A large collection of cowpea rhizobia strains was obtained from soil samples collected from either a semi-arid or a tropical
rain forest area located at about the same latitude in the north-eastern region of Brazil and evaluated for their intrinsic
antibiotic resistance to eight commercial antibiotics. The aim of this study was to correlate antibiotic resistance of native
rhizobia strains to edaphic-climatic factors as a way to establish suitable inoculants for specific areas. A large diversity
regarding intrinsic antibiotic resistance was found, and 17 clusters were identified as varying from sensitive to gradually
resistant up to 500 μg·ml–1 of the antibiotics tested. Clustering analysis did not show any pattern related to the geographic region where isolates have
been obtained. On the other hand, an increase in the antibiotic-resistant rhizobia population was associated with an increase
in soil P and Al contents. lsolates which were sensitive to spectinomycin, ampicillin, streptomycin, chloramphenicol and tetracycline
were present at higher rates in soils devoid of Al. Rhizobia bacteria producing mucus type I (fluid and capable of spreading
over the solid media) were found preferentially in soil with Al concentrations up to 36 mg·kg–1, diminishing quickly at higher levels.
Received: 2 May 1997 相似文献
The Melon yellows virus (MYV), a whitefly-transmitted closterovirus, is one of the major pathogens causing crop losses in protected melons in southeastern Spain. An accession of the wild Asiatic Cucumis melo ssp. agrestis (Cma) shows resistance to MYV infection. Results indicate the participation of two resistance mechanisms in this source: firstly, an antixenotic reaction against Trialeurodes vaporariorum, the disease vector, and secondly, resistance to the virus. The combined effect of these two mechanisms confers Cma a higher level of resistance, expressed as a delayed and milder infection.The genetics of resistance to the Melon yellows closterovirus have been studied in two families derived from Cma. As under natural infection conditions, the effect of antixenosis and virus resistance cannot be distinguished, a biometrical model that permits separation of the two resistance mechanisms operating in the same resistant source, has been proposed to determine genetic control of MYV resistance.The genetic analysis has been conducted by fitting the disease progress curves of each generation to the biometrical model instead of fitting the final disease ratios. The scoring of disease incidence over time allows for the comparison of data from assays conducted in different conditions (2 years/4 transplanting dates), thus reinforcing the analysis.The results agree with a simple control of the resistance to MYV derived from Cma, with incomplete penetrance of the gene and partial dominance of resistance. The effect of antixenosis on the spread of this plant virus is highly significant in Cma, but not in segregant generations.Since there do not exist crossability barriers between this accession and the cultivated melon, Cma could be readily used in breeding programmes to obtain melon varieties resistant to MYV. 相似文献
Plant growth is directly affected by soil water, soil aeration, and soil resistance to root penetration. The least limiting water range (LLWR) is defined as the range in soil water content within which limitations to plant growth associated with water potential, aeration and soil resistance to root penetration are minimal. The LLWR has not been evaluated in tropical soils. Thus, the objective of the present study was to evaluate the LLWR in a Brazilian clay Oxisol (Typic Hapludox) cropped with maize (Zea mays L. cv. Cargil 701) under no-tillage and conventional tillage. Ninety-six undisturbed soil samples were obtained from maize rows and between rows and used to determine the water retention curve, the soil resistance curve and bulk density. The results demonstrated that LLWR was higher in conventional tillage than in no-tillage and was negatively correlated with bulk density for values above 1.02 g cm−3. The range of LLWR variation was 0–0.1184 cm3 cm−3 in both systems, with mean values of 0.0785 cm3 cm−3 for no-tillage and 0.0964 cm3 cm−3 for conventional tillage. Soil resistance to root penetration determined the lower limit of LLWR in 89% of the samples in no-tillage and in 46% of the samples in conventional tillage. Additional evaluations of LLWR are needed under different texture and management conditions in tropical soils. 相似文献
Juveniles of the Caribbean scallops Argopecten nucleus and Nodipecten nodosus were suspended in pearl nets at densities of 25%, 30% and 50% coverage of the bottom of the net (BC) during days. Additionally, adults of A. nucleus (>35 mm) were placed in lantern nets at densities of 40%, 50% and 65% BC during days. Stocking density affected the growth of animals, with higher growth rates obtained at the lower densities. Although the survival of the two scallops was not influenced at the densities studied, the 15-day net changes, and culture in water of <28 °C produced better survival of A. nucleus and N. nodosus respectively. Predators of the families Cymatiidae, Grapsidae, Portunidae, Majidae and Xanthidae were found in the culture nets of juvenile A. nucleus , but during the adult phase they were absent. Predators exhibited a dense-independent behaviour in relation to scallops. There was not any association between predator frequency and survival of A. nucleus . 相似文献
The present study was conducted to define a methodology to produce and store small‐scale microalgae paste to be used in a mollusk hatchery. Microalgae were cultured in 500 L fiberglass tanks, under temperature of 20 ± 2 C, Guillard f/2 culture medium, and continuous light intensity of 203–226 μmol photons/m2/sec. Cultures were centrifuged at 2000 g at the exponential growth phase. Microalgae cell quality after centrifugation and during storage was determined by analyses with Evan’s blue stain and by counting the number of total marine bacteria. Treatments with and without additive were applied to the microalgae paste produced, which was distributed into 100 mL plastic containers, capped, and stored under refrigeration at 4 ± 1 C. Results indicated that in the Chaetoceros muelleri paste, centrifugation did not damage the cells and the number of total marine bacteria reduced significantly from 2.9 × 106 to 8.3 × 105 colony‐forming units per milliliter. Chaetoceros muelleri and Chaetoceros calcitrans pastes stored with addition of 0.1% ascorbic acid had a shelf life shorter than 2 wk. For the treatment without additive, results with Evan’s blue stain showed that cells (99%) remained viable until the sixth week of storage for C. muelleri and seventh week of storage for Skeletonema sp. and C. calcitrans. The number of bacteria did not increase during storage for C. calcitrans and Skeletonema (P > 0.05). For C. muelleri, an increase in bacteria (P < 0.05) was observed after the sixth week of storage. This study demonstrated the feasibility to produce and store microalgae paste for a period of 2–8 wk, which allows it to be used as food source and also optimizes the use of microalgae cultured in laboratory. 相似文献
This study investigates the effects of surface liming on soil attenuation radiation properties. For this, measurements of soil chemical attributes (pH, organic carbon, H+Al, Al3+, Ca2+, and Mg2+) and attenuation radiation parameters (mass attenuation coefficient, μm, atomic and electronic cross sections, σa and σe, effective atomic number and electron density, Zeff and Nel) were carried out. This aim was motivated by the fact that possible μm variation might cause as well variation in the determination of soil physical properties.
Materials and methods
The studied soil, classified as a Dystrudept sity-clay, is located in South Brazil. The trial consisted of five stripes, one of them under pasture and the remaining under no-till system (NTS). Lime rates of 0, 10, 15, and 20 t ha?1 were broadcast on the NTS soil surface. Disturbed soil samples were collected 30 months after liming at the top (0–10 cm) and subsoil (10–20 cm) layers. Soil chemical attributes were characterized following standard experimental procedures. The soil oxide composition, obtained by EDXRF analysis, was used to calculate μm for 241Am and 137Cs photon energies with XCOM computer code. μm values were employed to calculate σa, σe, Zeff, and Nel and to predict variations in soil bulk density (ρ) and total porosity (φ).
Results and discussion
Surface liming notably increased contents of soil pH, Ca2+, and Mg2+ while reduced H+Al and Al3+ at the top soil layer, where μm, σa, σe, and Zeff were also increased with the lime rates. However, at the subsoil layer, liming neither lessened soil acidity nor induced remarkable changes in the attenuation parameters. When using 137Cs photon energy, incoherent scattering totally dominated over the radiation interaction processes whereas photoelectric absorption and coherent scattering substantially contributed when 241Am photon energy was used. Therefore, the increasing in soil attenuation parameters at the top soil layer was more accentuated considering 241Am than 137Cs photon energy. Variation in μm caused considerable variation in ρ and φ only for 241Am photon energy.
Conclusions
The findings regarding the effect of μm variation induced by liming on the determination of soil physical properties are extremely relevant because traditionally, in the soil science area, μm values are calculated without considering any chemical modification to which the soil can be submitted. Bearing in mind that ρ and φ are important parameters from the agricultural and environmental points of view, not representative measurements of μm can lead to biased values of ρ and φ.
Powdery mildew symptoms and signs were observed on seedlings of Lagerstroemia speciosa within an ornamental plant nursery in Viçosa, State of Minas Gerais, Brazil. Signs were most prominent as intense mycelial growth and sporulation on the shoots, new branches and leaves, which led to tissue necrosis. Based on morphological characteristics and phylogenetic analyses of ITS and 28S rDNA sequences, the powdery mildew pathogen of L. speciosa was identified as Erysiphe australiana, a well‐known powdery mildew pathogen infecting Lagerstroemia spp. worldwide. To our knowledge, this is the first report of E. australiana causing powdery mildew on L. speciosa in Brazil. 相似文献