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ABSTRACT. The objetives of the study were to evaluate the agronomic efficiency of strains approved as inoculants for cowpea and of three new strains under selection, as well as to evaluate the influence of liming on the symbiosis and productivity. Two field experiments were conducted using cowpea (BRS Guariba) from June to September 2013. In the first experiment, a completely randomized block experimental design was used, with four replicates and seven treatments (INPA03-11B, UFLA03-84, UFLA03-153, UFLA03-154, UFLA03-164 and two controls). In the second experiment, a 7 x 2 factorial design was used, with the above described treatments being the first factor and liming as the second factor. Parameters evaluated were: nodule dry matter, shoot dry matter, shoot N content and accumulation, number of pods per plant, number of seeds per pod, weight of 100 seeds, grain yield and grain N content and accumulation. Strains UFLA03-153 and UFLA03-164 were more efficient for cowpea inoculation than the strains approved as inoculants, UFLA03-84 and INPA03-11B. Although the tested rhizobia strains and cultivar BRS Guariba are tolerant to soil acidity, productivity washigher when soil was limed. Yields obtained with fertilized and inoculated treatments were well above Maranhão state average.Keywords: Vigna unguiculata, Bradyrhizobium, acidity tolerance, efficiency, production system, biology nitrogen fixation.Inoculação com rizóbio e calagem aumentam a produtividade de feijão-caupi no Maranhão RESUMO. Os objetivos do estudo foram avaliar a eficiência agronômica de estirpes aprovadas como inoculantes para feijão-caupi e de três novas estirpes sob seleção, bem como avaliar a influência da calagem sobre a simbiose e a produtividade. Dois experimentos de campo foram conduzidos utilizando feijão-caupi (BRS Guariba) de junho a setembro de 2013. No primeiro experimento, o delineamento experimental utilizado foi em blocos ao acaso, com quatro repetições e sete tratamentos (INPA03-11B, UFLA03-84, UFLA03-153, UFLA03-154, UFLA03-164 e dois controles). No segundo experimento, um fatorial 7 x 2 foi utilizado, sendo o primeiro fator composto pelos tratamentos acima e a calagem o segundo fator. Os parâmetros avaliados foram: matéria seca de nódulos, matéra seca de parte aérea, teor e acúmulo de N na parte aérea, número de vagens por planta, número de grãos por vagem, peso de 100 sementes, rendimento de grãos e teor e acúmulo de N no grão. As estirpes UFLA03-153 e UFLA03-164 foram mais eficientes para a inoculação do feijão-caupi do que as estirpes aprovadas como inoculantes, UFLA03-84 e INPA03-11B. Apesar de as estirpes e da cultivar testadas serem tolerantes à acidez do solo, a produtividade de feijão-caupi foi maior quando o solo recebeu calagem. As produções obtidas com os tratamentos de inoculação fertilizados foram bem acima da média do estado do Maranhão.Palavras-chave: Vigna unguiculata, Bradyrhizobium, tolerância a acidez, eficiência, sistema de produção, fixação biológica de nitrogênio.

One of the most cultivated and consumed vegetables in Brazil is the common bean, Phaseolus vulgaris L. The symbiosis of this plant species with nitrogen-fixing bacteria that are adapted to the stresses commonly found in tropical soils can increase production. The aim of this study was to evaluate the symbiotic effectiveness of bacterial strains from soils under different land uses in the Amazon region. Further, rhizobia tolerance to acidity and aluminium and the involvement of some possible physiological mechanisms of such tolerance were also investigated. In assessing the efficiency of biological nitrogen fixation, inoculation with strains UFLA04-195, UFLA04-173 and UFLA04-202, belonging to the genus Rhizobium, resulted in greater plant growth, higher shoot nitrogen content and good nodulation compared to the inoculation with the strain CIAT 899 (R. tropici), and to the mineral nitrogen control or Burkholderia fungorum strains that nodulated or not bean plants. These efficient strains grew better at pH 5.0 than at pH 6.0 or pH 6.9; they also tolerated up to 1 mmol l(-1) of Al(3+) and showed an increased production of exopolysaccharides where the growing rates were less (pH 6.0 and pH 6.9). With respect to aluminium, the highest production of EPS produced greater tolerance to this element. Taken together, these results indicate that the strains evaluated in this study were tolerant to acidity and aluminium; they appeared to have developed resistance mechanisms such as EPS production and a resistant cell outer membrane (indicated by resistance to polymyxin and methyl violet). As these strains also gave increased yields of the host species, further studies on whether to recommend these strains as inoculants are already underway.

Management of biological nitrogen fixation in common bean still requires improvement. The objective of this study was to verify the compatibility of nitrogen fertilization with biological N 2 fixation to increase common bean yield and profitability. Four field experiments were performed in four municipalities of Minas Gerais, Brazil, during the 2009 and 2010 winter crop season. The 2009 crop experiments were set up under a no-till system and the 2010 crop was conducted using conventional tillage. A randomized block experimental design was used with four replications and seven treatments combining application rates of mineral N (at sowing and/or topdressing) and seed inoculation with Rhizobium tropici strain CIAT899. Inoculation with 20 kg ha -1 N-urea at sowing and seed inoculation does not interfere with nodule dry matter and promotes yield comparable to that observed with 80 kg ha -1 N-urea with economic profitability in both no-till and conventional tillage systems. These results show the possibility of economic savings with respect to N fertilizers, but also a significant ecological contribution by avoiding problems associated with misuse of these fertilizers, such as eutrophication of waterways and denitrification.

Cowpea (Vigna unguiculata) cultivation in northern and northeastern Brazil provides an excellent source of nutrients and carbohydrates for the poor and underprivileged. Production surplus leads to its consumption in other regions of Brazil and also as an export commodity.Its capacity to establish relationships with atmospheric nitrogen-fixing bacteria is crucial to the reduction of production costs and the environmental impact of nitrogen fertilizers. This study assessed the symbiotic efficiency of new strains of symbiotic nitrogen-fixing bacteria with cowpea and their tolerance to pH and aluminum. Twenty-seven strains of bacteria from different soils were evaluated under axenic conditions. These strains were compared to the following inoculant strains: INPA03-11B, UFLA03-84 and BR3267 and two controls that were not inoculated (with and without mineral nitrogen). Six strains and the three strains approved as inoculants were selected to increase the dry weight production of the aerial part (DWAP) and were tested in pots with soil that had a high-density of nitrogen-fixing native rhizobia. In this experiment, three strains (UFLA03-164, UFLA03-153, and UFLA03-154) yielded higher DWAP values. These strains grow at pH levels of 5.0, 6.0, 6.8 and at high aluminum concentration levels, reaching 10 9 CFU mL -1 .In particular UFLA03-84, UFLA03-153, and UFLA03-164 tolerate up to 20 mmol c dm -3 of Al +3.Inoculation with rhizobial strains, that had been carefully selected according to their ability to nodulate and fix N 2 , combined with their ability to compete in soils that are acidic and contain high levels of Al, is a cheaper and more sustainable alternative that can be made available to farmers than mineral fertilizers.

Several processes that promote plant growth were investigated in endophytic and symbiotic bacteria isolated from cowpea and siratro nodules and also in bacterial strains recommended for the inoculation of cowpea beans. The processes verified in 31 strains were: antagonism against phytopathogenic fungi, free-living biological nitrogen fixation, solubilization of insoluble phosphates and indole acetic acid (IAA) production. The resistance to antibiotics was also assessed. Sequencing of the partial 16S rRNA gene was performed and the strains were identified as belonging to different genera. Eight strains, including some identified as Burkholderia fungorum, fixed nitrogen in the free-living state. Eighteen strains exhibited potential to solubilize calcium phosphate, and 13 strains could solubilize aluminum phosphate. High levels of IAA production were recorded with L-tryptophan addition for the strain UFLA04-321 (42.3 μg mL⁻¹). Strains highly efficient in symbiosis with cowpea bean, including strains already approved as inoculants showed the ability to perform other processes that promote plant growth. Besides, these strains exhibited resistance to several antibiotics. The ability of the nitrogen-fixing bacteria to perform other processes and their adaptation to environmental conditions add value to these strains, which could lead to improved inoculants for plant growth and environmental quality.

Resumo -O objetivo deste trabalho foi avaliar o efeito do pH do meio de cultivo na eficiência simbiótica de estirpes de Rhizobium, em solo com e sem calagem. Foram realizados experimentos em casa de vegetação, com e sem calagem, e no campo, apenas com calagem. O delineamento experimental utilizado foi de blocos ao acaso com quatro repetições, e os tratamentos foram cinco estirpes de Rhizobium (UFLA 02-100, UFLA 02-68, UFLA 04-195, UFLA 04-202 e CIAT 899), cultivadas em meio de cultura 79 com diferentes valores de pH (5,0, 6,0 e 6,9), e testemunhas sem inoculação, com ou sem nitrogênio mineral. Foram avaliados número e massa de matéria seca de nódulos, massa de matéria seca da parte aérea, eficiência relativa, teor e acúmulo de nitrogênio na parte aérea e nos grãos, número de vagens por planta, número de grãos por vagem e rendimento e massa de 100 grãos. O valor de pH ideal para o meio de cultivo variou com as estirpes e com a variável analisada. Os tratamentos que receberam calagem foram superiores aos demais. No campo, independentemente do pH do inoculante, as populações nativas do solo e as estirpes introduzidas promoveram rendimento de grãos semelhante ao da testemunha com 70 kg ha -1 de N e ao da estirpe referência CIAT 899.Termos para indexação: Phaseolus vulgaris, Rhizobium, acidez, fixação biológica de nitrogênio, inoculante. Symbiosis of nitrogen fixing bacteria with common bean in different pH valuesAbstract -The objective of this study was to evaluate the effect of the culture medium pH in the symbiotic effectiveness of Rhizobium strains in soil, with and without liming. Assays were performed in greenhouse, with and without liming, and in the field, with liming. The experimental design was randomized blocks with four replicates, and treatments were five strains (UFLA 02-100, UFLA 02-68, UFLA 04-195, UFLA 04-202 and CIAT 899), grown in a 79 culture medium with different pH values (5.0, 6.0 and 6.9), and two controls without inoculation: with or without mineral nitrogen. Variables analyzed were number and weight of dry nodules, shoot dry matter, relative efficiency, nitrogen content and accumulation in shoots and grains, number of pods per plant, number of grains per pod, and yield and weight of 100 grains. The optimum pH value for the culture medium varied with the strains and the variable analyzed. Treatments with liming were superior to the others. In the field, regardless of the inoculant pH, the native soil populations and the introduced strains promoted grain yields similar to those of the control with 70 kg ha -1 of N and to those of the reference strain CIAT 899.

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