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La eficiencia del uso del nitrógeno (EUN) por los cultivos agrícolas es muy baja por pérdidas asociadas con su alta solubilidad y, en algunas condiciones de suelo, por alta volatilidad, lo que trae como consecuencia impactos negativos en la rentabilidad económica y la contaminación del medio ambiente. Se realizó un experimento de campo bajo riego en suelo del Bajío en Guanajuato, México para estudiar la eficiencia de asimilación del fertilizante nitrogenado por el cultivo de trigo (Triticum aestivum L.) mediante el fraccionamiento de su aplicación utilizando como trazador sulfato de amonio enriquecido con 3.0237 % de átomos en exceso de 15N. Las variedades de trigo de invierno evaluadas fueron Gema C2004, Nana F2007 y Urbina S2007; el 100 % de la dosis de fertilización fue 240 kg N ha-1 y se usaron 10 fraccionamientos en porcentaje a la siembra, encañe y espigamiento (00-00-00, 100-00-00, 00-100-00, 50-50-00, 00-50-50, 70-30-00, 30-70-00, 00-30-70, 00-70-30 y 33-33-33). El rendimiento y el porcentaje de Eficiencia del Uso del Nitrógeno (EUN) fueron mayores cuando el fertilizante N se fraccionó en 30-70-00, la EUN osciló entre el 19.68 %, cuando se fraccionó 100-00-00, y el 41.14 % cuando se fraccionó 30-70-00. Cuando no se aplicó fertilización de N en la siembra la EUN fue inferior al 25.72 %. En las prácticas locales se recomienda la fertilización 50-50-00, pero el rendimiento aumentó hasta 8.4 % y la EUN en grano hasta 8.32 % cuando el fraccionamiento fue 30-70-00. El fraccionamiento adecuado de la fertilización N es necesario para mejorar las tasas de asimilación del nitrógeno y la calidad del grano.

A study was undertaken to evaluate the role of Trichoderma strains on 15N-uptake by maize under greenhouse and field conditions. The Trichoderma strains were isolated from different maize production systems in Guanajuato State, Central Mexico. A total of 39 Trichoderma isolates were obtained, 23 correspond to Trichoderma harzianum (Intra-Genic-Segment). Some colonized the endorhizosphere of maize better than others; however, Trichoderma diversity did not correlate with the maize production system (rainfall vs. irrigation conditions). In greenhouse conditions, biomass production and 15N-uptake were equally variable; maize plants inoculated with selected Trichoderma species (high root colonization) and fertilized with 140 mg N kg-1 soil, showed similar increase in grain yield and 15N-uptake vs. those fertilized with 280 mg N kg-1 soil. Biomass and 15N-uptake directly correlated with the capacity of Trichoderma spp. to colonize the rhizosphere. Under field conditions, the N-fertilizer use efficiency was the highest when maize cv. P30G40 was inoculated with T. harzianum T35 at a N fertilization rate of 180 kg N ha-1 (78%). All measured parameters showed positive effects of inoculation under-scoring the feasibility of inoculants with these fungi based on Trichoderma to increase the N-fertilizer use efficiency applied to maize. Key words: Biofertilizers, N-fertilizer use efficiency, 15N isotope.

The aim of this research was to isolate and characterize rhizosphere bacteria of huizache ( Acacia farneciana ) and mesquite ( Prosopis glandulosa ), predominant vegetation surrounding a lead (Pb) recycling company. Bacteria were evaluated on their tolerance to Pb(NO 3 ) 2 ,andtheircapacity to produce indole-acetic acid (IAA) and siderophores using Salkowski and chrome azurol S (CAS) agar medium techniques, respectively. The bacterial isolates were identified through the 16S rRNA gene. Ten bacterial strains tolerated concentrations between 0.6 and 1.8 mM of Pb (NO 3 ) 2 , of which seven strains had IAA production of up to 60 μg/mL and five strains showed siderophore production. Qualitatively, the bacterial strains L2, L48 and L67 presented a more intense orange halo than the bacterial strains L138 and L23, which indicates a positive reaction of siderophores production. Additionally, the siderophores production of the hydroxamate type produced by the bacterial strains L2, L48, L67, L138 and L23 was variable, between 0.02 and 0.054 μg/mL. The results of this research allow concluding that there are bacterial strains around the lead (Pb) recycling company with a high potential for phytoremediation processes.

With the objective to examine maize plant response to whole rhizosphere microbial communities in terms of growth and nutrition, we performed a greenhouse pot experiment with full mineral fertilization except for N and P, which were applied in a fully factorial set-up (Without, with N, with P and with N+P). Rhizosphere microbial communities were measured in terms of mycorrhiza formation and biomarker fatty acids. Whole rhizosphere microbial communities promoted plant growth with single N fertilization (P limitation), whereas negative plant growth responses were observed without fertilization (N and P limitation), single P fertilization (N limitation) and dual N and P fertilization (no nutrient limitation). Whole rhizosphere microbial communities increased and decreased shoot N content without and with N fertilization, respectively. On the other hand whole rhizosphere microbial communities increased shoot P content both with and without P fertilization, though more without P. The observed plant growth and shoot nutrient response to whole rhizosphere microbial communities seems to be linked to the root association with arbuscular mycorrhizal fungi (AMF) and abundance of rhizosphere microorganisms. In conclusion, our results show that maize plant response to whole rhizosphere microbial communities in terms of growth and nutrition is strongly associated with N and P fertilization balance.

Objective: To evaluate the industrial quality of three sugarcane cultivars in a template cycle at the supply area of “Santa Rosalía de la Chontalpa” sugarcane mill. Design / Methodology / Approach: An experiment was established under a factorial design 3x3 (3 cultivars: CP 72-2086, MEX 79-431 and MEX 69-290; x 3 sampling dates: 330, 390 and 450 DDS, Spanish equivalent for days after sowing) on an Eutric Fluvisol soil. In each plantation, a sample of 10 stems with three replications was collected to determine the industrial quality by polarimetry. Results: The industrial quality of the evaluated cultivars only differed statistically in terms of the percentage of purity, MEX 79-431 was the one that presented the lowest value for this variable. At 450 DDS, the highest value was observed for °Brix (17.28), POL percentage (14.92), purity (86.44%). The values obtained in the present study for the quality of juice in the evaluated cultivars are within the range of the standard values established for Mexico. Limitations / Implications: Polarimetry is still the method used by most of the sugar mills in Mexico, even if other more environmental-friendly methodologies exist. Findings / Conclusions: The trend line that best fit to MEX 69-290 and MEX 79-431, for °Brix, POL and purity, was a linear polynomial and to CP 72-2086, a polynomial quadratic. Fresh stems and reducing sugars showed best fit with an inverse polynomial. °Brix presented strong and positive correlation with POL (R = 0.99**); and strong and negative with reducer sugars (R = -0.95**) and fresh stem humidity (R = -0.91**).

We have previously described that laboratory strains of Ustilago maydis, a fungal pathogen of maize and its ancestor teosinte, harbor an intracellular bacterium that enables the fungus to fix nitrogen. However, it is not clear whether other strains isolated from nature also harbor endosymbiotic bacteria, and whether these fix nitrogen for its host. In the present study, we isolated U. maydis strains from naturally infected maize. All the isolated strains harbored intracellular bacteria as determined by PCR amplification of the 16S rRNA gene, and some of them showed capacity to fix nitrogen. That these are truly bacterial endosymbionts were shown by the fact that, after thorough treatments with CuSO(4) followed by serial incubations with antibiotics, the aforementioned bacterial gene was still amplified in treated fungi. In all, these data support the notion that U. maydis-bacterium endosymbiosis is a general phenomenon in this species.

We have previously described that laboratory strains of Ustilago maydis, a fungal pathogen of maize and its ancestor teosinte, harbor an intracellular bacterium that enables the fungus to fix nitrogen. However, it is not clear whether other strains isolated from nature also harbor endosymbiotic bacteria, and whether these fix nitrogen for its host. In the present study, we isolated U. maydis strains from naturally infected maize. All the isolated strains harbored intracellular bacteria as determined by PCR amplification of the 16S rRNA gene, and some of them showed capacity to fix nitrogen. That these are truly bacterial endosymbionts were shown by the fact that, after thorough treatments with CuSO4 followed by serial incubations with antibiotics, the aforementioned bacterial gene was still amplified in treated fungi. In all, these data support the notion that U. maydis-bacterium endosymbiosis is a general phenomenon in this species.

Los cultivos absorben en promedio de 20 a 40% del fertilizante aplicado, el porcentaje restante se pierde del sistema agrícola por diversos mecanismos, ocasionando cuantiosas pérdidas económicas y contaminación ambiental. Entre los beneficios del uso de microorganismos en la agricultura está su capacidad para mejorar la asimilación de nutrimentos. Se realizó un experimento en campo cuyo objetivo fue evaluar el efecto de la inoculación de biofertilizantes bacterianos y fúngicos sobre la eficiencia de asimilación del fertilizante nitrogenado en el cultivo de trigo. El diseño experimental fue de bloques completos al azar con cuatro repeticiones y nueve tratamientos de biofertilización empleando la técnica de dilución isotópica de 15N para determinar el N en la planta derivado del fertilizante y el porcentaje de utilización del fertilizante N. La fertilización química produjo los mayores rendimientos de grano y asegura su calidad. La inoculación del trigo con HVA incrementó significativamente el rendimiento de grano hasta en 1 291 kg ha-1, la cantidad de N en la planta proveniente del fertilizante hasta en 15 kg y la eficiencia de utilización del fertilizante nitrogenado hasta en 11% en comparación con el testigo sin inocular. Hubo diferencias significativas en la interacción planta-microorganismo en la producción de biomasa y asimilación de N.

Burkholderia anthina XXVI is a rhizosphere bacterium isolated from a mango orchard in Mexico. This strain has a significant biological control activity against the causal agent of mango anthracnose, Colletotrichum gloeosporioides, likely through the production of siderophores and other secondary metabolites. Here, we present a draft genome sequence of B. anthina XXVI (approximately 7.7 Mb; and G + C content of 67.0%), with the aim of gaining insight into the genomic basis of antifungal modes of action, ecological success as a biological control agent, and full biosynthetic potential.

Anthropogenic activities have greatly increased heavy metal pollution worldwide. Due to inadequate waste management, mining is one of the chief causes. One particularly affected area in Mexico is the “La Zacatecana” Lagoon, in the municipality of Guadalupe, Zacatecas. From colonial times until the mid-nineteenth century, about 20 million tons of mine tailings were deposited at this site. Here, we catalogue the heavy metal content and their distribution in soils and sediments of La Zacatecana. The mobility of lead in soils was also assayed by sequential extraction. Concentrations of the different metals analysed were as follows: Pb > Cr > As > Ni > Hg > Cd. Site VIII accumulated the highest amount of Pb (3070 mg·kg−1) sevenfold more than the limit established by the Mexican standards for agricultural soils (i.e., 400 mg·kg−1). On the other hand, the contents of Cd, Cr, and Ni were within the levels accepted by the above normativity, set at 37, 280, and 1600 mg·kg−1, respectively. Concentrations of Hg and Pb were highest in the north-northwest zone of the lagoon and decreased towards the southeast. Except for Site VIII where 30% of the Pb was in an interchangeable form or bound to carbonates, most Pb in La Zacatecana soils was present in an unavailable form, associated with Fe-Mn oxides.

Bioremediation is an environmentally sound and cost-effective strategy to restore soils contaminated with petroleum oil. In this study, native bio-surfactant-producing bacteria able to use polycyclic aromatic hydrocarbons (PAHs: anthracene [ANT] or phenanthrene [PHE]) as their sole carbon source were isolated from soil contaminated with weathered petroleum oil (WPO) collected near “La Venta” gas processing complex, in Tabasco, Mexico. Bacteria able to grow in 200 ppm of ANT or PHE and having high bio-surfactant activity were selected. Included were bacteria identified as Sphingobium sp., Bosea sp., Pseudomonas sp., Rhodococcus sp. and Phenylobacterium sp. Different consortia of these bacteria ± bio-stimulation with glucose, ammonium nitrate, or both, were examined for their capacity to degrade either WPO or fresh crude petroleum oil (FCPO) in soil microcosms. No efficient short-term (24 days) degradation of WPO using bio-augmentation and bio-stimulation strategies was possible using two bacterial consortia conformed by combinations of these bacterial isolates. Best results were obtained with a consortium composed of Pseudomonas cloritidismutans and Rhodococcus qingshengii, identified by 16S rRNA sequencing, which led to an 86% reduction of FCPO contaminants in soil microcosms, 90 days after treatment. However, the bioremediation efficiency of this consortium was significantly reduced by moderate salinity levels in the soil. Although promising, these results highlight the adverse effect that weathering and/or salinity may have on the remediation of petroleum oil-contaminated soils.

The present research evaluated the redox ability of Cr6+ in the earthworm Eisenia foetida, and its effects on growth and survival. An assay was conducted with an increasing doses of K2Cr2O7 (0, 200, 400, 800, 1000, and 1200 mg K2Cr2O7 kg-1 substrate) applied to a vermicomposting system which was inoculated with 30 earthworms per chamber. Earthworm growth, estimated by weight, showed a quadratic response (p = 0.03) with the lowest estimated growth at a dose of 435 mg K2Cr2O7 kg-1. Chromium (Cr) concentration increments were positively associated to mortality in adults and cocoons (p < 0.01), and total Cr accumulation in earthworms (p < 0.01). The accumulation of chromium in worms, is an indicator of the capacity of worms to resist different doses of potassium dichromate and, the effect on accumulation of Cr (VI) in vermicomposting was higher (p = 0.11). The difference of potassium dichromate in worms could indicate the oxide reduction in E. fetidae. Key words: Redox chromium, growth, survival, Eisenia foetida.

Heavy metal pollution is a growing problem due to anthropic activities such as mining. The most hazardous metals in Mexico are mercury, arsenic, lead, and chromium. States like Zacatecas, Queretaro, Hidalgo, and San Luis Potosi have been affected by the pollution of soils with heavy metals. In this sense, phytoremediation has emerged as an alternative to solve this problem. In Mexico, plant species with metal accumulative capacity have been identified. These include Scirpus americanus, Typha latifolia, Jatropha dioica, Eichhornia crassipes and Amaranthus hybridus . Metal phytoextraction could be improved by coupling bioaugmentation of soil microorganism. Bacteria from the genus R hizobium, Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas and Pseudomonas , have shown promising results enhancing heavy metal phytoextraction, as well as the arbuscular fungi from the Glomus genus. However, more studies on endemic plants and microbial from contaminated sites are needed to optimize current treatment strategies.

In herbaceous plants grown in controlled environmental conditions nitrate (NO3−) uptake increases during the day and decreases in the night. The aim of this work was to measure NO3− uptake rates along the day-night cycle, in rose (Rosa hybrida L.) plants grown under controlled environmental conditions. Two independent experiments were conducted inside a growth chamber at 20 °C and 25 °C, using rose mini-plants cv. Texas, grown in a hydroponic nutrient film technique (NFT) set-up with at 3.0 mol m−3 NO3− concentration. Dry matter and nitrogen (N) accumulation were registered during growth and NO3− uptake rates were measured during a day-night cycle, using 15N as 15NO3−. In both experiments the hourly estimated N-NO3− accumulation rates are near to the measured uptake rates of 15NO3− and nitrate uptake decrease during the day and increase in the night, in contrast with the herbaceous plants. Results are discussed on the basis of N plant demand and carbohydrates availability.

A nivel mundial, la contaminacion por metales pesados es un problema que afecta la salud humana. Por su toxicidad y abundancia, los metales mas estudiados son el mercurio, el arsenico y el plomo. La toxicidad de los metales pesados en suelos esta asociada con la fraccion disponible que es asimilada por los seres vivos, la cual esta determinada por las caracteris ticas fisicoquimicas del suelo (principalmente pH, potencial redox y materia organica). Las tecnicas para la remediacion de suelos se basan en metodos fisicoquimicos y biologicos; estas ultimas llamadas biorremediacion , ya que aprovecha el potencial metabolico de organismos vivos (bacterias y hongos) para la descontaminacion. Los procesos mas usados en la biorremediacion son la sorcion, precipitacion, lixiviacion y volatilizacion de metales pesados. Sin embargo, son necesarios mas estudios sobre la diversidad microbiana de los sitios contaminados con metales pesados para encontrar cepas mejor adaptadas y con mayor capacidad de biorremediacion de estos contaminantes.

We observed that the maize pathogenic fungus Ustilago maydis grew in nitrogen (N)-free media at a rate similar to that observed in media containing ammonium nitrate, suggesting that it was able to fix atmospheric N2 . Because only prokaryotic organisms have the capacity to reduce N2 , we entertained the possibility that U. maydis was associated with an intracellular bacterium. The presence of nitrogenase in the fungus was analyzed by acetylene reduction, and capacity to fix N2 by use of (15) N2 . Presence of an intracellular N2 -fixing bacterium was analyzed by PCR amplification of bacterial 16S rRNA and nifH genes, and by microscopic observations. Nitrogenase activity and (15) N incorporation into the cells proved that U. maydis fixed N2 . Light and electron microscopy, and fluorescence in situ hybridization (FISH) experiments revealed the presence of intracellular bacteria related to Bacillus pumilus, as evidenced by sequencing of the PCR-amplified fragments. These observations reveal for the first time the existence of an endosymbiotic N2 -fixing association involving a fungus and a bacterium.

Manufacturing environment in tanning industry has been associated with human diseases caused by biological agents. This paper reports the microbiological contamination of the indoor environment at a group of tanneries in León, Guanajuato, México. The microorganisms present in the indoor environment of the tannery industry has been quantified and identified. The bacteria and fungi concentration in almost all cases exceeded the acceptable levels as dictated by Swedish standard and the limit values proposed by the American Industrial Hygiene Association. The indoor/outdoor ratio was determined, revealing poor air quality at the studied sites. This study has illustrated the presence of pathogens, hazardous to humans in indoor environments of tanneries. The identified bacteria by sequencing of 16S rDNA gene, belonged to families: Bacillaceae, Corynebacteriaceae, Enterobacteriaceae, Moraxellaceae, Nocardiopsaceae, Pseudomonadaceae, Staphylococcaceae. Some of which can be categorized as pathogens such as Acinetobacter calcoaceticus, Acinetobacter johnsonii, Nocardiopsis dassonvillei, Pantoea agglomerans, Pseudomonas putida and Staphylococcus gallinarum. In addition, Fungi genuses identified by microscopy were Aspergillus and Penicillium. The yeasts presumptively identified by ChromAgar medium were Candida krusei and Candida glabrata. Some of these identified microorganisms have been correlated with adverse human health effects, especially in individuals with immunocompromised system.

Se evaluó a través de la técnica isotópica de 15N el aporte de nitrógeno (N) proveniente de la pollinaza durante el desarrollo del cultivo de brócoli (Brassica oleracea L.). Se establecieron seis tratamientos a una dosis de 300 kg N ha-1producto de la combinación de fertilizante químico (N-fosfonitrato) y fertilizante orgánico (N-pollinaza): 1) 0 + 300; 2) 100 + 200; 3) 150 + 150; 4) 200 + 100; 5) 300 kg N-fosfonitrato ha−1 + 0 kg N-pollinaza ha−1; y 6) testigo sin fertilización N, en un diseño de bloques al azar con cinco repeticiones. Los resultados mostraron el mayor rendimiento en los tratamientos 200 kg N-fosfonitrato ha-1 + 100 kg N-pollinaza ha-1 y 300 kg N-fosfonitrato ha-1 (16 y 17 Mg ha-1, respectivamente). El mayor rendimiento de N total (347 kg N ha-1) ocurrió a 150 kg N-pollinaza. Los datos de 15N indican una baja asimilación de N-fertilizante, destacando los tratamientos donde se aplicó pollinaza con los valores menores (3,2-4,1%) en comparación con el tratamiento a base de N-fosfonitrato (28,7-48,0%). La absorción de N-pollinaza fue aproximadamente 29,5%. Los datos sugieren un efecto sinérgico entre el fertilizante químico y orgánico, ya que se observó una alta contribución del N del suelo dadas las constantes aplicaciones de pollinaza al sistema de producción en Tepeaca, Puebla. The nitrogen (N) contribution from chicken manure to broccoli crop (Brassica oleracea L.) was evaluate using the 15N isotopic technique. Six treatments combining chemical fertilizer (N-phosphonitrate) and organic fertilizer (N-chicken manure) at a total rate of 300 kg N ha-1 were established: 1) 0 + 300; 2) 100 + 200; 3) 150 + 150; 4) 200 + 100; 5) 300 kg N-phosphonitrate ha−1 + 0 kg N-chicken manure ha-1; and 6) an unfertilized N control. Five replicates per treatment were established under a statistical design of random blocks. The highest yield of broccoli (heads) corresponded to treatments 200 kg N-phosphonitrate ha-1 + 100 kg N-chicken manure ha-1 and 300 kg N-phosphonitrate ha-1 yielding 16 and 17 Mg ha-1, respectively. The highest value of total N-uptake (347 kg N ha-1) was observed in treatment corresponding to 150 kg N-phosphonitrate ha−1 + 150 kg N-chicken manure ha−1. The 15N data indicate low values of N-fertilizer uptake (3.2-4.1%) when manure was applied. The N-phosphonitrate alone treatment showed a higher value of N-fertilizer (28.7-48.0%) than treatment 200 kg N-phosphonitrate ha−1 + 100 kg N-chicken manure ha−1 (4.5-5.1%). The N uptake by plants from manure was 29.5% average. The combination of chemical and organic fertilizers resulted in a higher total N-uptake. The constant application of manure to soil under broccoli production in Puebla region resulted in a significant contribution of soil N to the broccoli crop. Fil: Daniel Torres Nava. Colegio de Postgraduados. Campus Puebla (México) Fil: Engelberto Sandoval Castro. Colegio de Postgraduados. Campus Puebla (México) Fil: Juan José Peña-Cabriales. Centro de Investigación y Estudios Avanzados (Guanajuato, México) Fil: José Antonio Vera-Núñez. Centro de Investigación y Estudios Avanzados (Guanajuato, México)

The effect of an arbuscular mycorrhizal fungi (AMF) consortium conformed by (Glomus intraradices, Glomus albidum, Glomus diaphanum, and Glomus claroideum) on plant growth and absorption of Pb, Fe, Na, Ca, and (32)P in barley (Hordeum vulgare L.) and sunflower (Helianthus annuus L.) plants was evaluated. AMF-plants and controls were grown in a substrate amended with powdered Pb slag at proportions of 0, 10, 20, and 30% v/v equivalent to total Pb contents of 117; 5,337; 13,659, and 19,913 mg Pb kg(-1) substrate, respectively. Mycorrhizal root colonization values were 70, 94, 98, and 90%, for barley and 91, 97, 95, and 97%, for sunflower. AMF inoculum had positive repercussions on plant development of both crops. Mycorrhizal barley absorbed more Pb (40.4 mg Pb kg(-1)) shoot dry weight than non-colonized controls (26.5 mg Pb kg(-1)) when treated with a high Pb slag dosage. This increase was higher in roots than shoots (650.0 and 511.5 mg Pb kg(-1) root dry weight, respectively). A similar pattern was found in sunflower. Plants with AMF absorbed equal or lower amounts of Fe, Na and Ca than controls. H. vulgare absorbed more total P (1.0%) than H. annuus (0.9%). The arbuscular mycorrizal consortium enhanced Pb extraction by plants.

Agaves are major biotic resources in arid and semi-arid ecosystems. Despite their ecological, economical and cultural relevance, many aspects of the microbial communities associated with agaves are still unknown. Here, we investigated the bacterial communities associated with two Agave species by 16S rRNA- Denaturing gradient gel electrophoresis fingerprinting and sequencing. We also evaluated the effects of biotic and abiotic factors in the structure of the bacterial communities. In parallel, we isolated and characterized diazotrophic bacteria associated with agaves, as Agave soils are characterized by their low nitrogen content. Our results demonstrate that in Agave, the structure of prokaryotic assemblages was mostly influenced by the community group, where the soil, episphere, and endosphere were clearly distinct. Proteobacteria (γ and α), Actinobacteria, and Acidobacteria were the dominant phyla. Bacterial communities in the episphere of agaves were mainly influenced by the host species, whereas in the endosphere were affected by the season. Fifteen bacterial taxa were common and abundant in the endosphere of both Agave species during the dry season. Notably, some of the confirmed diazotrophic strains belonged to this group, suggesting a possible beneficial role in planta.

Four thermophilic bacterial species, including the iron-reducing bacterium Geobacillus sp. G2 and the sulfate-reducing bacterium Desulfotomaculum sp. SRB-M, were employed to integrate a bacterial consortium. A second consortium was integrated with the same bacteria, except for Geobacillus sp. G2. Carbon steel coupons were subjected to batch cultures of both consortia. The corrosion induced by the complete consortium was 10 times higher than that induced by the second consortium, and the ferrous ion concentration was consistently higher in iron-reducing consortia. Scanning electronic microscopy analysis of the carbon steel surface showed mineral films colonized by bacteria. The complete consortium caused profuse fracturing of the mineral film, whereas the non-iron-reducing consortium did not generate fractures. These data show that the iron-reducing activity of Geobacillus sp. G2 promotes fracturing of mineral films, thereby increasing steel corrosion.

O objetivo deste trabalho é avaliar o consumo de nutrientes (CN) e exportação de nutrientes do campo através dos produtos colhidos (EN), visando estimar o balanço dos principais macronutrientes: nitrogênio (N), fósforo (P) e potássio (K) nos países da América Latina e Caribe (ALC). O balanço de nutrientes (BN) foi estimado pela quantidade de nutrientes que entra nos sistemas agrícolas pela adubação e fixação biológica de nitrogênio (FBN) em relação à quantidade que deixa o sistema pelos produtos colhidos, em cada um dos países da região. A região da ALC, com base em estatísticas oficiais correspondentes ao ano 2016, apresentou um balanço positivo de N, no entanto, em alguns países (Argentina, Bolívia e Paraguai) esse balanço foi negativo. A FBN é a principal fonte de N da agricultura latino-americana, respondendo por 62% (11,29 Mt N) do total de N (18,10 Mt N) que ingressa anualmente nos sistemas agrícolas. De um modo geral, o fornecimento de P via adubação na América Latina atenderam as exportações realizadas pelos produtos colhidos, com exceção dos países Argentina, Bolívia, Guatemala e México, os quais se utilizaram de reservas do solo para a nutrição das plantas. O K foi um dos nutrientes cuja aplicação não atende a demanda das culturas da região, apresentando balanços negativos em quase todos os países, exceto Brasil, Colômbia e Venezuela. Deve-se destacar que em muitos solos agrícolas na região, a disponibilidade de K ainda é alta. Os maiores cuidados com o uso de insumos, incluindo os fertilizantes, se dá com os cultivos de soja, milho, café, cana de açúcar e laranja, ficando o restante dependente da fertilidade natural dos solos, que em sua maioria já são pobres, colocando em risco a segurança alimentar.

Inoculation of mango trees with Burkholderia caribensis XV and Rhizobium sp. XXV led to mango growth promotion (dry biomass increased in root 89 %, stem 34 %, leaves 51 %, and foliar area 53 %), floral fate (floral buds 100 %), and increased number of flowers (100 %). Nitrogen content in leaves was similar in inoculated and noninoculated trees, around 1.4 % (optimal condition for floral induction). The total foliar nitrogen content increased significantly (56 %) when the microbial inoculation treatment was applied. In addition, the initial content of sucrose, glucose, and fructose in leaves was higher in the microbial inoculation treatment trees but decreased during the evaluated period. The sucrose content in the noninoculated trees presented similar dynamics compared to the microbial inoculation treatment trees, but glucose and fructose showed increased values compared to those of the microbial inoculation treatment. FLOWERING LOCUS T (FT) expression profiles normalized to ACTIN showed similar dynamics but different expression levels: RQ values of 0.03 and 0.05 for noninoculated and microbial inoculation treatments, respectively. In addition, FT expression profiles in microbial inoculation, normalized to the noninoculated treatment, showed an increased FT expression dynamic over time (up to RQ = 2.2), although a drastic decrease in the last sampling date, when all trees presented developed panicles and flowers, was observed. This FT upregulation was in accordance with the flowering induction in that treatment. Temperature had an important influence on mango flowering induction, which was observed for a 1-month period (~10 °C at night and 20 °C during the day). Bud growth that occurred during that period generated mixed and floral buds depending on the exposure time to these inductive temperatures, less than 2 weeks and more than 3 weeks, respectively. Data indicate that inoculation of mango trees with plant growth-promoting rhizobacteria (associated with this crop) is a potential alternative way to promote growth and induce flowering in mango, greatly reducing the high economical costs and environmental contamination associated with traditional agricultural practices.

Twenty isolates of Trichoderma were obtained from orchards located in three main mango-producing States in Mexico: Chiapas, Oaxaca, and Michoacan, which represent different agronomical management practices and levels of soil fertility. Phylogenetic analysis showed that Trichoderma isolates belong to the following taxa: Hypocrea lixii (10 isolates), Hypocrea jecorina (four isolates), Trichoderma asperellum (three isolates), Trichoderma spirale (two isolates), and Trichoderma brevicompactum (one isolate). The genus Hypocrea is the teleomorph (sexual) stage of the genus Trichoderma, anamorph stage. Seventeen Trichoderma isolates showed at least 67% growth inhibition against the phytopathogenic fungus Colletotrichum gloeosporioides ATCC MYA 456 and three Trichoderma isolates showed complete overgrowth of this pathogen. One member of this group, identified as T. asperellum T8a, was able to control C. gloeosporioides ATCC MYA 456 in vitro and in vivo, as well as five C. gloeosporioides isolates obtained from mango orchards from the State of Oaxaca. Assay of the lytic enzymes involved suggest that cellulases of T. asperellum T8a play a role in biological control against C. gloeosporioides ATCC MYA 456 more than chitinase or glucanase. Thus, native T. asperellum T8a associated with mango trees can be used to enhance mango production, controlling anthracnose through cellulase activity.

Dry wastes (dw) generated in processing mangoes, composed (in dry weight) mainly of soluble carbohydrates (71 ± 2%) and fiber (16 ± 1%), were evaluated as substrates in a “home-made” solid-state fermentation (using polyurethane foam as inert support matrix, various C:N ratios, moisture contents, and incubation periods) of Trichoderma asperellum T8a, a promising biological control agent against the mango pathogen Colletotrichum gloeosporioides (causal agent of anthracnose). Highest spore production (2.5 x 106 up to 76 ± 3 x 108 spores g-1 dw) occurred after 8 days of incubation [at 28 ± 1 °C, relative humidity of 85 ± 5%, photoperiod of 12h (540 Lux) - 12h (20 Lux)] at a C:N ratio of 26, and a moisture content of 78%. Scanning electron microscopy showed that T. asperellum T8a was able to grow on mango industrial wastes and into polyurethane foam. The extensive growth can be related to cellulases secreted by this fungus, liberating glucose from these wastes to its growth. Most (94 ± 1%) of the spores grown on mango industrial wastes survived storage at 4 °C for 7 days and were equally effective as those grown on potato dextrose agar medium (86 ± 4% viable) in biological control tests against C. gloeosporioides ATCC MYA 456. Results indicate the potential use of mango industrial wastes as substrates to produce T. asperellum T8a spores in situ (mango orchards) under a cheap “home-made” solid-state fermentation, reducing problems associated with wastes disposal and permitting the production of a biological control agent against C. gloeosporioides.

Colletotrichum gloeosporioides is the causal agent of anthracnose in mango. Burkholderia cepacia XXVI, isolated from mango rhizosphere and identified by 16S rDNA sequencing as a member of B. cepacia complex, was more effective than 6 other mango rhizosphere bacteria in inhibiting the model mango pathogen, C. gloeosporioides ATCC MYA 456. Biocontrol of this pathogen was demonstrated on Petri-dishes containing PDA by90 % reduction of surface colonization. The nature of the biocontrol metabolite(s) was characterized via a variety of tests. The inhibition was almost exclusively due to production of agar-diffusible, not volatile, metabolite(s). The diffusible metabolite(s) underwent thermal degradation at 70 and 121 °C (1 atm). Tests for indole acetic acid production and lytic enzyme activities (cellulase, glucanase and chitinase) by B. cepacia XXVI were negative, indicating that these metabolites were not involved in the biocontrol effect. Based on halo formation and growth inhibition of the pathogen on the diagnostic medium, CAS-agar, as well as colorimetric tests we surmised that strain XXVI produced a hydroxamate siderophore involved in the biocontrol effect observed. The minimal inhibitory concentration test showed that 0.64 μg ml(-1) of siderophore (Deferoxamine mesylate salt-equivalent) was sufficient to achieve 91.1 % inhibition of the pathogen growth on Petri-dishes containing PDA. The biocontrol capacity against C. gloeosporioides ATCC MYA 456 correlated directly with the siderophore production by B. cepacia XXVI: the highest concentration of siderophore production in PDB on day 7, 1.7 μg ml(-1) (Deferoxamine mesylate salt-equivalent), promoted a pathogen growth inhibition of 94.9 %. The growth of 5 additional strains of C. gloeosporioides (isolated from mango "Ataulfo" orchards located in the municipality of Chahuites, State of Oaxaca in Mexico) was also inhibited when confronted with B. cepacia XXVI. Results indicate that B. cepacia XXVI or its siderophore have the potential to be used as a biological control agent against C. gloeosporioides; thus diminishing environmental problems caused by the current practices to control this disease.

Rice is one of the most important food crops in the world. Abiotic stresses directly or indirectly affect the physiological status of rice and negatively alter its overall metabolism. Drought in rain-fed, lowland ecosystems is particularly harmful to growth and yield. As with many other food crops, drought tolerance in rice is a polygenic trait in which many genes localized in various quantitative trait loci (QTL) have additive effects on resistance to desiccation. For these reasons, breeding drought-tolerant rice is complex, time-consuming, and cost-intensive. Marker-assisted breeding (MAB) is a more efficient approach but requires comprehensive molecular linkage maps. Increased trehalose accumulation in various tissues correlates with exposure of the plant to drought. Elevated trehalose levels probably play roles in osmo-protection and the stabilization of various essential functions. Here, we document trehalose accumulation, the activity of the enzyme trehalase, and plant growth in three Mexican rice genotypes: two drought-tolerant accessions (Temporalero A95 and Sabanero A95) and one drought-sensitive variety (Morelos A98). Our data show that similar trehalose contents were found in roots of all three accessions under irrigated conditions, but that drought enhanced the synthesis of trehalose only in drought-tolerant varieties. Changes in the levels of non-structural sugars accompanied the accumulation of trehalose. Increased trehalase activities and photosynthetic capacities in drought-tolerant varieties was observed under stress conditions, which was accompanied by lower stomatal resistance and better water-holding capacity. We conclude that DNA markers built around trehalose metabolism can be used during MAB to study drought tolerance in rice and other crops.

N-fertilization dynamics and agronomic practices on a Vertisol in central Mexico were evaluated under irrigated conditions: (1) wheat-maize rotation with conventional tillage (CT) and burning of residues (W-M/CT/B, regional control); (2) wheat-beans rotation with CT and incorporation of residues into the soil (W-P/CT/I); (3) wheat-maize rotation with CT and incorporation of residues into the soil (W-M/CT/I); (4) maize-beans rotation bi-annual with CT and incorporation of residues into the soil (M-P/CT/Bi); and (5) wheat-maize, no tillage (NT) and residues left on the soil surface as mulch (W-M/NT/S). 15N and acetylene inhibition techniques were used to estimate N fertilizer efficiency and losses (N2 + N2O). Treatments received 240, 60, and 300 kg N ha−1 for spring maize, beans and winter wheat, as ammonium sulphate enriched with 5.468% atoms 15N excess. In the spring summer cycle, the fertilizer N recovery ranged from 27% for W-M/NT/S to 68% for M-P/CT/Bi. From the total N-fertilizer applied, only 3 to 9% remained in soil after harvest (W-M/NT/S and W-M/CT/I being the respective extremes). Unaccounted N-fertilizer ranged between 27 and 69%, the highest losses corresponding to W-M/NT/S treatment. Fertilizer N recovery in wheat varied from 19 to 37% (W-M/NT/S–W-M/CT/B). N-fertilizer remaining in soil was 14 to 24% (W-M/NT/S – W-M/CT/I). N2 and N2O emissions were higher in the no tillage system. Emissions ranged from 3 to 28 kg N ha−1 for W-P/CT/I and W-M/NT/S, respectively. The best treatments were those in which residues were incorporated resulting in N immobilization in top soil (0–15 cm), small N gas losses, and higher soil organic matter, these treatments were W-P/CT/I, W-M/CT/I.

Scirpus americanus Pers. occurs naturally in "San Germán," a pond that serves as a receptor of industrial wastewater in Guanajuato, México. This plant accumulates metals mainly in the root: concentrations (mg/kg) of Cr, As, Cd and Se were 970, 49, 41, and 85 respectively. Analysis of rhizosphere samples indicated bacterial population of 10(8) cfu g(-1) in media with 0.2 mM Cr(VI) and 10 mM sodium gluconate. Thirteen isolates were obtained and phylogenetic analyses (16S rRNA) indicated they corresponded to genera of Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas and Pseudomonas. Cr(VI) reduction was evaluated using the diphenyl carbazide method. The isolates accomplished 5-40% (20 microM) of reduction in assays of resting cell and tolerated 0.5-5.0 mM Cr(VI). Eight strains used nitrate and thirteen used iron and chromium as electron acceptors to grow under anaerobic conditions. Cr(VI) reduction by five strains occurred at pH values (7-9) and NaCl concentrations (0.5-1.0 M) in basal medium. A mixed culture of strains (S17 and S28) reached a chromium removal of 100% at 0.2 mM Cr(VI) initial concentration. Aerobically, this consortium was capable of 93.8% Cr(VI) reduction of 81 microg L(-1) Cr(VI) of the industrial effluent, indicating their possible use in environmental cleanup.

Pulsed-field gel electrophoresis and 16S rRNA hybridization experiments showed that multichromosome genome structures and very large genome sizes (6.46 to 8.73 Mb) are prevalent in novel plant-associated Burkholderia species. 15 N 2 isotope dilution assays revealed unambiguous diazotrophy in these novel species. nifH gene sequence analysis, often used to determine phylogenetic relatedness among diazotrophs, showed tight clusters of Burkholderia species, which were clearly distinct from those of other diazotrophs.

gThe use of leguminous plants in the agricultural systems of the humid tropics is an alternative means for providing N to soils considered unfertile and marginal. The objective of this study was to assess the biological nitrogen fixation (BNF) in the symbiosis Rhizobia-herbaceous legumes (Mucuna deeringiana, Canavalia ensiformis, Vigna umbellata, Cajanus cajan, Dolichos lablab, Arachis pintoi and Crotala-riajuncea) grown in acid soil amended with P fertilizer by the 15N-isoto-pic dilution technique (“A”-value) using maize, Brachiaria spp. and no nodulating common bean as reference crops. Results indicate a positive effect of P-fertilizer on parameters associated with BNF and biomass accumulation. The legumes Cajanus cajan and Canavalia ensiformis showed good performance in terms of dry weight of nodules (C. cajan = 758 and C ensiformis = 500 mg nodules plant−1 with 49 and 56 nodules plant−1, respectively) at flowering stage, and BNF (C. cajan = 170 and C ensiformis = 366 kg N ha−1 cycle−1) when 8...

Biological N fixation by symbiotic micro-organisms plays a key role in making atmospheric N largely available to other organisms and nodule formation by rhizobia in roots of many legume species represents a very important pathway (Postgate 1998). Estimates of the contribution of symbiotic fixation to total N content in soils range from 44 to 200 Tg y−1(Bøckman 1997).

Sucrose was the predominant sugar in seeds of fi ve cultivars of common bean (Phaseolus vulgaris L.) grown under controlled conditions. The concentration of soluble sugars was higher in seeds from nodulated plants than seeds from N-fertilized or control (uninoculated and unfertilized) plants. Trehalose was detected in seeds but only from nodulated plants, and average values ranged from undetectable levels in seeds from control plants to 18 mg kg −1 in seeds from nodulated plants. Nodulated plants also produced larger and heavier seeds of higher quality; sucrose and trehalose contents were highly correlated with germination rate and with the vigor index of artifi cially aged seeds. Seeds of nine common bean cultivars grown in the fi eld showed soluble sugar contents similar to those observed in seeds from nodulated plants raised in a growth chamber, except that the trehalose levels were signifi cantly higher (23–111 mg kg −1 ). Trehalose was not detected in seeds from nonnodulating control plants. Seed trehalose level was signifi cantly correlated with germination rate and with the vigor index of artifi cially aged seeds. Seed glucose concentrations were negatively related to all of the seed quality parameters used. In general terms, the recognition of the natural presence of trehalose in seeds has been limited to reports on the expression of the trehalose biosynthetic genes in a few plants. Our data show that in seeds produced by nodulated bean, plants trehalose occurs at high levels, suggesting that strategies to maximize nodulation may result in improvement of seed quality.

Earthworms and mulch can have positive or negative effects on mycorrhizae (fungus-roots) and N uptake by plants. In the present experiment, maize plants were grown under greenhouse conditions with or without tropical earthworms (Balanteodrilus pearsei) and mulch of velvetbean (Mucuna pruriens var. utilis). The formation of vesicles and hyphae of arbuscular-mycorrhizal (AM) fungi in roots and N uptake by maize plants was measured at harvest. The addition of earthworms and velvetbean reduced AM root colonization. Earthworms had no effect on plant root or shoot biomass. In the absence of velvetbean, earthworms reduced AM colonization, but when velvetbean was present, this effect disappeared. The addition of velvetbean mulch, on the other hand, had an effect on plant biomass (above- and belowground) and a positive effect on AM fungal colonization of roots in presence of worms, but a negative effect when worms were absent. When both M. pruriens and B. pearsei were added, shoot and root biomass and N concentrations increased. Vesicle formation was related to velvetbean mulch decomposition as well as the higher N concentration in maize roots. Management of mulch–earthworm interactions may be of value, particularly in low-input and organic agricultural systems, and deserves further investigation.

Nitrogenase activity and trehalose accumulation were measured in nodulated and non-nodulated common beans ( Phaseolus vulgaris ) that were exposed to drought. Plants were infected with the Fix + Rhizobium sp NGR234, or a Fix − derivative (NGRΩ fixF ), or high trehalose-producing, native rhizobia. Trehalose content increased significantly while acetylene reduction activity (ARA) decreased in the nodules of plants exposed to drought. Nevertheless, ARA decreased at a slower rate in nodules with high trehalose levels. Under water stress, nodules infected with NGRΩ fixF tended to accumulate more trehalose than nodules infected with wild-type NGR234 (9±0.1 vs 8±0.1 mg g −1 dw, respectively). Highest trehalose accumulations were registered in nodules of plants infected with native rhizobia (average 16 mg g −1 dw), and these plants also exhibited the highest relative water content (65%), while in plants infected with the NGRΩ fixF RWC was significantly lower (56%). Our results suggest that nodule trehalose may protect bacterial nitrogenase activity under drought conditions, and that both trehalose and biological nitrogen fixation (BNF) contribute to drought tolerance.

Fourteen thermotolerant and thermophilic bacterial isolates from a hot spring in Guanajuato State, Mexico, were tested for their ability to induce the corrosion of carbon steel in monocultures and, in selected cases, in mixed cultures and co-culture with a sulfate-reducing strain, SRB-M. Characterization by 16S rDNA showed that three of the thermophilic isolates (G9a, G9c, and G11) belong to the genus Bacillus , one (G2) showed homology with Bacillus as well as Geobacillus , and the SRB-M strain is closely related to Desulfotomaculum sp. Ten of the fourteen thermophilic and thermotolerant isolates promoted significantly more corrosion than the sterile controls. Under microaerobic batch-culture conditions at 55°C, SRB-M in monoculture did not show a corrosive effect measured as weight loss; in a mixed culture with G2, G9a, and G11, however, the final corrosion was enhanced 3.6 times when compared to sterile controls. Co-cultures of "G" isolates with SRB-M showed various effects on the final corrosion, rang...

Grain amaranth is an emerging crop that produces seeds having high quality protein with balanced amino-acid content. However, production is restricted by agronomic limitations that result in yields that are lower than those normally produced by cereals. In this work, the use of five different rhizobacteria were explored as a strategy to promote growth and yields in Amaranthus hypochondriacus cv. Nutrisol and A. cruentus cv. Candil, two commercially important grain amaranth cultivars. The plants were grown in a rich substrate, high in organic matter, nitrogen (N), and phosphorus (P) and under greenhouse conditions. Burkholderia ambifaria Mex-5 and B. caribensis XV proved to be the most efficient strains and significantly promoted growth in both grain amaranth species tested. Increased grain yield and harvest index occurred in combination with chemical fertilization when tested in A. cruentus. Growth-promotion and improved yields correlated with increased N content in all tissues examined. Positive effects on growth also occurred in A. cruentus plants grown in a poor soil, even after N and P fertilization. No correlation between non-structural carbohydrate levels in roots of inoculated plants and growth promotion was observed. Conversely, gene expression assays performed at 3-, 5- and 7-weeks after seed inoculation in plants inoculated with B. caribensis XV identified a tissue-specific induction of several genes involved in photosynthesis, sugar- and N- metabolism and transport. It is concluded that strains of Burkholderia effectively promote growth and increase seed yields in grain amaranth. Growth promotion was particularly noticeable in plants grown in an infertile soil but also occurred in a well fertilized rich substrate. The positive effects observed may be attributed to a bio-fertilization effect that led to increased N levels in roots and shoots. The latter effect correlated with the differential induction of several genes involved in carbon and N metabolism and transport.

En los años 40s, la región El Bajío, Guanajuato, México se posicionó como líder a nivel nacional en la producción de fresa. En 1966, esta región alcanzó una producción de 143 x 103 t, ocupando el segundo lugar en la producción mundial y reconociéndose como La Capital Mundial de la Fresa. Sin embargo, diversos factores entre los que destacan: 1) la falta de organización y capacitación tecnológica de los productores; 2) la dependencia de los Estados Unidos de América para adquirir la planta madre; y 3) la incidencia de plagas y enfermedades ocasionadas por el monocultivo, generaron un declive significativo en la producción. Esta baja productividad, así como la incapacidad de mantener la demanda externa, favorecieron el surgimiento de nuevas regiones productoras, destacando Michoacán y Baja California. El objetivo de este ensayo fue realizar un análisis retrospectivo sobre la producción de fresa en la región El Bajío estado de Guanajuato para identificar los puntos críticos a resolver en el afán de recuperar el liderazgo nacional, destacando las acciones siguientes: 1) promover la organización entre productores para adoptar sistemas de producción más rentables; 2) lograr la independencia del extranjero en la adquisición de planta madre, apoyándose en instituciones nacionales de investigación a fin de generar variedades propias para la región; 3) fortalecer la comercialización de la fresa, estimulando la creación de empresas que den valor agregado al producto; y 4) hacer un esfuerzo conjunto entre productores, autoridades, empresas y la ciudadanía en general para que a través de acciones coordinadas (crédito, capacitación, difusión, cultura, etc.) se consolide y fortalezca la cadena producto fresa y se vigorice a la fresa como ícono de la ciudad de Irapuato, Guanajuato.

“El Bajio” (ca. 5 × 105 ha) is one of the main cereal producing regions of Mexico. During the past 30 years, a cereal-cereal rotation has been practiced with constantly increasing annual rates of N-fertilizer application: from 120 kg N ha−1 in 1960 to 320 kg N ha−1 in 1998. Furthermore, in order to avoid overlap between the fall-winter and spring-summer cycles, crop residues are burned. A field experiment was conducted in Abasolo, Guanajuato, Mexico, to compare the effects of incorporating or burning sorghum crop residues on the availability of fertilizer N for a succeeding crop of winter wheat. The wheat received 320 kg N ha−1, either all applied at sowing or in two split applications of 160 kg each (at sowing and at 50 days after sowing), as ammonium sulphate enriched in 15N. In a parallel experiment, a fertilizer N balance was carried out using a single application at sowing of 320 kg N ha−1 as ammonium sulphate enriched in 15N. Dry matter and grain yields obtained ranged from 4 to 6 t ha−1 fo...

Nine cultivars of common bean were grown in the presence of a natural microflora without exogenous rhizobial inoculation. Nodules were harvested 30 days post planting (early flowering stage) and the presence of trehalose determined. Amounts present varied according to cultivar and were between 0.20 and 1.63 mg g -1 nodule dry weight. Rhizobial strains were isolated from the nodules of three selected cultivars (Canario 101, Flor de Mayo Bajio and Flor de Mayo 38). Trehalose levels in nodules produced after either mixed strain reinfection, or after axenic homologous reinfection or after axenic cross-reinfection could be manipulated by applying drought stress. Mixed reinfection nodules from stressed plants accumulated between two and six times the trehalose concentration found in non-stressed control plants. After axenic cross-reinfection up to 48-fold increases in nodule trehalose content were recorded during drought stress. Those cultivars exhibiting high nodule trehalose levels and/or a high degree of trehalose stimulation in response to drought stress also exhibited a high leaf relative water content and were also the most drought resistant. During drought stress nodule trehalase levels rose only slightly.

A field experiment under rainfed conditions was conducted in Durango, Mexico, to assess N2-fixation of three cultivars of common bean (Phaseolus vulgaris L.) using 15N-methodology. In addition, diversity of rhizobial isolates obtained from nodules of the different plant genotypes was evaluated by intrinsic antibiotic resistance (IAR), PCR using enterobacterial repetitive intergenic consensus (ERIC) primers, PCR-RFLP analysis of the 16S rRNA gene and multilocus enzyme electrophoresis (MLEE). Selected isolates were used to determine acetylene reduction and competitive ability under greenhouse conditions. The three cultivars tested did not show high variation in N2-fixation, the %Ndfa values ranged from 19 to 26%. Variability in N2-fixation efficiency among various native rhizobial isolates was very high and our results indicate that differences in competitive abilitiy exist also. PCR-RFLP of the 16S rRNA gene and MLEE revealed that most of the isolates belong to the species Rhizobium etli. Intrinsic antibiotic resistance analysis and ERIC-PCR showed high diversity among isolates. In contrast, our results using MLEE show low genetic diversity (H = 0.105).