Uptake of 32P from labelled organic matter,by mycorrhizal and non-mycorrhizal subterranean clover (Trifolium subterraneum L.)

An experiment was designed to study whether hyphae and colonized roots of arbuscular mycorrhiza have more direct access to P in organic matter than roots of non-mycorrhizal plants. Soil supplied with 0, 15 or 45 mg P kg^–1 was uniformly mixed with ³²P-labelled organic matter at four levels (0, 1, 2 and 5 g kg^–1) and inoculated with a mycorrhizal fungus or left uninoculated. Pots were incubated at 60% of field capacity for one week prior to sowing of clover, and plants were harvested after a growth period of 23 days. Mycorrhizal colonization increased shoot dry weight, P concentration and ³²P uptake at all P levels. Specific activity in plants was consistently higher than in corresponding soil. This indicates that the added ³²P never reached an equilibrium with inorganic P in the soil. P mineralized from organic matter thus had a residence time in the soil solution sh ort enought to partially avoid isotopic exchange and adsorption. Mycorrhizal colonization influenced specific activity of ³²P in plants from three of the nine combinations of P and labelled organic matter: At the lowest level of P the specific activity was highest in non-mycorrhizal plants, and at the intermediate level of P there was one treatment where mycorrhizal plants had the highest specific activity. These differences are discussed. Plant dry weight and P concentration did not respond to addition of organic matter, though soil extracts consistently contained higher amounts of inorganic P as a result of organic matter addition. The results suggest that mycorrhizal plants at an early growth stage utilize a substantially higher amount of P released from organic matter than non-mycorrhizal plants. This mycorrhizal advantage does not seem to be related to a mycorrhizal influence on mineralization.     

Contribution of the VA mycorrhizal hyphae in acquisition of phosphorus and zinc by maize grown in a calcareous soil

An investigation was carried out to test whether the mechanism of increased zinc (Zn) uptake by mycorrhizal plants is similar to that of increased phosphorus (P) acquisition. Maize (Zea mays L.) was grown in pots containing sterilised calcareous soil either inoculated with a mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe or with a mixture of mycorrhizal fungi, or remaining non-inoculated as non-mycorrhizal control. The pots had three compartments, a central one for root growth and two outer ones for hyphal growth. The compartmentalization was done using a 30-m nylon net. The root compartment received low or high levels of P (50 or 100 mg kg^–1 soil) in combination with low or high levels of P and micronutrients (2 or 10 mg kg^–1 Fe, Zn and Cu) in the hyphal compartments.Mycorrhizal fungus inoculation did not influence shoot dry weight, but reduced root dry weight when low P levels were supplied to the root compartment. Irrespective of the P levels in the root compartment, shoots and roots of mycorrhizal plants had on average 95 and 115% higher P concentrations, and 164 and 22% higher Zn concentrations, respectively, compared to non-mycorrhizal plants. These higher concentrations could be attributed to a substantial translocation of P and Zn from hyphal compartments to the plant via the mycorrhizal hyphae. Mycorrhizal inoculation also enhanced copper concentration in roots (135%) but not in shoots. In contrast, manganese (Mn) concentrations in shoots and roots of mycorrhizal plants were distinctly lower, especially in plants inoculated with the mixture of mycorrhizal fungi.The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorrhizal plants. The minimal hyphae contribution (delivery by the hyphae from the outer compartments) to the total plant acquisition ranged from 13 to 20% for P and from 16 to 25% for Zn.     

Depletion of soil mineral N by roots of Cucumis sativus L. colonized or not by arbuscular mycorrhizal fungi

Two experiments were conducted where Cucumis sativus were grown in uncompartmented pots either alone or in symbiosis with Glomus intraradices Schenck and Smith (Experiment 1) or Glomus sp. (Experiment 2) in order to investigate if root colonization by arbuscular mycorrhizal (AM) fungi has an effect on depletion of the soil mineral N pool. All pots were gradually supplied with 31 mg NH₄NO₃-N kg^-1 dry soil from 12–19 days after planting and an additional 50 mg (NH₄)₂SO₄-N kg^-1 dry soil (¹⁵N-labelled in Experiment 1) was supplied at 21 or 22 days after planting in Experiments 1 and 2, respectively. Dry weight of plant parts, total root length, mycorrhizal colonization rate and soil concentration of NH ₄ ⁺ and NO ₃ ^- were recorded at five sequential harvest events: 21, 24, 30, 35 and 42 days (Experiment 1) and 22, 25, 28, 31 and 35 days (Experiment 2) after planting. In Experiment 1, plants were also analysed for total content of N and ¹⁵N. The mycorrhizal colonization rate increased during time: from 25 to 40% in Experiment 1 and from 50 to 60% in Experiment 2. Plant dry matter accumulation was unaffected by mycorrhizal colonization, except in Experiment 1 where shoot dry weights were slightly increased and in Experiment 2 where root dry weights were slightly decreased compared to non-mycorrhizal control plants. The total root length was similar in the control and mycorrhizal treatments in Experiment 1, while it was decreased (20–30%) by mycorrhizal colonization in the last two harvest in Experiment 2. Mycorrhizal colonization affected the rate of depletion of soil mineral N in Experiment 1, where both NH ₄ ⁺ and NO ₃ ^- concentrations were markedly lower in the first two harvests, when plants were mycorrhizal. As the root length was similar in mycorrhizal and control treatments, this may indicate that the external AM hyphae contributed to the depletion of the soil mineral N pool. A similar pattern was observed in Experiment 2, although the effect was less pronounced. The ¹⁵N enrichment in mycorrhizal plants (Experiment 1) also indicated a faster NH ₄ ⁺ uptake than in the non-mycorrhizal controls in the first two harvests after application of the ¹⁵N-labelled N source. However, the external hyphae and roots seemed to have access to the same N sources as the ¹⁵N enrichment and total N content were similar in mycorrhizal and control plants at the end of the experiment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Accumulation and Speciation of Selenium in Plants as Affected by Arbuscular Mycorrhizal Fungus <Emphasis Type="BoldItalic">Glomus mosseae</Emphasis>

Effects of arbuscular mycorrhizal fungus (Glomus mosseae) on the accumulation and speciation of selenium (Se) in alfalfa, maize, and soybean were investigated by using Se(IV)-spiked soil. Mycorrhizal inoculation decreased Se accumulation in roots and shoots of all the plants at Se spiked level of 0 or 2 mg kg⁻¹, while an increased Se accumulation was observed in alfalfa shoots and maize roots and shoots at the spiked level of 20 mg kg⁻¹. Concentration of inorganic Se (especially Se(VI)) in roots and shoots of the three plants was much higher in mycorrhizal than non-mycorrhizal treatment. Mycorrhizal inoculation decreased the portion of total organic Se in plant tissues with the exception of alfalfa and maize shoots at Se spiked level of 20 mg kg⁻¹, in which organic Se portion did not reduced greatly (<5%) for mycorrhizal treatment. Mycorrhizal effects on alfalfa and maize were more obvious than on soybean in terms of root colonization rate, biomass, and Se accumulation.     

Growth and arsenic uptake by Chinese brake fern inoculated with an arbuscular mycorrhizal fungus

A split-root experiment investigated the effects of inoculation with the arbuscular mycorrhizal fungus Glomus mosseae and arsenic (As) addition on As uptake by Pteris vittata L. Either part or all of the root system was inoculated with G. mosseae or exposed to As addition (50 ml 1000 μmol L⁻¹ As 1 week before harvest). Mycorrhizal colonization substantially increased frond and root dry weight and P and As contents irrespective of As addition. Frond As contents in mycorrhizal plants were highest when the whole root system was exposed to As. Frond As concentrations and contents were higher when inoculation and As addition were in the same parts of the root system than when spatially separate. There were positive effects of arbuscular mycorrhiza inoculation on plant growth and As uptake, and inoculation of part of the roots seemed to be as effective as inoculation of the whole root system.     

Effects of vesicular-arbuscular mycorrhizae on growth and phosphorus and zinc nutrition of peanut (Arachis hypogaea L.) in an Oxisol from subtropical Australia

Peanut plants (cv. Shulamit) were grown in an Oxisol soil in pots in the glasshouse to assess effects of soil sterilization and inoculation with spores of vesicular-arbuscular mycorrhizal fungi (VAMF) on the response to five rates of phosphorus (0 to 240 kg P ha^–1) and two rates of zinc (0 and 10 kg Zn ha^–1) fertilizers.Both P and Zn nutrition were affected by VAMF activity but the dominant role of VAMF in this soil type was in uptake of P. In the absence of VAMF there was a clear threshold level of P application (60 kg P ha^–1) below which plants grew poorly, which resulted in a sigmoidal response of dry matter to applied P. The maximum response was not fully defined because dry matter production continued to increase up to 240 kg P ha^–1. Tissue P concentration of non-mycorrhizal plants increased linearly with P rate and was always significantly less than that in mycorrhizal plants.Mycorrhizal plants responded without threshold to increasing P rate, attaining maximum dry matter at 120 kg P ha^–1 in inoculated sterilized soil and at 30 kg P ha^–1 in nonsterile soil. These differences in maximal P rates and in the greater dry matter produced in sterile soil at high P rates were attributed to the negative effects of the root-knot nematodeMeloidogyne hapla in nonsterile soil.Plant weight did not respond to zinc fertilizer but tissue Zn concentration increased with applied Zn. Tissue Zn concentration and uptake were increased by VAMF.     

Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-μm nylon net. The root compartment received a uniform level of P (50 mg kg⁻¹ soil) in combination with low or high levels of P (50 or 150 mg kg⁻¹ soil) in the hyphal compartments. Plants were inoculated withGlomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected. Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels. Mycorrhizal contribution accounted for 76% of total P uptake at the low P level and 79% at the high P level, and almost all of this P was delivered by the hyphae from the outer compartment. In the non-mycorrhizal plants, the depletion of NaHCO₃-extractable P (Olsen-P) extended about 1 cm into the outer compartment, but in the mycorrhizal plants a uniform P depletion zone extended up to 11.7 cm (the length of the hyphal compartment) from the root surface. In the outer compartment, the mycorrhizal hyphae length density was high (2.5–7 m cm⁻³ soil) at the various distances (0–11.7 cm) from the root surface. Uptake rate of P by mycorrhizal hyphae was in the range of 3.3–4.3×10⁻¹⁵ mol s⁻¹ cm⁻¹.     

Growth response and phosphorus uptake of rye with long and short root hairs: Interactions with mycorrhizal infection

Plant growth and phosphorus (P) uptake of two selections of rye (Secale cereale L.) differing in length of root hairs, in response to mycorrhizal infection were investigated. Rye plants with short root hairs (SRH) had a greater length of root infected by Glomus intraradices (up to 32 m pot^–1) than those with long root hairs (LRH) (up to 10 m pot^–1). Application of P decreased the percentage of root length infected in both selections. In low-P soil, mycorrhizal infection increased shoot and root P concentration, especially in LRH plants. Generally, LRH had higher shoot dry weight than SRH plants. P uptake was increased both by LRH and by mycorrhizal infection. Differences in specific P uptake and P utilization efficiency between SRH and LRH plants were observed in non-mycorrhizal plants. With low P supply, P utilization efficiency (dry matter yield per unit of P taken up) of LRH plants increased with time. However, mycorrhizal infection reduced P utilization efficiency, particularly of SRH plants. SRH plants, which were agronomically less efficient (i.e. low dry matter yield at low P supply) were more responsive to either mycorrhizal infection or P addition than the LRH plants. No interaction was observed between mycorrhizal infection and root hair length.     

Uptake of cadmium from an experimentally contaminated calcareous soil by arbuscular mycorrhizal maize (<Emphasis Type="Italic"> Zea mays </Emphasis>L.)

We investigated uptake of Cd by arbuscular mycorrhizal (AM) maize inoculated with Glomus mosseae from a low-P sandy calcareous soil in two glasshouse experiments. Plants grew in pots containing two compartments, one for root and hyphal growth and one for hyphal development only. Three levels of Cd (0, 25 and 100 mg kg^–1) and two of P (20 and 60 mg kg^–1) were applied separately to the two compartments to assess hyphal uptake of Cd. Neither Cd nor P addition inhibited root colonization by the AM fungus, but Cd depressed plant biomass. Mycorrhizal colonization, P addition and increasing added Cd level led to lower Cd partitioning to the shoots. Plant P uptake was enhanced by mycorrhizal colonization at all Cd levels studied. When Cd was added to the plant compartment and P to the hyphal compartment, plant biomass increased with AM colonization and the mycorrhizal effect was more pronounced with increasing Cd addition. When P was added to the plant compartment and Cd to the hyphal compartment, plant biomass was little affected by AM colonization, but shoot Cd uptake was increased by colonization at the low Cd addition rate (25 mg kg^–1) and lowered at the higher Cd rate (100 mg kg^–1) but with no difference in root Cd uptake. These effects may have been due to immobilization of Cd by the fungal mycelium or effects of the AM fungus on rhizosphere physicochemical conditions and are discussed in relation to possible phytostabilization of contaminated sites by AM plants.     

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.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Benefit and cost analysis and phosphorus efficiency of VA mycorrhizal fungi colonizations with sorghum (Sorghum bicolor) genotypes grown at varied phosphorus levels

Sorghum [Sorghum bicolor (L.) Moench] was grown in a greenhouse in a low P (3.6 mg kg^-1) soil (Typic Argiudolls) inoculated with the vesicular-arbuscular mycorrhizal fungi (VMAF) Glomus fasciculatum and P added at 0, 12.5, 25.0, and 37.5 mg kg^-1 soil to determine the effects of VAMF-root associations on plant growth, benefit and cost analysis, and P efficiency (dry matter produced/unit P absorbed). Root colonization with VAMF and shoot growth enhancements decreased with increased soil P applications. Mycorrhizal plants were less P efficient than nonmycorrhizal plants. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants were considered the benefit derived by plants from VAMF-root associations. Shoot dry matter differences between mycorrhizal and nonmycorrhizal plants with similar P concentrations were considered the costs paid by plants for VAMF-root associations. Values of benefit and cost analysis for VAMF-root associations were highest when soil P was lowest and decreased with increasing P applications. Genotypic differences for calculated costs were pronounced, but not benefits. Benefit and cost analysis.may be helpful to evaluate host plant genotypes and VAMF species to optimize efficiencies of VAMF symbiosis in different soil environments.     

Effects of arbuscular mycorrhizal fungi and drought stress on growth and nutrient uptake of two wheat genotypes differing in drought resistance

 The effects of an arbuscular mycorrhizal (AM) fungus and drought stress on the growth, phosphorus, and micronutrient uptake of two wheat genotypes exhibiting differences in drought resistance were investigated. Plants were grown on a low P (4 mg kg^–1 soil) silty clay (Typic Xerochrept) soil-sand mix. Mycorrhizal infection was higher under well-watered than under dry soil conditions and the drought-resistant genotype CR057 had a higher mycorrhizal colonization than the drought-sensitive genotype CR006. Total and root dry matter yields and total root length were higher in mycorrhizal than in nonmycorrhizal plants of both genotypes. CR057 had higher total dry matter but not root dry matter than CR006 plants. The enhancement in total dry matter due to AM inoculation was 42 and 39% under well-watered and 35 and 45% under water-stressed for CR057 and and CR006, respectively. For both genotypes, the contents of P, Zn, Cu, Mn, and Fe were higher in mycorrhizal than in nonmycorrhizal plants and higher under well-watered than under dry soil conditions. The enhancement of P, Zn, Cu, Mn, and Fe uptake due to AM inoculation was more pronounced in CR006 than in CR057, particularly under water-stressed conditions. Thus CR006 benefitted from AM infection more than the CR057 under dry soil conditions, despite the fact that CR057 roots were highly infected. It appears that CR006 is more dependent on AM symbiosis than CR057. Accepted: 12 February 1997     

Vesicular-arbuscular mycorrhizal effectiveness in an acid soil amended with fresh organic matter

A greenhouse investigation was undertaken to determine the influence of fresh organic matter on the formation and functioning of vesicular-arbuscular mycorrhizal symbiosis in Leucaena leucocephala grown in an acid aluminum-rich ultisol. In soil not amended with fresh organic matter or lime, plants failed to grow. Mycorrhizal infection level, mycorrhizal effectiveness measured in terms of pinnule P content of L. leucocephala leaves and dry matter yield of the legume increased with increase in fresh organic matter. Although VAM colonization level and dry matter yield of L. leucocephala were significantly higher if the test soil was limed (7.2 cmole OH^–) than if amended with fresh organic matter, the latter was as effective as lime in off-setting the detrimental effect of aluminum on mycorrhizal effectiveness. The lower mycorrhizal colonization level and the lower dry matter yield noted in the soil treated with fresh organic matter appears to be related to the inadequacy of Ca in the soil amended with fresh organic matter. These observations are supported by the low calcium status of soil and plant tissues in the absence of lime. It is concluded that while fresh organic matter, in appropriate amounts, could protect sensitive plants and VAM symbiosis against Al toxicity in acid soils, maximum mycorrhizal inoculation effects are not likely to be attained unless the soils are also amended with Ca.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No 3740.     

Effects of the root-lesion nematode Pratylenchus vulnus and the vesicular-arbuscular mycorrhizal fungus Glomus mosseae on the growth of three plum rootstocks

The effects of Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on growth of Myrobalan 605, Marianna 2624 and San Julian 655-2 plum rootstocks were measured under shadehouse conditions in the field for two growing seasons (1990–91). Shoot dry weights were higher in the majority of the vesicular-arbuscular mycorrhizal (VAM) alone inoculated plants after both growing seasons. Root weights of mycorrhizal Myrobalan and Marianna were higher than root weights of the same rootstocks lacking mycorrhizae, inoculated with P. vulnus, and VAM in combination with the nematode. Mycorrhizal Marianna inoculated with the nematode showed a considerably higher final nematode population in relation to non-inoculated VAM treatments. No correlation was found in the number of nematodes per gram of root between mycorrhizal and non-mycorrhizal treatments. P. vulnus adversely affected the mycorrhizal colonization in Marianna, but not in Myrobalan and San Julian. Marianna appears to be more mycorrhizal dependent than the two other rootstocks.     

Effects of different N fertilizers on the activity of <Emphasis Type="Italic">Glomus mosseae</Emphasis> and on grapevine nutrition and berry composition

Grapevine N fertilization may affect and be affected by arbuscular mycorrhizal (AM) fungal colonization and change berry composition. We studied the effects of different N fertilizers on AM fungal grapevine root colonization and sporulation, and on grapevine growth, nutrition, and berry composition, by conducting a 3.5-year pot study supplying grapevine plants with either urea, calcium nitrate, ammonium sulfate, or ammonium nitrate. We measured the percentage of AM fungal root colonization, AM fungal sporulation, grapevine shoot dry weight and number of leaves, nutrient composition (macro- and micronutrients), and grapevine berry soluble solids (total sugars or °Brix) and total acidity. Urea suppressed AM fungal root colonization and sporulation. Mycorrhizal grapevine plants had higher shoot dry weight and number of leaves than non-mycorrhizal and with a higher growth response with calcium nitrate as the N source. For the macronutrients P and K, and for the micronutrient B, leaf concentration was higher in mycorrhizal plants. Non-mycorrhizal plants had higher concentration of microelements Zn, Mn, Fe, and Cu than mycorrhizal. There were no differences in soluble solids (°Brix) in grapevine berries among mycorrhizal and non-mycorrhizal plants. However, non-mycorrhizal grapevine berries had higher acid content with ammonium nitrate, although they did not have better N nutrition and vegetative growth.     

Influence of the fungicide pentachloronitrobenzene on VA-mycorrhizal and total root length and phosphorus uptake of oats (Avena sativa)

The effect of application of the fungicide pentachloronitrobenzene (PCNB) at levels between 2 and 50 mg kg^–1 soil on root growth, mycorrhizal infection and P uptake was studied in pot culture with oats (Avena sativa cv. Alfred) growing in a rendzina soil low in available P. The soil had been partially sterilized by X-ray, and half of the pots were inoculated with spores of the VAM-fungusGlomus mosseae (indigenous species).Soil irradiation (0.5 Mrad) did not decrease the levels of infection by VAM. Application of PCNB decreased the VAM-infected root length, at 50 mg PCNB kg^–1 soil VAM-infected root length was about 12% of the controls. Total root length, however, increased to about 126% of control values at PCNB rates up to 20 mg kg^–1 soil, but decreased to 89% of the controls at 50 mg kg^–1 soil. Total P-uptake decreased with increasing levels of PCNB and was linearly correlated with infected root length (r=0.92).The stimulation of root growth by PCNB at rates up to 20 mg kg^–1 soil is regarded as an indirect effect, brought about by suboptimal P-supply due to inhibition of VA-mycorrhiza. Conversely, the reduction of total root length at 50 mg PCNB kg^–1 soil is most likely a direct effect. Due to the phytotoxicity of the fungicide, the contribution of the indigenous VA-mycorrhiza to plant P uptake under field conditions cannot be determined by soil application of PCNB at rates sufficient for complete inhibition of VAM.As inhibition or absence of VAM may lead to compensatory root growth, mycorrhizal dependency ought to be calculated from the amounts of P taken up per unit root length in mycorrhizal and nonmycorrhizal plants, respectively.     

Mycorrhizal inoculant alleviates salt stress in<Emphasis Type="Italic"> Sesbania aegyptiaca</Emphasis> and<Emphasis Type="Italic"> Sesbania grandiflora</Emphasis> under field conditions: evidence for reduced sodium and improved magnesium uptake

A field experiment was conducted to examine the effect of the arbuscular mycorrhizal fungus Glomus macrocarpum and salinity on growth of Sesbania aegyptiaca and S. grandiflora. In the salt-stressed soil, mycorrhizal root colonisation and sporulation was significantly higher in AM-inoculated than in uninoculated plants. Mycorrhizal seedlings had significantly higher root and shoot dry biomass production than non-mycorrhizal seedlings grown in saline soil. The content of chlorophyll was greater in the leaves of mycorrhiza-inoculated as compared to uninoculated seedlings. The number of nodules was significantly higher in mycorrhizal than non-mycorrhizal plants. Mycorrhizal seedling tissue had significantly increased concentrations of P, N and Mg but lower Na concentration than non-mycorrhizal seedlings. Under salinity stress conditions both Sesbania sp. showed a high degree of dependence on mycorrhizae, increasing with the age of the plants. The reduction in Na uptake together with a concomitant increase in P, N and Mg absorption and high chlorophyll content in mycorrhizal plants may be important salt-alleviating mechanisms for plants growing in saline soil.     

Influence of arbuscular mycorrhiza on clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons

 The effect of arbuscular mycorrhiza (AM) on white clover and ryegrass grown together in a soil spiked with polycyclic aromatic hydrocarbons (PAH) was assessed in a pot experiment. The soil was spiked with 500 mg kg^–1 anthracene, 500 mg kg^–1 chrysene and 50 mg kg^–1 dibenz(a,h)anthracene, representing common PAH compounds with three, four and five aromatic rings, respectively. Three treatments and two harvest times (8 and 16 weeks) were imposed on plants grown in spiked soil: no mycorrhizal inoculation, mycorrhizal inoculation (Glomus mosseae P2, BEG 69) and mycorrhizal inoculation and surfactant addition (Triton X-100). Pots without PAH were also included as a control of plant growth and mycorrhizal colonization as affected by PAH additions. The competitive ability of clover vis-à-vis ryegrass regarding shoot and root growth was enhanced by AM, but reduced by PAH and the added surfactant. This was reflected by mycorrhizal root colonization which was moderate for clover (20–40% of total root length) and very low for ryegrass (0.5–5% of total root length). Colonization of either plant was similar in spiked soil with and without the added surfactant, but the PAH reduced colonization of clover to half that in non-spiked soil. P uptake was maintained in mycorrhizal clover when PAH were added, but was reduced in non-mycorrhizal clover and in mycorrhizal clover that received surfactant. Similar effects were not observed on ryegrass. These results are discussed in the context of the natural attenuation of organic pollutants in soils. Accepted: 12 June 2000     

Histological, Physiological and Biochemical Interactions between Vesicular-Arbuscular Mycorrhizae and Thielaviopsis basicola in Tobacco Plants

Interactions between the mycorrhizal fungus Glomus monosporum and the root rot pathogen Thielaviopsis basicola and their effects on tobacco plants were investigated over a 4 week period. Mycorrhizal tobacco plants, obtained by preinoculation with G. monosporum, showed a better tolerance to T. basicola than non-mycorrhizal seedlings. Root and leaf dry weights of mycorrhizal plants were greater than those of controls. Mycorrhizal plants inoculated with T. basicola showed higher root and leaf dry weights than non-mycorrhizal infected plants, but lower values than mycorrhizal plants which were not infected. No appreciable differences in free aminoacid composition were observed among the different treatments with two exceptions: proline content was higher in infected and mycorrhizal infected plants compared to control and mycorrhizal plants; arginine content was higher in infected and mycorrhizal infected plants compared to control and mycorrhizal plants; arginine content was higher in mycorrhizal plants than in all the other treatments. The mechanisms by which (VAM) fungi can reduce disease incidence and pathogen development are discussed.