorum isolates: DEMFc2 (PGPF) and DEMFc5 (DRMO), and a pathogenic strain DEMFc37 were studied in test tube experiments. The effect of water-suspended RBCs released from the rye root cap on the rate of macroconidia germination and hyphae (mycelial) growth of F. culmorum strains was also examined. It was found that root caps of 3-d-old rye seedlings (with the root length of 20 mm) were surrounded with a layer of RBCs generated in a number specific for this plant species of 1980 ± 30. Introduction of the macroconidia of the tested F. culmorum strains into the root zone of 3-d-old seedlings resulted, after 3 d of incubation, in the formation of mantle-like structures only in the rhizosphere of plants inoculated with the pathogenic DEMFc37 strain. The macroconidia were suspended in (1) water, (2) a water mixture with root caps deprived of RBCs, (3) Martin medium, (4) malt extract broth, and (5) a water mixture with rye RBCs, and their percentage germination was determined during 96-h incubation at 20 °C. Germination of the macroconidia of all the tested F. culmorum strains suspended in the rich growth media (Martin and malt extract broth) and in the mixture with RBCs was significantly speeded up. While only an average of 16.6 % of macroconidia suspended in water germinated after 96-h incubation, more than 90 % of those suspended in the growth media or in the mixture with RBCs germinated after 24 h of incubation. In all the treatments, the highest rate of macroconidia germination was found in suspensions of the pathogenic strain and the lowest in macroconidial suspensions of the PGPF strain. The stimulatory effect of RBCs was not specific to the pathogenic strain. Nevertheless, microscopic observation revealed that it was only in the suspension containing a mixture of rye RBCs and macroconidia of the pathogenic strain that after 48-h incubation compact clusters of hyphae and RBCs, resembling mantle-like structures found in the root zone of plants inoculated only with the pathogenic strain but not inoculated with DRMO and PGPF strain, were formed.     

Magnesium as NMDA receptor blocker in the traditional Chinese medicine Danshen

Aqueous extracts of the traditional Chinese medicine Danshen, the dried roots of Salvia miltiorrhiza Bunge (Labiatae), blocked N-methyl-D-aspartate (NMDA) evoked currents in cerebrocortical neurons in vitro. The block of the NMDA-evoked currents was voltage dependent and showed the negative slope conductance reminiscent of the effect of Mg2+ ions. Atomic absorption spectrophotometry (AAS) revealed that aqueous Danshen extracts contained approximately 9mM magnesium. Fractionation of the extracts by high performance liquid chromatography followed by patch clamp recording and AAS indicated that magnesium ions were present in two distinct fractions. One fraction contained approximately 5 mM magnesium and blocked NMDA-induced currents indicating that it contained mostly free Mg2+ ions, while a second fraction did not possess NMDA antagonist activity despite the presence of approximately 4 mM magnesium suggesting that Mg2+ in this fraction was mostly chelated. Following removal of the free Mg2+ by ion exchange chromatography, the previously observed block of the NMDA-induced currents was abolished. These data demonstrate that Danshen contains both free and chelated Mg2+. Free Mg2+ ions account for the NMDA antagonist activity of Danshen in vitro.     

Amelioration of Pb and Mn Toxicity to Arbuscular Mycorrhizal Fungus Glomus intraradices by Maize Root Exudates

The effect of maize root exudates on the toxicity of lead and manganese to arbuscular mycorrhizal fungus (AMF) Glomus intraradices was studied in vitro by observation of intraradical hyphae regrowth from colonised root segments. Higher heavy metal (HM) concentrations strongly reduced the hyphal growth, however, the inhibitory effect was to a large extent eliminated by the addition of maize root exudates to the media. However, the capacity of exudates to ameliorate HM toxicity was limited and did not operate when a threshold HM concentration was reached. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of fulvic acids on the toxicity of lead and manganese to arbuscular mycorrhizal fungus<Emphasis Type="Italic">Glomus intraradices</Emphasis>

The effect of fulvic acids (FA) on arbuscular mycorrhizal (AM) fungi and on the toxicity of lead and manganese toward these symbionts were demonstatedin vitro. Incubation of root segments colonized with the AM fungusGlomus intraradices in undiluted fraction of FA (813 mg/L carbon) decreased an outgrowth of intraradical hyphae. Diluted FA solutions (<271 mg/L C) did not influence the proportion of root segments bearing proliferating hyphae; solution containing 27.1 mg/L C even increased the proliferation. A decrease of heavy metal toxicity toward the fungus was observed when FA (81.3 mg/L C) were added to the solutions containing higher concentrations (≤100 μmol/L) of Mn and Pb; the positive effect of FA was not significant at higher concentrations of metals (0.5 mmol/L). A short-term cultivation of six different saprophytic microorganisms — three actinomycetes and three filamentous fungi — on the FA media (81.3 mg/L C) did not result in a modification of hyphal proliferation from the root segments subsequently incubated in these media.     

Characterization of the metal ion binding properties of the hepatitis C virus RNA polymerase

The hepatitis C virus nonstructural 5B protein (NS5B) protein has been shown to require either magnesium or manganese for its RNA-dependent RNA polymerase activity. As a first step toward elucidating the nature and the role(s) of the metal ions in the reaction chemistry, we have utilized endogenous tryptophan fluorescence to quantitate the interactions of magnesium and manganese ions with this protein. The association of either Mg(2+) or Mn(2+) ions with the enzyme resulted in a decrease in the intensity of the tryptophan emission spectrum. This decrease was used to determine the apparent dissociation constants for both ions. The apparent K(d) values for the binding of Mg(2+) and Mn(2+) ions to the free enzyme were 3.1 and 0.3 mm, respectively. Dual ligand titration experiments demonstrated that both ions bind to a single common site, for which they compete. The kinetics of real time metal ion binding to the NS5B protein were also investigated. Based on the results of our fluorescence and near-UV circular dichroism experiments, we show that NS5B undergoes conformational changes upon the binding of metal ions. However, this process does not significantly stimulate the binding to the RNA or NTP substrates. We envisage that the ion-induced conformational change is a prerequisite for catalytic activity by both correctly positioning the side chains of the residues located in the active site of the enzyme and also contributing to the stabilization of the intermediate transition state.     

硅素对水稻幼苗镉积累及抗胁迫应答的调节效应

施硅(Si)可以显著缓解镉(Cd)胁迫对水稻生长发育的毒害效应。本研究通过水培分根试验,研究了Si对水稻幼苗Cd积累及胁迫应答的调节效应。结果表明: Cd胁迫下水稻幼苗的生物量显著降低,加Si可以显著缓解Cd对水稻幼苗生长的抑制效应。水稻幼苗对Cd的吸收、转运和积累明显受到Si的影响,单侧根系Cd胁迫下加Si(Si-Cd+Si,Si-Cd)使根系对Cd的滞留系数达83.3%～83.6%,限制了Cd从根向地上部转移。单侧根系Cd胁迫下非胁迫侧加Si(Si-Cd)处理的植株对Cd的吸收和累积明显增加,尤其是根中Cd的积累量较单侧根系Cd胁迫下无Si(CK-Cd)处理增加了48.2%;而单侧根系Cd胁迫下双侧加Si(Si-Cd+Si)处理则显著降低了根和地上部对Cd的吸收,较CK-Cd处理分别降低了36.7%和54.9%。双侧Cd胁迫下单侧加Si(Cd-Cd+Si)则使根和地上部对Cd的吸收量显著减少,较双侧根Cd胁迫(Cd-Cd)处理分别降低了57.8%和46.5%。Cd胁迫下水稻幼苗根中含较高浓度的Si,加Si则使Cd胁迫下根和地上部积累更多的Si。加Si也影响了水稻幼苗对其他金属元素如钙(Ca)、镁(Mg)、锰(Mn)的吸收,Cd-Cd+Si处理显著增加了根系和地上部的Ca、Mg浓度,但Mn浓度的变化则因Cd胁迫程度而表现不同。加Si对Cd胁迫下根系超氧化物歧化酶(SOD)和过氧化物酶(POD)活性有一定的影响,尤其是Si-Cd处理的胁迫侧POD和非胁迫侧SOD活性显著上升,有利于清除Cd胁迫产生的氧自由基。总之,Si对Cd胁迫下水稻幼苗生长、Cd和Si等的吸收及根系的抗氧化反应有一定的调节效应,植株体内较高的Si浓度有利于增强植株对Cd的耐受性。     

The significance of the magnesium to manganese ratio in plant tissues for growth and alleviation of manganese toxicity in tomato (Lycopersicon esculentum) and wheat (Triticum aestivum) plants

Results are reported for tomato (Lycopersicon esculentum L. var. Ailsa craig) and wheat (Triticum aestivum L. cv. Mara) which demonstrate that increasing concentrations of Mg in the plant raises plant tolerance to Mn toxicity.Water culture experiments with tomato show that under conditions of high Mn supply (200 µM, Mn), not only does increasing Mg application (0.75 mM to 15 mM) depress Mn uptake, but the higher Mg concentrations in the shoot counteract the onset of Mn toxicity when the concentrations of Mn in the shoot are also high. The ratio of Mg: Mn in the tissues is a better indicator of the appearance of toxicity symptoms than Mn concentration alone. Toxicity symptoms were observed when the Mg:Mn ratio in the shoot tissue was from 1.13 to a value between 3.53 and 6.54. The corresponding Mg: Mn ratio in the older leaves was from 0.82 to between 2.27 and 3.51.For wheat grown in soil, analyses of leaves revealed that growth could be expressed by the following relationship: Y=A+B exp(-kX), where Y=growth, X=Mg:Mn ratio, A, B and k=constants. Growth was significantly reduced when the Mg:Mn ratio fell below 20:1. From a measurement of this ratio it is therefore possible to predict the appearance of Mn toxicity and its influence on growth.     

The causal agents of damping-off disease of buglosse from Iran

Iran is considered a major genetic for medicinal plant in the world. Because of this significant diversity and historical background in identification and utilization to remedy human and animal diseases, export of medicinal plant can help to strengthen local as well as natural economy. Buglosse (Fig. 1) is one of the most important and common medicinal plants in Iran and exist as Echium amoneum and Borago officinalis. This work was conducted in order to identify the causal agent(s) of damping off disease in buglosse. Plant disease samples were taken from Esfahan and Tehran provinces. Symptoms on original plant including root, crown rot, dark tissue, pith and hallow root were collected in order to isolate disease agent(s). Symptomatic root and crown tissues after surface sterilization with 96% ethanol were transferred on to PDA and WA media and also on moist filter paper in petri dishes. Two fungal colonies grew from tissue segments and spore culture was subsequently purified. The fungal isolate identified as Rhizoctonia solani based on the following test. Hyphal tip was removed from colony margin placed on PDA and PSA media and incubated in dark. Colony diameter of one hundred hyphae measured and nucleus was stained according to Bandoni (1979), Kronland and Stanghellini (1988). It was observed that in each cell of hyphae there are more than two nuclei. Single spore culture were obtained from macroconidia of Fusarium isolate. After 24 hr of incubation, growing single spore were transferred to KCL medium to detect spore chains. Fungal isolates transferred to PSA and PDA media for sporulation. After 7 days colonies appeared as white cream to pinkish on top and cream to dark pink at the bottom of petri dish with abundant micro and macro conidia. Colonies were snow white, felting shape, with ample causal hyphae on PSA medium. On KCL medium, fungal growth was superficial and colonies were colorless with long macroconidia and individual sausage-shape macroconidia being thinner one side and having maximum four septa. Microconidia were long double compartment round on both side, straight to slightly curved. Base on morphology and dimension of conidia and production of chlamidospore the funguses identify as Fusarium solani.     

Overexpression of an Arabidopsis magnesium transport gene, AtMGT1, in Nicotiana benthamiana confers Al tolerance

Aluminium (Al) toxicity is the most important limiting factor for crop production in acid soil environments worldwide. In some plant species, application of magnesium (Mg(2+)) can alleviate Al toxicity. However, it remains unknown whether overexpression of magnesium transport proteins can improve Al tolerance. Here, the role of AtMGT1, a member of the Arabidopsis magnesium transport family involved in Mg(2+) transport, played in Al tolerance in higher plants was investigated. Expression of 35S::AtMGT1 led to various phenotypic alterations in Nicotiana benthamiana plants. Transgenic plants harbouring 35S::AtMGT1 exhibited tolerance to Mg(2+) deficiency. Element assay showed that the contents of Mg, Mn, and Fe in 35S::AtMGT1 plants increased compared with wild-type plants. Root growth experiment revealed that 100 microM AlCl(3) caused a reduction in root elongation by 47% in transgenic lines, whereas root growth in wild-type plants was inhibited completely. Upon Al treatment, representative transgenic lines also showed a much lower callose deposition, an indicator of increased Al tolerance, than wild-type plants. Taken together, the results have demonstrated that overexpression of ATMGT1 encoding a magnesium transport protein can improve tolerance to Al in higher plants.     

Effect of metal ions on the activity of the catalytic domain of calcineurin

Calcineurin (CN) is a heterodimer, composed of a catalytic subunit (CNA) and a regulatory subunit (CNB). There are four functional domains present in CNA, which are catalytic domain (CNa), CNB-binding domain (BBH), CaM-binding domain (CBH) and autoinhibitory domain (AI). It has been shown previously that the in vitro activity of calcineurin is relied primarily on the binding of metal ions. Mn2+ and Ni2+ are the most crucial cation-activators for this enzyme. In order to determine which domain(s) in CN is functionally regulated by metal ions, the rat CNA alpha subunit and its catalytic domain (CNa) were cloned and expressed in E. coli. The effects of Mn2+, Ni2+ and Mg2+ on the catalytic activity of these purified proteins were examined. Our results demonstrate that all the metal ions tested in this study activated either CNA or CNa. However, the activation degree of CNa by the metal ions was much higher than that of CNA. In term of different metal ions, the activating extents to CNA and CNa were different. To CNA, the activating order from high to low was Mg2+ > > Ni2+ > Mn2+, but Mn2+ > Ni2+ > > Mg2+ to CNa. No effect of CaM/Ca2+ and CNB/Ca2+ on the activity of CNa was observed in our experiments. Moreover, a weak interaction (or untight coordination binding) between metal ions and the enzyme molecule was also identified. These results suggest that the activation of these enzymes by the exogenous metal ions might be via both regulating fragment of CNA (including BBH, CBH and AI) and catalytic domain (CNa), and mainly via regulating fragment to CNA and mainly via catalytic domain to CNa. The activating extents of metal ions via catalytic domain were higher than that via regulating fragment. The results obtained in this study should be very useful for understanding the molecular mechanism underlying the interaction between calcineurin and metal ions, especially Mn2+, Ni2+ and Mg2+.     

Growth characteristics of the slow-growing actinobacterium Frankia sp. strain CcI3 on solid media

Frankia sp. strains are slow-growing Actinobacteria that form N₂-fixing root nodules on certain angiosperm trees and shrubs. These organisms are generally cultured in liquid media as routine growth on solid media is impractical because of lengthy incubation periods. Liquid media is problematic for filamentous organisms because unicellular spores are difficult to isolate, and mutants are difficult to separate. The goal of this study was to optimize Frankia CcI3 growth on solid media. Starting with an appropriate basal medium containing pyruvate as a carbon source, various peptones were individually tested to determine if the time needed for colonies to appear could be reduced. Bacto™ Proteose Peptone #3 at 1.6 mg ml⁻¹ final concentration promoted the best growth with colonies visible after 3 days, compared with the 7- to 10-day incubation required for control unamended media. Gellan gum gave a more rapid growth response as compared with agar. Increased calcium concentrations were required to promote solidification of gellan gum. Sporulation occurred within 2 weeks of plating CcI3 with colonies changing color from cream to reddish-brown. Scanning and transmission electron micrographs revealed the architecture of colonial development. Colonies developed in a lens-shaped manner mainly by penetration into the gel. Older colonies had numerous empty hyphae with sporangia developing near or at the surface and with crystalline material, presumably pigment, interspersed among the hyphae. Occasional vesicles were seen mixed within the colonies. Colonial development of Frankia CcI3 is reminiscent of that described for other members of the filamentous Suborder Frankineae.     

Competitive ion binding to low density lipoproteins: an electron spin resonance study

The electron spin resonance (ESR) technique was used to evaluate binding constants for Ca(II) and Mg(II) in interaction with low density lipoprotein (LDL). The Ca(II) or Mg(II) ions competed with the paramagnetic Mn(II) ions for the same binding sites of two different classes on the LDL surface. For each ion competing with Mn(II), the solutions of eight non-linear competition equations were fit to the experimental titration curves, with two adjustable parameters, the two binding constants. The derived "intrinsic" values (the values corrected for the electrolyte-induced change of the surface potential) for "strong" binding sites for Ca(II) (170 +/- 85 M-1) and Mg(II) (60 +/- 30 M-1) differ significantly from the respective value for Mn(II) (760 M-1). The values for the "weak" binding sites (18 M-1, 15 M-1 and 10 M-1 for Mn(II), Ca(II) and Mg(II), respectively are in the range of the binding constants for these ions in interaction with model membranes.     

Are there benefits of simultaneous root colonization by different arbuscular mycorrhizal fungi?

Arbuscular mycorrhizal fungal (AMF) communities were established in pots using fungal isolates from a single field in Switzerland. It was tested whether multispecies mixtures provided more phosphorus and supported greater plant growth than single AMF species. Two host plants, medic (Medicago truncatula) and leek (Allium porrum), were inoculated with three AMF species (Glomus mosseae, G. claroideum and G. intraradices), either separately or in mixtures. The composition of the AMF communities in the roots was assessed using real-time PCR to determine the copy number of large ribosomal subunit genes. Fungal communities in the roots were usually dominated by one AMF species (G. mosseae). The composition of the communities depended on both plant identity and the time of harvest. Leek colonized by a mixture of G. claroideum and G. intraradices acquired more P than with either of the two AMF separately. Direct evidence is provided for functional complementarity among species within the AMF community colonizing a single root system. Competition among the species poses a major challenge in interpreting experiments with mixed inoculations, but this is greatly facilitated by use of real-time PCR.     

Analysis of acid-soil stress in sorghum genotypes with emphasis on aluminium and magnesium interactions

Acid-soil stress in 12 sorghum (Sorghum bicolor (L.) Moench) genotypes was attributed mainly to aluminium (Al) toxicity. Root damage and magnesium (Mg) deficiency are two independent aspects of plant sensitivity to Al, either in acid soil or in nutrient solution. At moderate soil acidity, Mg deficiency dominantly limited growth whilst at high acidity root damage overruled the effect of Mg deficiency on the growth response. In nutrient solutions containing Al, increased Mg supply improved both root development and Mg nutrition of plants, whereas increased calcium (Ca) supply, or nutrition with ammonium (NH₄) instead of nitrate (NO₃), alleviated root damage but amplified Mg deficiency. At lowered pH the syndrome of Al toxicity was more profound. The implications of Mg-Al interactions, root damage, Mg supply and genotype selection are elucidated.