With more than 30 million doses of rhizobial inoculants marketed per year, it is probable that Brazilian agriculture benefits more than any other country from symbiotic N z fixation. As a result of strain-selection programs, 142 strains of rhizobia are officially recommended for use in commercial inoculants for ninety-six leguminous crops. In this study, sixty-eight of these elite strains were characterized by rep-PCR with the BOX-primer. Reproducibility of the DNA profiles was confirmed, suggesting efficacy of BOX-PCR both for control of quality of inoculants and for preliminary characterization of rhizobial culture collections. Strains of different species never showed similarity higher than 70% in the BOX-PCR analysis, however, some strains of the same species fit into more than one cluster, and correlation between BOX-PCR products and l6S rRNA sequences was low (7.6%). On the other hand, a polyphasic approach -20%:80% of BOX-PCR:16S rRNA which correlated well with the l6S rRNA analysis (95%), and provided higher definition of the genotypes, resulting in clearer indications of the taxonomic groups -might expedite rhizobial diversity studies.
RESUMONeste estudo foram avaliados atributos qualitativos e quantitativos da microbiota do solo, visando monitorar alterações por diferentes manejos do solo e das culturas. As avaliações foram feitas em um ensaio a campo, conduzido há 14 anos em Londrina, PR, sob plantio convencional (PC) ou plantio direto (PD) e com sucessão (S) (soja/trigo) ou rotação (R) (tremoço/milho/aveia-preta/soja/trigo/soja/ trigo/soja) de culturas, quando todos os sistemas estavam com soja no estádio de florescimento pleno. Os incrementos no C e N da biomassa microbiana (CBM e NBM) no PD foram de 114 e 157 %, respectivamente, em comparação ao PC; além disso, o quociente metabólico (qCO 2 ) foi inferior em 37 % no PD, indicando maior eficiência metabólica da microbiota do solo. Não foram detectadas diferenças nesses atributos em função dos sistemas de rotação e sucessão de culturas. A diversidade genética da comunidade bacteriana total do solo foi superior no PD e inferior no PC com sucessão de culturas. Em relação à fixação biológica do N 2 , a massa, o N total e a fração de N-ureídos acumulados na parte aérea e a eficiência dos nódulos em fixar N 2 foram superiores no PD. A diversidade genética dos rizóbios foi afetada, principalmente, pelo manejo das culturas, sendo superior com a rotação, provavelmente pelo maior número de espécies de plantas. Contudo, com a rotação ocorreu decréscimo na eficiência do processo de fixação biológica do N 2 , o que pode estar relacionado com os teores mais elevados de N no solo, ou com a menor
O objetivo deste trabalho foi identificar genes candidatos da subfamília de fatores transcricionais HD-Zip I que contribuem para a tolerância à seca em soja. Foram avaliados trifólios de soja de cultivar tolerante (Embrapa 48) e suscetível à seca (BR 16), sob três níveis de deficit hídrico: ausência, moderado (-1,5 MPa) e severo (-3,0 MPa). Pela análise dos promotores, foi identificada a presença de possíveis elementos cis-regulatórios relacionados à resposta à seca, nos três genes avaliados (GmHB6, GmHB13 e GmHB21). No entanto, não houve padrão de distribuição específico associado à maior tolerância do genótipo à seca. Com a análise comparativa, foram identificados seis elementos cis-regulatórios potencialmente envolvidos na indução da expressão gênica sob seca. O gene GmHB13 foi exclusivamente induzido pela seca no genótipo tolerante, e o gene GmHB6 apresentou redução da expressão somente no genótipo suscetível. Já o gene GmHB21, apresentou aumento da expressão em ambos os genótipos. O gene GmHB13 é um importante elemento na regulação do mecanismo de tolerância à seca em soja, na cultivar tolerante Embrapa 48.
The objective of this work was to assess yield losses due to stalk rot in corn (Zea mays) hybrids, as well as to identify the main pathogenic fungi responsible for the disease during the off-season, in the state of Tocantins, Brazil. Two field experiments were carried out, one with five hybrids, in 2015, and another with four, in 2016. Ears of healthy and stalk rot-infected plants were collected from each plot, at harvest. Ear size, grain and ear weights, and grain moisture content were measured. From each diseased plant, a stalk piece with two to three nodes was removed for pathogen identification and quantification. The most common pathogens were: Fusarium graminearum, Stenocarpella maydis, and Macrophomina phaseolina. All yield parameters were significantly lower in stalk rot-infected plants, with average losses of 30.6%, in 2015, and of 34.3% in 2016. The highest yield losses are observed in hybrids with the highest grain/ear weight ratio.
Artificial drying, although costly, ensures a safe storage and preserves post-harvest grain quality. Alternatively, a more costeffective measure is to delay harvest to promote natural drying, but this management may increase the risk of mycotoxigenic ear rot fungi contamination. The objective was to evaluate the effect of increasing maize harvest delay on fungal disease and fumonisin levels. Three maize hybrids (BRS1035, Attack and DKB390 YG) were sown in the field to study the effect of five harvest delays (15, 30, 45, 60 and 75 days) after the optimum date (18% grain moisture). There was a significant trend of increasing the incidence of kernel rot and total fumonisins in the grains when delaying harvest, but fungal incidence and mycotoxin contamination varied with the hybrid. The main fungi detected in the grain samples were Fusarium verticillioides and Stenocarpella maydis. The hybrid DKB390 YG showed significantly lower incidence of F. verticillioides contamination and lower fumonisin accumulation in the grains than the other hybrids. The hybrid Attack was the least susceptible to kernel rot incidence. Our data shows that delaying harvest for minimizing drying costs may increase the risk of mycotoxin contamination in maize in the tropics of Brazil.
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