Comparative effects of vesicular-arbuscular mycorrhizal inoculation and phosphorus fertilization on growth and phosphorus uptake of maize (Zea mays L.) and sorghum (Sorghum bicolor L.) plants under drought-stressed conditions

Maize (Zea mays L.) and sorghum (Sorghum bicolor L.) Moench (local variety called Masakwat) plants were grown in a sterilized low-P soil in the greenhouse for 12 weeks. Each plant species was either mycorrhizal with vesicular-arbuscular mycorrhizal (VAM) fungi, non-mycorrhizal but minimally fertilized with soluble P, or non-mycorrhizal but highly fertilized with soluble P. Drought stress was imposed after 4 weeks at weekly intervals. Under unstressed conditions, leaf area, shoot dry weights, xylem pressure, and soil water potentials were similar for VAM and the two non-mycorrhizal P-fertilized treatments but each of the VAM-infected species had a greater total root length. Total P uptake was similar for the maize treatments but higher for VAM than non-mycorrhizal P-fertilized sorghum treatments. Under drought-stressed conditions, the growth parameters and soil water potential were similar for all maize treatments but they were reduced by mycorrhizal inoculation in sorghum. Greater water extraction occurred in drought-stressed mycorrhizal sorghum. In both plant species, total P uptake and P uptake per unit root length (including unstressed species) were significantly enhanced in non-mycorrhizal P-fertilized treatments compared with the mycorrhizal treatment. Except for the root dry weight of sorghum plants, there were no differences in the growth parameters and P uptake between minimally and highly P-fertilized non-mycorrhizal treatments for either maize or sorghum. The increased total root length in drought-stressed mycorrhizal sorghum plants and the similar infected root lengths in unstressed and drought-stressed sorghum plants may have caused high C partitioning to drought-stressed mycorrhizal roots and therefore caused the reduced growth parameters in mycorrhizal plants compared to the non-mycorrhizal P-fertilized counterparts. The results indicate that P fertilization in addition to mycorrhizal inoculation may improve the drought tolerance of maize and sorghum plants.     

The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae

Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.     

Inoculation with arbuscular mycorrhizal fungus Acaulospora mellea decreases Cu phytoextraction by maize from Cu-contaminated soil

A pot culture experiment was carried out to study the growth of and Cu uptake by maize (Zea mays) inoculated with or without arbuscular mycorrhizal (AM) fungus Acaulospora mellea in sterilized soil with different Cu amounts added (0, 100, 200, 400, 800 mg kg⁻¹). Root colonization rates were significantly lower with the addition of 400 and 800 mg kg⁻¹ Cu. AM inoculation increased shoot dry weights at 200 and 400 mg kg⁻¹ Cu added but showed no effects at other levels, while increased root dry weights at all Cu addition levels except 800 mg kg⁻¹. Compared with the nonmycorrhizal plants, shoot Cu concentrations in mycorrhizal plants were higher when no Cu was added but lower at other levels, while root Cu concentrations were lower at 400 and 800 mg kg⁻¹ Cu added but not affected at other levels. Thus, shoot Cu uptake in mycorrhizal plants increased with no Cu added but decreased at other levels, while mycorrhizal effects on root Cu uptake varied. Compared with nonmycorrhizal controls, Cu uptake efficiency and phytoextraction efficiency in mycorrhizal plants were higher when no Cu was added but lower at other levels, and Cu translocation efficiency was lower at all Cu addition levels. AM inoculation improved shoot and root P nutrition at all Cu addition levels. Soil pH was higher in mycorrhizal treatment than in the control when 200 mg kg⁻¹ or more Cu was added. These results indicate that A. mellea ZZ may be not suitable for Cu phytoextraction by maize, but shows a potential role in phytostabilization of soil moderately polluted by Cu.     

Water status and stomatal behaviour of cowpea,Vigna unguiculata (L.) Walp,plants inoculated with two Glomus species at low soil moisture levels

The effects of arbuscular mycorrhizal (AM) fungi on water status and stomatal behaviour of cowpea, Vigna unguiculata (L.) Walp. cv. B89-504, under water-stressed conditions in the greenhouse were studied. The 3 × 2 experimental design included two levels of mycorrhizal colonisation (Glomus mosseae, Glomus versiforme) and non-mycorrhizal control treatment and two soil moisture levels (well-watered pots and pots allowed to dry). Relative water content and leaf water potential values were higher in well-watered mycorrhizal and non-mycorrhizal plants than in water-stressed mycorrhizal and non-mycorrhizal plants. AM species had no significant effect on leaf osmotic potential, stomatal conductance and leaf transpiration in both well watered and water-stressed plants. The values of stomatal conductance and leaf transpiration were high during the vegetative stage and low during the flowering stage. These responses which can be related to the age of the plant suggest that mycorrhizal colonisation did not affect stomatal closure of cowpea plants during water stress. The decrease in plant growth and dry matter production in both mycorrhizal and non-mycorrhizal plants shows that drought resistance in cowpea was unaffected by mycorrhiza in the vegetative phase.     

Brassinosteroid (BR) and arbuscular mycorrhizal (AM) fungi alleviate salinity in wheat

This study was conducted to assess the impacts of brassinosteroide (BR), arbuscular mycorrhizal (AM) fungi, Glomus mosseae and their interactions on salt stress tolerance in Triticum aestivum L. After foliar spraying of mycorrhizal and non-mycorrhizal plants by 5 µM epibrassinolide, they were subjected to 0 and 150 mM sodium chloride (NaCl) for 2 weeks. The experiment was conducted in a randomized complete block design, replicated 4 times. Our results showed a probable potential of BR and/or AM fungi in improving salt tolerance of plants. Total phenol and proline content increased in BR and/ or AM treatments. AM fungi promoted plant growth, including leaf area, shoot and root dry weights, and lengths under saline condition. Moreover, BR improved growth parameters except root dry weights and lengths. This study indicated that BR and/or AM fungi may contribute to improve salt tolerance of the plant.     

Tolerance of eastern gamagrass to excess aluminum in acid soil and nutrient solution

Eastern gamagrass, Tripsacum dactyloides L., has been reported to tolerate a wide variety of soil conditions, including drought, flooding, and acidity, but its specific tolerance to aluminum (Al) has not been tested. One strain of this species, PMK Select Lot 94 SFG‐1, was tested for its tolerance to excess Al in an acid, Al‐toxic Tatum subsoil (clayey, mixed, thermic, Typic Hapludult) and in nutrient solutions containing Al. Roots were able to penetrate unfertilized Tatum subsoil at pH levels as low as 4.1–4.2 (1:1 soil‐water), at Al saturations of 64 to 77% of CEC, and to tolerate Al concentrations in nutrient solution that would be lethal for many crop plants. For example, with 4 mg Al L^‐1 and a final solution pH of 4.67, shoot and root dry weights were 75 and 76%, respectively, of those with no Al. Even with 24 mg Al L^‐1 and a final solution pH of 4.13, shoot and root dry weights were 45 and 46%, respectively, of those for the no Al check treatment. Hence, this strain of gamagrass shows promise for use on soils having acidic, Al‐toxic subsoil layers that act as root barriers and predispose plants to injury by drought. Roots of gamagrass are also reported to penetrate hard clay pans and to create root channels for subsequent crops that lack this ability. Current studies indicate that the strain tested was susceptible to a chlorosis resembling iron (Fe) deficiency when grown in a Jiffy Mix potting mixture or with excess Al in nutrient solutions. Hence, gamagrass is tentatively being classified as a calcifuge [Al tolerant‐Fe‐inefficient]. In the current experiment, considerable plant to plant variability was noted regarding susceptibility to this chlorosis factor and to a purpling symptom resembling phosphorus (P) deficiency. Results indicate that an exhaustive screening of gamagrass populations could identify strains that are more suitable for specific soil situations.     

Changes in soil aggregation and glomalin-related soil protein content as affected by the arbuscular mycorrhizal fungal species Glomus mosseae and Glomus intraradices

Arbuscular mycorrhizal (AM) fungi are key organisms of the soil/plant system, influencing soil fertility and plant nutrition, and contributing to soil aggregation and soil structure stability by the combined action of extraradical hyphae and of an insoluble, hydrophobic proteinaceous substance named glomalin-related soil protein (GRSP). Since the GRSP extraction procedures have recently revealed problems related to co-extracting substances, the relationship between GRSP and AM fungi still remains to be verified. In this work the hypothesis that GRSP concentration is positively correlated with the occurrence of AM fungi was tested by using Medicago sativa plants inoculated with different isolates of Glomus mosseae and Glomus intraradices in a microcosm experiment. Our results show that (i) mycorrhizal establishment produced an increase in GRSP concentration - compared to initial values - in contrast with non-mycorrhizal plants, which did not produce any change; (ii) aggregate stability, evaluated as mean weight diameter (MWD) of macroaggregates of 1-2 mm diameter, was significantly higher in mycorrhizal soils compared to non-mycorrhizal soil; (iii) GRSP concentration and soil aggregate stability were positively correlated with mycorrhizal root volume and weakly correlated with total root volume; (iv) MWD values of soil aggregates were positively correlated with values of total hyphal length and hyphal density of the AM fungi utilized.The different ability of AM fungal isolates to affect GRSP concentration and to form extensive and dense mycelial networks, which may directly affect soil aggregates stability by hyphal enmeshment of soil particles, suggests the possibility of selecting the most efficient isolates to be utilized for soil quality improvement and land restoration programs.     

红三叶草丛枝菌根对有机磷的吸收

The capacities of two arbuscular mycorrhizal (AM) fungi, Glomus mosseae and Glomus versderme, tomineralize added organic P were studied in a sterilized calcareous soil. Mycorrhizal (inoculated with either of the AM fungi) and non-mycorrhizal red clover (Trghlium pmtense L.) plants were grown for eight weeksin pots with upper root, central hyphal and lower soil compartments. The hyphal and soil compartmentsreceived either organic P (as Na-phytate) or inorganic P (as KH₂PO₄) at tbe rate of 50 mg P kg^-1. No P wasadded to the root compartments. Control pots received no added P. Yields were higher in mycorrhizal than innon-mycorrhizal clover. Mycorrhizal inoculation doubled shoot P concentration and more than doubled total P uptake of plaflts in P-amended soil, irrespective of the form of applied P. The mycorrhizal contributionto inorganic P uptake was 80% or 76% in plants inoculated with G. mosseae or G. verefforme, respectively. Corresponding values were 74% and 82% when Na-phytate was applied. In the root compartments of the mycorrhizal treatments, the proportion of root length infected, hyphal length density and phosphatase activity were all higher when organic P was applied than when inorganic P was added.     

EFFICIENCY OF ELEMENTAL SULFUR ON MYCORRHIZAE IN THE YIELD INCREASE OF WHEAT

Mycorrhizal infection enhances phosphorus (P) nutritional status of plants under P-deficient soil condition. The yield responses of wheat to sulfur (S) alone, phosphorus (P) alone, and combined application of sulfur and phosphorus in mycorrhizal and non-mycorrhizal treatments in calcareaous non-sterilized Karaburun soil (sub-group Typic Xerorthent) were investigated. The elemental sulfur enhanced the mycorrhizal efficiency that resulted in increased shoot, root, and grain yield dry matter in comparison to mycorrhiza treatment alone. This new finding to increase the efficiency of the mycorrhizae by the addition of 100 mg kg^?1 elemental sulfur to the soil suggests that elemental sulfur may play an important role for the ecologic agriculture. It can be specifically important for increasing the efficiency of mycorrhizal infection in well-aerated P deficient soil.     

Nickel-tolerant Brevibacillus brevis and arbuscular mycorrhizal fungus can reduce metal acquisition and nickel toxicity effects in plant growing in nickel supplemented soil

The growth of clover (Trifolium repens ) and its uptake of N, P and Ni were studied following inoculation of soil with Rhizobium trifolii, and combinations of two Ni-adapted indigenous bacterial isolates (one of them was Brevibacillus brevis) and an arbuscular mycorrhizal (AM) fungus (Glomus mosseae). Plant growth was measured in a pot experiment containing soil spiked with 30 (Ni I), 90 (Ni II) or 270 (Ni III) mg kg⁻¹ Ni-sulphate (corresponding to 11.7, 27.6 and 65.8 mg kg⁻¹ available Ni on a dry soil basis). Single inoculation with the most Ni-tolerant bacterial isolate (Brevibacillus brevis) was particularly effective in increasing shoot and root biomass at the three levels of Ni contamination in comparison with the other indigenous bacterial inoculated or control plants. Single colonisation of G. mosseae enhanced by 3 fold (Ni I), by 2.4 fold (Ni II) and by 2.2 fold (Ni III) T. repens dry weight and P-content of the shoots increased by 9.8 fold (Ni I), by 9.9 fold (Ni II) and by 5.1 fold (Ni III) concomitantly with a reduction in Ni concentration in the shoot compared with non-treated plants. Coinoculation of G. mosseae and the Ni-tolerant bacterial strain (B. brevis) achieved the highest plant dry biomass (shoot and root) and N and P content and the lowest Ni shoot concentration. Dual inoculation with the most Ni-tolerant autochthonous microorganisms (B. brevis and G. mosseae) increased shoot and root plant biomass and subtantially reduced the specific absorption rate (defined as the amount of metal absorbed per unit of root biomass) for nickel in comparison with plants grown in soil inoculated only with G. mosseae. B. brevis increased nodule number that was highly depressed in Ni I added soil or supressed in Ni II and Ni III supplemented soil. These results suggest that selected bacterial inoculation improved the mycorrhizal benefit in nutrients uptake and in decreasing Ni toxicity. Inoculation of adapted beneficial microorganisms (as autochthonous B. brevis and G. mosseae) may be used as a tool to enhance plant performance in soil contaminated with Ni.     

Hyphal translocation and uptake of sulfur by vesicular-arbuscular mycorrhizae of onion

Translocation of S by external hyphae of Glomus fascieulatus, a vesicular-arbuscular (VA) mycorrhizal fungus, was demonstrated. When tracers were injected 8 cm from onion roots in soil chambers, both ³⁵S and ³²P appeared in roots of mycorrhizal plants. Neither radionuclide was present in non-mycorrhizal plants.In a second soil-chamber experiment, mycorrhizal onions took up more ³⁵S per unit dry weight than non-mycorrhizal controls when ³⁵S was injected into soil chambers in a region 3–6 cm from roots. Severing of external hyphae between the application area and the roots reduced the concentration of ³⁵S in tops of mycorrhizal plants but not in roots. Volatile ³⁵S per unit dry weight collected from all plants in each treatment was highest in the mycorrhizal-hyphae intact treatment, and higher in the mycorrhizal-hyphae severed treatment than the non-mycorrhizal treatment. Movement of ³⁵S in soil from the area of application to roots was similar for all treatments.Increased uptake of S from soil by VA mycorrhizal plants can result from hyphal translocation of S to infected roots by external mycorrhizal hyphae.     

The influence of inorganic nutrient fertilization on the growth,nutrient composition and vesicular-arbuscular mycorrhizal colonization of pretransplant rice (Oryza sativa L.) plants

Summary The effects of P, N and Ca+Mg fertilization on biomass production, leaf area, root length, vesiculararbuscular mycorrhizal (VAM) colonization, and shoot and root nutrient concentrations of pretransplant rice (Oryza sativa L.) plants were investigated. Mycorrhizal plants generally had a higher biomass and P, N, K, Ca, Mn, Fe, Cu, Na, B, Zn, Al, Mg, and S shoot-tissue nutrient concentrations than non-mycorrhizal plants. Although mycorrhizal plants always had higher root-tissue nutrient concentrations than non-mycorrhizal plants, they were not significantly different, except for Mn. N fertilization stimulated colonization of the root system (colonized root length), and increased biomass production and nutrient concentrations of mycorrhizal plants. Biomass increases due to N were larger when the plants were not fertilized with additional P. P fertilization reduced the colonized root length and biomass production of mycorrhizal plants. The base treatment (Ca+Mg) did not significantly affect biomass production but increased the colonized root length. These results stress the importance of evaluating the VAM rice symbiosis under various fertilization regimes. The results of this study suggest that pretransplant mycorrhizal rice plants may have a potential for better field establishment than non-mycorrhizal plants.     

Take-all disease is systemically reduced in roots of mycorrhizal barley plants

The systemic effect of root colonization by the arbuscular mycorrhizal fungus Glomus mosseae on infection of barley by Gaeumannomyces graminis var. tritici (Ggt) was studied. In split-root systems of barley one side was inoculated with G. mosseae and the other side was inoculated with Ggt.Root infection by Ggt was systemically reduced when barley plants showed high degrees of mycorrhizal root colonization, whereas a low mycorrhizal root colonization exhibited no effect on Ggt infection. Our results show a clear systemic bioprotectional effect depending on the degree of root colonization by the mycorrhizal fungus. At a higher mycorrhizal colonization rate the concentration of salicylic acid (SA) was increased in roots colonized by the mycorrhizal fungus but no systemic increase of SA could be measured in non-mycorrhizal roots of mycorrhizal plants, indicating that the systemic bioprotectional effect against Ggt is not mediated by salicylic acid.     

The effect of soil liming on shoot development, root growth, and cluster root activity of white lupin

 The effects of a limed soil upon root and shoot growth of white lupin (Lupinus albus L.) were investigated using soil tubes and pots. After 75 days in the soil tubes, the combined taproot and lateral root dry weight in limed soil (2.5% CaO w/w) was significantly less than in neutral pH soil (by 57%). However, the dry weight and numbers of cluster roots remained comparable between the treatments, demonstrating for the first time that the cluster roots respond differently to the rest of the root system. Cluster roots accounted for 17% of the total root biomass in neutral soil, increasing significantly to over 30% in limed soil. When grown for 43 days in pots containing soil with different additions of lime (0.5–2.5% CaO w/w), soil citrate concentrations were higher than in the neutral pH soil treatment in all except the 2.5% lime treatment, in which they were lower. In both experiments, shoot dry weights were lower in the presence of the limed soil compared with those in the neutral pH soil. Although a reduction in shoot dry weight was not apparent at 21 days in the limed-soil tubes, the initiation of fewer mainstem leaf primordia indicated a slower shoot development than occurred in the neutral soil. Plants grown in the limed-soil tubes showed leaf yellowing and some chlorosis within 9 days. At the final harvest, the shoot phosphorus and manganese concentrations were significantly lower in plants grown in limed soil compared with those in the neutral pH soil, whereas the concentration of calcium was higher. Received: 11 October 1999     

Hydrolytic enzyme activities in maize (Zea mays) and sorghum (Sorghum bicolor) roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices

Two strains of Gluconacetobacter diazotrophicus (Pal 5, UAP5541) and the arbuscular mycorrhizal fungus Glomus intraradices increased both the shoot and root dry weight of sorghum 45 days after inoculation, whereas they had no effect on the shoot and root dry weight of maize. Co-inoculation (Gluconacetobacter diazotrophicus plus Glomus mosseae) did not increase the shoot and root dry weight of either plant. There was a synergistic effect of Gluconacetobacter diazotrophicus on root colonization of maize by Glomus intraradices, whereas an antagonistic interaction was observed in the sorghum root where the number of Gluconacetobacter diazotrophicus and the colonization by Glomus intraradices were reduced. Plant roots inoculated with Gluconacetobacter diazotrophicus and Glomus intraradices, either separately or together, significantly increased root endoglucanase, endopolymethylgalacturonase and endoxyloglucanase activities. The increase varied according to the plant. For example, in comparison with non-inoculated plants, there were higher endoglucanase (+328%), endopolymethylgalacturonase (+180%) and endoxyloglucanase (+125%) activities in 45-day old co-inoculated maize, but not in 45-day old sorghum. The possibility is discussed that hydrolytic enzyme activities were increased as a result of inoculation with Gluconacetobacter diazotrophicus, considering this to be one of the mechanisms by which these bacteria may increase root colonization by AM fungi.     

Comparative Study of Substrate-Based and Commercial Formulations of Arbuscular Mycorrhizal Fungi in Romaine Lettuce Subjected to Salt Stress

To compare the effect of substrate-based and commercial arbuscular mycorrhizal fungi (AMF) in salt stress tolerance of Romaine lettuce a bifactorial analysis was carried out. Under non-saline conditions, only plants inoculated with formulation 1 stimulated shoot weight but not related with greater root AMF colonization. Phosphorus and potassium concentrations in leaves were improved by mycorrhizal association. Irrigation with 100 mM sodium chloride (NaCl) did not affect leaf relative water content and we observed no osmotic adjustment in leaves from non-mycorrhizal plants. However, root dry biomass and its starch content decreased, while leaf starch and root soluble sugar concentrations were enhanced. Lettuce inoculated with formulation 2 and substrate-based Glomus intraradices showed the highest root colonization percentages. Nevertheless, none of the mycorrhizal treatments induced a significant improvement on growth of lettuce subjected to salt stress. Romaine lettuce seems to be a moderately tolerant variety to salinity and therefore, the contribution of AMF was minimized.     

Changes in the rhizospheric pH induced by arbuscular mycorrhiza formation in onion (Allium cepa L.)

Rhizospheric pH changes induced by arbuscular mycorrhiza formation in onion plants fertilized either with NO₃^? or NH₄⁺ were studied. The pH changes promoted by either mycorrhizal or non-mycorrhizal roots were studied by means of a non-destructive technique using the pH indicator bromocresol purple. Results showed that the pH changes observed depended on i) the symbiotic status of the root and ii) the N form amended to the soil. When growing in a NH₄⁺-supplied soil, mycorrhizal onion roots produced more intense and wider acidification halos than non-mycorrhizal plants did. These differences were maintained throughout the whole experiment (60 days). NO₃^?-supplied mycorrhizal roots initially promoted a more intense alkalinization on their surface, compared to the control roots (30 days); however, at the end of the experiment (60 days), intense acidification halos were observed in the mycorrhizosphere, whereas this acidification was almost absent in the non-mycorrhizal rhizosphere. The link between these mycorrhiza-induced pH changes in the soil and the higher efficiency in the exploitation of nitrogen in the rhizosphere by the arbuscular-mycorrhizal plants is discussed.     

Tolerances of wheat germplasm to acid subsoil

Aluminum toxicity is a major growth limiting factor for plants in many acid soils of the world. Correcting the problem by conventional liming is not always economically feasible, particularly in subsoils. Aluminum tolerant plants provide an alternative and long‐term supplemental solution to the problem. The genetic approach requires the identification of Al tolerance sources that can be transferred to cultivars already having desirable traits. Thirty‐five cultivars and experimental lines of wheat (Triticum aestivum L. em. Thell) were screened for Al tolerance on acid Tatum soil (clayey, mixed thermic, typic Hapludult) receiving either 0 or 3500 mg CaCO₃/kg (pH 4.1 vs. pH 7.1). Entries showed a wide range of tolerance to the acid soil. On unlimed soil at pH 4.3, absolute shoot dry weights differed by 5‐fold, absolute root dry weights by 6.5‐fold, relative shoot weights (wt. at pH 4.3/wt. at pH 7.1 %) by 4.7‐fold and relative root dry weights by 7‐fold. Superior acid soil (Al) tolerance of ‘BH‐1146’ from Brazil and extreme sensitivities of cultivars ‘Redcoat’ (Indiana, USA) and ‘Sonora 63’ (Mexico) were confirmed. Seven experimental (CNT) lines from Brazil showed a range of acid soil tolerance but were generally more tolerant than germplasm from Mexico and the USA. One line, ‘CNT‐1’, was equal to BH‐1146 in tolerance and may be useful in transferring Al tolerance to existing or new cultivars. Five durum cultivars (Triticum, durum, Desf.) were extremely sensitive to the acid Tatum subsoil at pH 4.3 compared with pH 7.1.     

Distribution of protozoa in scots pine mycorrhizospheres

The distribution of heterotrophic flagellates, naked amoebae, testate amoebae and ciliates was investigated in habitats created by Scots pine-Paxillus involutus and -Suillus bovinus ectomycorrhizospheres. The protozoa living on plant and fungal surfaces preferred the non-mycorrhizal pine roots over mycorrhizal roots or external mycelium. The testate amoebae were more abundant on external mycelium than on mycorrhizae regardless of the mycorrhizal fungal species. Numbers of protozoa were higher in the different habitats provided by S. bovinus mycorrhizospheres when compared with P. involutus mycorrhizospheres. Interestingly, the quality of the bacterial flora as food for the protozoa was affected by the mycorrhizal fungi even in the soils adjacent to non-mycorrhizal root tips of pine. These results demonstrate that mycorrhizal fungi create habitats differently suitable for protozoa living in boreal forest soil.     

Mycorrhizal phosphorus enhancement of plants in undisturbed soil differs from phosphorus uptake stimulation by arbuscular mycorrhizae over non-mycorrhizal controls

Previous greenhouse and field studies have shown arbuscular mycorrhizal (AM) plants usually have greater P uptake and growth when raised in undisturbed soil compared to soil disturbed between plantings, such as by tillage. We report here for the first time that AM fungi able to stimulate shoot P uptake in experimental comparisons to non-mycorrhizal plants differ in their ability to bring about similar responses in undisturbed soil compared to disturbed soil. This outcome indicates a difference in functional character between the two stimulation processes. Three isolates of AM fungi were tested for growth promotion of maize (Zea mays L.) in pots in a soil disturbance experiment that included non-mycorrhizal controls. All three fungi colonized roots well and promoted shoot P uptake compared to non-inoculated controls, but only Glomus mosseae was able to stimulate growth in undisturbed soil compared to disturbed soil. This effect was seen when Glomus mosseae was alone or in combination with Gigaspora margarita. However, the presence of Glomus aggregatum in combination with Glomus mosseae prevented any stimulation, presumably due to domination by Glomus aggregatum. The ability of AM fungi to be beneficial to plants in comparison to non-mycorrhizal situations likely relates to the spread of mycelium in the soil and the capacity for nutrient transfer to the root. The ability of an AM fungus to promote growth in undisturbed soil appears to be related to these features and, in addition, a capacity for persistence and retention of functional capacity of the extraradical mycelium from one plant generation to the next.