Percentage Of Arbuscular Mycorrhizal Colonization Research Articles

Arbuscular mycorrhizal association in Conocarpus erectus (Combretaceae) in mangroves from Yucatán, México

Background: Soil flooding and salinity can be limiting for arbuscular mycorrhizal fungi (AMF), yet they are found in mangrove ecosystems. Arbuscular mycorrhizal association could influence the functioning of mangrove ecosystems, but little is known about its roll and balance in these dynamic systems.
 Question: What is the status of the arbuscular mycorrhizal association in C. erectus under natural conditions?
 Species study: Conocarpus erectus is a tree or shrub species that establishes in mangrove ecosystems and is tolerant to elevated levels of salinity and flooding.
 Study site and dates: Three zones representing a range of conditions of salinity were selected in the Ría Lagartos Biosphere Reserve in Mexico, as well as two contrasting collecting seasons in 2009 and 2010: northwind season and drought season. 
 Methods: Roots were sampled from C. erectus plants in each zone to evaluate the percentage of arbuscular mycorrhizal colonization and rhizospheric soil samples were taken to evaluate AMF spore density and species richness. 
 Results: The highest percentage of root colonization was recorded in the site of highest salinity during the northwind season, with this value decreasing in the drought season. The same seasonal pattern was observed in spore density. The highest richness was recorded in the site of lowest salinity in both seasons. A total of 16 AMF species were identified. 
 Conclusions: The results suggest that seasonality, in its relation with soil salinity and soil water availability, can influence the development and symbiotic association of the AMF in mangrove associated communities dominated by C. erectus .

Selection of a suitable plant for phytoremediation in mining artisanal zones

ABSTRACTA study was undertaken with the aim of identifying a suitable plant for the phytoremediation of metal-polluted soil from an artisanal mining area in Ecuador. Three zones including a natural zone (NZ), abandoned zone (AZ) and intensively mined zone (IZ) were selected. Three common native plants grown in the three zones were identified and collected, including Miconia zamorensis, Axonopus compressus and Erato polymnioides. The percentage of arbuscular mycorrhizal colonization that benefits their own survival in polluted soil was analyzed in the root samples of these candidate species. Analysis of the soils and plants collected from the different zones showed that the concentrations of Pb, Zn, Cu and Cd were comparatively lower in the NZ, higher in the AZ and IZ, and highest in the AZ for all the metals. The concentration of all these metals in plant tissues was the highest in E. polymnioides. The data analysis including the metal accumulation index, bioconcentration factor and translocation factor strongly identified E. polymnioides as a hyperaccumulator plant suitable for phytoremediation.

Seasonal Dynamics of Arbuscular Mycorrhizal Fungi (AMF) in Forest Trees of Chittagong University Campus in Bangladesh

Status of Arbuscular Mycorrhizal (AM) colonization in seven tree species (Albizia saman, Acacia auriculiformis A. Cunn. ex Benth., Albizia lebbeck, Chickrassia tabularis A. Juss., Eucalyptus camaldulensis Dehnn., Gmelina arborea (Roxb) DC, Swietenia macrophylla King.) collected from the hilly areas of Chittagong University (CU) was investigated. Roots and rhizosphere soil samples were collected in different seasons (pre-monsoon, monsoon and post monsoon). Percentage of AM colonization in root and number of spores/100 gm dry soil were assessed. The result of the investigation reveals that the intensity and percentage of AM colonization varied in different forest tree species in different seasons. In this study, maximum AM colonization and spore population were found in pre-monsoon and minimum were in monsoon season. The intensity of colonization was maximum in C. tabularis (74.43%) in pre-monsoon, A. lebbeck (69.45%) in monsoon and S. macrophylla (67.8%) in post monsoon seasons and minimum in A. auriculiformis (53.75%) during pre-monsoon, A. saman (24.4%) in monsoon and A. saman (19.36%) in post monsoon. The number of spores found per 100 g dry soil ranged between 164-376 during pre-monsoon, 27-310 during monsoon and 194-299 in post monsoon season. Out of six recognized genera of AM fungi, Glomus, Sclerocystis, Entrophospora, Scutellospora, Acaulospora and other unidentified spores were observed.

Arbuscular mycorrhizal and septate endophyte fungal associations in lycophytes and ferns of south India

We studied extent and type of arbuscular mycorrhizal (AM) and septate endophytic (SE) fungal associations in five lycophytes and 50 ferns collected from Eastern and Western Ghats regions. Of the 54 species and one variety (belonging to 31 genera) examined; 54 taxa had AM association and AM fungal structures were absent in Marsilea quadrifolia. This is the first report of AM and SE fungal status for 26 species each. Of the 55 taxa examined, AM morphology has been evaluated for the first time in 51 species. The hydrophytic fern Salvinia molesta was mycorrhizal and non-mycorrhizal at different sites. All the epiphytic and saxicolous species examined were mycorrhizal. The percentage of AM colonization ranged from 22.23 (Christella parasitica) to 82.20 (Adiantum lunulatum) in ferns and 53.46 (Selaginella bryopteris) to 84.34 (Selaginella sp.) in lycophytes. Epiphytic life-forms had the maximum average AM colonization levels, whereas aquatic life-forms had the minimum colonization levels. The percentage root length colonized by septate fungi ranged between 0.59 in Ophioglossum reticulatum and 16.36 in Pteris pellucida. The root length with AM and SE fungal structures as well as their total colonization significantly varied among the taxa examined. Most of the lycophytes and ferns had intermediate-type of AM morphology with a few exhibiting Paris-type. AM fungal spore numbers ranged from 1.0 (Angiopteris evecta, Pteridium aquilinum) to (Nephrolepis exaltata) 9.3 spores per 25 g soil and varied significantly among taxa. AM fungal spore morphotypes belonging to Claroideoglomus, Funneliformis, Glomus and Rhizophagus were recorded.

Arbuscular mycorrhizal colonization of Sorghum vulgare in presence of root endophytic fungi of Myrtus communis

A total of 150 isolates of endophytic fungi were obtained from roots of Myrtus communis grown in the Mediterranean area. Seven different endophytic fungi taxa, all belonging to the Ascomycota phylum were found. Arbuscular mycorrhizal (AM) fungi, belonging to the Glomus genus, were also present in roots of M. communis. None of the root endophytic fungi tested, except Drechslera sp., colonized the root of sorghum. Among all fungi tested, only Phoma schachtii and Cylindrocarpon destructans significantly increased the shoot dry weight of sorghum grown in sterilized soil. Phomopsis columnaris, P. schachtii, Eucasphaeria sp. and Cylindrocarpon pauciseptatum increased the dry matter of sorghum grown in unsterilized soil. The percentage of AM colonization of sorghum was increased significantly only when Ph. columnaris, P. schachtii, the two strains of Bionectria ochroleuca, Eucasphaeria sp. and C. pauciseptatum were applied to unsterilized soil. The number of CFUs of endophytic fungi inoculated in sorghum rhizosphere was similar throughout the experiment, except B. ochroleuca-1 whose population increased at the end of the experiment either in sterilized or in unsterilized soil. No significant effect of the endophytic fungi isolated from M. communis on shoot and root P, K, Ca, Na, Mg, Cu and Zn concentrations of plants grown in sterilized or unsterilized soils, except Eucasphaeria sp. which increased the shoot P concentration of sorghum grown in unsterilized soil.

Effects of Arbuscular Mycorrhizal Inoculation and Phosphorus Application on Yield and Nutrient Uptake of Yam

To be sustainable, production in the traditional yam cropping system, faced with declining soil fertility, could benefit from yam–arbuscular mycorrhizal (AM) symbiosis, which can improve nutrient uptake, disease resistance, and drought tolerance in plants. However, only limited information exists about AM colonization of yam. A pot experiment was conducted to collect information on the response of two genotypes (Dioscorea rotundata accession TDr 97/00903 and D. alata accession TDa 297) to AM inoculation (with and without) and phosphorus (P) (0, 0.05, 0.5, and 5 mg P kg–1 soil). Factorial combinations of the treatments were arranged in a completely randomized design with four replicates. The percentage of AM colonization was significantly lowered at 5 mg P kg–1 soil rate in mycorrhizal plants of both genotypes. TDr 97/00903 showed more responsiveness to AM inoculation than TDa 297. The greatest AM responsiveness for tuber yield (52%) was obtained at 0.5 mg P kg–1 soil rate for TDr 97/00903. Mycorrhizal inoculation significantly increased root dry weight and tuber yield of TDr 97/00903 with the greatest values obtained at the 0.5 mg P kg–1 soil rate. Arbuscular mycorrhizal inoculation did not lead to significant (P < 0.05) changes in root length and area. Phosphorus application significantly increased the shoot dry weight and root diameter of TDa 297. Uptake of P was greatest at 0.5 mg P kg–1 soil in both genotypes and was significantly influenced by AM inoculation. Nitrogen (N) and potassium (K) uptake were greatest in mycorrhizal plants at 0.05 mg P kg–1 soil for TDr 97/00903 but at 0.5 mg P kg–1 soil of nonmycorrhizal plants of TDa 297. The increased tuber yield and nutrient uptake observed in the mycorrhizal plants indicate the potential for the improvement of nutrient acquisition and tuber yield through AM symbiosis.

Arbuscular mycorrhizal fungi associated with tangerine (Citrus reticulata) in Chiang Mai province, northern Thailand, and their effects on the host plant

There are many tangerine (Citrus reticulata) orchards in northern Thailand. These orchards are supplied with different levels of fertilizers. The objective of this study is to investigate arbuscular mycorrhizal (AM) fungi associated with tangerine in Chiang Mai province, northern Thailand, and the effect of AM fungi on the growth of the air layered tangerine variety 'Sai Num Phung' with different levels of nitrogen (N) and phosphorus (P) fertilizers in a pot experiment. Percentage of AM colonization in the tangerine roots and spore density in the rhizosphere varied significantly with the available P levels in the orchard soil. Means of root colonization and spore density were significantly depressed at >500 mg P/kg soil. Twenty-two species of AM fungi were found to be associated with tangerine in orchards of the Chiang Mai province. The effects of AM fungi, and N and P fertilizers on air layered tangerine plants were investigated in pots for ten months. AM fungi increased growth of the host plant especially in pots with N but without P fertilizers. AM fungi increased concentrations of P and Mg in leaves of tangerine. Application of N and P fertilizers depressed root colonization of AM fungi in the pot experiment. This study has shown that a wide range of AM fungi is associated with tangerine in commercial orchards, but with high levels of N and P fertilizers the increase in growth of tangerine trees due to the association with AM fungi may be limited.

Arbuscular Mycorrhizal Fungi Increased Plant Growth and Nutrient Concentrations of Milkwood Tropical Tree Species Alstonia scholaris Under Greenhouse Conditions

The objective of this study was to determine the effect of five arbuscular mycorrhizal (AM) fungi on the early growth of Alstonia scholaris (milkwood) seedlings. The seedlings were inoculated with Glomus clarum Nicholson &amp; Schenk, Gigaspora decipiens Hall &amp; Abbott, Glomus sp. ACA Tulasne &amp; Tulasne , Entrophospora sp. Ames &amp; Scheneider, and Glomus sp. ZEA Tulasne &amp; Tulasne, and uninoculated (control) under greenhouse conditions. Percentage of AM colonization, plant growth, survival rate, mycorrhizal dependency (MD), shoot nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations were measured after 150 days. Survival rates were higher in the AM-colonized seedlings at 150 days after transplantation than those in the control seedlings. Mycorrhizal Dependency (MD) values were 80, 78, 79, 78 and 78% in A. scholaris inoculated with G. clarum, G. decipiens , Glomus sp. ACA, Entrophospora sp . , and Glomus sp. ZEA, respectively. Shoot N, P, K, Ca and Mg content of the seedlings were increased by AM fungi as much as 82-86, 81-86, 81-86, 88-91 and 85-90%, respectively. The percentage of AM colonization of A. scholaris ranged from 64 to 91 %. Colonization by five AM fungi increased plant height, diameter, total fresh weight, total dry weight and total length root. Glomus clarum was more effective in improving nutrient content and plant growth of A. scholaris than G. decipiens, Entrophospora sp ., Glomus sp. ZEA and Glomus sp. ACA. Total root length of A. scholaris ranged from 1,180 to 1,310 cm. The results suggest that AM fungi can accelerate the establishment of the seedling stocks of A. scholaris . This finding would contribute to the effort of establishing A. scholaris plantation.

Arbuscular mycorrhizal fungi increased early growth of two nontimber forest product species Dyera polyphylla and Aquilaria filaria under greenhouse conditions

Nontimber forest products (NTFPs) represent an important source of income to millions of people in tropical forest regions, but some NTFP species have decreased in number and become endangered due to overexploitation. There is increasing concern that the planting stocks of Dyera polyphylla and Aquilaria filaria are not sufficient to sustain the yield of NTFPs and promote forest conservation. The objective of this study was to determine the effect of two arbuscular mycorrhizal (AM) fungi, Glomus clarum and Gigaspora decipiens, on the early growth of two NTFP species, D. polyphylla and A. filaria, under greenhouse conditions. The seedlings of both species were inoculated with G. clarum or G. decipiens, or uninoculated (control) under greenhouse conditions. Percentage of AM colonization, plant growth, survival rate, and nitrogen (N) and phosphorus (P) concentrations were measured after 180 days of growth. The percentage of AM colonization of D. polyphylla and A. filaria ranged from 87 to 93% and from 22 to 39%, respectively. Colonization by G. clarum and G. decipiens increased plant height, diameter, and shoot and root dry weights. Shoot N and P concentrations of the seedlings were increased by AM colonization by as much as 70-153% and 135-360%, respectively. Survival rates were higher in the AM-colonized seedlings at 180 days after transplantation than in the control seedlings. The results suggest that AM fungi can accelerate the establishment of the planting stocks of D. polyphylla and A. filaria, thereby promoting their conservation ecologically and sustaining the production of these NTFPs economically.

Ectomycorrhizal colonization of Alnus acuminata Kunth in northwestern Argentina in relation to season and soil parameters

The objective of this study was to determine patterns of arbuscular mycorrhizal (AM) colonization of Alnus acuminata Kunth at two natural forests in relation to soil parameters at two different seasons (autumn and spring). The soil parameters studied were field capacity, pH, electrical conductivity, available P, total N and organic matter. The percentage of AM colonization was estimated and correlated to soil properties and to two different seasons. The results indicate that the percentage of AM colonization varied among soil types and was higher in spring than autumn. A significant positive correlation was found between AM colonization and electrical conductivity, organic matter and total Nitrogen. Results of this study provide evidence that AM colonization of A. acuminata can be affected by some soil parameters and seasonality.

Arbuscular mycorrhizal colonization of the dominant plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji

Arbuscular mycorrhizal (AM) colonization was observed on four plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji. The AM colonization of the dominant species, Polygonum cuspidatum, contradicts the conclusion that Polygonaceae are often regarded as being non-mycorrhizal species. The secondary dominant species, Polygonum weyrichii var. alpinum, formed no mycorrhizas. The roots of Cirsium purpuratum, Clematis stans and Campanula punctata ssp. hondoensis, showed a higher percentage of AM colonization than P. cuspidatum. AM colonization and spore density in the rhizosphere soil of P. cuspidatum significantly decreased as elevation increased. AM colonization in roots of Cirsium purpuratum and Clematis stans also tended to decrease with increased altitudes. Cirsium purpuratum and Campanula punctata ssp. hondoensis formed single structural types of Arum- and Paris-type, respectively, whereas P. cuspidatum and Clematis stans formed both Arum- and Paris-type morphologies.

Mycorrhizal response and successional status in 80 woody species from south Brazil

Arbuscular mycorrhizal (AM) fungi colonization and response were studied in seedlings of 80 native woody species belonging to different successional groups from the Tibagi River Basin, Paraná State, south Brazil. This study includes data from 43 native woody species already published. The results with 80 species did not differ from the results of the 43 species. The experiment was carried out in a greenhouse in plastic bags filled with a mix of subsoil (85%) and sand (15%), inoculated or not with spores of native AM fungi obtained from rhizosphere soil of different native tree species in an area with natural vegetation dominated by woody pioneer species. The successional groups were represented by 16 pioneer, 20 early secondary, 29 late-secondary and 15 climax species. The AM response and colonization in the greenhouse were 5.9 and 4.2 times greater in the early successional species than in the late-successional species, respectively. Seedlings of 49 woody species were collected in the interior under the canopy of the tropical forest of the Mata dos Godoy State Park and in a cleared area dominated by woody pioneer species. The percentage of AM colonization in the field was 54.9, 40.4, 7.2 and 3.1 for the pioneer, early secondary, late-secondary and climax species, respectively. The response to AM inoculation was strongly and directly related to AM colonization in the greenhouse and field and inversely related to seed weight. The AM colonization in the greenhouse was strongly and directly related to AM colonization in field. The late-successional species showed lower AM colonization and response than early successional species. The accentuated mycotrophism of the early successional species may be involved in their establishment, growth, survival and early forest structuring on low-fertility soils.

Interaction between the soil yeast Rhodotorula mucilaginosa and the arbuscular mycorrhizal fungi Glomus mosseae and Gigaspora rosea

The effect of the soil yeast, Rhodotorula mucilaginosa LBA, on Glomus mosseae (BEG n°12) and Gigaspora rosea (BEG n°9) was studied in vitro and in greenhouse trials. Hyphal length of G. mosseae and G. rosea spores increased significantly in the presence of R. mucilaginosa. Exudates from R. mucilaginosa stimulated hyphal growth of G. mosseae and G. rosea spores. Increase in hyphal length of G. mosseae coincided with an increase in R. mucilaginosa exudates. No stimulation of G. rosea hyphal growth was detected when 0.3 and 0.5 ml per petri dish of yeast exudates was applied. Percentage root length colonization by G. mosseae in soybean ( Glycine max L. Merill) and by G. rosea in red clover ( Trifolium pratense L. cv. Huia) was increased only when the soil yeast was inoculated before G. mosseae or G. rosea was introduced. Beneficial effects of R. mucilaginosa on arbuscular mycorrhizal (AM) colonization were found when the soil yeast was inoculated either as a thin agar slice or as a volume of 5 and 10 ml of an aqueous solution. R. mucilaginosa exudates (20 ml per pots) applied to soil increased significantly the percentage of AM colonization of soybean and red clover.

INTRASPECIFIC AM FUNGAL VARIATION CONTRIBUTES TO PLANT–FUNGAL FEEDBACK IN A SERPENTINE GRASSLAND

Feedback between plants and arbuscular mycorrhizal (AM) fungi can affect species diversity in plant and fungal communities. Feedback depends on (1) some specificity between plants and fungi and (2) fungi exhibiting specificity either improving (positive feedback) or decreasing (negative feedback) host performance relative to other fungi. Associations between AM fungi and plant species in a serpentine grassland dominated by Andropogon gerardii, Schizachyrium scoparium, Sorghastrum nutans, and Sporobolus heterolepis were examined, and their performance consequences were evaluated. Specificity was determined from AM fungal spore abundance under plants in the field and from trap cultures established by inoculating greenhouse plants with field-collected roots containing fungal material. Seven AM fungal species were unevenly distributed among plant species, with differences in total spore numbers and evenness. In the field, Gigaspora gigantea exhibited specificity to Sporobolus compared to Andropogon and Sorghastrum, and Glomus microcarpum exhibited specificity to Schizachyrium compared to Andropogon and Sporobolus. In trap cultures, Glomus etunicatum exhibited specificity to Andropogon compared to Sorghastrum, and an unidentified species of Glomus exhibited specificity to Sorghastrum compared to Sporobolus. The AM fungal community associated with Smilax rotundifolia, a grassland invader, was not qualitatively different from those of grasses. In the greenhouse, plant and fungal performance was examined as a function of three host plant species (excluding Sporobolus), four AM fungal species, and the same three plant species on which the fungi had been collected in the field. Fungal species affected host plant biomass and fungal performance, measured by AM root length and percentage of colonization. Compared to other fungal species, Gigaspora gigantea increased plant biomass and had a higher percentage of colonization and AM root length on both Andropogon and Schizachyrium. Glomus etunicatum, which exhibits specificity on Andropogon, depressed growth of Andropogon relative to other fungi, implicating negative feedback. Schizachyrium had reduced biomass when inoculated with Glomus microcarpum, its specific fungus. Additionally, fungi originating on a conspecific host depressed plant growth compared to fungi collected from other host species, an effect strongest for Andropogon. Percentage of AM colonization on Andropogon was also greater for fungi originating on Andropogon. The latter two effects serve as another example of negative plant–fungal feedback and could represent intraspecific fungal variation within this community. Corresponding Editor: S. H. Faeth

Interactions of arbuscular-mycorrhizal fungi and Bacillus strains and their effects on plant growth, microbial rhizosphere activity (thymidine and leucine incorporation) and fungal biomass (ergosterol and chitin)

The effects of two Bacillus strains ( Bacillus pumillus and B. licheniformis) on Medicago sativa plants were determined in single or dual inoculation with three arbuscular-mycorrhizal (AM) fungi and compared to P-fertilization. Shoot and root plant biomass, values of thymidine and leucine incorporation as well as ergosterol and chitin in rhizosphere soil were evaluated to estimate metabolic activity and fungal biomass, respectively, according to inoculation treatments. For most of the plant parameters determined, the effectiveness of AM fungal species was influenced by the bacterial strain associated. Dual inoculation of Bacillus spp. and AM fungi did not always significantly increase shoot biomass compared to single AM-colonized plants. The most efficient treatment in terms of dry matter production was the dual Glomus deserticola plus B. pumillus inoculation, which produced similar shoot biomass and longer roots than P-fertilization and a 715% (shoot) and 190% (root length) increase over uninoculated control. The mycorrhizas were more important for N use-efficiency than for P use-efficiency, which suggests a direct mycorrhizal effect on N nutrition not mediated by P uptake. Both chemical and biological treatments affected thymidine and leucine incorporation in the rhizosphere soil differently. Thymidine was greater in inoculated than in control rhizospheres and B. licheniformis was more effective than B. pumillus in increasing thymidine. Non-inoculated rhizospheres showed the lowest thymidine and leucine values, which shows that indigenous rhizosphere bacteria increased with introduced inocula. The highest thymidine and leucine values found in P-fertilized soils indicate that AM plants are better adapted to compete with saprophytic soil bacteria for nutrients than P-amended plants. Chitin was only increased by coinoculation of B. licheniformis and G. intraradices. B. pumillus increased ergosterol (indicative of active saprophyte fungal populations) in the rhizosphere of AM plants and particularly when colonized by G. mosseae. The different AM fungi have different effects on bacterial and/or fungal saprophytic populations and for each AM fungus, this effect was specifically stimulated or reduced by the same bacterium. This is an indication of ecological compatibilities between microorganisms. Particular Glomus–bacterium interactions (in terms of effect on plant growth responses or rhizosphere population) do not seem to be related to the percentage of AM colonization. The effect on plant growth and stimulation of rhizosphere populations, as a consequence of selected microbial groups, may be decisive for the plant establishment under limiting soil conditions.

Compatibility of a wild type and its genetically modified Sinorhizobium strain with two mycorrhizal fungi on Medicago species as affected by drought stress

The effect of double inoculation with two strains of Sinorhizobium meliloti [the wild type (WT) strain GR4 and its genetically modified (GM) derivative GR4(pCK3)], and two species of arbuscular mycorrhizal (AM) fungi ( Glomus deserticola and Glomus intraradices) was examined in a microcosm system on three species of Medicago ( M. nolana, M. rigidula, M. rotata). Two water regimes (80 and 100% water holding capacity, WHC) were assayed. The efficiency of each AM fungus increasing plant growth, nutrient content, nodulation and water-stress tolerance was related to the Sinorhizobium strains and Medicago species. This indicates selective and specific compatibilities between microsymbionts and the common host plant. Differential effects of the mycorrhizal isolates were not associated with their colonizing ability. Nodulation and mycorrhizal dependency (MD) changed in each plant genotype in accordance with the Sinorhizobium strain and AM fungi involved. Generally, Medicago sp. MD decreased under water-stress conditions even when these conditions did not affect AM colonization (%). Proline accumulation in non-mycorrhizal plant leaves was increased by water stress, except in M. rotata plants. Differences in proline accumulation in AM-colonized plants suggest that both the AM fungus and the Sinorhizobium strain were able to induce different degrees of osmotic adjustment. Mycorrhizal plants nodulated by the WT strain accumulated more proline in M. rigidula and M. rotata under water stress than non-mycorrhizal plants. Conversely, mycorrhizal plants nodulated by the GM strain accumulated less proline in response to both AM colonization and drought. These results indicated changes in the synthesis of this nitrogenous osmoregulator product associated with microbial inoculation and drought tolerance. Mycorrhizal plants nodulated by the GM Sinorhizobium strain seem to suffer less from the detrimental effect of water stress, since under water limitation relative plant growth, percentage of AM colonization, root dry weight and the highest R/S ratio remained the same. The fact that GM nodulated plants are better adapted to drought stress could be of practical interest and the management of GM microorganism inoculation may be crucial for biotechnological approaches to improving crop yield in dry environments.

Effects of climatic factors on native arbuscular mycorrhizae and Meloidogyne exigua in a Brazilian rubber tree (Hevea brasiliensis) plantation

The root‐knot nematode Meloidogyne exigua and arbuscular mycorrhizal (AM) fungi may both occur in the roots of Brazilian rubber trees (Hevea brasiliensis). AM fungi may stimulate plant growth whereas nematodes usually reduce it. Variations of native AM fungi and M. exigua populations in soil and roots of rubber trees were studied for one year in a Brazilian plantation. The number of AM spores in the soil was generally greater in the rainy season than in the dry season, although AM colonization of roots was unaffected by season. During the dry season, numbers of juveniles and eggs of M. exigua in roots were lower than in the rainy season. A site without nematodes in the soil or roots showed the greatest numbers of AM spores in soil and highest AM colonization of roots. A negative correlation was observed between the percentage of AM colonization and the number of second‐stage juveniles in soil and second‐stage juveniles and eggs in roots. Microscope observations revealed (i) tissue specificity for each of the microorganisms in the roots, with a cortical location of mycorrhizae and a mainly vascular cylinder location of nematodes, and (ii) that Gigaspora was the most abundant AM genus in the plantation soil.