ABSTRACT. Agar media with insoluble phosphates are used for semi-quantitative screening of phosphate-solubilizing bacteria based on the solubilization halo formed around the colonies. We show that the volume of the culture medium (15, 20, and 30 mL) and the inoculation method (toothpick vs microdrop) strongly influence the diameter of the solubilization halo, and this should be considered in advance selection of the isolates most efficient in this process.

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 N2, 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.

Microbial solubilisation of low soluble inorganic phosphates is an important process contributing for the phosphorus available to plants in tropical soils. This study evaluates the ability of inoculant strains for tropical legumes to solubilise inorganic phosphates of low solubility that are found in tropical soils. Seven strains of Leguminosae nodulating bacteria (LNB) were compared with one another and with a non-nodulating positive control, Burkholderia cepacia (LMG 1222T). Four of the strains are used as inoculants for cowpeas (Vigna unguiculata) (Bradyrhizobium sp. UFLA 03-84; Bradyrhizobium elkani INPA 03-11B and Bradyrhizobium japonicum BR3267) or for common beans (Phaseolus vulgaris) (Rhizobium tropici CIAT 899T). Rhizobium etli UFLA 02-100 and Rhizobium leguminosarum 316C10a are also efficient nodulators of beans and Cupriavidus taiwanensis LMG 19424T nodulates on Mimosa pudica. Two experiments, with solid and liquid media, were performed to determine whether the strains were able to solubilise CaHPO4, Al(H2PO4)3 or FePO4.2H2O. On solid GELP medium none of the strains dissolved FePO4.2H2O, but LMG 1222, UFLA 03-84 and CIAT 899 solubilised CaHPO4 particularly well. These strains, along with LMG 19424 and BR 3267, were also able to increase the solubility of Al(H2PO4)3. In liquid GELP medium, LMG 1222 solubilised all phosphate sources, but no legume nodulating strain could increase the solubility of Al(H2PO4)3. The strains CIAT 899 and UFLA 02-100 were the only legume nodulating bacteria able to solubilise the other phosphate sources in liquid media, dissolving both CaHPO4 and FePO4.2H2O. There was a negative correlation between the pH of the culture medium and the concentration of soluble phosphate when the phosphorus source was CaHPO4 or FePO4.2H2O. The contribution of these strains to increasing the phosphorus nutrition of legumes and non-legume plant species should be investigated further by in vivo experiments.

O objetivo deste trabalho foi avaliar a qualidade fisiológica de sementes da sempre-viva (Syngonanthus elegans (Bong.) Ruhland) colhidas em diferentes épocas e locais e submetidas ao armazenamento. Capítulos de S. elegans foram colhidos em duas áreas de ocorrência natural no Campus II da Universidade Federal dos Vales do Jequitinhonha e Mucuri - UFVJM, em Diamantina, MG, em um Neossolo Quartzarênico Órtico típico (local 1) e Neossolo Quartzarênico Hidromórfico típico (local 2), em cinco épocas (04/07/05, 19/07/05, 22/08/05, 21/09/05 e 20/10/05) e, posteriormente, submetidos a testes para avaliação do vigor e da germinação da semente (Experimento 1). Os capítulos colhidos em 04/07/05 (locais 1 e 2) foram acondicionados em sacos de polietileno transparente e armazenados no Laboratório de Sementes em temperatura ambiente (20º±2ºC) ou em geladeira (5º±2ºC), por zero, quatro, oito e 12 meses. Após esses períodos, as sementes foram novamente avaliadas quanto ao vigor e à germinação (Experimento 2). O delineamento experimental foi o inteiramente casualizado, no esquema fatorial 5x2 (época x local de colheita) para o Experimento 1; e o esquema fatorial 4x2x2 (armazenamento x local de colheita x temperatura de armazenamento) para o Experimento 2, com quatro repetições. A qualidade fisiológica da semente de S. elegans variou em função da época e do local de colheita. Durante o armazenamento, o vigor e a germinação não variaram com o local de colheita e a temperatura de armazenamento. O armazenamento por 12 meses em geladeira ou temperatura ambiente melhorou a qualidade fisiológica da semente.

The objective of this study was to evaluate the physiological quality of star-flower (Syngonanthus elegans (Bong.) Ruhland) seeds produced at different locations, harvested at different times and then stored under different conditions and for different periods of time. Capitula of S. elegans were colected in two areas of star-flowers plants natural occurrence on the Campus II of the Federal University of the Valleys of Jequitinhonha and Mucuri, in Diamantina, state of Minas Gerais, Brazil, in a typical Orthic Quartzarenicenic Neossoil (location 1) and a typical Hydromorphic Quartzarenic Neossoil (location 2) at 07/04/05, 07/19/05, 08/22/05, 09/21/05, and 10/20/05) and later submitted to germination and vigor tests (Experiment 1). The capitula taken on 07/04/05 (locations 1 and 2) were placed in transparent polyethylene bags and stored either in a seed laboratory at room temperature (20±2ºC) or in a refrigerator (5±2ºC), for zero, four, eight, and 12 months. After these periods, the seeds were evaluated again for germination and vigor (Experiment 2). A completely random design in a 2x5 factorial was used (harvest time and production location) for Experiment 1; and a 4x2x2 factorial scheme (storage time x location x storage temperature) for Experiment 2, with four replications. The physiological quality of S. elegans seeds varied in function of time and location of production. During storage, germination and vigor did not vary with production location and storage temperature. Storage for 12 months in a refrigerator or at room temperature resulted in higher percentages of normally germinating seeds.